DROUGHT AND DROUGHT TOLERANCE

Drought tolerance is a nebulous term that becomes more nebulous the mo re closely we look at it, much as a newspaper photograph does when vie wed through a magnifying glass. From the vantage point of an ecologist the features that distinguish xerophytic from mesophytic vegetation a re 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 contras t is that the traits we associate with xerophytes typically concern su rvival during drought, whereas with crops we are concerned with produc tion - and insofar as the term ''drought tolerance'' has any useful me aning in an agricultural context, it must be defined in terms of yield in relation to a limiting water supply. Further, with the well-develo ped major crop plants, those of us trying to increase water-limited yi eld would be pleased to achieve improvements of just a few percent in environments that are highly variable in their water supply. This vari ability often means that several seasons are required to demonstrate t he advantages of an allegedly improved cultivar. Traits that confer dr ought tolerance in such circumstances are subtle, and may manifest the mselves in some types of drought but not in others. Indeed the most in fluential characters often have no direct connection to plant water re lations at all, as I elaborate on below. I will concentrate on the agr icultural rather than the natural environment (although there are no d oubt lessons for us still to learn from analysing the behaviour of nat ural vegetation - see Monneveux, this volume), and will argue that dro ught tolerance is best viewed at an ontogenetic time scale - i.e. at t he 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 variabl es at this time scale. Occasionally though, rapid changes in the envir onment, such as a sudden large rise in air temperature and humidity de ficit, perhaps associated with hot dry winds, make appropriate short-t erm physiological and biochemical responses essential for the survival of the crop. These short term responses may be amenable to cellular a nd sub-cellular manipulation, especially if the sudden environmental d eterioration occurs at especially sensitive stages in development such as pollen meiosis or anthesis. Purists insist that ''drought'' is a m eteorological term, that refers only substantial to periods in which r ainfall 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 plant s suffer reduced growth or yield because of insufficient water supply, or because of too large a humidity deficit despite there being seemin gly adequate water in the soil.