[CO2-exchange in amphistomatic leaves : 2. A comparison between the diffusive CO2-exchange of both leaf surfaces of zea mays and the viscous flow of volume in the porometer].

The steady-state values of the diffusive CO2-exchange of both leaf surfaces and the porometer flow (≙ flow of volume through the leaf at 100 mm column of water pressure difference) were measured. Leaves of different age from Zea mays were used for the study. The regression curves for upper surface/porometer flow and lower surface/porometer flow were fitted as polynomes. The correlation coefficients were calculated. CO2-exchange of the upper leaf surface is highly significant correlated with the porometer flow. A small flow of volume through the leaf is always linked with a small diffusive CO2-exchange of the upper leaf surface. The correlation between the diffusive CO2-exchange of the lower surface and the porometer flow is not significant and the variation of these values is considerable. In asymmetric amphistomatic leaves the leaf surface with the higher stomatal resistance limits the flow of volume under porometer conditions. Consequently the porometer flow alone cannot be a parameter for the CO2-exchange. When the porometer flow is very small a considerable diffusive CO2-uptake takes place. In the porometer the CO2-uptake of a leaf consists of two components: the uptake from the viscous flow of volume through the leaf and the diffusive uptake from the airstream flowing through the lower chamber of the porometer. The diffusive part depends on the relation between the stomatal opening of the two leaf surfaces and the CO2-gradient. In Zea mays the stomata of the lower surface open more quickly than those of the upper surface. When the stomata of the upper surface finally just begin to open and the first porometer flow can be measured, the diffusive CO2-uptake through the already opened stomata of the lower surface reaches 20 mg dm(-2) h(-1), which is 2/3 of the final value. The fact that the number of stomata per mm(2) leaf surface increases with the height of insertion is interpreted as being characteristic of a more xeromorphous leaf. Accordingly, the conclusion seems to be evident that the decreasing stomatal opening of the upper leaf surface described here is a xeromorphous adaptation as well, which influences the relation transpiration/CO2-assimilation favourably. Though the higher inserted leaves are amphistomatic in regard to their anatomy, physiologically they react as if they were hypostomatic.