The order of priority for supply of assimilate and nitrogen between individual grains of the wheat ear was studied by determining their accumulation of dry matter and nitrogen when the supply was varied by shading and defoliation, alone and combined, from 2 weeks after anthesis.The production of assimilates by untreated plants was surplus to the requirements of grain filling. Treatments influenced the distribution of dry matter so as to minimize effects on grain.In one cultivar, there was no well-defined pattern of individual grain responses to treatments. In another, the data were consistent with the predominance of an effective parallel linkage of spikelets to sources, although a series-type linkage assumed some importance when there was severe shortage of assimilate (60% reduction in grain growth); upper spikelets were then more seriously affected. Within spikelets, a series effect was more evident, grains being increasingly affected by severe shortage with progression from base to apex of the spikelets; when the overall grain growth reduction was about 60%, growth of first, second, and third grains in a central spikelet was reduced by about 50, 60, and 70% respectively. There were differences between shading and defoliation in their effects on distribution of assimilate, and these differences were consistent with preferential distribution of shoot assimilate to second grains and central spikelets.The third grain of three.grain spikelets was characteristically lower in nitrogen than the others, and this difference was increased by even moderate levels of shortage.It is suggested that the fast growth rate of the second grain in central spikelets is due to its capacity for growth rather than to a favourable position vis· a-vis the vascular system.
The work reported here was done to explore the extent to which the mature weight of a grain is determined by (i) its potential for growth, defined as its intrinsic capacity to accumulate dry matter, and (ii) the resistance to assimilate transport imposed by the vascular system of the ear. Estimation of growth potential was attempted by observing the effects of systematic patterns of grain removal on the mature weights of grains remaining, these being compared with weights of matched grains from intact ears. Resistance to transport of assimilate was inferred from the apparent order of priority between grains for the supply of assimilate, as revealed by comparing their weights when assimilate supply was either normal, or reduced by plant shading.When neighbouring grains were removed, those remaining usually grew larger to an extent that indicated growth potential appreciably in excess of that utilized in intact ears under the most favourable conditions. Although grains within a spikelet of an intact ear attain quite different weights, the experiments suggested that their differing potentials for growth seemed to play only a minor role in this, and that the major influence was the relative ease with which assimilate could reach the grains; this depended largely on the distance of the grains from the spike rachis. Comparing between spikelets, the difference found in intact ears between grains in the same spikelet location tended to persist when some grains were removed from each spikelet, indicating a possible role of growth potential as a controlling influence. This may be partly due to the sequence of morphogenesis, established as early as the double ridge stage.Although the removal of competing grains within a spikelet usually enhanced the growth of the one remaining, this was not always so; there was evidence from one experiment that removal of competing grains towards the spikelet apex represented the removal of some beneficial influence.The bearing of the results on possible limitations to grain yield are discussed.
The effects of vernalization and photoperiod on times from planting of seedlings to ear emergence were measured in 68 Australian and 49 overseas varieties of wheat, comprising a broad spectrum of genetic material, in a glasshouse in Canberra (latitude 35�S). Vernalization was carried out by growing germinated seedlings in the dark at 1-2�C for 6 weeks. Long photoperiods (16 h) separated unvernalized plants into two distinct groups, corresponding to commonly recognized spring and winter types. Responses to vernalization were generally small under natural photoperiods (11-15 h), but much more pronounced in long photoperiods, particularly with winter wheats. In a second experiment, 24 varieties of wheat gave widely different responses to vernalization treatments. With 8 weeks' vernalization and long photoperiods, all varieties reached ear emergence within 66 days, but in some winter wheats 4 weeks treatment had little effect and 6 weeks gave incomplete vernalization. Under the conditions of these experiments, Australian wheats showed a wide range of responses to photoperiod and a narrow range of responses to vernalization compared with overseas varieties. The need to investigate the control of flowering time in obtaining varieties suited to the high-rainfall zone of Australia is discussed.
In this paper, experiments are described which examine the effect of requirement for assimilates by the ear on the rate of net photosynthesis in leaves of wheat (Triticum aestivum L.). Different levels of requirement were achieved by various levels of sterilization of florets just before anthesis, which resulted in a range of grain numbers per ear, and by inhibiting photosynthesis of the intact ear by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Only the ear and two uppermost leaves of the main shoot were considered, all the lower leaves and tiller leaves being excised when the experimental treatments were imposed. In two experiments, tiller regrowth was permitted during the experimental period, while in a third, new tillers were defoliated regularly.The response of leaf photosynthesis to the level of assimilate requirement by the ear was influenced by the treatment of the vegetative tillers. Thus, the net photosynthesis rate of the flag leaf was decreased by a reduction in grain number, or increased by inhibition of photosynthesis in the ear, only when the vegetative tillers were kept defoliated; when these tillers were allowed to re-grow normally, there was no influence of ear treatment on leaf photosynthesis. Temporal changes in leaf photosynthesis were consistent with this response pattern, i.e., when tillers were defoliated, the initial high rates of photosynthesis persisted for much longer.In the experiment where photosynthesis was influenced by the requirement for assimilate in the ear, the variation occurred through change in stomatal conductance on the abaxial surface of the leaf. This surface has a lesser conductance to CO2 exchange than the adaxial surface. The implication of this finding to rapid methods of plant screening is discussed.
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