Estimation of Whole-Plant Resistance to Gaseous Exchange Independent of Leaf Temperature Measurement

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Abstract
For studies into the uptake of mercury vapor by wheat (Triticum aestivum), a simple theory and plant chamber were employed to estimate total leaf resistance of whole plants to water vapor exchange. The estimates were independent of leaf temperature, for which mean values were indirectly determined. The approach involved the measurement, at steady-state conditions, of the net change in water vapor flux per unit of leaf surface (.DELTA.qv) in response to a small induced change in absolute humidity (.DELTA.CA). Assuming that total leaf resistance (rL) was constant and that change in leaf temperature (TL) was negligible, total leaf resistance was calculated from the equation, RL .apprxeq. .DELTA.CA/.DELTA.qv (sec cm-1). While the assumptions concerning rL and TL may or may not be correct, evidence is presented which indicates that such assumptions did not significantly alter estimates of rL from their true values for changes in ambient relative humidity ranging from 0.011-0.074. Total leaf resistance of groups of whole plants estimated in this manner did not differ for ambient temperatures of 17, 25 and 33.degree. C. Mean values of rL ranged from 83 se cm-1 in darkness to 2.4 s cm-1 at an illumination of 12.8 klx.