The gas exchange characteristics of 24 genotypes of Lycopersicon esculentum Mill. and one of L. minutum were measured with an infrared gas analyzer and dew point hygrometer in an open system. Net carbon exchange (NCE) and transpiration rate were measured at 50, 100, 150, and 300 μ1 1 −1 CO 2 , and a regression of NCE versus internal lead (CO 2 ) estimates was calculated. The slope of the regression curve at the CO 2 compensation point was used as the measure of carboxylation efficiency (CE). Significant genotypic differences for CE were obtained. Differences in CE did not appear to be due to differences in diffusive resistance defined as the sum of the boundary layer resistance ( r a ) and the stomatal plus cuticular resistance ( r 1 ). There was no correlation ( r = -0.07) between ( r a + r 1 ) and CE. Within groups with nonsignificantly different means for ( r a + r 1 ) there were genotypes with extremes for CE. The zero CO 2 intercept has been used as an indication of photorespiration. Application of this method revealed a strong inverse relationship between CE and the intercept value, indicating either that photorespiration is related directly to CE or that this method is unreliable for estimating photorespiration. The fact that the variation in CE occurs at light saturation suggests that the observed differences in CE and rates of NCE are determined either by: ( a ) the concentration and/or kinetic properties of the photochemical reaction centers and associated electron transfer components as they affect the supply of NADPH and ATP and consequently the levels of Calvin cycle intermediates; or ( b ) the concentration and/or kinetic properties of ribulose 1,5-diphosphate carboxylase.
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