Sensitivity of leaf gas exchange to atmospheric drought, soil drought, and water-use efficiency in some Mediterranean Abies species

The responses of C02 assimilation rate (A), transpiration rate (E), and leaf conductance g) to increasing leaf to air water vapor concentration difference (delta W) were investigated (i) using excised shoots from mature trees of Abies alba, Abies cephalonica, Abies marocana, and Abies nordmanniana and (ii) in situ on a mature tree of Abies bornmulleriana. Gas-exchange responses to increasing soil drought were also studied in plants of A. bornmulleriana, A. cephalonica, and Cedrus atlantica. Stable carbon isotope composition measurements were carried out on annual growth rings of A. bornmulleriana to estimate the time-integrated values of the ratio of intercellular leaf (Cj) to ambient (Ca) C02 concentration. Increasing delta W around the shoots reduced A and g in such a way that either Cj remained constant or its decrease was not pronounced enough for the changes in A to be accounted for by changes in g only. This suggests a direct effect of delta W on photosynthesis. The different Abies species showed clear differences in water-use efficiency. Abies cephalonica and A. marocana had lower water costs of C02 assimilation (E/A) than A. nordmanniana and A. alba. It has also been shown that A. cephalonica and A. marocana are characterized by an optimal stomatal control of leaf gas exchange. Stomata closed very rapidly in A. bornmulleriana in response to water supply being withheld, even prior to there being any important decrease in leaf predawn water potential. The stomatal response in C atlantica was more gradual. In A. bornmulleriana, drought adaptation appears to be linked to the ability to avoid internal water stress, whereas drought adaptation in C. atlantica involves the ability to tolerate internal water stress. The high stomatal sensitivity in A. bornmulleriana is also supported by the isotopic carbon composition data, as shown by the substantial interannual variations in the estimates of Cj/ca' ranging from 0.48 for the dryest years to 0.61 for the rainy years.