Changes in stomatal behaviour in the calcicole Leontodon hispidus due to the disruption by ozone of the regulation of apoplastic Ca²⁺ by trichomes

Abstract.

Regulation of the concentration of Ca²⁺ in the apoplast of plants is essential in order to allow Ca²⁺-dependent processes, such as Ca²⁺-mediated signal transduction in stomatal guard cells, to function correctly. This is particularly important for plants growing with high levels of Ca²⁺ in the rhizosphere. Recently, we have shown that in two calcicoles, Leontodon hispidus L. and Centaurea scabiosa L., trichomes play a key role in this regulatory process. Ozone is known to have a marked effect on plant Ca²⁺ homeostasis. Therefore, we have examined the effect of this pollutant on the regulation by trichomes of apoplastic Ca²⁺ in the calcicole L. hispidus. Treatment with 100 nl l^–1 ozone resulted in a reduction in stomatal conductance of approximately 25% in plants grown with 15 mM Ca²⁺ in the rhizosphere. Analysis of total Ca²⁺ levels revealed that these changes in stomatal behaviour reflect a decrease in the ability of trichomes to sequester Ca²⁺. The amount of Ca²⁺ present in the trichome tip cell was reduced by approximately 38%. This was accompanied by an increase in the levels of Ca²⁺ in the guard cells and other tissues of the leaf. These data suggest that ozone has a detrimental effect on the ability of trichomes to regulate the concentration of apoplastic Ca²⁺ in L. hispidus, resulting in altered stomatal behaviour, and hence gaseous exchange, possibly due to the disruption of guard-cell Ca²⁺-mediated signal transduction. This has important implications for the growth and survival of plants growing in Ca²⁺-rich soils.