Wall yield threshold and effective turgor in growing bean leaves

摘要

The rate of cell enlargement depends on cell-wall extensibility (m) and on the amount of turgor pressure (P) which exceeds the wall yield threshold (Y). The difference (P-Y) is the growth-effective turgor (Pe). Values ofP, YandPehave been measured in growing bean (Phaseolus vulgarisL.) leaves with an isopiestic psychrometer, using the stress-relaxation method to deriveY. When rapid leaf growth is initiated by light,P, YandPeall decrease. Thereafter, while the growth rate declines in maturing leaves,Ycontinues to decrease andPeactually increases. These data confirm earlier results indicating that the changes in light-stimulated leaf growth rate are primarily controlled by changes inm, and not by changes inPe. Seedlings incubated at 100 relative humidity have increasedP, but this treatment does not increase growth rate. In some casesYchanges in parallel withP, so thatPeremains unchanged. These data point out the importance of determiningPe, rather than justP, when relating cell turgor to the growth rate.