Greater efficiency of water use in poplar clones having a delayed response of mesophyll conductance to drought

Improvement of water use efficiency is a key objective to improve the sustainability of cultivated plants, especially fast growing species with high water consumption like poplar. It is well known that water use efficiency (WUE) varies considerably among poplar genotypes, and it was recently suggested that the use of the mesophyll-to-stomatal conductance ratio (g(m)/g(s)) would be an appropriate trait to improve WUE. The responses of 7-week-old cuttings of four hybrid poplar clones and one native Balsam poplar (Populus balsamifera L.) to a water stress-recovery cycle were examined to evaluate the relation between the g(m)/g(s) ratio and transpiration efficiency (TE), a leaf-level component of WUE. A contrasting g(s) response to water stress was observed among the five clones, from stomatal closure early on during soil drying up to limited closure in Balsam poplar. However in the hybrids, the decline in g(m) was consistently delayed by a few days compared with g(s). Moreover, in the most water use-efficient hybrids, the recovery following rehydration occurred faster for g(m) than for g(s). Thus, the delay in the response of g(m) to drought and its faster recovery upon rewatering increased the g(m)/g(s) of the hybrids and this ratio scaled positively with TE. Our results support the use of the g(m)/g(s) ratio to select genotypes with improved WUE, and the notion that breeding strategies focusing mainly on stomatal responses to soil drying should also look for a strong curvilinearity between net carbon assimilation rate and g(s), the indication of a significant increase in g(m)/g(s) in the earlier stages of stomatal closure.