Interacting Effects of Photoperiod and Photosynthetic Photon Flux on ...
Interacting Effects of Photoperiod and Photosynthetic Photon Flux on ...
Interacting Effects of Photoperiod and Photosynthetic Photon Flux on ...
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J. AMER. SOC. HORT. SCI. 121(2):264–268. 1996.<br />
<str<strong>on</strong>g>Interacting</str<strong>on</strong>g> <str<strong>on</strong>g>Effects</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>Photoperiod</str<strong>on</strong>g> <str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>Photosynthetic</str<strong>on</strong>g><br />
<str<strong>on</strong>g>Phot<strong>on</strong></str<strong>on</strong>g> <str<strong>on</strong>g>Flux</str<strong>on</strong>g> <strong>on</strong> Net Carb<strong>on</strong> Assimilati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> Starch<br />
Accumulati<strong>on</strong> in Potato Leaves<br />
Gary W. Stutte <str<strong>on</strong>g>and</str<strong>on</strong>g> Neil C. Yorio<br />
Dynamac Corporati<strong>on</strong>, Mail Code DYN-3, Kennedy Space Center, FL 32899<br />
Raym<strong>on</strong>d M. Wheeler<br />
Office <str<strong>on</strong>g>of</str<strong>on</strong>g> Biomedical Operati<strong>on</strong>s, The Nati<strong>on</strong>al Aer<strong>on</strong>autics <str<strong>on</strong>g>and</str<strong>on</strong>g> Space Administrati<strong>on</strong>, Kennedy Space<br />
Center, FL 32899<br />
Additi<strong>on</strong>al index words. advanced life support, c<strong>on</strong>trolled ecological life support system, CELSS, carbohydrate metabolism,<br />
c<strong>on</strong>trolled envir<strong>on</strong>ment, photosynthesis, Solanum tuberosum<br />
Abstract. The effect <str<strong>on</strong>g>of</str<strong>on</strong>g> photoperiod (PP) <strong>on</strong> net carb<strong>on</strong> assimilati<strong>on</strong> rate (A ) <str<strong>on</strong>g>and</str<strong>on</strong>g> starch accumulati<strong>on</strong> in newly mature<br />
net<br />
canopy leaves <str<strong>on</strong>g>of</str<strong>on</strong>g> ‘Norl<str<strong>on</strong>g>and</str<strong>on</strong>g>’ potato (Solanum tuberosum L.) was determined under high (412 ∝mol·m –2 ·s –1 ) <str<strong>on</strong>g>and</str<strong>on</strong>g> low ( 263<br />
∝mol·m –2 ·s –1 ) photosynthetic phot<strong>on</strong> flux (PPF) c<strong>on</strong>diti<strong>on</strong>s. The A decreased from 13.9 to 11.6 <str<strong>on</strong>g>and</str<strong>on</strong>g> 9.3 μmol·m net –2 ·s –1 , <str<strong>on</strong>g>and</str<strong>on</strong>g><br />
leaf starch increased from 70 to 129 <str<strong>on</strong>g>and</str<strong>on</strong>g> 118 mg·g –1 drymass (DM) as photoperiod (PP) was increased from 12/12 to 18/<br />
6, <str<strong>on</strong>g>and</str<strong>on</strong>g> 24/0, respectively. L<strong>on</strong>ger PP had a greater effect with high PPF c<strong>on</strong>diti<strong>on</strong>s than with low PPF treatments, with high<br />
PPF showing greater decline in A . <str<strong>on</strong>g>Photoperiod</str<strong>on</strong>g> did not affect either the CO compensati<strong>on</strong> point (50 μmol·mol net 2 –1 ) or CO2 saturati<strong>on</strong> point (1100–1200 μmol·mol –1 ) for A . These results show an apparent limit to the amount <str<strong>on</strong>g>of</str<strong>on</strong>g> starch that can<br />
net<br />
be stored (≈15% DM) in potato leaves. An apparent feedback mechanism exists for regulating A under high PPF, high<br />
net<br />
CO <str<strong>on</strong>g>and</str<strong>on</strong>g> l<strong>on</strong>g PP, but there was no correlati<strong>on</strong> between A <str<strong>on</strong>g>and</str<strong>on</strong>g> starch c<strong>on</strong>centrati<strong>on</strong> in individual leaves. This suggests<br />
2, net<br />
that maximum A cannot be sustained with elevated CO c<strong>on</strong>diti<strong>on</strong>s under l<strong>on</strong>g PP (≥12 hours) <str<strong>on</strong>g>and</str<strong>on</strong>g> high PPF c<strong>on</strong>diti<strong>on</strong>s.<br />
net 2<br />
If a physiological limit exists for the fixati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> transport <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong>, then increasing photoperiod <str<strong>on</strong>g>and</str<strong>on</strong>g> light intensity under<br />
high CO c<strong>on</strong>diti<strong>on</strong>s is not the most appropriate means to maximize the yield <str<strong>on</strong>g>of</str<strong>on</strong>g> potatoes.<br />
2<br />
L<strong>on</strong>g durati<strong>on</strong> space missi<strong>on</strong>s will likely require the use <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
bioregenerative life-support systems to generate oxygen, purify<br />
water, remove carb<strong>on</strong> dioxide, produce food, <str<strong>on</strong>g>and</str<strong>on</strong>g> recycle waste<br />
materials (MacElroy <str<strong>on</strong>g>and</str<strong>on</strong>g> Bredt, 1985; Ols<strong>on</strong> et al., 1988; Salisbury<br />
<str<strong>on</strong>g>and</str<strong>on</strong>g> Bugbee, 1988). Potatoes, which have been identified as a<br />
c<str<strong>on</strong>g>and</str<strong>on</strong>g>idate crop for inclusi<strong>on</strong> in a c<strong>on</strong>trolled ecological advanced<br />
life-support system (CELSS), have been the subject <str<strong>on</strong>g>of</str<strong>on</strong>g> study for<br />
several years (Tibbitts et al., 1993). Within a CELSS, total irradiance<br />
has been suggested to be the largest limitati<strong>on</strong> to crop<br />
productivity (Salisbury <str<strong>on</strong>g>and</str<strong>on</strong>g> Bugbee, 1988; Wheeler <str<strong>on</strong>g>and</str<strong>on</strong>g> Tibbitts,<br />
1986a, 1986b). We can expect to improve yield by increasing the<br />
net daily integral <str<strong>on</strong>g>of</str<strong>on</strong>g> photosynthetically active radiati<strong>on</strong> (PAR)<br />
through higher irradiance or l<strong>on</strong>ger photoperiod.<br />
It is generally accepted that l<strong>on</strong>g photoperiods inhibit tuber<br />
