The Physiology of Flowering Plants - KHAM PHA MOI

346 RESISTANCE TO STRESSFig: 13:1 Generalized scheme forthe sequence of events initiated bystress on plant cells. Sensitive plantsincapable of hardening sufferdeleterious effects which can befatal. In plants capable of hardening,the perception of stress, which maybe via a damage effect, leads to aswitch in gene activity and hence tochanges in cell organization andphysiology which counteract thedeleterious effects (curved upwardarrows).respiration respectively, and heavy metals in Chapter 4, with mineralnutrition.Hardening towards stress follows the same pattern in all cases, asillustrated in very general terms in Fig. 13.1. There is first perceptionof the stress; this may or may not be via the damage caused.This is followed by a signal transduction chain, also termed asignal cascade, leading to a changed pattern in gene transcriptionand in the synthesis of proteins that can counteract the deleteriouseffects of the stress, either directly or via their products if theproteins are enzymatic. Plant reactions to stress have proved to beextremely complex, with one stress able to evoke several reactionchains, with cross-reactions. On the other hand, the three factors,water deficit, low temperature and high temperature, might appearunrelated, but in fact their effects on plants show common featuresand they share (parts of) signal transduction chains.13.3 Water-deficit stressWater-deficit stress (water stress, desiccation stress, drought) isdefined as exposure to a low external C. Except in the most humid