Durum Wheat Genotypes Differ in Their Mesophyll Conductance Response to Drought and Temperature.

In order to mitigate the impacts of global warming and maintain sustainable crop production, selection for water-use efficient and heat tolerant crops is imperative.  Increased conductance to CO₂ from the intercellular air spaces to the chloroplasts (mesophyll conductance, g_m) should increase photosynthetic rate without a water use cost, resulting in increased leaf intrinsic water-use efficiency (WUE_i).  Genetic variation in g_m has been demonstrated within cereal crops, and g_m has been shown to be reduced by drought in some species.  Further, the temperature sensitivity of g_m varies considerably between species.  If increased g_m is to be included in breeding programs for increased water-use efficiency, the genotype ranking for the trait must be determined under a range of environmental conditions.  Using genotypes of durum wheat grown under well-watered and water-limited conditions, we assessed the response of g_m to drought and short-term temperature changes and the influence of g_m on WUE_i.  Drought reduced g_m in all but one genotype, and drought significantly altered the temperature response of g_m by reducing the thermal optima.  As expected drought increased WUE_i, driven by reduced stomatal conductance, but the degree of response differed between genotypes.  g_m and WUE_i responded similarly to leaf temperature under well-watered conditions.  In general, increased g_m facilitated supply of CO₂ to chloroplasts to maintain photosynthetic rate as stomatal conductance decreased in response to increasing leaf temperature. However, the g_m influence on WUE_i varied between genotypes and with water availability.