INFLUENCE OF ULTRAVIOLET-B (UV-B) RADIATION ON PHOTOSYNTHETIC AND GROWTH-CHARACTERISTICS IN FIELD-GROWN CASSAVA (MANIHOT-ESCULENTUM CRANTZ)

The effects of ultraviolet-B (UV-B between 290 and 320nm) on photosynthesis and growth characteristics were investigated in field grown cassava (Manihot esculentum Crantz). Plants were grown at ambient and ambient plus a 5.5 kJ m-2 d-1 supplementation of UV-B radiation for 95d. The supplemental UV-B fluence used in this experiment simulated a 15% depletion in stratospheric ozone at the equator (0-degrees-N). Carbon dioxide exchange, oxygen evolution, and the ratio of variable to maximum fluorescence (F(v)/F(m)) were determined for fully expanded leaves after 64-76d of UV-B exposure. All plants were harvested after 95d of UV-B exposure, assayed for chlorophyll and UV-B absorbing compounds, and separated into leaves, petioles, stems and roots. Exposure to UV-B radiation had no effect on in situ rates of photosynthesis or dark respiration. No difference in the concentration of UV-B absorbing compounds was observed between treatments. A 2-d daytime diurnal comparison of F(v) to F(m) ratios indicated a significant decline in F(v)/F(m) ratios and a subsequent increase in photoinhibition under enhanced UV-B radiation if temperature or PPF exceeded 35-degrees-C or 1800 mumol m-2 s-1, respectively. However, UV-B effects on fluorescence kinetics appeared to be temporal since maximal photosynthetic rates as determined by oxygen evolution at saturated CO2 and PPF remained unchanged. Although total biomass was unaltered with UV-B exposure, alterations in the growth characteristics of cassava grown with supplemental UV-B radiation are consistent with auxin destruction and reduced apical dominance. Changes in growth included an alteration of biomass partitioning with a significant increase in shoot/root ratio noted for plants receiving supplemental UV-B radiation. The increase in shoot/root ratio was due primarily to a significant decrease in root weight (-32%) with UV-B exposure. Because root production determines the harvestable portion of cassava, UV-B radiation may still influence the yield of an important tropical agronomic species, even though photosynthesis and total dry biomass may not be directly affected.