The Physiology of Flowering Plants - KHAM PHA MOI

TERMINOLOGY AND CONCEPTS 345situations, what is considered a stress depends on individual judgement.Key terms and concepts applied in discussions of stress areexplained below.Stress resistance (hardiness): the ability to endure an externallyapplied stress, e.g. the ability to survive a low external C(water potential).Stress avoidance: the ability to prevent an externally appliedstress from producing an equivalent internal stress in theplant, e.g. the ability to maintain a high cellular water contenteven when the external C is low.Stress tolerance: the ability to survive an internal stress,e.g. the ability to survive a low cellular water content.Resistance to a stress can thus be achieved by stress avoidance, bystress tolerance, or a combination of both to various degrees.Hardening (acclimation): the development of stress resistance,stimulated by subjection to mild and/or gradually increasingstress.Some species are capable of hardening towards a particular stress,others are not. Hardening is a vital process, for plants potentiallyresistant towards some stress do not maintain the state of resistancecontinuously; for instance, perennials of climates with acold winter are resistant towards subzero temperatures only duringthe cold season, undergoing annual cycles of hardening andde-hardening. Thus a species may be considered hardy towards agiven stress, but an individual of that species may not be hardy ata particular time.Throughout this chapter, reference is made to ‘resistant’ or ‘tolerant’as contrasted with ‘sensitive’ plants and tissues, and to ‘high’or ‘severe’ as opposed to ‘low’, ‘moderate’ or ‘mild’ levels of stress.These terms are relative, depending on the system in question andon the criteria used to assess resistance. An investigator comparingthe effects of chilling at 5 8C on two plant species, and finding thatspecies A survived without damage, while species B suffered somelesions to its leaves, might describe species A as chilling resistant andspecies B as sensitive, using leaf damage as the criterion. But a secondworker comparing the effects of the same treatment on species Band species D, which was killed outright, could judge B to be resistant,on the criterion of survival. Literature should be consulted withsuch considerations in mind.This chapter deals with stresses resulting from water deficit (includingsalinity), low temperature and high temperature. Additionalstresses experienced by plants are excessive light, UV radiation,chemical pollutants (including atmospheric ozone and sulphurdioxide, and heavy metals in soil or water) and anaerobiosis. Lackof space precludes detailed consideration of these, but some referenceis made in other chapters: excessive light and anaerobiosis areconsidered in Chapter 2 in connection with photosynthesis and