Photosynthesis and Stomatal Light Responses in Snap Beans Exposed to Hydrogen Sulfide and Ozone

Abstract

Apparent photosynthesis and leaf stomatal conductance were measured in well watered, field grown snap beans (Phaseolus vulgaris L. cv ‘GV50’) fumigated with various levels of H₂S alone and in the presence of 0.072 ppm ozone. Mean concentrations of H₂S were 0, 0.74, 3.25, and 5.03 ppm. Plants were fumigated from 1000 to 1400 hr (PUT) each day beginning at the first trifoliate leaf stage. The youngest fully expanded leaves were sampled after 18 ± 2 days of fumigation. The plants had 10-11 leaves and were flowering with about 80% of the flowers open. Hydrogen sulfide at 0.74 ppm increased maximum stomatal conductance (C _max) by 25% and maximum apparent photosynthesis (P _max) by 10%. Higher concentrations of H₂S depressed stomatal opening and CO₂ uptake, and O₃ plus H₂S depressed stomatal and photosynthetic response more than H₂S alone. At the highest H₂S concentration plus O₃, C _max was 41 %, and Pmax was 52% of the control. Both C_max and P _max responded similarly to the 8 treatments but the difference between maximum and minimum responses was greater for conductance. As pollutant stress increased, photosynthesis ceased to respond linearly to increasing conductance at lower conductance values indicating that mesophyll resistance to CO₂ transfer was more limiting than CO₂ diffusion through the stornata at higher light intensities. The initial slope of the curves of both conductance and photosynthesis as a function of irradiance decreased with increasing stress, suggesting decreased sensitivity to light. This indicates an increased sluggishness in the guard cells, which could be related to induced leaf senescence caused by the pollutants. It also indicates a loss of photochemical conversion efficiency resulting from loss of chlorophyll and development of necrotic areas on the leaves of the more stressed plants.