The effect of supplemental ultraviolet-B (UV-B) irradiation on Phaseolus vulgaris, cv. contender leaf cell enlargement

The effect of UV-B on leaf growth long has been debated. Early work sometimes suggested that UVB, especially in small amounts, enhanced leaf growth. Later experiments more frequently showed that UV-B inhibited leaf growth. Other analyses have held that UV-B may have little or no effect on leaf size. The confusion concerning UV-B's effect on leaf growth may have arisen because experimenters did not separate the leaf expansion phase characterized primarily by cell division from the leaf expansion phase marked by cell enlargement. When the two phases of leaf growth have been differentiated, reductions in leaf expansion due to UV-B induced decreases in cell division have become apparent. The effect of UV-B on leaf cell enlargement has remained less clear. Using a protocol perfected by Elizabeth Van Volkenburgh to isolate the cell enlargement phase of Phaseolus vulgaris leaf expansion from the cell division phase, we have found that high levels of UV-B inhibit leaf cell enlargement while low levels of UV-B promote leaf cell enlargement. Moreover, we have seen that UV-B-induced reductions in leaf cell enlargement are significantly (p < 0.01) correlated both with reductions in phenylpropanoid compounds and increases in leaf apoplastic pH. Decreases in phenylpropanoid compounds may increase leaf susceptibility to UV-B-induced damage. Then, in accordance with the acid growth hypothesis, it is possible that UV-B-associated increases in apoplastic pH lead to decreased cell wall elasticity and thus reduced growth.