Changes in Leaf Photosynthesis With Plant Ontogeny and Relationships With Yield Per Ear in Wheat Cultivars and 120 Progeny

Six wheat cultivars, and 120 random progeny derived from composite crosses among these cultivars and 10 others, were grown in a glasshouse and changes in photosynthesis of the main-shoot leaves determined throughout ontogeny. Photosynthetic rates for each leaf generally showed a short-term rise after ligule emergence, a plateau, and then a linear decline as the leaf aged. Both peak photosynthesis, and the rates of reduction in photosynthesis with aging, increased with successive leaves, and then either stabilized or declined in the upper leaf positions. Leaves also increased in area in a power progression with plant ontogeny. The combined effect of these factors was that successive leaves fixed progressively more carbon during their lives. Leaf emergence was essentially linear with time. Fast leaf emergence was linked with small leaves and a greater final leaf number. Whilst there was a negative correlation between area per leaf and photosynthetic rate, in terms of carbon fixation per leaf the benefits of larger leaves far outweighed the negative effects of reduced rates. Rates of aging of leaves and areas of leaves accounted for the major differences in carbon fixation per leaf among cultivars. There was no correlation between peak photosynthetic rates of leaf 3 and the flag leaf so that any screening for high rates could not be done on seedling leaves. In any event, this could be of doubtful benefit because yield per ear was unrelated to peak photosynthetic rates of the flag leaf. However, yield per ear was correlated with flag leaf area (P<0.05), rate of decline of flag leaf photosynthesis with age (P<0.001, inverse), and most closely with cumulative carbon production by the flag leaf during its life (P<0.001).