Effects of Water Vapor Pressure Deficit on Photochemical and Fluorescence Yields in Tobacco Leaf Tissue

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Abstract
The relationship between photochemical quantum yield (.PHI.s) and fluorescence yield have been investigated in leaf tissue from Nicotiana tabacum using CO2 exchange and a modulated fluorescence measuring system. The quantum yield of CO2 fixation at 1.6% (v/v) O2 and limiting irradiance was reduced 20% by increasing the mean H2O vapor pressure deficit (VPD) from 9.2 to 18.6 mbars. As [CO2) and irradiance were varied, the intrinsic quantum yield of open photosystem II units (.PHI.s/qQ where qQ is the photochemical fluorescence quenching coefficient) declined linearly with the degree of nonphotochemical fluorescence quenching. The slope and y-intercept values for this function were significantly reduced when the mean VPD was 18.4 millibars relative to 8.9 millibars. Susceptibility of the leaf tissue to photoinhibition was unaffected by VPD. Elevated O2 concentrations (20.5% v/v) reduced the intrinsic quantum yield of net CO2 uptake due to the occurrence of O2-reducing processes. However, the relative effect of high VPD compared to low VPD on intrinsic quantum yield was not dependent on the O2 level. This suggests that the Mehler reaction does not mediate the response of quantum yield to elevated VPD. The results are discussed with regard to the possible role of transpiration stress in regulating dissipation of excitation by electron transport pathways other than noncyclic electron flow supporting reduction of CO2 and/or O2.