initiati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> promote potato shoot growth (Gregory, 1965).<br />
However, it has been reported that tuber initiati<strong>on</strong> is inhibited <strong>on</strong>ly<br />
at low light levels (Tibbitts et al., 1993; Wheeler et al., 1991).<br />
Therefore, yields are typically higher for field grown plants produced<br />
under l<strong>on</strong>g, rather than short photoperiods with the relatively<br />
high total irradiance received under l<strong>on</strong>g day c<strong>on</strong>diti<strong>on</strong>s<br />
(Wheeler <str<strong>on</strong>g>and</str<strong>on</strong>g> Tibbitts, 1986b) .<br />
Since the photoperiod resp<strong>on</strong>se <str<strong>on</strong>g>of</str<strong>on</strong>g> potato tuberizati<strong>on</strong> was<br />
initially reported (Auchter <str<strong>on</strong>g>and</str<strong>on</strong>g> Harley, 1924; Garner <str<strong>on</strong>g>and</str<strong>on</strong>g> Allard,<br />
1923), a role for starch in the regulati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> photosynthesis, <str<strong>on</strong>g>and</str<strong>on</strong>g> thus<br />
yield, has been postulated. Although subject to intense investiga-<br />
Received for publicati<strong>on</strong> 8 May 1995. Accepted for publicati<strong>on</strong> 19 Sept. 1995. This<br />
research was c<strong>on</strong>ducted under the auspices <str<strong>on</strong>g>of</str<strong>on</strong>g> Biomedical Operati<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> Research<br />
Office, John F. Kennedy Space Center, Fla. We wish to thank Elizabeth Stryjewski,<br />
Cheryl Mackowiak, <str<strong>on</strong>g>and</str<strong>on</strong>g> Lisa Ruffe <str<strong>on</strong>g>of</str<strong>on</strong>g> the Dynamac corporati<strong>on</strong> for their support<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> this project. Menti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a trademark, proprietary product or vendor does not<br />
c<strong>on</strong>stitute an endorsement <str<strong>on</strong>g>of</str<strong>on</strong>g> the product by either The Dynamac Corporati<strong>on</strong>, or<br />
NASA. The cost <str<strong>on</strong>g>of</str<strong>on</strong>g> publishing this paper was defrayed in part by the payment <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
page charges. Under postal regulati<strong>on</strong>s, this paper therefore must be hereby marked<br />
advertisement solely to indicate this fact.<br />
ti<strong>on</strong>, the effects <str<strong>on</strong>g>of</str<strong>on</strong>g> irradiance, CO 2 c<strong>on</strong>centrati<strong>on</strong>, <str<strong>on</strong>g>and</str<strong>on</strong>g> temperature<br />
<strong>on</strong> starch accumulati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> photosynthetic acti<strong>on</strong> have not been<br />
fully explained at the whole-plant level (Geiger <str<strong>on</strong>g>and</str<strong>on</strong>g> F<strong>on</strong>dy, 1991;<br />
Herold, 1980; Neales <str<strong>on</strong>g>and</str<strong>on</strong>g> Incoll, 1968).<br />
Leaf necrosis has been reported in potato exposed to c<strong>on</strong>tinuous<br />
light (Wheeler <str<strong>on</strong>g>and</str<strong>on</strong>g> Tibbitts, 1986b). Cao <str<strong>on</strong>g>and</str<strong>on</strong>g> Tibbitts (1991)<br />
reported that starch was not correlated to c<strong>on</strong>tinuous light damage<br />
in susceptible cultivars. Starch accumulati<strong>on</strong> in potato leaves<br />
(Lorenzen <str<strong>on</strong>g>and</str<strong>on</strong>g> Ewing, 1992) was greater under l<strong>on</strong>g photoperiods<br />
than short photoperiods, even with the presence <str<strong>on</strong>g>of</str<strong>on</strong>g> a str<strong>on</strong>g sink<br />
(e.g., exp<str<strong>on</strong>g>and</str<strong>on</strong>g>ing tubers). Although yield enhancement under l<strong>on</strong>g<br />
days is generally assumed to be related to greater carb<strong>on</strong> assimilati<strong>on</strong>,<br />
there are few reports <str<strong>on</strong>g>of</str<strong>on</strong>g> the effect <str<strong>on</strong>g>of</str<strong>on</strong>g> photoperiod <strong>on</strong> net carb<strong>on</strong><br />
assimilati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> potato leaves.<br />
Our objectives were to establish whether l<strong>on</strong>g photoperiod at<br />
different PPF levels would affect net carb<strong>on</strong> assimilati<strong>on</strong> rate (A net )<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> potato leaves, <str<strong>on</strong>g>and</str<strong>on</strong>g> to determine whether photoperiod affects the<br />
c<strong>on</strong>centrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> starch in the leaves.<br />
Materials <str<strong>on</strong>g>and</str<strong>on</strong>g> Methods<br />
Plant growing c<strong>on</strong>diti<strong>on</strong>s. ‘Norl<str<strong>on</strong>g>and</str<strong>on</strong>g>’ potato (Solanum tuberosum),<br />
an early maturing, c<strong>on</strong>tinuous light tolerant cultivar was used in this<br />
experiment. Plantlets were initiated from in vitro nodal culture <str<strong>on</strong>g>and</str<strong>on</strong>g><br />
grown using a recirculating nutrient film technique (NFT) with a<br />
modified half strength Hoagl<str<strong>on</strong>g>and</str<strong>on</strong>g>’s soluti<strong>on</strong> (Wheeler et al., 1990).<br />
Plant growth chambers (model M12; EGC, Chagrin Falls, Ohio)<br />
were c<strong>on</strong>figured to provide either 600 μmol·m –2 ·s –1 PPF (High Light,<br />
HL) or 300 μmol·m –2 ·s –1 PPF (Low Light, LL) with four 400 W metal<br />
halide lamps (Pro-Arc, Venture Lighting, Clevel<str<strong>on</strong>g>and</str<strong>on</strong>g>, Ohio). The<br />
mean PPF valves at canopy level during testing periods are provided<br />
in the results.<br />
Envir<strong>on</strong>mental c<strong>on</strong>diti<strong>on</strong>s in each plant growth chamber c<strong>on</strong>sisted<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> an initial 12 h light/12 h dark (12/12) photoperiod (PP)<br />
