MIDDAY CLOSURE OF STOMATA IN AGING TOBACCO LEAVES

Abstract

The average stomatal widths in the lower epidermis of tobacco leaves at a given stem position in a crop were estimated repeatedly through a number of days during the growing season using a calibrated infiltration method. Stomata started opening more than an hour before morning twilight. Stomata in newly matured leaves stayed wide open until dark but with increasing leaf age the stomata closed earlier, as early as late morning, and this midday closure was then followed by reopening in the afternoon. The known closing effect of high light intensity was confirmed. There was also a significant daily correlation between stomatal width and soil moisture content both of which decreased through the season. Leaf moisture stress was assumed to be accentuated with leaf age. Comparisons of light saturation curves for the afternoon closure in old leaves indicated the presence of a more limited light reaction than that evident in morning opening or in the photoactive closure in younger leaves. Reasons are given for proposing that a well-known light reaction supports an active stomatal opening mechanism.