A thirty percent increase in UV-B has no impact on photosynthesis in well-watered and droughted pea plants in the field

It has been suggested that field experiments which increase UV-B irradiatio n by a fixed amount irrespective of ambient light conditions ('square-wave' ), may overestimate the response of photosynthesis to UV-B irradiation. In this study, pea (Pisum sativum L.) plants were grown in the field and subje cted to a modulated 30% increase in ambient UK summer UV-B radiation (weigh ted with an erythemal action spectrum) and a mild drought treatment. UV-A a nd ambient UV control treatments were also studied. There were no significa nt effects of the UV-B treatment on the in situ CO2 assimilation rate throu ghout the day or on the light-saturated steady-state photosynthesis. This w as confirmed by an absence of UV-B effects on the major components contribu ting to CO2 assimilation; photosystem II electron transport, ribulose 1,5-b isphosphate regeneration, ribulose 1,5-bisphosphate carboxylase/oxygenase c arboxylation, and stomatal conductance. In addition to the absence of an ef fect on photosynthetic activities, UV-B had no significant impact on plant biomass, leaf area or partitioning. UV-B exposure increased leaf flavonoid content. The UV-A treatment had no observable effect on photosynthesis or p roductivity. Mild drought resulted in reduced biomass, a change in partitio ning away from shoots to roots whilst maintaining leaf area, but had no obs ervable effect on photosynthetic competence. No UV-B and drought treatment interactions were observed on photosynthesis or plant biomass. In conclusio n, a 30% increase in UV-B had no effects on photosynthetic performance or p roductivity in well-watered or droughted pea plants in the field.