264 J. AMER. SOC. HORT. SCI. 121(2):264–268. 1996.
with a matching thermoperiod <str<strong>on</strong>g>of</str<strong>on</strong>g> 20/16 ± 0.2C <str<strong>on</strong>g>and</str<strong>on</strong>g> c<strong>on</strong>stant<br />
relative humidity <str<strong>on</strong>g>of</str<strong>on</strong>g> 65% ± 6%. Atmospheric CO 2 c<strong>on</strong>centrati<strong>on</strong>s<br />
were maintained at 1200 ± 10 μmol·mol –1 with an infrared gas<br />
analyzer (Anarad AR600, Santa Barbara, Calif.) <str<strong>on</strong>g>and</str<strong>on</strong>g> dedicated<br />
computer c<strong>on</strong>trol system. Daily records <str<strong>on</strong>g>of</str<strong>on</strong>g> water use, acid additi<strong>on</strong>s<br />
for pH c<strong>on</strong>trol, <str<strong>on</strong>g>and</str<strong>on</strong>g> nutrient replenishment soluti<strong>on</strong> additi<strong>on</strong>s<br />
were maintained. Weekly measures <str<strong>on</strong>g>of</str<strong>on</strong>g> photosynthetic phot<strong>on</strong> flux<br />
(PPF) at the plant canopy were taken. At harvest, plants were<br />
separated into shoots (leaves <str<strong>on</strong>g>and</str<strong>on</strong>g> stems), roots, <str<strong>on</strong>g>and</str<strong>on</strong>g> tubers. Fresh<br />
mass (FM) <str<strong>on</strong>g>of</str<strong>on</strong>g> tissues was recorded before oven drying for 72 h at<br />
70C for dry mass (DM) determinati<strong>on</strong>.<br />
<str<strong>on</strong>g>Photoperiod</str<strong>on</strong>g> treatments. At 56 days after planting (DAP), the<br />
photoperiod was changed from 12/12 to 18 h light/6 h dark (18/6).<br />
The plants were allowed to adapt to the photoperiod for 36 h before<br />
gas exchange <str<strong>on</strong>g>and</str<strong>on</strong>g> starch measurements were determined. At 63<br />
DAP the photoperiod was changed to 24 h light/0 h dark (24/0).<br />
Following each photoperiod adjustment, the photoperiod was<br />
cycled to 12/12. It is recognized that the age <str<strong>on</strong>g>of</str<strong>on</strong>g> the plant was<br />
different during each photoperiod treatment, but care was taken to<br />
sample leaves <str<strong>on</strong>g>of</str<strong>on</strong>g> similar phylogenetic age.<br />
<str<strong>on</strong>g>Photosynthetic</str<strong>on</strong>g> measurements. Diurnal measurements <str<strong>on</strong>g>of</str<strong>on</strong>g> the<br />
net CO 2 assimilati<strong>on</strong> rate (A net ) were performed <strong>on</strong> recently mature<br />
single leaves in the upper canopy using a closed leaf cuvette<br />
photosynthesis system (Model LI-6200; Li-Cor, Lincoln, Neb.)<br />
The A net vs. CO 2 internal (A Ci ) curves <str<strong>on</strong>g>and</str<strong>on</strong>g> CO 2 compensati<strong>on</strong> point<br />
were determined at about 1 h after lights came <strong>on</strong> for each <str<strong>on</strong>g>of</str<strong>on</strong>g> the<br />
photoperiod tests (McDermitt et al, 1988). Fresh mass (FM), leaf<br />
area (LA), <str<strong>on</strong>g>and</str<strong>on</strong>g> dry mass (DM) were obtained for the porti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
leaves measured during the carb<strong>on</strong> exchange rate (CER) tests. This<br />
tissue was subsequently used for starch analysis.<br />
Starch analysis. Starch c<strong>on</strong>centrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> potato leaves used for<br />
diurnal photosynthetic measurements were analyzed using a modificati<strong>on</strong><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> the method <str<strong>on</strong>g>of</str<strong>on</strong>g> Wang <str<strong>on</strong>g>and</str<strong>on</strong>g> Stutte (1992). Oven-dried<br />
tissue was ground with a mortar <str<strong>on</strong>g>and</str<strong>on</strong>g> pestle, <str<strong>on</strong>g>and</str<strong>on</strong>g> samples (20–100<br />
mg) were extracted three times for 10 min each in boiling 80%<br />
ethanol to remove soluble sugars <str<strong>on</strong>g>and</str<strong>on</strong>g> chlorophyll. The samples<br />
were centrifuged, <str<strong>on</strong>g>and</str<strong>on</strong>g> pellets were resuspended in 0.1 M acetate<br />
buffer, pH 4.6, <str<strong>on</strong>g>and</str<strong>on</strong>g> boiled for 10 min to swell <str<strong>on</strong>g>and</str<strong>on</strong>g> gelatinize the<br />
pellets. St<str<strong>on</strong>g>and</str<strong>on</strong>g>ards <str<strong>on</strong>g>of</str<strong>on</strong>g> potato starch (Sigma Chemical Co., St. Louis)<br />
up to 4.0 mg were run with the samples. To digest the starch,<br />
amyloglucosidase was added to samples <str<strong>on</strong>g>and</str<strong>on</strong>g> st<str<strong>on</strong>g>and</str<strong>on</strong>g>ards <str<strong>on</strong>g>and</str<strong>on</strong>g> incubated<br />
in a water bath at 55C for 30 min. The supernatant was<br />
assayed colorimetrically for c<strong>on</strong>versi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> starch to glucose using<br />
J. AMER. SOC. HORT. SCI. 121(2):264–268. 1996.<br />
glucose oxidase (Sigma Diagnostics, St. Louis). When original<br />
digests yielded starch c<strong>on</strong>centrati<strong>on</strong>s greater than 4.0 mg/sample,<br />
the procedure was repeated <strong>on</strong> the undigested pellet <str<strong>on</strong>g>and</str<strong>on</strong>g> starch<br />
c<strong>on</strong>centrati<strong>on</strong>s from multiple digests were summed together. The<br />
final starch c<strong>on</strong>centrati<strong>on</strong> per sample was expressed as mg starch/<br />
g leaf DM or mg starch/cm 2 LA.<br />
Results<br />
Net carb<strong>on</strong> assimilati<strong>on</strong><br />
Light intensity. As expected, A net <str<strong>on</strong>g>of</str<strong>on</strong>g> recently mature leaves<br />
grown under HL c<strong>on</strong>diti<strong>on</strong>s was higher than A net <str<strong>on</strong>g>of</str<strong>on</strong>g> leaves grown<br />
under LL c<strong>on</strong>diti<strong>on</strong>s (Table 1). The dark period respirati<strong>on</strong> rate<br />
(Rs) <str<strong>on</strong>g>of</str<strong>on</strong>g> the HL leaves was higher than Rs <str<strong>on</strong>g>of</str<strong>on</strong>g> the LL leaves. The<br />
