Avenues for genetic modification of radiation use efficiency in wheat.

Radiation use efficiency (RUE) of a crop is a function of several interacting physiological phenomena, each of which can be tackled independently from the point of view of genetic improvement. Although wheat breeding has not raised RUE substantially, theoretical calculations suggest room for improvement. Selection for higher rates of leaf photosynthesis at saturating light intensities (Amax) has not resulted in improved RUE of crops, perhaps in part because most leaves in a canopy are not light-saturated. However, higher Amax may be observed as a pleiotropic effect of other yield-enhancing genes (e.g. genes for reduced height). Genetic transformation of Rubisco to double its specificity for CO2 would theoretically increase Amax by perhaps 20%, and some evidence suggests that photosynthesis at sub-saturating light intensities would also be improved. However, photo-protection may be jeopardized if capacity for oxygenase activity is impaired. Photosynthetic rate of the whole eanopy can be enhanced by manipulation of leaf angle, which is under relatively simple genetic control, and possibly by manipulating leaf-N distribution throughout the canopy. Genetic diversity for adaptation of lower canopy leaves (e.g. changes in chlorophyll a:b ratio) to reduced light intensity observed in some crops needs to be investigated in wheat. Improved RUE may be achieved by increasing sink demand (i.e. kernel number) if excess photosynthetic capacity exists during grain filling, as suggested by a number of studies in which source-sink balance was manipulated. Some evidence suggests that improved sink strength may be achieved by lengthening the duration of the period for juvenile spike growth. Balancing source- and sink-strength is a complex genetic challenge since a crop will change between source and sink limitation as conditions vary during the day, and with phenological stage. Improved RUE will be partly a function of a genotype's ability to buffer itself against changes in its environment to match the demand imposed by its development. Analysis of the physiological basis of genotype by environment interactions may indicate avenues for genetic improvement. The genetic control of photosynthetic regulation may be elucidated in the future through the application of genomics. However, given a lack of specific knowledge on the genetic basis of RUE, empirical selection is currently the most powerful tool for detecting favourable genetic interactions resulting from crosses between lines with superior photosynthetic traits and other high yielding characteristics. Selection for superior segregants can be accelerated using rapidly measured physiological selection traits, such as stomatal conductance or canopy temperature depression.