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.

This paper describes the extraction of water by field crops of sunflower and sorghum from deep podzolic profiles during a drying cycle encompassing most of the growth period. The work was done to evaluate sunflower as a prospective dryland crop for southern Australia, and data are presented for two contrasting years. Sunflower extracted water more rapidly from all levels of the measured profile (0-2 m). Both species drew similar amounts from the top 1 m, but sunflower took two to three times more than sorghum from 1-2 m, the difference being greater in the year of greater evaporative load. Sunflower exhausted the soil of available water to 150-175 cm, sorghum only to 50 cm, taking progressively less from each successive layer down the profile. Extraction of available water from 0-2 m by sunflower was estimated to be 92%, and by sorghum 64%. In the absence of rainfall during its growth period, dryland sunflower is strongly dependent on the presence of water at depth - for example, at 50% flowering the crop was already drawing two-thirds of its water from below 1 m. This dependence limits the frequency with which the crop may be grown in southern Australia and regions similarly characterised by low effective summer rainfall, high evaporative load, and uncertainty in the time needed for a recharge of the profile sufficient to grow sunflower again. In such environments, sunflower may better be regarded as an opportunity crop than as one featuring regularly in farm rotations.

Light stimulated the incorporation of [14C]sucrose into the starch of developing wheat kernels cultured in liquid medium. The light response curve saturated at about one-tenth full sunlight. Surgical removal of the green-layer of the inner pericarp, or inhibition of its photosynthesis by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, eliminated the light response. Experiments with different gases above the culture medium suggested that photosynthetic generation of oxygen by the green- layer was a possible cause of the response. Accumulation of label from [14C]sucrose into the alcohol-soluble fraction of the grain was slightly light-sensitive. This effect also saturated at about one-tenth full sunlight but there was no evidence that it was related to the photosynthetic behaviour of the green-layer. These observations are discussed in relation to in vitro culturing methodology, to the layered structure of wheat starch granules, and to the role of the green-layer of the inner pericarp of the wheat kernel.

Experiments were designed to examine whether drought imposed on plants between sowing and flowering endows them with adaptations which enable them to cope more effectively with drought occurring during grain growth. Specifically, the character sought was adaptation persistence. In two experiments, one in a growth chamber, the other in a glasshouse, plants of two-rowed and six-rowed barleys, and of durum and aestivum wheats, were grown in canopies in 1-metre-deep pots filled with soil, so that the development of water stress might approximate that in the field. Various drying cycles were imposed during the vegetative phase, after which plants were rewatered and allowed to fill their grain during a further drying cycle. During the initial drying cycles there were morphological changes, including changes in area per leaf, leaf area per plant, specific leaf weight and in the numbers of plant organs; the plants to which water was applied sparingly had better water-use efficiency than those with a plentiful supply. On rewatering, however, the previously droughted plants became prodigal in their water use and in the final drying cycle there was little evidence that the earlier responses improved the efficiency of water use either per unit dry matter gain or per unit grain produced. Thus any adaptations were not persistent. Indeed, in general, plants which were heavier at anthesis (i.e. those which had lost more water by this stage), used water most efficiently in grain production. This result is discussed in relation to current photosynthesis and the remobilization of reserves during grain-filling. 14C data indicated that barley plants exposed to drought during grain growth did not retranslocate more stored materials than those which were frequently watered during this stage.

This paper explores the possible role of photosynthesis by individual glumes in influencing the growth rates of individual grains of the wheat ear. Specifically we determined: (1) the photosynthetic capacities of individual glumes (i.e. nonflowering glumes, lemmas, and paleas) of an awned wheat cultivar; (2) the pattern of translocation of 14C-Iabelled assimilate from individual glumes to individual grains; and (3) the time course of translocation of label from glumes and flag leaf to the grain.The lemmas were photosynthetically the most active of the glumes; over the greater part of grain filling they accounted for some 15% of whole-plant photosynthesis; non-flowering glumes and paleas accounted for 5-6% and 2-5% respectively. Towards maturity the lemmas decreased in importance in relation to other glumes, particularly paleas. The photosynthetic efficiency (as measured by 14C fixed per unit weight) of glumes was less than half of that of flag and penultinlate leaves in the early stages, but declined more slowly with time. The nearest grain was the preferred but not exclusive sink for the 14C-Iabelled assimilate from any glume. Movement of label was always towards the apex and on the same side of the spikelet as the glume, if there was a grain towards the apex on that side;if not, movement across the spikelet, still towards the apex, occurred. This caused an accumulation of label in the distal grains of spikelets. Labelled assimilate from the glumes was detected in the grains within 10 min of the commencement of exposure and the rate of increase in activity was maxinlal in about 1 hr; corresponding times for flag leaf label were 1 and 2-3 hr.The distribution of glume assimilate between the grains provided no explanation for differences in their growth rates; there was evidence that the grains themselves exerted a controlling influence.