Photosynthesis, Dark Respiration, and Growth of Rumex patientia L. Exposed to UV-B (280-315 nm) Irradiance Corresponding to Reduced Atmospheric Ozone Concentrations
Net photosynthesis, dark respiration, chlorophyll concentrations and growth were determined for Rumex patientia L. exposed to UV-B radiation corresponding to reduced atmospheric ozone concentrations. The hypothesis of whether reciprocity is maintained in the response of R. patientia to polychromatic UV-B radiation was tested. On the basis of the relationships derived from these studies, a simulation model was developed for the prediction of photosynthesis and growth of R. patientia exposed to UV-B radiation corresponding to any atmospheric ozone reduction.
Photosynthetic rates were found to be depressed after two hours exposure to UV-B irradiance simulating a 0.18 atm•cm ozone column when the sun is at 30° from the zenith. During t his initial exposure period, partial stomatal closure was implicated in the suppression of photosynthesis. However, after one day exposure, substantial increases in photosynthetic resistances apart from stomatal diffusion resistance occurred in the UV-irradiated plants and no differences in stomatal diffusion resistance were apparent between UV-irradiated and control plants. Dark respiration rates were slightly higher in those plants exposed to UV radiation.
Leaf expansion of B-. patientia was substantially repressed but only during the initial few days of exposure. Thereafter, leaf expansion was similar in the UV-irradiated and control plants. A reduction in total plant dry weight and leaf area xiv of approximately 50 percent occurred after 22 days treatment while chlorophyll concentrations remained unaltered. Time of leaf initiation was shown to be delayed in those plants exposed to UV-B radiation. Leaf longevity was decreased with increased UV radiation but accelerated whole-plant senescence and death was not observed.
Photosynthetic rates determined through the ontogeny of the third leaf of R. patientia exposed to four levels of UV irradiance were found to be depressed as a function of the accumulated biologically effective UV irradiation. Thus, reciprocity was demonstrated between 6350 and 3175 J biologically effective UV irradiation.
Results of the simultion model showed that under reduced atmospheric ozone concentrations, suppression of photosynthesis and leaf growth would be more severe during mid-summer (i.e. June) than would occur during the March to early May period. This results from smaller solar angles from the zenith and lower prevailing ozone concentrations prevalent during June.
A validation test of the model was made with photosynthetic data obtained during a field study with R. patientia exposed to UV-B radiation corresponding to a 38 percent atmospheric ozone reduction (0.18 atm•cm when the sun was at 30° from the zenith). This validation test showed a reasonable correspondence between the measured and predicted photosynthetic rates.
R. patientia was selected as the test species for this study because (1) it is reasonably sensitive to UV radiation as determined in preliminary studies evaluating approximately 20 native and agricultural plant species, (2) it is normally exposed to full sunlight in its natural habitat, and (3) individual leaves are relatively long-lived (about 60 days) and are not normally shaded by other leaves of the same plant. Although this species probably represents one of the more sensitive plants to UV radiation, it would be this group of sensitive plants that would be initially affected under conditions of reduced atmospheric ozone. If more resistant plants with long-lived plant parts also accumulate UV radiation damage as was shown to occur in R. patientia, over sufficient periods of time even these species might be significantly impacted under conditions of reduced atmospheric ozone.