Water use efficiency in C-3 cereals under Mediterranean conditions: a review of physiological aspects

In this review, we will discuss physiological traits of C-3 cereals related to water use efficiency (WUE) in Mediterranean environments, from leaf (WUEinstantaneous) to crop level (WUEyield or 'water productivity'). First, we analyse the WUEinstantaneous and the possible trade-off between improving this parameter and growth/yield performance. Ways to ameliorate WUE without penalties are discussed. We also analyse in what cases breeding by high or low WUEinstantaneous is a suitable criterion to maintain grain yield under drought (Mediterranean) conditions. This question is approached in the framework of carbon isotope discrimination, (Delta C-13), the main indirect parameter used to integrate (at time and space scale) the WUEinstantaneous in C-3 plants. A negative correlation between these two parameters has been confirmed by several studies. The relationship between Delta C-13 and grain yield, however, is more complex, and may differ from one environment to another. In Mediterranean conditions with moderate or no water stress, a positive correlation between Delta C-13 and grain yield is found in barley and wheat, whereas in 'stored-water' crops (such as in some regions of Australia), lower Delta C-13 (i.e. higher WUEinstantaneous) is associated with higher grain yield, particularly in more stressful conditions. These apparent inconsistencies and their possible implications for plant breeding are discussed. One physiological trait that has received minor attention in attempts to improve WUEinstantaneous is the role of ear photosynthesis. Ears of barley and durum wheat have a higher WUEinstantaneous than the flag leaf, both in well-watered and in drought conditions. The underlying causes of the higher WUEinstantaneous of ears are not fully understood, but their refixation capacity (i.e. the capacity to re-assimilate respired carbon dioxide) could be important. Although the genotypic variability of this trait has not been extensively studied, some data support the idea that variation in refixation capacity may be attributable to genetic factors. At the crop level, decreasing soil evaporation is a crucial factor in efforts to improve the WUEyield in Mediterranean conditions, and fast initial growth of the crop (i.e. early vigour) seems to be relevant. In wheat, modern varieties with dwarfing genes (giberellic acid - insensitive) have higher yields but, concomitantly, they have lower initial growth performance. Recently, semi-dwarf cultivars (giberellic acid - sensitive) with high grain yield and simultaneously high early vigour were found, opening new avenues to increase WUEyield in wheat. The negative effects of futile water loss by cuticular and nocturnal transpiration are also commented. Finally, we discuss some agronomic practices (in particular, 'deficit irrigation' systems) linked to physiological traits that confer higher WUEyield,, in particular, in the cases of Mediterranean regions.