Mapping intercellular CO2 mole fraction (Ci) in Rosa rubiginosa leaves fed with abscisic acid by using chlorophyll fluorescence imaging -: Significance of Ci estimated from leaf gas exchange

Imaging of photochemical yield of photosystem II (PSII) computed from leaf chlorophyll fluorescence images and gas-exchange measurements were performed on Rosa rubiginosa leaflets during abscisic acid (ABA) addition. In air ABA induced a decrease of both the net CO2 assimilation (A(n)) and the stomatal water vapor conductance (g(s)). After ABA treatment, imaging in transient nonphotorespiratory conditions (0.1% O-2) revealed a heterogeneous decrease of PSII photochemical yield. This decline was fully reversed by a transient high CO2 concentration (7400 mu mol mol(-1)) in the leaf atmosphere. It was concluded that ABA primarily affected A(n) by decreasing the CO2 supply at ribulose-1,5-bisphosphate carboxylase/oxygenase. Therefore, the A(n) versus intercellular mole fraction (C-i) relationship was assumed not to be affected by ABA, and images of C-i and g(s) were constructed from images of PSII photochemical yield under nonphotorespiratory conditions. The distribution of g(s) remained unimodal following ABA treatment. A comparison of calculations of C-i from images and gas exchange in ABA-treated leaves showed that the overestimation of C-i estimated from gas exchange was only partly due to heterogeneity. This overestimation was also attributed to the cuticular transpiration, which largely affects the calculation of the leaf conductance to CO2, when leaf conductance to water is low.