Threshold response of mesophyll CO2 conductance to leaf hydraulics in highly transpiring hybrid poplar clones exposed to soil drying

Mesophyll conductance (g(m)) has been shown to impose significant limitations to net CO2 assimilation (A) in various species during water stress. Net CO2 assimilation is also limited by stomatal conductance to water (g(sw)), both having been shown to co-vary with leaf hydraulic conductance (K-leaf). Lately, several studies have suggested a close functional link between K-leaf, g(sw), and g(m). However, such relationships could only be circumstantial since a recent study has shown that the response of g(m) to drought could merely be an artefactual consequence of a reduced intercellular CO2 mole fraction (C-i). Experiments were conducted on 8-week-old hybrid poplar cuttings to determine the relationship between K-leaf, g(sw), and g(m) in clones of contrasting drought tolerance. It was hypothesized that changes in g(sw) and K-leaf in response to drought would not impact on g(m) over most of its range. The results show that K-leaf decreased in concert with g(sw) as drought proceeded, whereas g(m) measured at a normalized C-i remained relatively constant up to a g(sw) threshold of similar to 0.15mol m(2) s(1). This delayed g(m) response prevented a substantial decline in A at the early stage of the drought, thereby enhancing water use efficiency. Reducing the stomatal limitation of droughted plants by diminishing the ambient CO2 concentration of the air did not modify g(m) or K-leaf. The relationship between gas exchange and leaf hydraulics was similar in both drought-tolerant and drought-sensitive clones despite their contrasting vulnerability to stem cavitation and stomatal response to soil drying. The results support the hypothesis of a partial hydraulic isolation of the mesophyll from the main transpiration pathway.