Environmental stresses such as drought (Lauteri et al., 1997), salt stress (Bongi and Loreto, 1989), and leaf aging (Loreto et al., 1994) reduce conductance to CO 2 diffusion in the leaf mesophyll (mesophyll conductance). No information exists about possible increases of mesophyll conductance, such as when the stresses are alleviated. One obstacle to the investigation of this possibility is that mesophyll conductance reduction is frequently associated with the impairment of biochemical and photochemical characteristics of the leaf. The former is generally permanent, whereas the latter may recover slowly. However, it was recently shown that low salt accumulation (leaf Na concentration less than 15 mg g Ϫ1 ) primarily affects the conductance to CO 2 diffusion in spinach (Spinacia oleracea L.) leaves (Delfine et al., 1998). A coordinate reduction in stomatal and mesophyll conductance decreased the chloroplast CO 2 concentration of salt-stressed spinach. This, in turn, caused an inhibition of photosynthesis that was not associated with changes in biochemical or photochemical capacity when salt accumulation in the leaves was two to three times that of the controls.Mesophyll conductance reduction is also frequently associated with changes in leaf anatomy (Longstreth and Nobel, 1979;Bongi and Loreto, 1989;Evans et al., 1994;Syvertsen et al., 1995). This is likely to be a permanent effect, at least when leaf thickness is involved. However, low salt accumulation did not increase but slightly decreased the thickness of spinach leaves (Delfine et al., 1998). On the other hand, salt accumulation caused a 25% reduction of the intercellular spaces in the mesophyll of spinach leaves with respect to the controls. This could have caused a more tortuous path for CO 2 directed toward the chloroplast and was suggested to be responsible for the observed photosynthesis reduction associated with low mesophyll conductance in salt-stressed leaves (Delfine et al., 1998).The objectives of this work were to understand, under conditions that do not affect relevantly the biochemical and photochemical capacity of salt-stressed leaves, and are not able to change leaf anatomy significantly: (a) whether the reduction of mesophyll conductance can be reversed by alleviating the salt stress, and (b) how important changes in mesophyll conductance are in determining photosynthesis limitation.
MATERIALS AND METHODS
Plant Material and Experimental ConditionsFour groups of 30 spinach (Spinacia oleracea L. cv Matador) plants were grown in 3-dm 3 pots containing a mixture of soil, peat, and sand (1:1:1). When five to six leaves were fully expanded, the first group of plants (control) was grown under optimal water conditions by daily restoring the water lost through evapotranspiration. Evapotranspiration was estimated by weighing the pots daily. The second group of plants (salt stressed) was irrigated for 50 d with saline water (containing 1% [w/v] NaCl) when evapotranspiration was restored. The third group of plants (rewatered) was irrigated with saline w...
