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Errors as small as 1 C in the measurement of leaf temperature (T(leaf)) are shown to cause significant changes in the estimated value of the stomatal resistance (expressed in terms of total resistance to water vapor transfer, summation operatorrH(2)O). The effect increases as T(leaf) increases and as ambient relative humidity increases, if other conditions are maintained constant. The effect on the key CO(2) exchange parameter, the intracellular (or mesophyll) resistance, r(int), tends to be small under open stomata conditions but increases rapidly as stomatal closure occurs, particularly if the true value of r(int) is relatively small.Errors in the determination of the ambient water vapor and CO(2) concentrations can also significantly affect the calculated value of summation operatorrH(2)O and r(int). The effect on summation operatorrH(2)O and summation operatorrCO(2) increases as the ratio of the inlet/outlet concentration departs from unity and also increases as the assumed leaf-air concentration difference decreases. The combined effect on r(int) tends to be less than the individual effects on summation operatorrH(2)O and summation operatorrCO(2) since both are in the same direction.

A model is presented which enables gas exchange data to be used to partition the intracellular resistance to leaf photosynthesis into carboxylation and transport components. A basic assumption is that the over-all kinetics of the carboxylation reaction fit the Michaelis-Menten equation.The model was tested for cotton (Gossypium hirsutum L., var. Deltapine Smoothleaf), where photorespiration was suppressed by using gas mixtures containing less than 1.5% oxygen. It was concluded that the transport resistance formed the major component of the intracellular resistance for the plants studied. However, in some cases the major intracellular factor limiting photosynthesis, at an ambient CO(2) concentration of 600 ng cm(-3), was the carboxylation system, which was close to saturation.

Cotton plants (Gossypium hirsutum L., var. Deltapine Smooth Leaf) were grown under controlled environmental conditions over a range of day/night temperatures from 20/15 to 40/35 C. Their photosynthetic characteristics were then measured over a comparable temperature range. Net photosynthesis tended stongly to be greatest, and intracellular resistance to CO(2) transport to be lowest, when the measurement temperature corresponded to the daytime growth temperature, suggesting pronounced acclimation of the plants to the growth temperature. The preferred growth temperature was close to the 25/20 C regime, since net photosynthesis of these plants, regardless of measurement temperature, was higher and intracellular resistance lower, than in plants from any other regime.Ribulose diphosphate carboxylase activity per unit leaf area was highest in plants grown at 25/20 C, but did not show pronounced changes with growth temperature. Glycolate oxidase activity decreased and NADH-malate dehydrogenase activity tended to increase with increasing growth temperature. In contrast, changes in carbonic anhydrase activity with growth temperature showed a general similarity to changes in photosynthetic rate. This may suggest that the "chemical resistance" component of the intracellular resistance bears a relationship to the amount of carbonic anhydrase in the leaf.

Substantial errors can be introduced to estimates of leaf resistance (r(l)) obtained from diffusion porometers unless precautions are taken to reduce the leaf-cup temperature difference (T(leaf) - T(cup)) to acceptable levels prior to measurement. When T(leaf) > T(cup), underestimation of leaf resistance occurs; the reverse applies when T(leaf) < T(cup).The effect is most pronounced under open-stomata conditions and declines as stomatal resistance increases. Under typical measurement conditions, T(leaf) - T(cup) values of the order of 1 C induce a reduction in the ratio of the apparent to true leaf resistance to about 0.8 when the leaf resistance is low (r(l) = 1-2 sec cm(-1)). When T(leaf) - T(cup) = 5 C, the ratio drops below 0.5. Under high leaf resistance conditions (r(l) = 10-50 sec cm(-1)) the comparable ratios are approximately 0.9 and 0.7, respectively.