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Environmental and Stress Physiology

Photosynthesis Decrease and Stomatal Control of Gas Exchange in Abies alba Mill. in Response to Vapor Pressure Difference

Station de Sylviculture et de Production, Institut National de la Recherche Agronomique, Champenoux, 54280 Seichamps, France, Centre de Recherches Forestières de Nancy, Champenoux, 54280 Seichamps, France

The responses of steady state CO₂ assimilation rate (A), transpiration rate (E), and stomatal conductance (g_s) to changes in leaf-to-air vapor pressure difference (W) were examined on different dates in shoots from Abies alba trees growing outside. In Ecouves, a provenance representative of wet oceanic conditions in Northern France, both A and g_s decreased when W was increased from 4.6 to 14.5 Pa KPa^–1. In Nebias, which represented the dry end of the natural range of A. alba in southern France, A and g_s decreased only after reaching peak levels at 9.0 and 7.0 Pa KPa^–1, respectively. The representation of the data in assimilation rate (A) versus intercellular CO₂ partial pressure (C_i) graphs allowed us to determine how stomata and mesophyll photosynthesis interacted when W was increased. Changes in A were primarily due to alterations in mesophyll photosynthesis. At high W, and especially in Ecouves when soil water deficit prevailed, A declined, while C_i remained approximately constant, which may be interpreted as an adjustment of g_s to changes in mesophyll photosynthesis. Such a stomatal control of gas exchange appeared as an alternative to the classical feedforward interpretation of E versus W responses with a peak rate of E. The gas exchange response to W was also characterized by considerable deviations from the optimization theory of IR Cowan and GD Farquhar (1977 Symp Soc Exp Biol 31: 471-505).