Interactive effects of ozone and soil volume on Plantago major

Plantage major L. ssp. major usually grows on trampled paths, tracks and gateways, and its root system is often restricted by compaction or by a limited volume of soil. In this heterogeneous environment, individuals close to each other will experience variable degrees of root restriction. As some populations of P. major have been demonstrated to be sensitive to ozone, the aim of this study was to investigate the effect of root restriction on the response of P. major to ozone. It was hypothesized that root restriction should reduce the effects of ozone. If this is true it would have implications for evolution of ozone resistance. A single population of P. major was grown in controlled-environment chambers and exposed to either charcoal/Purafil-filtered air or to ozone (70 nl O-3 l(-1) for 7 h d(-1)). Plants were grown in pots of four different volumes: 0.1751; 0.3651; 2.2801; and 5.8301. Four harvests were performed, at 21, 37, 51 and 91 d from germination. At the final han est, the total nitrogen content of leaf tissue was analysed and compared with that of plants collected from the field. Stomatal conductance (g(s)) and net carbon assimilation (A) rate were also recorded during development. Plants grown in the smallest pots exhibited severely impaired growth, lower g(s) and lower A than non-restricted plants. Root growth was less impaired than shoot growth. However, only plants in the smallest pots resembled those found in the field, which implies that realistic assessment of ozone effects on this species should involve a degree of root restriction. Appearance and leaf nitrogen content of leaves suggested that root restriction may have caused nitrogen deficiency. Contrary to the initial hypothesis, exposure to ozone reduced the growth of plants in all sizes of pot. Stomatal conductance was consistently lower in plants exposed to O-3, but there was no effect of ozone on A.