JASP 3 -- 1985.pdf - International Herbage Seed Group

More documents

Recommendations

Info

44 JOURNAL OF APPLIED SEED PRODUCTION, VOL. 3, 19851978). The percentage of florets setting seed is more or lessindependent of spikelet position, but where lodging occursthere is a marked profile of decreasing seed production perspikelet from the base to the tip of the ear (Burbidge et al.,1978). The latter observation matches the normal pattern offloret production in branching inflorescences foot and meadowfescue where the branches at the base of the inflorescencedevelop many more florets than those in the mid and apicalregions (Ryle, 1964). This difference in floret distribution inthese species is particularly evident in inflorescences borneby summer produced tillers compared with those of tillersappearing in early spring.An analysis of reproductive development at the level of thetiller apex allows some explanation for the differences inspikelet and floret production in inflorescences of tillers ofdifferent age. As temperate grasses such as L. perenne initiateinflorescences in long days following vernalization(Langer, 1979), tillers appearing in the summer initiatespikelet primordia at more or less the same time as laterappearing tillers and so have only a slightly earlier date ofinflorescence emergence (Anslow, 1963; Ryle, 1964). Thusdifferences in spikelet number between tillers arising at differenttimes primarily reflect differences in the size of thetiller apex, i.e. the availability of axillary sites for spikeletprimordia to be laid down. As leaf primordia tend to accumulatewith time in vegetative tillers (Langer, 1979) and asspikelet primordia arise in the axils of leaf primordia, then atthe onset of spikelet initiation tillers appearing in summerand autumn will have additional sites for spikelets to developcompared with later appearing tillers (Ryle, 1964). The degreeto which the apex continues to elongate to producefurther sites for spikelet primordia is also importanUn determiningfinal spikelet number, but it is not clear if thisdiffers in tillers appearing at different times of the year.In field experiments the application of nitrogen fertilizer inthe spring has relatively little effect on spikelet productionbut greatly influences floret production, the greater the supplythe higher the number of florets per spikelet (Hi,ll andWatkin, 1975; Hebblethwaite and Ivins, 1977, 1978.) Ittherefore seems likely that the greater floret number of spikeletsfrom summer appearing tillers, compared with laterappearing tillers, reflects the greater availability of nitrogenwithin the older and correspondingly larger tillers during thecritical period of floret differentiation. On the other hand, asshading at this time has been found to reduce the number offlorets produced per spikelet (Ryle, 1966, 1967) differencesin floret production may also reflect the availability of assimilatefor floret development, with older tillers able tocontribute assimilate more readily than smaller, youngertillers. This would particularly be the case if the latter becomeshaded within the crop canopy; alternatively someassimilate utilized by the differentiating apex may be derivedfrom the seasonal decline in fructan level that occurs prior tothe commencement of spring growth (Pollock and Jones,1979; Pollock, 1981), in which case younger tillers might beexpected to have a smaller pool of accumulated fructanreserve than older tillers.Seed Set and Seed DevelopmentAs previously described many florets are unproductive.The reasons for this are not readily resolved as there is littleinformation on the proportion of florets that are innatelysterile, or those that are fertile but are not successfully pollinatedor fertilized, or those that become fertilized but abortembryos or developing seeds. The situation is further complicatedby the fact that some mature seeds may be lost byshedding prior to harvest. These uncertainties can only beresolved by detailed observations on the development andSpike lets per inflorescenceMS Tl T2 T3 T4 TS-617-11Florets per spikeletMS T1 T2 T3 T4 TS-6 17-11100Seed yieldperinflorescence(mg) 50Florets per inflorescenceMS T1 T2 T3 T4 TS-6 17-11Seeds per inflorescenceMS T1 T2 T3 T4 TS-6 17-11O MS Tl T2 T3 T4 TS-6 17-11Figure 1. The components of seed yield and seed yield perinflorescence in relation to the identity of primary tillers in L.perenne ( + S.E.). MS = main shoot; T1- T11 =primary tillersarising from the axillary buds of main shoot leaves 1-11.
