Drought tolerance is a nebulous term that becomes more nebulous the more closely we look at it, much as a newspaper photograph does when viewed through a magnifying glass. From the vantage point of an ecologist the features that distinguish xerophytic from mesophytic vegetation are clear. We can all tell that a cactus is more drought tolerant than a carnation. But when we look at crop plants, the features that confer drought tolerance are far from clear. The main reason for the contrast is that the traits we associate with xerophytes typically concern survival during drought, whereas with crops we are concerned with production - and insofar as the term ''drought tolerance'' has any useful meaning in an agricultural context, it must be defined in terms of yield in relation to a limiting water supply. Further, with the well-developed major crop plants, those of us trying to increase water-limited yield would be pleased to achieve improvements of just a few percent in environments that are highly variable in their water supply. This variability often means that several seasons are required to demonstrate the advantages of an allegedly improved cultivar. Traits that confer drought tolerance in such circumstances are subtle, and may manifest themselves in some types of drought but not in others. Indeed the most influential characters often have no direct connection to plant water relations at all, as I elaborate on below. I will concentrate on the agricultural rather than the natural environment (although there are no doubt lessons for us still to learn from analysing the behaviour of natural vegetation - see Monneveux, this volume), and will argue that drought tolerance is best viewed at an ontogenetic time scale - i.e. at the time scale of the development of the crop - weeks to months for an annual crop. The timing of the main developmental changes, like floral initiation and flowering, and the rate of development of leaf area in relation to the seasonal water supply, are the most important variables at this time scale. Occasionally though, rapid changes in the environment, such as a sudden large rise in air temperature and humidity deficit, perhaps associated with hot dry winds, make appropriate short-term physiological and biochemical responses essential for the survival of the crop. These short term responses may be amenable to cellular and sub-cellular manipulation, especially if the sudden environmental deterioration occurs at especially sensitive stages in development such as pollen meiosis or anthesis. Purists insist that ''drought'' is a meteorological term, that refers only substantial to periods in which rainfall fails to keep up with potential evaporation. Within the spirit of this meeting it is appropriate to interpret the term more loosely than this definition, and to define it as circumstances in which plants suffer reduced growth or yield because of insufficient water supply, or because of too large a humidity deficit despite there being seemingly adequate water in the soil.
