Effect of drought on leaf gland secretion of the mangrove Avicennia germinans L.

This study assessed the effects of salinity changes over space and time upon leaf gland secretion in Avicennia germinans trees growing naturally in an area featuring markedly seasonal rainfall. Soil pi (approximate to soil psi; MPa) during the wet season was -0.95+/-0.05 and -2.12+/-0.08 at low and high salinity sites, respectively. During the dry season, these values decreased to -3.24+/-0.09 at low salinity and to -5.75+/-0.06 at high salinity. Consequently, predawn and midday plant water potential were lowered during drought at both sites. The rates of secretion (mmol m(-2) h(-1)) increased during drought from 0.91+/-0.12 during the wet season to 1.93+/-0.12 at low salinity, and from 1.69+/-0.12 during the wet season to 2.81+/-0.15 at high salinity. Conversely, stomatal conductance (g(s)) was lowered by both salinity and drought. As xylem osmolality increased during drought, secretion tended to rise exponentially, and g(s) decayed hyperbolically. Thus. a trade-off is obtained between enhancement in salt secretion and control of water loss suggested by g(s).