greater Rs <str<strong>on</strong>g>of</str<strong>on</strong>g> the HL treatment may partially result from the greater<br />
specific leaf weight (SLW) <str<strong>on</strong>g>of</str<strong>on</strong>g> the HL than LL leaves.<br />
<str<strong>on</strong>g>Photoperiod</str<strong>on</strong>g>. Changing the PP from 12/12 to 18/6 resulted in a<br />
16% decrease in the instantaneous A net <str<strong>on</strong>g>and</str<strong>on</strong>g> a 23% decrease in Rs.<br />
Exposing the plants to c<strong>on</strong>tinuous lighting c<strong>on</strong>diti<strong>on</strong>s for 36 h<br />
resulted in a 33% decrease from 12/12 in A net (Table 1). Rs was not<br />
measured for the 24/0 treatment.<br />
Diurnal resp<strong>on</strong>se. The A net <str<strong>on</strong>g>of</str<strong>on</strong>g> recently mature leaves did not vary<br />
significantly during the light cycle in either the 12/12 or 18/6<br />
photoperiod treatments (Fig. 1). In c<strong>on</strong>trast, variati<strong>on</strong> in A net increased<br />
when leaves were exposed to c<strong>on</strong>tinuous lighting c<strong>on</strong>diti<strong>on</strong>s.<br />
Under c<strong>on</strong>tinuous illuminati<strong>on</strong>, individual leaves began to exhibit<br />
morphological changes, including localized chlorosis <str<strong>on</strong>g>and</str<strong>on</strong>g> necrosis,<br />
which could result in increased variati<strong>on</strong> in A net between leaves. In<br />
additi<strong>on</strong>, the leaves exhibited slight epinasty <str<strong>on</strong>g>and</str<strong>on</strong>g> the accumulati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
anthocyanin <strong>on</strong> the abaxial surface <str<strong>on</strong>g>of</str<strong>on</strong>g> the leaf. The morphological<br />
resp<strong>on</strong>ses were similar to the c<strong>on</strong>tinuous illuminati<strong>on</strong> injury described<br />
by Cao <str<strong>on</strong>g>and</str<strong>on</strong>g> Tibbitts (1991). The A net values <str<strong>on</strong>g>of</str<strong>on</strong>g> leaves grown<br />
under HL c<strong>on</strong>diti<strong>on</strong>s were comparable to A net values previously<br />
reported for plants <str<strong>on</strong>g>of</str<strong>on</strong>g> comparable ages <str<strong>on</strong>g>and</str<strong>on</strong>g> grown under similar<br />
envir<strong>on</strong>mental c<strong>on</strong>diti<strong>on</strong>s (Dwelle et al., 1983).<br />
CO 2 compensati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> saturati<strong>on</strong>. CO 2 resp<strong>on</strong>se curves were<br />
determined for each <str<strong>on</strong>g>of</str<strong>on</strong>g> the treatments <str<strong>on</strong>g>and</str<strong>on</strong>g> normalized to 400<br />
μmol·m –2 ·s –1 PPF based <strong>on</strong> PPF resp<strong>on</strong>se curves (Fig. 2). The light<br />
resp<strong>on</strong>se curves obtained from individual leaves used in these<br />
studies was linear (y = 0.0436x – 3.826; r 2 = 0.961) over the PPF<br />
range <str<strong>on</strong>g>of</str<strong>on</strong>g> 65 to 650 μmol·m –2 ·s –1 . A net values <str<strong>on</strong>g>of</str<strong>on</strong>g> the HL treatment<br />
were greater than the LL treatment at 12/12. The HL <str<strong>on</strong>g>and</str<strong>on</strong>g> LL<br />
treatment A net rates were similar at 18/6 <str<strong>on</strong>g>and</str<strong>on</strong>g> A net <str<strong>on</strong>g>of</str<strong>on</strong>g> HL treatments<br />
Table 1. <str<strong>on</strong>g>Effects</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> photosynthetic phot<strong>on</strong> flux (PPF) <str<strong>on</strong>g>and</str<strong>on</strong>g> photoperiod <strong>on</strong> net carb<strong>on</strong><br />
assimilati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> starch accumulati<strong>on</strong> in recently mature ‘Norl<str<strong>on</strong>g>and</str<strong>on</strong>g>’ potato leaves. Values<br />
represent the mean <str<strong>on</strong>g>of</str<strong>on</strong>g> all measurements taken during either the light or dark cycle for a<br />
respective treatment.<br />
Carb<strong>on</strong> assimilati<strong>on</strong> Starch c<strong>on</strong>cn<br />
(μmol CO 2 /m 2 per sec) (mg·g –1 DM)<br />
Treatment Light cycle Dark cycle<br />
PPF<br />
Light cycle Dark cycle<br />
Low lightz 9.92 ± 0.37y –0.86 ± 0.14 110.32 ± 5.87 83.50 ± 15.21<br />
High light 12.57 ± 0.69 –1.29 ± 0.10<br />
<str<strong>on</strong>g>Photoperiod</str<strong>on</strong>g><br />
105.31 ± 5.50 90.03 ± 10.90<br />
12/12 13.87 ± 0.87 –1.17 ± 0.05 70.02 ± 8.46 64.10 ± 7.02<br />
18/6 11.63 ± 0.68 –0.90 ± 0.18 129.23 ± 2.69 132.08 ± 8.06<br />
24/0 9.27 ± 0.38 --- x<br />
117.57 ± 3.21 ---<br />
z Low light = 263 ( ± 4.3) μmol·m –2 ·s –1 PPF <str<strong>on</strong>g>and</str<strong>on</strong>g> high light = 412 ( ±10.2) μmol·m –2 ·s –1 PPF<br />
at canopy level.<br />
y Mean ± st<str<strong>on</strong>g>and</str<strong>on</strong>g>ard error.<br />
x No dark period.<br />
265
Fig. 1. Effect <str<strong>on</strong>g>of</str<strong>on</strong>g> PPF <strong>on</strong> diurnal changes in net carb<strong>on</strong> assimati<strong>on</strong> rate <str<strong>on</strong>g>of</str<strong>on</strong>g> newly<br />
mature ‘Norl<str<strong>on</strong>g>and</str<strong>on</strong>g>’ potato leaves exposed to different photoperiods. The mean<br />
PPF values were 263 <str<strong>on</strong>g>and</str<strong>on</strong>g> 412 μmol·m –2 ·s –1 in the low <str<strong>on</strong>g>and</str<strong>on</strong>g> high light chambers,<br />
respectively.<br />
was less than LL at 24/0 PP (Fig. 2). The CO 2 compensati<strong>on</strong> point<br />
was about 50 μmol·mol –1 <str<strong>on</strong>g>and</str<strong>on</strong>g> the CO 2 saturati<strong>on</strong> point was<br />
between 1100 <str<strong>on</strong>g>and</str<strong>on</strong>g> 1200 μmol·mol –1 , irrespective <str<strong>on</strong>g>of</str<strong>on</strong>g> light intensity<br />
or photoperiod treatment. The CO 2 resp<strong>on</strong>se curves were c<strong>on</strong>sistent<br />
