Effect of foliar application of antitranspirant on photosynthesis and water relations of pepper plants under different levels of CO2 and water stress

Strategies such as foliar application of antitranspirants have the potential to regulate transpiration, but often, the limitation of CO2 exchange as a result of reduced stomatal conductance can impair this beneficial effect. Elevated ambient [CO2] could significantly improve CO2 diffusion while effectively reducing transpiration. In this experiment, we examined the response of sweet pepper (Capsicum annuum L.) to the foliar application of antitranspirant (AT) under two [CO2] (380 and 2000 μmol mol−1) and two drought intensities (4 or 8 d without irrigation). The results showed that stomatal conductance and transpiration were reduced, while AT impaired photosynthesis at standard, but not at elevated [CO2] of fully irrigated plants. This effect was already apparent after 4 d of drought. Drought had a minor impact on chlorophyll fluorescence (Fv/Fm). Additionally, root respiration was increased at elevated [CO2] but, after 8 d of drought, it was higher for plants treated with AT than for non-sprayed plants. Leaf water potential was affected more by drought at ambient compared to elevated [CO2], and, especially after 8 d of drought, AT minimized the reductions in leaf water potential. Leaf concentrations of proline and starch were affected by both [CO2] and AT, especially after 8 d of drought. Moreover, increasing [CO2] promoted the accumulation of starch, but led to decreases in the tissue concentrations of the soluble organic osmolytes, and hence diminished osmotic adjustment after 8 d of water withholding, relative to ambient [CO2]. This study indicates that, in addition to the reported beneficial effect of elevated [CO2] on drought stress, AT could significantly improve drought tolerance in sweet pepper plants.