Intrinsic water use efficiency (WUEintr), the ratio of photosynthesis to stomatal conductance to water, is often used as an index for crop water use in breeding projects. However, WUEintr conflates variation in these two processes, and thus may be less useful as a selection trait than knowledge of both components. The goal of the present study was to determine whether the contribution of photosynthetic capacity and stomatal conductance to WUEintr varied independently between soybean genotypes and whether this pattern was interactive with mild drought. Photosynthetic capacity was defined as the variation in WUEintr that would occur if genotypes of interest had the same stomatal conductance as a reference genotype and only differed in photosynthesis; similarly, the contribution of stomatal conductance to WUEintr was calculated assuming a constant photosynthetic capacity across genotypes. Genotypic differences in stomatal conductance had the greatest effect on WUEintr (26% variation when well watered), and was uncorrelated with the effect of photosynthetic capacity on WUEintr. Thus, photosynthetic advantages of 8.3% were maintained under drought. The maximal rate of Rubisco carboxylation, generally the limiting photosynthetic process for soybeans, was correlated with photosynthetic capacity. As this trait was not interactive with leaf temperature, and photosynthetic capacity differences were maintained under mild drought, the observed patterns of photosynthetic advantage for particular genotypes are likely to be consistent across a range of environmental conditions. This suggests that it is possible to employ a selection strategy of breeding water-saving soybeans with high photosynthetic capacities to compensate for otherwise reduced photosynthesis in genotypes with lower stomatal conductance.
Climate models predict more frequent and more severe extreme events (e.g. heat waves, extended drought periods) in Europe during the next decades. The response of plants to elevated temperature is a key issue in this context. Stomatal regulation is not only relevant for the diffusion of CO2 from the ambient air into the leaves, but it plays also an important role for the control of transpiration and leaf cooling. The regulation of stomatal aperture by the water status (hydroactive and hydropassive feed-back) and by internal CO2 availability (CO2 feed-back) are well documented in the literature, while the response of the stomates to elevated temperature was far less considered in the past.
Photosynthesis is negatively affected by elevated temperature, but the water loss via transpiration may still be high. In the experiments reported here, bean leaf segments were incubated in darkness floating on water in the range from 20 to 50°C and then analyzed immediately by taking a photograph with a digital microscope. Stomatal aperture was measured on these pictures in order to quantify stomatal opening. After the incubation for 30 min, the opening was 0.66, 2.76 and 4.28 µm at 23, 30 and 35°C respectively. This opening at elevated temperature was fully reversible. Abscisic acid (0.1 µM) in the incubation medium shifted the temperature for stomatal opening to higher values. It can be concluded that elevated temperature stimulates stomatal opening regardless of the CO2 assimilation status and that there is a trade-off between leaf cooling on one hand and limiting water loss during drought periods on the other hand.