Rapid Changes in Transpiration in Plants

Abstract

IN the recent discussion in Nature under this heading^1–3, Rufelt^1,3 has adopted Ivanoff's⁴ hypothesis that the transient increase in transpiration rate observed when a leaf is detached in air is caused “by a sudden release of the water stress in the conducting elements, which means that it is caused by an increase in the water supply to the leaf”¹. He has further supposed that the increase in transpiration obtained after bathing the roots of intact plants with mannitol solution⁵, or 1 M sodium chloride¹, is also due to the release of water stress in the plant, making the subsidiary hypothesis that the mannitol or sodium chloride causes an increase in the water permeability of the roots¹. He concludes that this last effect is peculiar to the roots¹, and since he writes of “the permeability barrier in the root”³ he must postulate that the transpiration stream here passes through the cells, though in the whole of the rest of the plant he supposes it to move in the cell walls only, except when it is in the xylem. He would appear to have obtained no data for the behaviour of the stomata in his experiments; his discussion of the supposed absence of ‘mechanical effects’ and the different permeabilities of guard cells and epidermal cells therefore seems too great an extrapolation from his results—transpiration rate can only be taken as a measure of stomatal diffusive conductance when there is a constant potential difference across the stomata.