Enhancement of photosynthesis in Sorghum bicolor by ultraviolet radiation

We assessed the influence of ultraviolet radiation (UV) on net photosynthetic CO₂ assimilation rate (P_n) in Sorghum bicolor, with particular attention to examining whether UV can enhance P_n via direct absorption of UV and absorption of UV-induced blue fluorescence by photosynthetic pigments. A polychromatic UV response spectrum of leaves was constructed by measuring P_n under different UV supplements using filters that had sharp transmission cut-offs from 280 to 382 nm, against a background of non-saturating visible light. When the abaxial surface was irradiated, P_n averaged 4.6% higher with the UV supplement that cut-off UV at 311 nm, compared to lower and higher UV wavelength supplements. This former supplement differed from higher wavelength supplements by primarily providing more UV between 320 and 350 nm. To assess the possibility of direct absorption of UV by photosynthetic pigments, we measured the absorbance of extracted chlorophylls. Chlorophyll a had absorbance peaks at 340 and 389 nm that were 49 and 72% of that at the sorét peak. Chlorophyll b had absorbance peaks at 315 and 346 nm that were both 35% of that at the sorét peak. Since the epidermis transmits some UV, the strong UV absorbance of chlorophyll implies a potential role for irradiance beyond the bounds of the conventionally defined photosynthetically active radiation waveband (400-700 nm). To assess the role of absorption of UV-induced blue fluorescence, we measured the UV-induced fluorescence excitation and emission spectra of leaves. Abaxial excitation peaked at 328 nm, while emission peaked at 446 nm. In this analysis, we used our abaxial fluorescence excitation spectrum and the UV photosynthetic inhibition spectrum of Caldwell et al. (1986) to weight the UV irradiance with each cut-off filter, thereby estimating the potential contribution of UV-induced blue fluorescence to photosynthesis and the inhibitory effects of UV irradiance on photosynthesis, respectively. With a non-saturating visible background, we estimate that the absorption of UV-induced blue fluorescence and the direct absorption of UV by photosynthetic pigments maximally enhanced photosynthesis by about 1% each with the UV supplement that cut-off UV at 311 nm. We suggest that a portion of the incident UV on the S. bicolor leaves was used to drive photosynthesis.