Exposure of a grass-clover mixture to ozone in open-top chambers - Effects on yield, quality and botanical composition

A field-grown grass-clover mixture was exposed to four different levels of ozone in open-top chambers: filtered air, filtered air and non-filtered air with two different levels of ozone added. There was also an ambient air treatment with no chambers. The same pasture was exposed to ozone for two consecutive growing seasons. Three cuts per season were made. There was a negative relationship between yield and ozone concentration, and a significant regression was obtained between total dry weight yield for all six harvests and the Accumulated exposure Over Threshold 40 nl l(-1) (AOT40) for the whole exposure period. Analysis of variance did not reveal significant ozone effects on quality parameters (fibre content, energy content, protein, Ca, Fe, Mo). Fibre content and botanical composition were, however, influenced by enclosure of the plants in the open-top chambers. The importance of the ozone concentration gradient above a field crop for the estimation of yield loss is discussed. It Is hypothesised that the lower ozone sensitivity in pasture compared with wheat is due to the fact that pasture is harvested before natural senescence, while wheat is exposed during this ozone sensitive phase of plant development.