The influence of CO and O , singly and in combination, on gas exchange, growth and nutrient status of radish (Raphanus sativus L.).

Five days after emergence radish (Raphanus sativus L. ev. Cherry Belle) plants were transferred to a phytotron at the GSF München, where they were exposed in four large controlled climate chambers to two atmospheric concentrations of CO , ('ambient', daily means of ∼ 385 μmol ; elevated, daily means of ∼ 765 μmol mol ) and two O regimes ('non-polluted' air, 24 h mean of 20 nmol mol ; polluted air, 24 h mean of 73 nmol mol ). Leaf gas-exchange measurements were made at intervals, and visible O damage, effects on growth, dry matter partitioning and mineral composition were assessed at a final whole-plant harvest after 27 d. In 'non-polluted air' CO enrichment resulted in a progressive stimulation in A , whilst there was a decline in g which decreased E (i.e. improved WUE ). The extra carbon fixed in elevated CO stimulated growth of the root (+ hypocotyl) by 43 %, but there was no significant effect on shoot growth or leaf area. Moreover, a decline in SLA and LAR in CO -enriched plants suggested that less dry matter was invested in leaf area expansion. Tissue concentrations of N, S, P, Mg and Ca were lower (particularly in the root + hypocotyl) in elevated CO , indicating that total uptake of these nutrients was not affected by CO , and there was an increase in the C:N ratio in root (+ hypocotyl) tissue. In contrast, O depressed A , (∼ 26%) and induced slight stomatal closure, with the result that WUE, declined. All plants exposed to 'polluted' air developed typical visible symptoms of O injury, and effects on carbon assimilation were reflected in reduced growth, with shoot growth maintained at the expense of the root. In addition, O increased the P and K concentration in shoot and root (+ hypocotyl) tissue, indicating enhanced uptake of these nutrients from the growth medium. However, there was no affect of O on tissue concentrations of N, S, Mg and Ca. Interactions between the gases were complex, and often subtle. In general, elevated CO counteracted (at least in part) the detrimental effects of phytotoxic concentrations of O , whilst conversely, O reduced the impact of elevated CO . Moreover, there were indications that cumulative changes in source: sink relations in O -exposed plants may limit plant response to CO -enrichment to an even greater extent in the long-term. The future ecological significance of interactions between CO and O are discussed.