Genotypic, environmental and leaf anatomical effects on mesophyll conductance in wheat (Triticum aestivum L.)

Abstract

Improvements in water-use efficiency are required to maintain and improve wheat yield for future climates. The conductance to CO2 diffusion from intercellular airspace to chloroplasts has been termed mesophyll conductance, and significantly limits photosynthetic rate. Increased mesophyll conductance has the potential to improve leaf-level water-use efficiency through increases in photosynthetic rate. For estimating mesophyll conductance in wheat cultivars under a range of growth and measurement conditions, I used gas exchange measurements in combination with discrimination against 13CO2. My objectives were; to assess existing genetic variation in mesophyll conductance among selected wheat cultivars, to quantify the effect of environmental changes on mesophyll conductance and to determine the anatomical basis of variation in mesophyll conductance and its effect on water-use efficiency. Mesophyll conductance varied two-fold among Australian wheat cultivars. A significant decline in mesophyll conductance as leaves aged was related to a decline in the surface area of chloroplasts exposed to intercellular air spaces. Mesophyll conductance also responded to environmental conditions, declining with increasing CO2 concentration and under drought, but increasing as light increased. The association between mesophyll conductance and photosynthetic rate is stronger compared to that between mesophyll conductance and stomatal conductance. Consequently selecting for increased mesophyll conductance in wheat will increase photosynthetic rate and water-use efficiency, so long as stomatal conductance and proportional allocation of carbon to grain remain the same.