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

Musil, C.F., Kgope, B.S., Chimphango, S.B.M., Dakora, F.D.
Environmental and experimental botany 2003 v.50 no.3 pp. 197-210
Mediterranean climate, Podalyria calyptrata, absorption, anthropogenic activities, carbon dioxide, carboxylation, carotenoids, chlorophyll, correlation, ecosystems, income, leaves, legumes, nitrates, nitrogen content, nutrient use efficiency, ozone depletion, photosynthesis, ribulose-bisphosphate carboxylase, sand, seedlings, soil, stomatal conductance, ultraviolet radiation
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-BBE. Conversely, nodulated plants that received nitrate supplements displayed a linear reduction in stomatal conductance (gS) but non-linear asymptotic reductions in light-saturated net CO2 assimilation rate (Asat), apparent carboxylation efficiency (ACE) and nitrogen use efficiency (NUE) with increased UV-B(BE). RuBP regeneration limitations on CO2 assimilation were not apparent, since Asat at saturating internal leaf CO2 concentrations displayed insignificant depressions with increased UV-BBE. 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 Asat were negatively correlated with leaf N content, much of which probably constituted assimilated nitrate according to the less negative δ15N 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.