The Physiology of Flowering Plants - KHAM PHA MOI

366 RESISTANCE TO STRESSare believed to refold unfolded proteins. Thus many HSP appear to havechap erone activ ity ( Section 13.3. 4), as d o some COR and LEA proteins.Another HSP, ubiquitin, tags proteins to which it binds for degradation,and it is thought that this is how heat-damaged proteins are marked forhydrolysis.The production of HSP is a transient phenomenon and the synthesisof ‘normal’ proteins is resumed after a period; in soybean(Glycine max (soja)) treated at 40 8C this occurs after six hours, theoriginal mRNA being preserved in an inactive form while the mRNAfor HSP is being translated. On return to below the threshold temperature,the HSP synthesis ceases and thermotolerance is lost. Pearlmillet (Pennisetum glaucum) given daily 50 8C heat shocks undergoesdaily cycles of acclimation, with HSP synthesis and dehardening(Howarth 1991). Although the HSP are long-lived – e.g. one HSPfrom pea has a half-life of 52 days – new synthesis seems to berequired each time the temperature is raised. The presence of previouslysynthesized HSP fails to maintain thermotolerance at lowertemperatures.Some enzymes from thermotolerant plants are more heat-stablethan the corresponding enzymes from heat-sensitive species, andincreases in the thermostability of some enzymes during heat hardeninghave been noted, but this is only occasional.The properties of membrane lipids are also implicated in heattolerance, the lipids from more tolerant or hardened tissues beingmore saturated. With a rise in temperature, membrane fluidityincreases because of increased thermal motion of the molecules; inan excessively fluid membrane, membrane proteins are no longerheld in the precise spatial arrangement needed for normal membranefunction: in effect the membrane melts. A high level of lipidsaturation raises the melting point and confers more rigidity on themembranes.13.6 Relationships between different types of stressresistance: cross-toleranceThe resistance to high temperature of a number of temperate-zoneevergreens shows two annual maxima. One is in the summer asexpected; the second coincides with the midwinter maximum ofresistance to cold and water-deficit stress. Hardening against coldor water stress increases thermotolerance; hardening against freezinghardens against water-deficit stress and vice versa. Furthermore,during the midwinter maximum resistance towards freezing, waterdeficit and high temperature, plants also show high resistancetowards other injurious factors such as toxic chemicals and lack ofO 2 . This is known as cross-tolerance. Individual resistance genesmay be activated via more than one pathway, and one stress signalmay activate several pathways, resulting in complex networks