with A net values obtained during the diurnal sampling (Table<br />
1).<br />
Starch analysis<br />
Light intensity. The mean leaf starch c<strong>on</strong>centrati<strong>on</strong> during<br />
either the light or dark cycles was not affected by the light intensity<br />
treatment (Table 1).<br />
<str<strong>on</strong>g>Photoperiod</str<strong>on</strong>g>. Increasing the PP from 12/12 to 18/6 significantly<br />
increased the leaf starch c<strong>on</strong>centrati<strong>on</strong> during the light (84%) <str<strong>on</strong>g>and</str<strong>on</strong>g><br />
dark (106%) cycles. Increasing the PP from 12/12 to 24/0 resulted<br />
in a similar increase (68%) over 12/12 in leaf starch c<strong>on</strong>centrati<strong>on</strong><br />
during the light cycle (Table 1).<br />
Diurnal resp<strong>on</strong>se. Under 12/12 PP, diurnal cycling <str<strong>on</strong>g>of</str<strong>on</strong>g> leaf starch<br />
c<strong>on</strong>centrati<strong>on</strong> was observed, with accumulati<strong>on</strong> occurring during the<br />
light cycle, <str<strong>on</strong>g>and</str<strong>on</strong>g> remobilizati<strong>on</strong> occurring during the dark cycle (Fig.<br />
3). With the 18/6 PP, c<strong>on</strong>centrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> leaf starch at the end <str<strong>on</strong>g>of</str<strong>on</strong>g> the dark<br />
Fig. 2. Effect <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 c<strong>on</strong>centrati<strong>on</strong> <strong>on</strong> net carb<strong>on</strong> assimilati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> newly mature<br />
‘Norl<str<strong>on</strong>g>and</str<strong>on</strong>g>’ potato leaves exposed to different photoperiods. The mean PPF values<br />
were 263 <str<strong>on</strong>g>and</str<strong>on</strong>g> 412 μmol·m –2 ·s –1 in the low <str<strong>on</strong>g>and</str<strong>on</strong>g> high light chambers, respectively.<br />
cycle increased as the length <str<strong>on</strong>g>of</str<strong>on</strong>g> the dark cycle decreased, although the<br />
change was small. Under c<strong>on</strong>tinuous lighting, the c<strong>on</strong>centrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
starch in the leaves was about 12% <str<strong>on</strong>g>of</str<strong>on</strong>g> dry mass. Under a 12/12 PP, the<br />
starch c<strong>on</strong>centrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> leaves ranged from 20 to 120 mg/g leaf DM<br />
under LL c<strong>on</strong>diti<strong>on</strong>s, but A net was c<strong>on</strong>stant at 12 μmol·m –2 s –1 during<br />
the light cycle. In c<strong>on</strong>trast, both the starch c<strong>on</strong>centrati<strong>on</strong> (85–120 mg/<br />
g) <str<strong>on</strong>g>and</str<strong>on</strong>g> A net (8–12 μmol·m –2 ·s –1 ) in the 24/0 LL treatment were<br />
relatively c<strong>on</strong>stant (Fig. 4). Although there was a trend towards lower<br />
A net <str<strong>on</strong>g>and</str<strong>on</strong>g> higher starch c<strong>on</strong>centrati<strong>on</strong>s with increasing photoperiod<br />
(Fig. 4), there was no statistical correlati<strong>on</strong> between starch c<strong>on</strong>centrati<strong>on</strong><br />
<str<strong>on</strong>g>and</str<strong>on</strong>g> A net <str<strong>on</strong>g>of</str<strong>on</strong>g> individual leaves (r 2 = 0.129).<br />
Discussi<strong>on</strong><br />
These short-term single leaf studies <str<strong>on</strong>g>and</str<strong>on</strong>g> whole plant studies<br />
(Stutte et al., 1993) suggest that carb<strong>on</strong> assimilati<strong>on</strong> is maximized<br />
at 1100 μmol·mol –1 CO 2 at both LL <str<strong>on</strong>g>and</str<strong>on</strong>g> HL c<strong>on</strong>diti<strong>on</strong>s. Dwelle<br />
(1985) reported that single leaf A net was saturated at about 1200<br />
μmol·m –2 ·s –1 at ambient CO 2 c<strong>on</strong>centrati<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> that the A net values<br />
ranged from 14–16 μmol·m –2 ·s –1 , which were c<strong>on</strong>sistent with A net<br />
266 J. AMER. SOC. HORT. SCI. 121(2):264–268. 1996.
Fig. 3. Effect <str<strong>on</strong>g>of</str<strong>on</strong>g> PPF <strong>on</strong> diurnal changes in leaf starch c<strong>on</strong>centrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> newly mature<br />
‘Norl<str<strong>on</strong>g>and</str<strong>on</strong>g>’ potato leaves exposed to different photoperiods. The mean PPF values<br />
were 263 <str<strong>on</strong>g>and</str<strong>on</strong>g> 412 μmol·m –2 ·s –1 in the low <str<strong>on</strong>g>and</str<strong>on</strong>g> high light chambers, respectively.<br />
values obtained in these experiments. Light resp<strong>on</strong>se curves obtained<br />
from individual leaves used in these studies indicated that<br />
the resp<strong>on</strong>se was linear (r 2 = 0.961) over the range tested <str<strong>on</strong>g>and</str<strong>on</strong>g> that<br />
light saturati<strong>on</strong> had not been reached.<br />
There were no significant differences in either A net or leaf starch<br />
c<strong>on</strong>tent <str<strong>on</strong>g>of</str<strong>on</strong>g> measurements obtained in 12/12 treatments, suggesting<br />
that the adaptive resp<strong>on</strong>ses were not associated with age <str<strong>on</strong>g>of</str<strong>on</strong>g> the<br />
plant. This result is c<strong>on</strong>sistent with other reports in the literature.<br />
Although Wolf (1993) reported that an exp<str<strong>on</strong>g>and</str<strong>on</strong>g>ing leaf has greater<br />
A net <str<strong>on</strong>g>and</str<strong>on</strong>g> starch c<strong>on</strong>tent than a fully exp<str<strong>on</strong>g>and</str<strong>on</strong>g>ed leaf, in potato, the<br />
differences between the recently mature leaves were not statistically<br />
significant. Frier (1977) reported that A net <str<strong>on</strong>g>of</str<strong>on</strong>g> the first fully<br />
exp<str<strong>on</strong>g>and</str<strong>on</strong>g>ed leaf increased following tuber initiati<strong>on</strong> but remained<br />
c<strong>on</strong>stant during tuber bulking. All photoperiod treatments were<br />
