Photosynthesis of ozone-sensitive and -resistant Phaseolus vulgaris genotypes under ambient ozone and moderate heat stress

Physiological responses from sensitive (S156) and resistant (R123) genotypes of ozone bioindicator, snap bean, were investigated after exposing the plants to cumulative, phytotoxic ozone amounts. Daily course of gas-exchange parameters showed delayed stomatal response in S156 leaves to environmental changes comparing to the response of R123 leaves. Potential photosynthetic quantum conversion, Stern-Volmer nonphotochemical quenching (NPQ), and maximum photochemical efficiency of PSII (Fᵥ/Fₘ) values changed differently in the two genotypes between the first and last measuring days. We concluded that the higher ozone sensitivity originated at least partly from inferior regenerating and/or antioxidant capacity. Experimental protocol proved to be determinant on chlorophyll fluorescence parameters: Fᵥ/Fₘand NPQ declined at midday, and only the sensitive leaves showed a slight increase in NPQ between 12 h and 16 h. We explained these results by moderately high temperatures and shade-adapted state of our experimental plants under substantial ozone stress. On the base of temperature dependence of minimal fluorescence yield (F₀), critical temperature proved to be higher than 32.7°C for Phaseolus vulgaris under these conditions. We found a strong linear correlation between NPQ and nonphotochemical quenching of F₀, indicating that NPQ was determined mostly by energy-dependent quenching (qE). The qEis the light-harvesting complex located component of NPQ and depends on the amount of zeaxanthin molecules bound in PSII proteins. Thus, difference between daily courses of NPQ in the two genotypes was probably due to different ways of utilization of the zeaxanthin pool under the interactive effect of ozone and moderate heat stress.