INTRODUCTION

Although there has been increasing awareness in the past decade of the potential significance of xylem embolisms for plant water relations, there is still an incomplete understanding of the importance of cavitations and consequent xylem embolism as a major factor in plant adaptation to dry environments. In this chapter we attempt to provide a theoretical basis for consideration of the significance of xylem embolisms and the different ways by which different plant species survive and grow in drought conditions. It might be expected that any loss of functional xylem as a result of xylem embolism would be disadvantageous. It is possible, however, that adaptation to dry conditions may involve either mechanisms that minimise the occurrence of cavitations, or mechanisms that minimise their detrimental consequences. In general it seems likely that more ‘conservative’ behaviours that minimise cavitations (for example by reducing water loss through stomatal closure) will also reduce the chances of high productivity. Circumstances where some controlled cavitation may be advantageous in terms of productivity are identified and appropriate stomatal control functions are outlined.