Cereal flag leaf adaptations for grain yield under drought: knowledge status and gaps

Selection for drought-tolerant cereal varieties has successfully moved to screening for grain yield under stress. Grain yield is the culmination of the process of grain filling, which in turn is closely linked to flag leaf functionalities. For grain filling to occur under drought, either a relatively uncompromised or a favorably reprogrammed functioning of the flag leaf is required. However, knowledge is limited on how effectively flag leaves can function under stress conditions or what adaptations could allow such functioning. The information on rice flag leaf function and/or adaptation under drought is critically limited, while rice continues to be the crop with the highest potential to alleviate hunger and poverty. In fact, other cereal crops are equally important in maintaining regional food baskets and these too suffer intermittently from different intensities and kinds of drought. Patchy information is available on the morpho-anatomical, physiological and biochemical aspects of flag leaves under drought; even this is dispersed within different cereals, with studies predominantly on wheat. Hence, a reasonable understanding of the function of flag leaf under drought is lacking for any cereal. Importantly, very few reports exist on the molecular and mechanistic understanding of any known adaptations of flag leaf function under drought. Here we review the existing information on cereal flag leaf function under drought and highlight the need to better understand its characteristics/adaptations, especially at the molecular level. Novel drought-tolerant breeding material generated through selection for yield under stress can be a useful resource to underpin the mechanistic basis of the contribution of flag leaves to such yield. Improved knowledge can then be used for providing dependable markers (morphological, anatomical, physiological, biochemical and/or molecular) for robust flag leaves, leading to efficient and judicious use of resources for screening broader germplasm collections.