Abstract
Diurnal and seasonal water relations were determined in leaves of Couepia colyandra Aubl., Thouinidium decandrum Radlk., Randia armata L. (evergreens); Trichilia trifolia L., Thouinia paocidentata Poit. (dry season deciduous); Jacquinia pungens A. Gray, Coccoloba liebmannii Lindau, and Forchhammeria pallida Liebm. (wet season deciduous) in a tropical dry-deciduous forest of the western Coast of Mexico, where the rainy season is from June to October. The difference in the minimal value of leaf water potential (.psi.) experienced by the wet-deciduous species between the dry and the rainy season was on average -1.42 MPa, with absolute values of -1.4 and -3.0 MPa for February and April, respectively. This difference was only 0.48 MPa in the evergeeen species. The minimum value of .psi. of the wet-deciduous species fell from -2.5 MPa and -1.2 MPa in February for J. pungens and C. liebmannii, respectively, to -3.7 and -3.0 MPa in April. The difference in the minimum values of turgor potential registered between the wet and the dry season was considerably less than the difference registered in .psi.. The maximum value of the modulus of elasticity (.epsilon.max) varied widely between species and time of the year. The three wet-deciduous species showed the highest values of .epsilon.max, ranging from 35-50 MPa in November to 20-25 MPa in February. The seasonal change in .epsilon.max of the evergreen species showed a similar trend but at a lower level (23-25 MPa in November, to 8-12 in July). The dry-deciduous species showed the lowest .epsilon.max ranging from 7 to 11 MPa in November but with little change in February. Evergreen and wet-deciduous species showed a higher capaicty to tolerate water stress than the dry-deciduous species.