Photosynthesis, chlorophyll fluorescence, and yield of snap bean (Phaseolus vulgaris L.) genotypes differing in sensitivity to ozone

Understanding the impact of pollutant ozone (O3) is a concern for agricultural production. This work was undertaken as the first comparative study of the effects of O3 on the photosynthetic processes and yield of three snap bean (Phaseolus vulgaris L.) genotypes with known differences in sensitivity to O3 (S156, R123 and R331). Previous information showed R123 and R331 to be tolerant and S156 sensitive. The purpose was to identify physiological subsystems that may mediate those differences in sensitivity. Plants were grown in environmentally controlled field chambers with four levels of O3 (0, 15, 30 and 60 nmol mol−1). Net assimilation (A) and fluorescence were measured throughout the growing season and yield data were collected at physiological maturity. All genotypes were tolerant of low O3 (<30 nmol mol−1) but the highest O3 significantly reduced the yield in all three, with R331 and S156 being equally sensitive on a unit exposure basis. Yield reductions were correlated with A, especially during pod filling. No genotype showed any significant response of stomatal conductance (gs) indicating equal O3 fluxes into the leaves in all genotypes. Mesophyll conductance (gm) was affected in S156 only, where it was reduced by 55% at 60 nmol mol−1 O3. There was an upward trend in F0, and a downward trend in the variable fluorescence ratio (Fv/Fm) with increasing O3 for S156 but not for the other genotypes. S156 was the only genotype to show significant decreases in photochemical quenching (qp) and R123 the only one to show significant decreases in non-photochemical quenching (qn). The sequence of loss of Rubisco content and/or activity and changes in gm, F0, and Fv/Fm could not be resolved in time and may all have been the result of generalized tissue destruction rather than sequential attack on individual subsystems. S156 had the highest photosynthetic rate in clean air but appeared to have no significant capacity to protect Rubisco from attack or to up-regulate Rubisco activity at high O3, thus there was no reserve capacity, while R123 was able to maintain both Rubisco activity and A within narrow ranges. These data suggest that S156 has comparatively little anti-oxidant capacity and/or is deficient in its ability to regulate Rubisco activity. For future studies the best contrasts for resolving questions of physiological sensitivity to O3 would be obtained from R123 and S156.