imposed during this period <str<strong>on</strong>g>of</str<strong>on</strong>g> tuber bulking.<br />
It should be noted that the photoperiod treatments were imposed<br />
<strong>on</strong> the same plants throughout the experiment, <str<strong>on</strong>g>and</str<strong>on</strong>g> as a<br />
c<strong>on</strong>sequence the plants were <str<strong>on</strong>g>of</str<strong>on</strong>g> different ages when the gas<br />
exchange <str<strong>on</strong>g>and</str<strong>on</strong>g> starch measurements were obtained. However, the<br />
short-term photoperiod treatments were started well after the<br />
J. AMER. SOC. HORT. SCI. 121(2):264–268. 1996.<br />
Fig. 4. Effect starch c<strong>on</strong>centrati<strong>on</strong> <strong>on</strong> net carb<strong>on</strong> assimulati<strong>on</strong> rate <str<strong>on</strong>g>of</str<strong>on</strong>g> newly mature<br />
‘Norl<str<strong>on</strong>g>and</str<strong>on</strong>g>’ potato leaves grown in either high or low light c<strong>on</strong>diti<strong>on</strong>s. The mean<br />
PPF values were 263 <str<strong>on</strong>g>and</str<strong>on</strong>g> 412 μmol·m –2 ·s –1 in the low <str<strong>on</strong>g>and</str<strong>on</strong>g> high light chambers,<br />
respectively.<br />
period <str<strong>on</strong>g>of</str<strong>on</strong>g> tuber initiati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> rapid vegetative development. A net ,<br />
A Ci <str<strong>on</strong>g>and</str<strong>on</strong>g> leaf starch measurements were obtained from individual<br />
leaves that had been returned to 12/12 photoperiod between<br />
treatments. The A net for the 12/12 PP at 56, 63, <str<strong>on</strong>g>and</str<strong>on</strong>g> 77 DAP were<br />
13.8, 14.1, <str<strong>on</strong>g>and</str<strong>on</strong>g> 13.3 μmol·m –2 ·s –1 , respectively, 1 h after the <strong>on</strong>set<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> the light cycle. These values were not significantly different.<br />
Wheeler et al. (1991) grew Norl<str<strong>on</strong>g>and</str<strong>on</strong>g> potatoes in a factorial<br />
experiment involving CO 2 c<strong>on</strong>centrati<strong>on</strong>, photoperiod, <str<strong>on</strong>g>and</str<strong>on</strong>g> light<br />
intensity. Total biomass <str<strong>on</strong>g>and</str<strong>on</strong>g> yield increased whenever any two<br />
variables (photoperiod, PPF, or CO 2 c<strong>on</strong>centrati<strong>on</strong>) were increased,<br />
but declined when all three were increased. This suggests that<br />
optimum envir<strong>on</strong>mental c<strong>on</strong>diti<strong>on</strong>s (e.g., light intensity, photoperiod,<br />
<str<strong>on</strong>g>and</str<strong>on</strong>g> CO 2 c<strong>on</strong>centrati<strong>on</strong>) derived from short-term experiments<br />
are not directly transferable to integrated, l<strong>on</strong>g-term producti<strong>on</strong><br />
systems. Our results showed a similar, but more pr<strong>on</strong>ounced<br />
effect. Whenever photoperiod was increased, there was a marked<br />
decline in A net , which was most pr<strong>on</strong>ounced under HL c<strong>on</strong>diti<strong>on</strong>s.<br />
The capacity <str<strong>on</strong>g>of</str<strong>on</strong>g> the leaves to return to pretreatment A net <str<strong>on</strong>g>and</str<strong>on</strong>g> starch<br />
levels suggest that the observed resp<strong>on</strong>ses are physiological <str<strong>on</strong>g>and</str<strong>on</strong>g><br />
not shock resp<strong>on</strong>ses.<br />
The maximum starch c<strong>on</strong>centrati<strong>on</strong>s were obtained in the HL<br />
treatments at shorter photoperiods than the LL treatments. This<br />
suggests that A net may be an indirect indicator <str<strong>on</strong>g>of</str<strong>on</strong>g> sink dem<str<strong>on</strong>g>and</str<strong>on</strong>g>, as<br />
described under field c<strong>on</strong>diti<strong>on</strong>s by Dwelle (1985), but does not<br />
appear to correlate with either sink dem<str<strong>on</strong>g>and</str<strong>on</strong>g> or starch storage under<br />
more optimal c<strong>on</strong>trolled envir<strong>on</strong>ment c<strong>on</strong>diti<strong>on</strong>s. The accumulati<strong>on</strong><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> starch in the leaves was not correlated with differences in<br />
vascular development between the HL <str<strong>on</strong>g>and</str<strong>on</strong>g> LL treatments, <str<strong>on</strong>g>and</str<strong>on</strong>g> is<br />
c<strong>on</strong>sistent with Moorby (1978), who established that phloem<br />
development did not limit tuber bulking <str<strong>on</strong>g>of</str<strong>on</strong>g> potato.<br />
One must be cautious extrapolating these short-term changes in<br />
267
photoperiod to l<strong>on</strong>g-durati<strong>on</strong> exposure tests. Increasing the PP<br />
from 12/12 to 18/6 resulted in a 16.1% decrease in A net , but<br />
exposure to c<strong>on</strong>tinuous lighting resulted in a 33.2% decrease in<br />
A net . Assuming that A net was at maximum potential in the leaf,<br />
increasing PP would initially appear to be an advantage in overall<br />
carb<strong>on</strong> assimilati<strong>on</strong>. Increasing PP from 12/12 to 18/6 under HL<br />
c<strong>on</strong>diti<strong>on</strong>s would increase the net daily integral (mmol CO 2 fixed/<br />
m 2 per day) from 550 mmol to 735 mmol; a 33.7% increase in total<br />
carb<strong>on</strong> assimilated. Similarly, c<strong>on</strong>tinuous illuminati<strong>on</strong> would result<br />
in the daily fixati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> 800 mmol <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong>, a 45% increase<br />
over 12/12 photoperiod. However, Norl<str<strong>on</strong>g>and</str<strong>on</strong>g> was unable to sustain<br />
these rates <str<strong>on</strong>g>of</str<strong>on</strong>g> A net under l<strong>on</strong>g PP in a closed envir<strong>on</strong>ment without<br />
significant tissue damage occurring (Stutte et al., 1993), a result<br />
