Drought and air pollution affect nitrogen cycling and free radical scavenging inPinus halepensis(Mill.)

Abstract

Fumigation of Aleppo pines with episodes of O₃ (up to 110 nl l⁻) causes immediate depressions of in vivo nitrate reductase (NaR) activities, slightly delayed reductions in the rates of ethene emissions (typical of O₃ plants), steady accumulations of total polyamines (although putrescine declines), and increases in pool sizes of reduced glutathione (GSH) and ascorbate in current year needles. Severe droughting produces smaller plants with reduced stomatal conductance and CO₂ assimilation rates as well as lower protein contents. Their roots have low rates of nitrate uptake but virtually no root NaR activities, while levels of shoot activities and NaR-associated proteins are unaffected although they have no substrate. Less severe droughting allows a restricted uptake of nitrate which is still reflected in reduced NaR activities, protein and total N contents, but the additional presence of O₃ (up to 120 nl l⁻¹) has no interactive effect on N cycling. Drought and O₃ together, however, depress CO₂ assimilation still further, which can not be accounted for by additional stomatal closure, but the interactive effects of drought and air pollution reduce levels of total phenols, GSH and ascorbate which, combined with a 12-fold reduction in glutathione reductase-(GR)-associated proteins, point to an increased susceptibility of Aleppo pines to photoinhibition as a reason for their current decline in Mediterranean areas.