RA 00110.pdf - OAR@ICRISAT

The conservative nature of qD is extremely valuable
for modeling productivity in dry areas.
Synthesis
Whereas temperature and solar radiation set the
upper limit for productivity of a given cultivar in wet
climates, extractable water and saturation deficit set
this limit in dry environments. Relations have been
calculated between W and D for pearl millet at two
extreme values of extractable water corresponding
to the range defined by Russell (1978) for soils at
Hyderabad (Fig. 6). The total mass of pearl millet in
postrainy seasons is unlikely to rise above 10 t ha -1
and will generally be much smaller.
Table 3 . M a i n factors d e t e r m i n i n g f i n a l standing d r y
weight.
(a) S o i l water n o t l i m i t i n g
Stand
I I
I I I
(b) S o i l water l i m i t i n g
Stand
I V
V
0.50
0.42
W
0.22
0.12
P
0.83
0.80
E 4
0.31
0.36
e 3
0.61
0.56
0.60
0.35
1. W = dry matter production.
2. f = fraction of mean daily insolation intercepted by the canopy.
3. e = amount of dry matter formed per unit radiation intercepted
(conversion coefficient).
4. E = amount of water that the crop extracts from the soil.
5. D = saturation vapor pressure deficit.
Limitations to
Productivity—Summary
Productivity and its causative factors, for the stands
in Table 1, are expressed as a fraction of stand I, the
most productive (Table 3). For the two stands for
which water was not limiting, W was reduced mainly
through effects on the conversion coefficient (e);
mean fractional interception was reduced by only
20%, a consequence of unknown factors mainly
20
affecting maximum leaf area. The reduction in e
(0.61) for stand I I , which grew in a similar atmospheric
environment to stand I, was entirely related to
senescence after anthesis. This effect was by far the
largest limiting productivity in moist conditions.
The additional reduction in e (0.56) for stand I I I may
have been caused by the effect of D on photosynthesis
referred to earlier.
For the two stands, IV and V, for which water was
limiting, W was reduced through effects both of
extracted water (E) and of D operating via qD. In
the extreme case of stand V for which productivity
was reduced by a factor of about 8, both E and q in
Equation 3 were reduced by factors of about 2.8. The
difference in W (a factor of almost 2) between stands
IV and V growing in very different environments but
extracting a similar amount of water was predominantly
caused by a difference in D.
10
200 mm
95 mm
0
0 2.0 4.0
S a t u r a t i o n d e f i c i t (kPa)
Figure 6. Modeled relation between final dry mass of
pearl millet and saturation deficit for representative
soils. Numbers by curves show volumes of extractable
water in the top 1.5 m of soil.
Partition of Assimilate
In wet climates, the amount of dry matter allocated
to the organs that constitute yield can be considered
in terms of the factors in Equation 1, with the addition
of a fraction (p) defined as the fraction of total
dry matter ( T D M ) allocated to the relevant organ.
As the process of allocation usually coincides with
an associated developmental phase, its duration is
strongly governed by temperature above a base and
can be defined as a thermal duration (Ong 1983b,
1984; G.R. Squire, personal communication). In
controlled environments at Nottingham, the partition
fraction changed little (about 0.4) over the
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