THE INFLUENCE OF CO2 AND O-3, SINGLY AND IN COMBINATION, ON GAS-EXCHANGE, GROWTH AND NUTRIENT STATUS OF RADISH (RAPHANUS-SATIVUS L)

Five days after emergence radish (Raphanus sativus L. cv. Cherry Belle) plants were transferred to a phytotron at the GSF Munchen, where they were exposed in four large controlled climate chambers to two at atmospheric concentrations of CO2 ('ambient', daily means of almost-equal-to 385-mu-mol mol-1; elevated, daily means of almost-equal-to 765-mu-mol 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(s) 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), (almost-equal-to 26 %) and induced slight stomatal closure, with the result that WUE(i) 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.