Nitrate additions enhance the photosynthetic sensitivity of a nodulated South African Mediterranean-climate legume (Podalyria calyptrata) to elevated UV-B

Nitrate deposition from anthropogenic activities into nutrient impoverished soils of Mediterranean ecosystems represents a significant income to their N economy, which may potentially increase the sensitivity of those typically UV-B (280-315 nm) resilient plants from these ecosystems with superior photosynthetic rates especially to increases in solar UV-B flux due to ozone depletion. This proposal was examined by exposing nodulated Podalyria calyptrata seedlings for 6-months in nitrate deficient and nitrate replete sand culture to biologically effective UV-B (UV-B-BE) supplements approximating 40 and 77% above clear-sky background (control) at an outdoor site. Leaf photosynthesis and chemical composition of purely symbiotic plants were unaffected by increased UV-B-BE. Conversely, nodulated plants that received nitrate supplements displayed a linear reduction in stomatal conductance (g(S)) but non-linear asymptotic reductions in light-saturated net CO2 assimilation rate (A(sat)), apparent carboxylation efficiency (ACE) and nitrogen use efficiency (NUE) with increased UV-B-BE. RuBP regeneration limitations on CO2 assimilation were not apparent, since A(sat) at saturating internal leaf CO2, concentrations displayed insignificant depressions with increased UV-B-BE. Nor was there any suggestion of diminished light absorption capacity of antenna complexes, or of photosynthetic inhibition due to starch accumulation, since leaf chlorophyll and carotenoid contents and non-structural carbohydrate concentrations were insignificantly altered by increased UV-B-BE. Also, there seemed unlikely photosynthetic inhibition due to reduced allocation of N to Rubisco with increased UV-B-BE in the nitrate-fed plants, since both ACE and A(sat) were negatively correlated with leaf N content, much of which probably constituted assimilated nitrate according to the less negative delta(15) N values. We suggest that the molecular processes that rendered P. calyptrata plants receiving nitrate supplements more sensitive to photosynthetic inhibition by increased UV-B-BE may be related indirectly to their more active metabolic state, this apparent from their elevated respiratory and photosynthetic rates, rather than to any direct UV-B effects on CO2 uptake and fixation. (C) 2003 Elsevier B.V. All rights reserved.