Stomatal resistance, transpiration, and relative water content as influenced by soil moisture stress

The effect of increasing soil moisture stress on stomatal resistance, transpiration, and relative water content of cucumber, beans and tomato has been investigated under controlled conditions. The results demonstrated that stomatal resistance can be used as a tool by which the soilwater-plant relationship can be predicted. Transpiration rates exhibited an initial rise as the soil moisture decreased, followed by a steady reduction. No significant changes in the relative water content of the leaves were observed until a severe soil moisture stress took place. For each species a linear relationship was obtained between the stomatal diffusive resistance (rsd) and the ratio of the vapour pressure difference between leaf surface and bulk air(Ae) over transpiration (E). The external resistances to water vapour (ra) obtained from this relationship were 7.2,4.4 and 2.8 sec cm-1 for cucumber, tomato and bean leaves, respectively. An essentially linear relationship was obtained between rsd and viscous air flow. The intercellular resistance (r,) obtained from extrapolation of the curves, were 12.0, 8.7 and 6.6 sec cm"1 for cucumber, bean and tomato, respectively. The external resistance calculated from the sensible heat transfer is well below ra values in all species. The relatively high values of ra and r, have been discussed.