SummaryTwenty-one lines representing wild progenitors and cultivated wheats at the diploid, tetraploid, and hexaploid levels were compared during growth under favourable conditions (2l/l6°C, l6-hr days of high intensity natural light, and nonlimiting nutrient supply).Grain weight, which varied 20-fold between lines, increased with increase in ploidy and with the shift from wild to cultivated forms, particularly at the diploid level. The area of individual leaves and the total leaf area of seedlings were proportional to the weight of the grains sown. In turn there was a close relation between the area of the largest leaf on the main stem and the weight of the ear and of the individual grains it supported. Evolution in wheat has thus involved a parallel increase in leaf and grain size, but this has been coupled with a progressive reduction in the rate of photosynthesis per unit leaf area. At low light intensities there was little difference between species in their rate of photosynthesis, but light saturation was approached at lower intensities in the more advanced forms. Photorespiration was also higher in the primitive wheats. Total photosynthesis per flag leaf blade was greatest in the wild and cultivated tetraploids and the cultivated hexaploids, due to their greater leaf size. All wild species showed a rapid fall in flag leaf photosynthesis during grain development, whereas in many of the cultivated wheats the flag leaf rate rose during most rapid grain development. The rate of ear photosynthesis depended on the presence of awns and the number of spikelets, and was highest in the durum wheats and in Triticum dicoccoides.The duration and rate of grain growth has tended to increase during evolution in wheat. Ear photosynthesis was largely sufficient for grain needs in Aegilops speltoides, but the need to import carbohydrates from the leaves has increased in modern wheats, as shown by the increased movement of 14C-Iabelled assimilates to the ear from the flag leaf and from the leaf below it. The loss in dry weight from the stems is also greater in modern wheats. Movement of assimilates to the roots, stem, and tillers has been correspondingly reduced during evolution, and the proportion of the shoot weight in the grain at harvest has increased.

Temperature effects on the growth and yield of wheat (cv. Gamenya) were studied in controlled environments under three day/night temperature regimes (viz. 25/20, 20/15 and 15/10°C) and at three stages of development, viz. vegetative, ear development and grain growth stages. The most important temperature effects were found during the ear development phase. Plants grown at low temperature at this time had long culms, large flag leaves and more potentially fertile florets in each spikelet. The number of florets which produced harvestable grains, and the weight of these grains at maturity, were affected by temperature during the grain growth stage. Temperature prior to floral initiation was not of major importance to final ear weight in this variety, but it did have an effect on the number of mature ears present at harvest. Grain weight per ear at maturity was found to be highly correlated with the number of grains set (r = 0.96), and hence variation in grain number accounted for most of the variation in ear grain weight. In those treatments where grain numbers were not markedly depressed by the temperature treatments, a hlgh positive correlation was found between flag leaf area duration and total grain yield (r = 0.73).

The objective of the present work was to examine to what extent increase in cell size has contributed to the parallel increases in leaf and grain size in the course of evolution in wheat. Eighteen lines were chosen to represent wild and cultivated wheats at the diploid and tetraploid levels, cultivated hexaploid wheats, and the two Aegilops species likely to have contributed the B and D genomes. All plants were grown at 21/16�C under natural light. The penultimate leaves and the basal grains from central spikelets were selected for comparison. The projected area of separated mesophyll cells from leaves was 1.5–2 times larger in the cultivated tetraploid and hexaploid wheats than in the diploids, and correlated positively but weakly with leaf blade area (r = 0.50), and negatively with photosynthetic rate in Triticum species (r = -0.66). Cell size in the endosperm bore no relation to cell size in the aleurone layer or mesophyll, or to grain volume. Aleurone cell size, however, correlated positively with both grain volume (r = 0.82) and mesophyll cell size (r = 0.79). Increase in grain weight during evolution has not involved increase in either specific gravity of the mature grain or endosperm cell size. Presumably increase in endosperm cell number has been the major factor. With increase in grain size during evolution there has been a fall in percentage nitrogen in the grain.

The cross-sectional area of the phloem and the number of vascular bundles at the top of the main stem were determined in 22 diploid, tetraploid, and hexaploid wheats and related wild species grown at 21{16�C in 16-hr days of high light intensity.

Some characteristics of photosynthesis in seven species of Triticum and two of Aegilops were examined. Differences between species in the rate of net photosynthesis per unit area were more pronounced in later formed leaves than in earlier ones, flag leaves showing the greatest range of photosynthesis rates. The range in flag leaf net photosynthetic rates was greatest for plants grown under high light intensity. Net photosynthetic rates of flag leaves of diploid species of Triticum, measured under high light intensity, increased progressively with increase in light intensity during growth, whereas the rates for the Aegilops species and the tetrapbid and hexaploid lines of Triticum reached their maxima at intermediate light intensity during growth.Under one set of environmental conditions during growth, a diffusion resistance study of flag leaf photosynthesis revealed that both the gas phase resistance and "residual" ("mesophyll" or "intracellular") resistance contribute to the observed differences in photosynthetic rate. When the average values of resiJual resistance for each genotype were plotted against the corresponding gas phase resistance a positive correlation, to which all but three of the lines adhered, was found, an increase of 0·5 s cm-1 in gas phase resistance being associated with an increase of I . 0 s cm-1 in residual resistance.When adaptaticJn to cifferent light intensities occurred over a prolonged period, the stomatal density on the flag leaf of two diploid lines and of a hexapbid (T. spelta) line adapted upwards to high light but did not do so for T. aestivum. However, the adaptation was small and of minor significance to the overall photosynthetic adaptation. Variation in stomatal density was not a major determinant of variation in stomatal resistance.Specific leaf weight bore no consistent relation with either photosynthetic rate or residual resistance.