Seasonal changes in photosynthesis of trees in the flooded forest of the Mapire River

We studied the flood tolerance of five tree species growing in the flooded forest adjacent to the Mapire river, in SW Venezuela, Mean photosynthetic r ate and leaf conductance were 11 mu mol m(-2) s(-1) and 700 mmol m-2 s(-1), respectively. Xylem water potential ranged from -0.08 to -1.15 MPa, Based on leaf gas exchange as a criterion of tolerance to flooding, two response patterns were identified: (1) decreasing photosynthetic rate with increasin g flooding and leaf conductance (Psidium ovatifolium Berg. ex Desc., Campsi andra laurifolia Benth., Symmeria paniculata Benth. and Acosmium nitens (Vo g.) Benth); and (2) independence of photosynthesis and leaf conductance fro m flooding (Eschweilera tenuifolia (Berg.) Miers,). In the first response p attern, declining photosynthetic rate with flooding may be interpreted as a sign of reduced flood tolerance, whereas the second response pattern may i ndicate increased flood tolerance. An increase in xylem water potential wit h depth of water column was found for all species (with the possible except ion of P. ovatifolium), indicating that flooding does not cause water stres s in these trees. Submerged leaves that had been under water for between fo ur days and four months generally had photosynthetic rates and leaf conduct ances similar to those of aerial leaves, indicating maintenance of photosyn thetic capacity under water. Daily positive oscillations in glucan content in submerged leaves of P. ovatifolium and C. laurifolia suggest that submer ged leaves do not represent a sink for photosynthates produced by aerial le aves.