The influence of CO₂ and O₃ , 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 CO₂ , ('ambient', daily means of ∼ 385 μmol^-1 ; elevated, daily means of ∼ 765 μmol mol^-1 ) and two O₃ regimes ('non-polluted' air, 24 h mean of 20 nmol mol^-1 ; polluted air, 24 h mean of 73 nmol mol^-1 ). Leaf gas-exchange measurements were made at intervals, and visible O₃ 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' CO₂ enrichment resulted in a progressive stimulation in A_sat , whilst there was a decline in g which decreased E (i.e. improved WUE_i ). The extra carbon fixed in elevated CO₂ 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 CO₂ -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 CO₂ , indicating that total uptake of these nutrients was not affected by CO₂ , and there was an increase in the C:N ratio in root (+ hypocotyl) tissue. In contrast, O₃ depressed A_sat , (∼ 26%) and induced slight stomatal closure, with the result that WUE, declined. All plants exposed to 'polluted' air developed typical visible symptoms of O₃ injury, and effects on carbon assimilation were reflected in reduced growth, with shoot growth maintained at the expense of the root. In addition, O₃ 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 O₃ on tissue concentrations of N, S, Mg and Ca. Interactions between the gases were complex, and often subtle. In general, elevated CO₂ counteracted (at least in part) the detrimental effects of phytotoxic concentrations of O₃ , whilst conversely, O₃ reduced the impact of elevated CO₂ . Moreover, there were indications that cumulative changes in source: sink relations in O₃ -exposed plants may limit plant response to CO₂ -enrichment to an even greater extent in the long-term. The future ecological significance of interactions between CO₂ and O₃ are discussed.