Xerophytic implications of increased sclerophylly: interactions with water and light in Hakea psilorrhyncha seedlings

Abstract

Hakea psilorrhyncha R. M. Barker seedlings were subjected to a two-way interaction experiment with two levels of water availability and two levels of light. Physiological drought was imposed by adding 6000 MW polyethylene glycol (PEG), at an osmotic potential of −0·56 MPa, in a continuous-flow drip irrigation system. Unstressed plants (-PEG) were watered with distilled water (osmotic potential of −0·01 MPa). Seedlings were grown under natural light (120–500 μmol m⁻² s⁻¹ sunlight at midday +L) or shaded (<150 μmol m⁻² s⁻¹ sunlight at midday; −L. Plant morphology tissue water relations and carbon isotope composition (δ¹³C) were measured after 12 wk growth. The smallest leaves had the highest mass per unit area (LMA, an index of sclerophylly) and were present in the +PEG+L treatment, whereas the largest, thickest leaves had the lowest density and were produced in the −PEG+L treatment. Plants in shade were smaller and less sclerophyllous. There was a linear decrease in osmotic potential at full turgor and turgor loss point, and an increase in elastic modulus and δ¹³C, as level of sclerophylly increased, with −PEG−L leaves at one extreme and +PEG+L at the other. We conclude that high levels of sclerophylly induced by low water availability and high light intensity are associated with substantial drought tolerance in H. psilorrhyncha.