Nitrogen status affects UV-B sensitivity of cucumber

Abstract

The effects of an increase in ultraviolet-B radiation (UV-B, 280–320 nm) on growth, photosynthesis and photoinhibition of a UV-B sensitive variety of cucumber (Cucumis sativus L. cv. Poinsett) were investigated at four levels of applied nitrogen (N, 0.5, 2.0, 5.0 and 10.0 mol m^-3). Plants were grown for 33 days in a UV-B transparent greenhouse at ambient and ambient +25% biologically effective UV-B radiation (UV-B_BE). A modulated lamp system controlled supplemental UV-B levels; average doses of UV-B_BE were 2.5 ± 0.8 (ambient) and 3.1 ± 1.0 (ambient +25%) kJ m^-2 d^-1. The doses are comparable to the expected change in UV-B_BE between 1979 and 1999 under similar weather conditions at this latitude. Plant biomass, height, and leaf N, chlorophyll and leaf carbon content increased with increasing amounts of applied N. At the highest level of N application, supplemental levels of UV-B caused a near identical decrease in leaf area (20%), height (28%) and total biomass (20%) compared with plants grown under ambient UV-B conditions. There was no such effect when N stress was imposed. Apparent quantum yield (&phis;_app), dark respiration and photosynthetic rate were unaffected by the supplemental UV-B and photodestruction of reaction centres was not evident. However, there was a significant increase in photoinhibition at midday in the N-replete plants. In leaves with N content <3%, supplemental levels of UV-B caused the concentration of UV-B absorbing compounds to increase significantly (72% at the lowest level of applied N). The epidermis was thicker at all levels of N under supplemental UV-B. The stress response to low N may confer secondary protection against UV-B damage and changes in N status due to fertilisers or pollution may lead to changes in UV-B sensitivity.