Water relations of potato leaves, I. Diurnal changes, gradients in the canopy, and effects of leaf-insertion number, cultivar and drought

Abstract

Potatoes (Solanum tuberosum L.) were grown in containers under a rain shelter for measurement of leaf water relations. Leaf water potentials (ψ) were measured with a pressure chamber. Osmotic potentials (π) were measured with a freezing point depression osmometer or with a vapour pressure osmometer after freezing and thawing of the leaves. Turgor (P) was obtained from the difference between ψ and π.At the middle of cloudless days and in the absence of water stress, ψ declined to about -1 MPa and P to 0·1 MPa (in this paper no allowance is made for apoplastic dilution). Gradients in ψ and its components along a stem were measured. Opposite gradients in ψ and π were found in transpiring plants. These were explained by decrease in π with leaf-insertion number.At full turgidity a representative value for P is 0·65 MPa and that for the bulk volumetric modulus of elasticity 7·5 MPa; turgor loss typically occurred at a relative water content of 0·92 when ψ was -0·8 MPa.Leaf water relations did not differ between the cvs Bintje, Kennebec and Saturna which cultivars are known to differ in drought resistance. Changes in π due to drought (osmotic adjustment) were maximally 0·25 MPa in leaves that had appeared before the stress period. Transient drought led to markedly higher π values after relief of stress in leaves that had appeared during stress (in comparison to controls). Measurements of leaf water relations are probably not very effective to identify drought resistant genotypes. © 1988 Annals of Botany Company.