THE INFLUENCE OF LIGHT AND WATER DEFICIT UPON DIFFUSIVE RESISTANCE OF LEAVES OF VICIA FABA L.

Abstract

SUMMARYThe resistance of the upper (ru) and lower (r1,) surface of the leaf to diffusion of water vapour in Vicia faba was influenced differently by light and water deficit. ru saturated at about 100 Wm‐2 while r1, saturated at about 13 Wm‐2. At a given light intensity r1, was always smaller than r1. The difference in the density of stomata between the two surfaces of the leaf alone does not explain the magnitude of the difference between ru and r1, for V. faba and other species. Impressions of the stomata of the two surfaces at different light intensities indicated that the stomata on the lower surface were more widely open than those of the upper surface.When stomatal response to light was not saturated, increase in leaf‐water deficit was accompanied by increase in ra and r1. Under saturating light intensities however r1, increase in leaf‐water deficit did not affect ru and r1, until leaves were flaccid and pressure potential close to zero; although the stomata on the upper surface closed at slightly higher water potentials than those of the lower surface. A dry treatment lasting 2 days, at the end of which the leaf was flaccid, affected the subsequent performance of the stomata while the leaf‐water deficit had been eliminated. This after‐effect was much greater on ru than on r1, and a complete recovery of the stomata on the upper surface took more than 2 days.Larger differences between ru and r1, values are associated with larger lower to upper stomatal density ratios in a number of species. However r1, the ratios of ru/r1, are much larger than the corresponding stomatal density ratios. It appears that in V. faba, and in a number of other species, the stomata of the lower surface are more widely open that those of the upper surface either at equal light intensity or when leaves are illuminated from above.