The Photosynthetic Role of Ears in C3 Cereals: Metabolism,Water Use Efficiency and Contribution to Grain Yield

This review concerns ear photosynthesis and its contribution to grain filling in C₃ cereals. Ear photosynthesis is quantitatively important to grain filling, particularly in dry areas where source (i.e., assimilate) limitations can occur. Compared to the flag leaf, ear photosynthesis exhibits higher water stress tolerance. Several factors could be involved in the ear's “drought tolerance.” First, although degree of C₄ metabolism in ear parts has been reported, current evidence supports only typical C₃ metabolism. Second, recycling of respired CO₂ (i.e., refixation) could have considerable impact on final crop yield by preventing loss of CO₂. Because refixation of CO₂ is independent of atmospheric conditions, water use efficiency (measured as total ear photosynthesis divided by transpiration) could be higher in the ear than in the flag leaf. Moreover, ear parts (in particular awns) show higher relative water content and better osmotic adjustment under water stress compared to the flag leaf. This capacity, in addition to persistence of photosynthetic components under drought (delayed senescence), might help the ear to continue to fix CO₂ late in the grain filling period.