Elevated Ultraviolet-B Radiation Affects Sweetpotato Growth and Yield.

To understand the consequences of raising levels of ultraviolet-B radiation (UV-B) reaching the Earth’s surface on an important storage root crop, sweetpotato, an experiment was conducted using sunlit plant growth chambers under three levels of UV-B, 0, 5 and kJ m^-2.  Ultraviolet-B treatments were imposed from 16 days after planting for plants grown at optimum water and nutrient and temperature (30/22°C, day/night) conditions.  During the treatment period, vine lengths and leaf numbers were recoded. Photosynthesis, relative injury to cell membranes, phenolic and pigment contents were measured also. At final harvest, root number (storage and adventitious) and dry weights of plants parts were determined, 70 days after treatment. Even though total root numbers were not affected by UV-B levels, 3 and 9% fewer storage roots were recorded at 5 and 10 kJ of UV-B, respectively. Total plant weight decreased by 20% at 5 kJ and 26% at 10 kJ of UV-B compared to 0kJ.  Among the plant components dry weights, the greatest decreases were recorded for storage roots, 27% at 5 kJ and 31% at 10 kJ compared to UV-B and leaves, 31% at 5 kJ and 28% at 10 kJ of UV-B compared to 0 UV-B. The decreases for stems were 5 and 17% at 5 kJ and 10 kJ of UV-B, respectively, compared to 0 UV-B.  In addition, plant grown at higher UV-B levels produced 10-13% more UV-B screening compounds such as phenolic compounds in the leaves compared to 0 UV-B. Decreased plant component dry weights were due to lower photosynthesis, particularly in the aged leaves and smaller leaf area under UV-B treatments compared to the zero UV-B. Future studies should screen genotypes for UV-B tolerance and to develop cultivars tolerant to current and possible future increases in UV-B radiation levels across the sweetpotato growing areas.