WATER STATUS AND DEVELOPMENT OF TROPICAL TREES DURING SEASONAL DROUGHT

Bud break, shoot growth and flowering of trees involve cell expansion, known to be inhibited by moderate water deficits. In apparent contrad iction to physiological theory, many trees flower or exchange leaves d uring the 6 month-long, severe dry season in the tropical dry forest o f Guanacaste, Costa Rica. To explore this paradox, changes in tree wat er stats during the dry season were monitored in numerous trees. Water potential of stem tissues (PSI(stem)) was obtained by a modification of the pressure chamber technique, in which xylem tension was released by cutting defoliated branch samples at both ends. During the early d ry season twigs bearing old, senescent leaves generally had a low leaf water potential (PSI(leaf)), while PSI(stem) varied with water availa bility. At dry sites, PSI(stem) was very low in hardwood trees (<-4 MP a), but near saturation (>-0.2 MPa) in lightwood trees storing water w ith osmotic potentials between -0.8 and -2.1 MPa. At moist sites trees bearing old leaves rehydrated during drought; their PSI(stem) increas ed from low values (<-3 MPa) to near saturation, resulting in differen ces of 3-4 MPa between PSI(stem) and PSI(leaf). Indirect evidence indi cates that rehydration resulted from osmotic adjustment of stem tissue s and improved water availability due to extension of roots into moist subsoil layers. In confirmation of physiological theory, elimination of xylem tension by leaf shedding and establishment of a high solute c ontent and high PSI(stem) were prerequisites for flowering and bud bre ak during drought.