Adverse effects of ambient ozone on watermelon yield and physiology at a rural site in Eastern Spain

Thf study reported was conducted to establish the impacts of photochemical oxidants (primarily ambient ozone, O-3) on the yield of watermelon (Citrull us lanatus) at a site on the east coast of Spain. Fruit yield and quality w ere monitored in plots established in a commercial watermelon field exposed , in open-top chambers (OTCs), to nonfiltered air (NFA; near-ambient levels of ozone) or charcoal-filtered air (decreased levels of photochemical oxid ants including O-3; CFA), or to ambient air (AA), during the 1988 and 1989 growing seasons. Ambient levels of O-3 were found to exceed present UN-ECE (United Nations Economic Commission for Europe; Convention on Long-Range Tr ansboundary Air Pollution) critical level guidelines for the protection of crop yield by approx. twofold in 1988 and by approx. fivefold in 1989. Plan ts exposed ro NFA and XX developed visible O-3 injury on the upper surface of sun-exposed older leaves, and fruit yield (annual marketable fruit weigh t and number) was found tu be depressed in OTCs ventilated with NFA in comp arison with those receiving CFA. Consistent with inter-annual variations in O-3 exposure, greater yield losses were experienced in 1989 (39%) than in 1988 (19%), an effict mediated predominantly by a decline in fruit number r ather than average fruit weight. Exposure to ambient levels of ozone also s lightly decreased fruit quality (4-8% decline in soluble solids content). L eaf gas exchange measurements made in the held in 1988 revealed effects of O-3 on fruit yield and quality to be associated with a decline in the net C O2 assimilation rate per unit leaf area under light saturation (A(sat)) and stomatal conductance to water vapour (g(s)), and enhanced rates of dark re spiration. A/c(1) curves (where A is the net CO2 assimilation rare per unit leaf area and c(1) is the mole fraction of CO2 in the leaf intercellular a ir space) constructed for plants grown in laboratory-based closed chambers, and exposed to an accumulated O-3 exposure similar to that experienced by plants in the field, suggested that the likely cause of the decline in phot osynthetic capacity was (1) a decrease in the amount and/or activity of Rub isco and (2) an impaired capacity for regeneration of ribulose 1,5-bisphosp hate, which was not mediated through changes in the photochemical efficienc y of photosystem II (F-v/F-m, where F-v is variable chlorophyll a fluoresce nce and F-m is maximum chlorophyll a fluorescence). No shift in the relativ e stomatal limitation to photosynthesis was observed under the influence of O-3, suggesting that the decline in g(s) induced by the pollutant in both field and laboratory was the result, and not the cause, of the decrease in A(sat). Ozone exposure also caused a decrease in C isotope discrimination ( approx 0.5 parts per thousand), a shift that revealed a departure from pred icted theory based on supporting leaf gas exchange measurements. The study demonstrates that ambient levels of photochemical oxidants on the Spanish M editerranean coast are high enough to adversely influence the yield and phy siology of an economically important crop grown in the region, and the magn itude of the effects was greater than would he predicted from exposure-yiel d-loss relationships for grain and fodder crops in central and northern Eur ope. Implications for the derivation of UN-ECE ozone standards are discusse d, along with the mechanistic basis of the observed yield decreases.