previously reported by Cao <str<strong>on</strong>g>and</str<strong>on</strong>g> Tibbitts (1991).<br />
‘Norl<str<strong>on</strong>g>and</str<strong>on</strong>g>’ potato has been successfully grown under c<strong>on</strong>diti<strong>on</strong>s<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> high CO 2 (1200 μmol·mol –1 ), high light (690 μmol·m –2 ·s –1 PPF)<br />
<str<strong>on</strong>g>and</str<strong>on</strong>g> 12/12 photoperiods in the Biomass Producti<strong>on</strong> Chamber at<br />
Kennedy Space Center (KSC). Preliminary results from <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
these tests (Stutte et al., 1993) are c<strong>on</strong>sistent with the inability <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
plants to sustain high levels <str<strong>on</strong>g>of</str<strong>on</strong>g> A net under l<strong>on</strong>g photoperiods. About<br />
18 h into the light cycle (i.e., 6 h after the former <strong>on</strong>set <str<strong>on</strong>g>of</str<strong>on</strong>g> the dark<br />
cycle) A net began to decline, <str<strong>on</strong>g>and</str<strong>on</strong>g> c<strong>on</strong>tinued to decrease at a c<strong>on</strong>stant<br />
rate until a dark cycle was imposed. The physiological tolerance <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
the populati<strong>on</strong> to c<strong>on</strong>tinuous light was not reported.<br />
In c<strong>on</strong>trolled envir<strong>on</strong>ments, especially bioregenerative life<br />
support systems, the rate <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> fixati<strong>on</strong> needs to be maximized<br />
without damaging plant tissues. The evidence suggests reciprocity<br />
between photoperiod <str<strong>on</strong>g>and</str<strong>on</strong>g> light intensity in the plant’s ability to fix<br />
<str<strong>on</strong>g>and</str<strong>on</strong>g> transport carb<strong>on</strong>. Overall yield increases may be achieved<br />
through increased photoperiod if PPF is reduced so that unloading<br />
can be maintained. C<strong>on</strong>versely, light intensity may be increased to<br />
achieve higher short term A net rates if a sufficient dark cycle is<br />
provided (≥12 h) for the unloading <str<strong>on</strong>g>and</str<strong>on</strong>g> transport <str<strong>on</strong>g>of</str<strong>on</strong>g> starch reserves<br />
to occur.<br />
If a physiological limit exists for the fixati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> transport <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
carb<strong>on</strong>, then increasing photoperiod <str<strong>on</strong>g>and</str<strong>on</strong>g> light intensity under high<br />
CO 2 c<strong>on</strong>diti<strong>on</strong>s may not be the most appropriate means <str<strong>on</strong>g>of</str<strong>on</strong>g> maximizing<br />
either carb<strong>on</strong> assimilati<strong>on</strong> or yield in potato. The results<br />
reported here suggest that there is a physiological limit to carb<strong>on</strong><br />
unloading capacity <str<strong>on</strong>g>and</str<strong>on</strong>g> that ‘Norl<str<strong>on</strong>g>and</str<strong>on</strong>g>’ potato has short-term mechanisms<br />
for feedback c<strong>on</strong>trol <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> assimilati<strong>on</strong>. The physiological<br />
mechanism <str<strong>on</strong>g>of</str<strong>on</strong>g> this feedback is unknown, but does not appear to<br />
be directly associated with starch storage. These results suggest<br />
that total irradiance is not limiting producti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> potato, as has been<br />
suggested by several authors (Salisbury <str<strong>on</strong>g>and</str<strong>on</strong>g> Bugbee, 1988; Wheeler<br />
<str<strong>on</strong>g>and</str<strong>on</strong>g> Tibbitts, 1987), but that producti<strong>on</strong> is limited by the potato<br />
leaves ability to either load, or unload the products <str<strong>on</strong>g>of</str<strong>on</strong>g> photosynthesis.<br />
Literature Cited<br />
Auchter, E.C. <str<strong>on</strong>g>and</str<strong>on</strong>g> C.P. Harley. 1924. Effect <str<strong>on</strong>g>of</str<strong>on</strong>g> various lengths <str<strong>on</strong>g>of</str<strong>on</strong>g> day <strong>on</strong><br />
development <str<strong>on</strong>g>and</str<strong>on</strong>g> chemical compositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> some horticultural plants.<br />
Proc. Amer. Soc. Hort. Sci. 21:199–214.<br />
Cao, W. <str<strong>on</strong>g>and</str<strong>on</strong>g> T.W. Tibbitts. 1991. Physiological resp<strong>on</strong>ses in potato plants<br />
under c<strong>on</strong>tinuous irradiati<strong>on</strong>. J. Amer. Soc. Hort. Sci. 116: 525–527<br />
Dwelle, R.B. 1985. Photosynthesis <str<strong>on</strong>g>and</str<strong>on</strong>g> photoassimilate partiti<strong>on</strong>ing, p.<br />
35–58. In: P.H. Li (ed.). Potato physiolgy Academic Press, Orl<str<strong>on</strong>g>and</str<strong>on</strong>g>o, Fla.<br />
Dwelle, R.B., P.J. Hurley, <str<strong>on</strong>g>and</str<strong>on</strong>g> J.J. Pavek. 1983. Photosynthesis <str<strong>on</strong>g>and</str<strong>on</strong>g><br />
stomatal c<strong>on</strong>ductance <str<strong>on</strong>g>of</str<strong>on</strong>g> potato cl<strong>on</strong>es (Solanum tuberosum L.). Plant<br />
Physiol. 72:172–176.<br />
Frier, V. 1977. The relati<strong>on</strong>ship between photosynthesis <str<strong>on</strong>g>and</str<strong>on</strong>g> tuber growth<br />
in Solanum tuberosum L. J. Expt. Bot. 28:999–1007.<br />
Garner, W.W. <str<strong>on</strong>g>and</str<strong>on</strong>g> H.A. Allard. 1923. Further studies <strong>on</strong> photoperiodism:<br />
The resp<strong>on</strong>se <str<strong>on</strong>g>of</str<strong>on</strong>g> plants to relative length <str<strong>on</strong>g>of</str<strong>on</strong>g> day <str<strong>on</strong>g>and</str<strong>on</strong>g> night. J. Agr. Res.<br />
23:871–920.<br />
Geiger, D.R. <str<strong>on</strong>g>and</str<strong>on</strong>g> B.R. F<strong>on</strong>dy. 1991. Regulati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> allocati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g><br />
