Variation in the amount of foliage on woody shoots and its effects on water relations parameters derived from pressure–volume curves

Abstract

The interpretation of pressure–volume curves for woody plant material is complicated by the use of foliated twigs to derive estimates of the water relations parameters of just the foliage. In this study, the ratio of foliage to twig water in shoots of loblolly pine (Pinustaeda) was varied systematically by defoliating twigs to varying degrees prior to constructing pressure–volume curves. Results show that variation in the ratio of foliage to twig water can influence interpretation of the curves. Reducing the amount of foliage causes decreases in the estimates of apoplasmic water content. Estimates of osmotic potential at full turgor and water potential at turgor loss were less affected. When foliated and fully defoliated twigs were compared, foliated twigs had lower water potentials at the same relative water contents. This may partially explain some of the differences in the parameter estimates among the treatments. If care is taken to avoid large differences in relative foliage weight, twigs can be used to estimate water relations characteristics of the foliage. For loblolly pine though, estimates of apoplasmic water content in twigs will be lower than in single fascicles. If single fascicles are used, more samples will have to be taken to obtain the same degree of confidence about the mean as for twigs, since the coefficient of variation is higher.