THE EFFECT OF VARIATION IN SOIL-WATER AVAILABILITY, VAPOR-PRESSURE DEFICIT AND NITROGEN NUTRITION ON CARBON ISOTOPE DISCRIMINATION IN WHEAT

Carbon isotope discrimination (DELTA) is an integrative measure of leaf transpiration efficiency and has been proposed as a select criterion for greater water-use efficiency in breeding programs for water-limited environments. Here we assess the effects of variation in soil water status, vapour pressure deficit and nitrogen nutrition on the value of DELTA measured in plant dry matter and the relative magnitudes of environmental and genotypic variation in DELTA among conventional wheat cultivars. Experiments were done using container- and field-grown plants. Two genotypes, cv. Cleopatra and Yaqui 50E, were grown in large (23 L) containers to simulate field conditions. Plants were subjected to contrasting watering regimes, to different levels of atmospheric demand (by growing the plants outdoors and varying sowing time) and to two levels of nitrogen nutrition (equivalent to 150 and 30 kg N ha-1). A field experiment using eight genotypes was conducted at Moombooldool in south-west New South Wales, which has an annual rainfall total and distribution typical of much of the south-east wheat belt. Changes in DELTA over the course of the season were followed by sampling recently expanded plant parts. In field-grown plants DELTA measured in dry matter fell by 5x10(-3) between early-formed leaves and the grain. A similar change (7x10(-3)) was observed in container-grown plants. For both field- and container-grown plants, environmental effects on DELTA were attributed to stomatal closure in response to declining soil water and/or increasing vapour-pressure deficit. Low nitrogen nutrition of container-grown plants, which reduced above-ground dry matter at maturity and leaf area at flag leaf emergence by 30%, had a small but variable effect on the value of DELTA. In the field experiment, variation among genotypes in DELTA of different plant parts was always significant, and was typically c. 1.8X10(-3). Genotype ranking for DELTA changed with different plant parts, but the magnitude of genotype x environment interaction was small in relation to genetic variation in DELTA. Changes in ranking mainly occurred in the latter half of the season. These were attributed primarily to differences in the rate and extent of soil drying among genotypes. Variation in the extent of soil water depletion measured at anthesis was positively correlated with DELTA of plant parts laid down early in the season.