partiti<strong>on</strong>ing: Status <str<strong>on</strong>g>and</str<strong>on</strong>g> research agenda, p. 1–9. In: J.L. B<strong>on</strong>nemain, S.<br />
Delrot, W.J. Lucas, <str<strong>on</strong>g>and</str<strong>on</strong>g> J.J. Dainty. (eds.). Recent advances in phloem<br />
transport <str<strong>on</strong>g>and</str<strong>on</strong>g> assimilate compartmentati<strong>on</strong>. Ouest ed., Nantes, France.<br />
Gregory, L.E. 1965. Physiology <str<strong>on</strong>g>of</str<strong>on</strong>g> tuberizati<strong>on</strong> in plants (tubers <str<strong>on</strong>g>and</str<strong>on</strong>g><br />
tuberous roots), p. 1328–1354. In: Encyclopedia <str<strong>on</strong>g>of</str<strong>on</strong>g> plant physiology.<br />
vol. 15. Springer-Verlag, Berlin.<br />
Herold, A. 1980., Regulati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> photosynthesis by sink activity—The<br />
missing link. New Phytol. 86:131–144.<br />
Lorenzen, J.H. <str<strong>on</strong>g>and</str<strong>on</strong>g> E.E. Ewing. 1992. Starch accumulati<strong>on</strong> in leaves <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
potato (Solanum tuberosum L.) during the first 18 days <str<strong>on</strong>g>of</str<strong>on</strong>g> photoperiod<br />
treatment. Ann. Bot. 69:481–484<br />
MacElroy, R.D. <str<strong>on</strong>g>and</str<strong>on</strong>g> J. Bredt. 1985. Current c<strong>on</strong>cepts <str<strong>on</strong>g>and</str<strong>on</strong>g> future directi<strong>on</strong><br />
in CELSS, p. 1–9. In: Life Support Systems in Space Travel. NASA<br />
C<strong>on</strong>f. Publ. 2378, M<str<strong>on</strong>g>of</str<strong>on</strong>g>fett field, Calif.<br />
McDermitt, D.K., J.M. Norman, J.T. Davis, T.M. Ball, T.J. Arkebauer,<br />
J.M. Welles, <str<strong>on</strong>g>and</str<strong>on</strong>g> S.R. Romer. 1988. CO 2 resp<strong>on</strong>se curves can be<br />
measured with a field-portable closed loop photosynthesis system. Proc.<br />
Intl. Symp. <strong>on</strong> forest tree physiology, Nance, France.<br />
Moorby, J., 1978. The physiology <str<strong>on</strong>g>of</str<strong>on</strong>g> growth <str<strong>on</strong>g>and</str<strong>on</strong>g> tuber yield, p. 153–95.<br />
In: P.M. Harris (ed.). The potato crop: The scientific basis for improvement.<br />
Wiley, New York.<br />
Neales, T.F. <str<strong>on</strong>g>and</str<strong>on</strong>g> L.D. Incoll. 1968. The c<strong>on</strong>trol <str<strong>on</strong>g>of</str<strong>on</strong>g> leaf photosynthesis rate<br />
by the level <str<strong>on</strong>g>of</str<strong>on</strong>g> assimilate c<strong>on</strong>centrati<strong>on</strong> in the leaf: A review <str<strong>on</strong>g>of</str<strong>on</strong>g> the<br />
hypothesis. Bot. Rev. 34:107–125.<br />
Ols<strong>on</strong>, R.L., M.W. Ales<strong>on</strong>, <str<strong>on</strong>g>and</str<strong>on</strong>g> T.J. Slavin. 1988. CELSS for advanced<br />
manned missi<strong>on</strong>s. HortScience 23:275–286.<br />
Salisbury, F.B. <str<strong>on</strong>g>and</str<strong>on</strong>g> B.G. Bugbee. 1988. C<strong>on</strong>trolled ecological life support<br />
system use <str<strong>on</strong>g>of</str<strong>on</strong>g> higher plants. NASA C<strong>on</strong>f. Publ. 2231, M<str<strong>on</strong>g>of</str<strong>on</strong>g>fett Field,<br />
Calif.<br />
Stutte, R.W., R.M. Wheeler, <str<strong>on</strong>g>and</str<strong>on</strong>g> B.V. Peters<strong>on</strong>. 1993. Characterizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
populati<strong>on</strong> CO 2 exchange <str<strong>on</strong>g>of</str<strong>on</strong>g> potato grown in KSC’s biomass producti<strong>on</strong><br />
chamber. 28:467 (Abstr.)<br />
Tibbitts, T.W., W. Cao, <str<strong>on</strong>g>and</str<strong>on</strong>g> R.M. Wheeler. 1993. Growth <str<strong>on</strong>g>of</str<strong>on</strong>g> potatoes for<br />
CELSS. Final Rpt.-NCC2-301. NASA Coop. Agreement.<br />
Wang, Z. <str<strong>on</strong>g>and</str<strong>on</strong>g> G.W. Stutte. 1992. The role <str<strong>on</strong>g>of</str<strong>on</strong>g> carbohydrates in active<br />
osmotic adjustment in apple under water stress. J. Amer. Soc. Hort. Sci.<br />
117:816–823.<br />
Wheeler, R.M., C.L. Mackowiak, J.C. Sager, W.M. Knott, <str<strong>on</strong>g>and</str<strong>on</strong>g> C.R.<br />
Hinkle. 1990. Potato growth <str<strong>on</strong>g>and</str<strong>on</strong>g> yield using nutrient film technique.<br />
Amer. Potato J. 67:177–187.<br />
Wheeler, R.M. <str<strong>on</strong>g>and</str<strong>on</strong>g> T.W. Tibbitts. 1986a. Utilizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> potatoes for life<br />
support systems in space. I. Cultivar–photoperiod interacti<strong>on</strong>s. Amer.<br />
Potato J. 63:315–323<br />
Wheeler, R.M. <str<strong>on</strong>g>and</str<strong>on</strong>g> T.W. Tibbitts. 1986b. Growth <str<strong>on</strong>g>and</str<strong>on</strong>g> tuberizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
potato (Solanum tuberosum L.) under c<strong>on</strong>tinuous light. Plant Physiol.<br />
80:801–804<br />
Wheeler, R.M. <str<strong>on</strong>g>and</str<strong>on</strong>g> T.W. Tibbitts, 1987. Utilizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> potatoes for life<br />
support systems in space: III. Productivity at successive harvest dates<br />
under 12 h <str<strong>on</strong>g>and</str<strong>on</strong>g> 24 h photoperiods. Amer. Potato J. 64:311–320.<br />
Wheeler, R.M., T.W. Tibbitts, <str<strong>on</strong>g>and</str<strong>on</strong>g> A.H. Fizpatrick. 1991. Carb<strong>on</strong> dioxide<br />
effects <strong>on</strong> potato growth under different photoperiods <str<strong>on</strong>g>and</str<strong>on</strong>g> irradiance.<br />
Crop Sci. 31:1209–1213.<br />
Wolf, S. 1993. Effect <str<strong>on</strong>g>of</str<strong>on</strong>g> leaf age <strong>on</strong> photosynthesis, carb<strong>on</strong> transport, <str<strong>on</strong>g>and</str<strong>on</strong>g><br />
carb<strong>on</strong> allocati<strong>on</strong> in potato plants. Potato Res. 36:253–262.<br />
268 J. AMER. SOC. HORT. SCI. 121(2):264–268. 1996.