JOURNAL OF APPLIED SEED PRODUCTION, VOL. 3, 1985 45fate of individual florets with time from the emergence of theinflorescence to its maturation.Agronomic studies on the effects of lodging on seed production,where seed yield may be reduced by as much as60%, have however provided some pertinent data on thecapacity of florets to produce harvestable seed (Burbidge etal., 1978). Examination of florets from peak anthesis toharvest has revealed that about 60% of the florets set seedwithin three weeks (i.e. they are successfully pollinated andfertilized), but that by the final harvest after 4-5 weeks only40-50% of these yield seed. Thus many more seeds are setthan are harvested and the major proportion of this loss can beattributed to the abortion of developing seeds. It is not clearprecisely which seeds are most prone to abortion, i.e. theirposition within the various spikelets.In general the reduction in the number of seeds that initiallydevelop to a level that can be supported to maturityseems a common feature of the reproductive biology of manyspecies, and may account for the relative constancy of seedsize observed in plants growing in a range of stressed conditions(Harper, 1977). Silvertown (1982) has suggested thatflorets which abort their developing seeds or fruits play animportant role in reproduction by acting as 'pollen donors'and thereby maximizing the pollen production of the inflorescence,their female function being redundant. It thus seemsprobable that the loss of developing seeds by abortion isregulated by some internal control system and Burbidge et al.( 1978) suggested that this might be related to a shortfall in thesupply of substrates to sustain seed growth, or to the involvementof a hormonal mechanism inhibiting the growth anddevelopment of certain seeds within each spikelet.Within an individual spikelet in rye grass there is a markeddecline in the capacity of florets to set seed from the basal tothe distal floret, with basal florets tending to produce heavierseeds than distal ones (Anslow, 1963, 1964; Burbidge et al.,1978). These patterns of development suggest that there maybe competition for assimilate or mineral nutrients within thespikelet, with the earlier maturing basal florets developing atthe expense of distal florets, i.e. the latter may be moresusceptible to seed abortion. There is some evidence thatspike lets with relatively few florets have a higher percentageseed set at harvest than spike lets with a higher floret numberand this may be explained in terms of reduced competitionfor resources within the spikelet; for example, the greaterseed setting capacity of florets from inflorescences of tillersproduced over winter compared with those from tillers appearingin summer, and the observations of Hebblethwaiteand Ivins (1978) where the delayed application of nitrogenfertilizer in the spring results in an improvement in seed set inspikelets with fewer florets than those produced by controlinflorescences. In general, increasing the supply of nitrogenreduces the percentage seed set in field crops of ryegrass(Spiertz and Ellen, 1972; Hebblethwaite and Ivins, 1977) butthis response is confounded by increased lodging. Neverthelessany increase in competition between florets within theinflorescence at the higher nitrogen levels seems likely toreflect the need for assimilate rather than any other substratefor seed development. There is some evidence to support thisview in that shading the crop in the spring reduces seed setInflorescence Appearancef---------lAnthesis1-----iSeed-filling10 20 30 40Days from inflorescence appearance50 60Figure 2. The fixation of 1 4C0 2by the inflorescence and leavesof L. perenne with time from inflorescence appearance ( +S.E.). Whole tiller exposed to 1 4C0 2for 10 minutes and thenharvested. IIIII , inflorescence; -A- , flag leaf; - - A- - , flagleaf sheath; 0 , penultimate leaf; o , upper internode; •remaining stem. (From Ong et al., 1978a).(Spiertz and Ellen, 1972) but there seem to have been fewspecific studies on this aspect of development in grasses,whereas in temperate cereals the results of glasshouse andfield work show that grain set is greatly reduced by shadingduring anthesis and the immediate post-anthesis period(Wardlaw, 1975).The Assimilate Supply for Seed GrowthThe production of assimilate by the various parts of theflowering tiller and its supply to developing seeds can beevaluated by measuring the uptake of t4C0 2and followingthe distribution of t4C-assimilate from source to sink. In afield study of the assimilatory capacity of the inflorescenceand leaves of ryegrass, Ong et al. (1978a) found that theinflorescence was the major assimilatory organ of the floweringtiller accounting for almost half of the t4CQ 2fixed by thetiller during anthesis (Fig. 2). This proportion increased withtime as leaf laminae progressively senesced. With the exceptionof the developing seeds all parts of the inflorescenceshowed significant photosynthetic capacity, the lemmas andpaleas contributed 40% of the t4C0 2 fixation at anthesis, theglumes 20%, and the rachis the remainder. There was significantexport of t4C-assimilate from the inflorescence after 24hours as also reported in ryegrass by Clemence andHebblethwaite (1984) and as found for Poa annua by Ongand Marshall (1975), but over 70% of the t4C in the floweringtiller was retained by the inflorescence, and the majority of
Page 1 and 2: Journal of AppliedSeed ProductionVo
Page 3 and 4: 'Grasslands Maku' lotus (Lotus pedu
Page 5 and 6: JOURNAL OF APPLIED SEED PRODUCTION,
Page 13: JOURNAL OF APPLIED SEED PRODUCTION,
Page 17: The Effect of Time of Application o
Page 20 and 21: 18 JOURNAL OF APPLIED SEED PRODUCTI
Page 26 and 27: Immaturity as a Cause of Low Qualit
Page 30 and 31: Seed Dormancy and Germination of Sw
Page 44 and 45: Seminar on Floret Site UtilizationW
Page 52 and 53: Floret Site Utilization in Grasses:
Page 68: 66 JOURNAL OF APPLIED SEED PRODUCTI

JASP 3 -- 1985.pdf - International Herbage Seed Group

Create successful ePaper yourself

Delete template?

Save as template?