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

48 JOURNAL OF APPLIED SEED PRODUCTION, VOL. 3, 1985ducing their susceptibility to abortion.It is clear that precise physiological studies are required tocharacterize the assimilate requirements of developing seeds.Information on the pattern of assimilate supply with time toindividual seeds within a spikelet is needed to identify anydifferences between those seeds which abort, presumably theyoungest and most distal within the spikelet, and those thatmaintain their growth to maturity. However, as the pattern ofassimilate distribution may be markedly influenced by thehormonal relations of the sink (Wareing, 1977, 1979), thenthe hormonal background of individual developing seedsmay be the critical feature that determines their fate. Thiscould form the basis of a mechanism of hormonal inhibitionof seed development (Evans et al., 1975) and this possibilityneeds to be resolved. It is important to note that even wherethe application of plant growth regulators increases seednumbers per inflorescence the percentage of florets that setseed is at best only about two-thirds of the total (Hamptonand Hebblethwaite, 1985b). Thus considerable yield potentialremains to be realized.Seed SizeSeeds that grow to maturity follow a characteristic patternof rapid increase in size and fresh weight in the first 10 daysafter fertilization, the period when they are most likely to besusceptible to abortion, and this is followed by a furtherperiod of dry weight increase during which reserves areaccumulated and the percentage water content of the seeddeclines (Hill, 1980). The maximum dry weight of seeds isreached at the end of this stage, approximately four weeksafter peak anthesis in perennial ryegrass. The final weight ofan individual seed depends mainly on its position within theinflorescence, although the earlier appearing inflorescencesof tillers produced in the summer tend to produce slightlylarger seeds than those of tillers produced later in the year(Anslow, 1964). Basal spikelets produce slightly larger seedsthan upper ones, but within a spikelet the basal florets tend toproduce much larger seeds than more distal florets. Thispattern reflects the relative activity of the individual seedsinks within the inflorescence and may well be related to theonset of fertilization of individual florets. Thus, seeds developingin upper florets within a spikelet compete poorly withthose in lower positions for mineral nutrients and assimilatemoving into the spikelet; on the other hand, if the lemma andpalea associated with each growing seed supply a substantialproportion of the carbohydrate required for growth (Ong etal., 1978a), then differences in the duration of the seedfillingperiod may be important in accounting for differencesin size between adjacent seeds within a spikelet.The results of field and glasshouse studies show that thelevel of nitrogen supply, or mineral nutrient regime in general,have relatively little effect on mean seed weight(Hebblethwaite and Ivins, 1977; Ong et al., 1978c). Overallit seems clear that variation in seed size is a minor componentof the yield structure of the grass inflorescence, and thisfollows the general pattern displayed by most plants, namelythat seed size tends to be relatively constant in a range ofconditions.CONCLUSIONSThe large difference between potential and actual seedproduction of the grass inflorescence is due to the poorcapacity of florets to yield mature seeds. This under-utilizationof floret potential is associated with the lodging of thecrop but even when this is reduced there is still a largediscrepancy between the numbers of florets and seeds. Thephysiological factors underlying the poor productivity offlorets are not fully understood but the evidence to datesuggests firstly, that many florets do not become pollinatedor fertilized, and secondly that a high proportion of developingseeds receive insufficient assimilate to sustain their growthat a critical period of their development, and as a result theyabort. The latter situation seems to be related to the demandsfor assimilate elsewhere in the plant and this aspect requiresmore detailed investigation. There is scope to increase thepotential seed yield further, for example by increasing theproportion of high yielding early-appearing tillers with a highnumber of florets per spikelet, but this approach is unrealisticuntil its potential can be realized. Currently the use of growthregulators to reduce lodging offers the best approach toimproving the productivity of florets in field crops, and alsoprovides a useful physiological tool to analyze the factorsunderlying this improvement.REFERENCES1. Anslow, R. C. 1963. Seed formation in perennial ryegrass. I.Anther exsertion and seed set. J. Br. Grassl. Soc. 18:90-96.2. Anslow, R.C. 1964. Seed formation in perennial ryegrass. II.Maturation of seed. J. Br. Grassl. Soc. 19:349-357.3. Austin, R.B., J.A. Edrich, M.A. Ford, and R.D. Blackwell.1977. The fate of dry matter, carbohydrates and 14 C lost fromthe leaves and stems of wheat during grain filling. Ann. Bot.41:1309-1321.4. Burbidge, A., P.D. Hebb1ethwaite, and J.D. Ivins. 1978.Lodging studies in Lolium perenne grown for seed. 2. Floretsite utilization. J. Agric. Sci., Cambridge 90:269-274.5. Clemence, T.G.A., and P.D. Hebb1ethwaite. 1984. An appraisalof ear, leaf and stem 14 C0 2 assimilation, t4 C­assimilate distribution and growth in a reproductive seed cropof amenity Lolium perenne. Ann. Appl. Bioi. 105:319-327.6. Col viii, K.E., and C. Marshall. 198 L The patterns of growth,assimilation of '4C0 2and distribution of t4C-assimilate withinvegetative plants of Lolium perenne at low and high density.Ann. Appl. Bioi. 99:179-190.7. Colvill, K.E., and C. Marshall. 1984. Tiller dynamics andassimilate partitioning in Lolium perenne with particularreference to flowering. Ann. Appl. Bioi. 104:543-557.8. Darwinkel, A. 1978. Patterns of tillering and grainproduction of winter wheat at a wide range of plant densities.Netherlands J. Agric. Sci. 26:383-398.9. Evans, L.T., I.F. Wardlaw, and R.A. Fischer. 1975. Wheat.pp. 101-149. In L.T. Evans (ed.) Crop Physiology: Some casehistories. Cambridge University Press.10. Griffiths, D.J., J. Lewis, and E.W. Bean. 1980. Problems ofbreeding for seed production in grasses. pp. 37-49. In P.D.Hebblethwaite (ed.) Seed production. Butterworths, London.11. Hampton, J.G., and P.D. Hebblethwaite. 1983. Yield componentsof the perennial rye grass (Lolium perenne L.) seedcrop. J. Appl. Seed Prod. 1:23-25.