Leaf temperature effects on measurements of diffusive resistance to water vapor transfer.

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.