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Rates of net photosynthesis of the flag leaves of 15 genotypes of wheat and related species were measured throughout their life, using intact leaves on plants grown in the field. At the stage when rates were maximal, they were in general highest for the diploid species, intermediate for the tetraploidspecies and lowest for Triticum aestivum (means of 38, 32 and 28 mg CO2 dm−2 h−1 respectively). Rates were strongly negatively correlated with leaf area, leaf width and the mean plan area per mesophyll cell and positvely correlated with stomatal frequency and number of veins per mm of leaf width. The differences among species in these attributes were mainly related to ploidy level. It was not possible to determine the relative importance of each anatomical feature, though the changes in stomatal frequency had only slight effects on stomatal conductance and the observed differences in rates of photosynthesis were much greater than would be expected from those in stomatal conductance alone. There was genetic variation in rates of light dependent oxygen evolution of isolated protoplasts and intact chloroplasts but no difference attributable to ploidy. The mean rate, 91 μmol O2 mg−1 chlorophyll h−1, equivalent to 3.9 mg CO2mg-1 chlorophyll h-1 was considerably less than the rate of photosynthesis in comparable intact leaves, which was 7.2 mg CO2 mg−1 chlorophyll h−1. The total above-ground dry matter yields were least for the wild diploids T. urartu and T. thauodar and the wild tetraploid T. dicoccoides, but the other wild diploids produced as much dry matter as the hexaploids. The prospects of exploiting differences in photosynthetic rate in the breeding of higher yielding varieties are discussed.
Austin et al. (Mon,) studied this question.