Effects of ultraviolet-B radiation on common bean (Phaseolus vulgaris L.) plants grown under nitrogen deficiency

Abstract

This work reports on the significance of UV-B absorbing compounds and DNA photorepair in protecting bean plants from UV-B radiation under nitrogen restriction. Bean plants grown in sterile vermiculite and irrigated periodically with a nutrient solution containing 12 or 1 mM of nitrate were irradiated with 22 mu W cm(-2) of UV-B, 4 h daily during 10 days after the first trifoliate leaf was developed. This intensity was equivalent to 3.2 kJ m(-2) per day, approximately. PAR fluence rate was 350 +/- 50 mu mol quanta m(-2) s(-1). Control plants did not receive UV-B irradiation. Leaf expansion was negatively affected by both nitrate restriction and UV-B irradiation. This decrease was paralleled by a significant increase in starch, which was exacerbated by the combined action of both factors. Combined action of low nitrogen and UV-B also negatively affected the CO2 assimilation rate and the stomatal conductance. Formation of UV-B absorbing compounds was significantly increased by both UV-B irradiation and nitrogen restriction and this increase was exacerbated by the combination of both factors. No significant increase in dimer formation was detected in irradiated plants at the UV-B dose used. Significant dimer formation was only obtained by using very high UV-B intensities. This suggests that under an irradiation level of 22 mu W cm(-2) of UV-B, which is close to natural conditions, protective mechanisms such as pigment screening and DNA photorepair were probably sufficient to prevent any dimer formation in leaves. (C) 2007 Elsevier B.V. All rights reserved.