LEAF AGING AND PLATEAU EFFECTS ON SEASONAL PRESSURE VOLUME RELATIONSHIPS IN 3 SCLEROPHYLLOUS QUERCUS SPECIES IN SOUTH-EASTERN USA

1. Seasonal plant and tissue-water relations were examined in fully expanded twigs of three sclerophyllous, evergreen Quercus species (Q. chapmanii, Q. inopina and Q. myrtifolia) in south-central Florida, USA. In these species we observed 'plateau effects' when obtaining pressure-volume curves. Such effects may cause significant interpretational errors in osmotic potentials and elastic moduli, and be confounded with seasonal changes in these parameters. 2. New leaves are produced and mature in late-March to early-May of each year. Older leaves or twigs sampled between September 1989 and February 1990 generally exhibited higher conductance to water vapour diffusion (g(wv)) and lower osmotic potentials (psi(pi)) than did new leaves sampled in May 1990. 3. However, over this 5-month period without significant drought older leaves of Q. inopina and Q. myrtifolia exhibited significant increases in psi(pi) and decreases in g(wv), which may be attributed to leaf age rather than environment. Q. chapmanii also had declining g(wv), but it displayed decreases then increases in psi(pi) that were correlated with temperature changes during this period. 4. Maximum bulk modulus of elasticity (epsilon) of 23.2 to 26.5 MPa in the study species were among the highest values reported for Quercus species in North America. 5. Despite not artificially rehydrating twigs prior to pressure-volume analysis, 55-73% of the samples exhibited plateaux from excess apoplastic water during the wetter sampling dates. No plateaux were observed during the driest period. The general effects of plateaux on estimating pressure-volume parameters included obscuring the true turgor loss point, increasing psi(pi) at full turgor and decreasing epsilon and the relative water content at zero turgor. Correcting for plateaux greatly reduced errors in pressure-volume curves associated with non-constancy of apoplastic water above the turgor loss point.