UNDERSTANDING PHOTOSYNTHESIS, PIGMENT AND GROWTH-RESPONSES INDUCED BY UV-B AND UV-A IRRADIANCES

Plant response to UV-B (0.290-0.320 mu m) irradiation in controlled environments has been difficult to assess, possibly because plants also respond to UV-A (0.320-0.400 mu m) and visible radiation. Photosynthetic dysfunction is often reported, but effects on photosynthetic pigments have been equivocal. Because UV-A/blue radiation is involved in pigment synthesis, the experimental UV-A irradiation was controlled and this study was conducted under high ambient photosynthetic photon flux (mid-day PPF greater than or equal to 1400 mu mol m(-2) s(-1)). Two biologically effective UV-B irradiances (10.7 and 14.1 kJ m(-2) day(-1)) were utilized and the UV-A irradiances were matched in controls (similar to 5 and 9 kJ m(-2) day(-1)). Normal and two mutant pigment isolines (chlorophyll-deficient, flavonoid-deficient) of soybean cultivar Clark were utilized for comparisons. Many pigment/growth variables exhibited a statistical interaction between spectral quality and quantity. UV-A/blue photoregulation was demonstrated in the UV-A controls. The pigment/growth pattern observed at the lower UV-B irradiance was interpreted as a photosystem II response similar to shade adaptation, suggesting phytochrome involvement in UV-B irradiation responses. On the other hand, two variables most commonly observed to manifest UV-B-induced effects-decreased photosynthesis and increased leaf flavonoid content-exhibited no interactions due to UV exposure or spectral quality. In general, the observed response patterns indicated either moderation of UV-B-induced responses by UV-A/blue radiation, or coaction between them, and provides an explanation for the common failure to demonstrate fluence-related responses in UV-B experiments.