Relationships between carbon isotope discrimination, water use efficiency and transpiration efficiency for dryland wheat

Abstract

Carbon isotope discrimination (-) has been shown to be negatively correlated with water use efficiency for wheat cultivars grown in the glasshouse. In the field this negative correlation has been confirmed for peanut but it has yet to be confirmed for wheat. Indeed, several field studies on wheat have shown positive (rather than negative) relationships between dry matter production and -. The aim of this study was to determine the relationship between - and water use efficiency for wheat grown in a dryland environment characterized by winterlspring-dominant rainfall and terminal drought. Eight genotypes chosen to give a range in - of c. 2.0x10-3 were grown on a red earth at Moombooldool in the Riverina region of New South Wales. Water use and above-ground dry matter (DM) were measured over the course of the season. Water use was partitioned into transpiration and soil evaporation and values of crop water use efficiency (WET) and transpiration efficiency ( WT) calculated. To account for the effect on WT of seasonal changes in the vapour pressure deficit of the air (D), crop coefficients (k) were derived by multiplying WT by the transpiration-weighted average daytime value of D for each genotype. During the preanthesis period, when there was little limitation of soil water supply on growth, there was a positive relationship between DM and -, as observed previously. The relationship between WET and - also had a positive (though non-significant) trend, but the relationship between k and - was negative, i.e. once the effects of variation in the ratio T/ET and seasonal changes in D were accounted for, the negative correlation between water use efficiency and - re-emerged. This apparent conflict between WET and k arose because genotypes with high - values developed their leaf area faster, with two important consequences. First, high - genotypes transpired more of their water supply during the winter when D was low and the exchange of water for CO2 more efficient. Second, transpiration made up a greater proportion of total water use by high - genotypes. The relationship between water use efficiency and - was further complicated as the crops depleted the soil water store after anthesis. During this period DM production tended to be greater in low - genotypes that had conserved soil water in the preanthesis period. However, DM production also remained high for two high - genotypes. The cause of this variation in post-anthesis growth among high - genotypes was not established.