RELATIONSHIPS BETWEEN CARBON-ISOTOPE DISCRIMINATION, WATER-USE EFFICIENCY AND TRANSPIRATION EFFICIENCY FOR DRYLAND WHEAT

Carbon isotope discrimination (DELTA) 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 DELTA. The aim of this study was to determine the relationship between DELTA and water use efficiency for wheat grown in a dryland environment characterized by winter/spring-dominant rainfall and terminal drought. Eight genotypes chosen to give a range in DELTA of c. 2.0 x 10(-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 (W(ET)) and transpiration efficiency (W(T)) calculated. To account for the effect on W(T) of seasonal changes in the vapour pressure deficit of the air (D), crop coefficients (k) were derived by multiplying W(T) 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 DELTA, as observed previously. The relationship between W(ET) and DELTA also had a positive (though non-significant) trend, but the relationship between k and DELTA 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 DELTA re-emerged. This apparent conflict between W(ET) and k arose because genotypes with high DELTA values developed their leaf area faster, with two important consequences. First, high DELTA 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 DELTA genotypes. The relationship between water use efficiency and DELTA was further complicated as the crops depleted the soil water store after anthesis. During this period DM production tended to be greater in low DELTA genotypes that had conserved soil water in the preanthesis period. However, DM production also remained high for two high DELTA genotypes. The cause of this variation in post-anthesis growth among high DELTA genotypes was not established.