We studied the flood tolerance of five tree species growing in the flooded forest adjacent to the Mapire river, in SW Venezuela. Mean photosynthetic rate and leaf conductance were 11 &mgr;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 increasing flooding and leaf conductance (Psidium ovatifolium Berg. ex Desc., Campsiandra laurifolia Benth., Symmeria paniculata Benth. and Acosmium nitens (Vog.) Benth); and (2) independence of photosynthesis and leaf conductance from flooding (Eschweilera tenuifolia (Berg.) Miers.). In the first response pattern, declining photosynthetic rate with flooding may be interpreted as a sign of reduced flood tolerance, whereas the second response pattern may indicate increased flood tolerance. An increase in xylem water potential with depth of water column was found for all species (with the possible exception of P. ovatifolium), indicating that flooding does not cause water stress in these trees. Submerged leaves that had been under water for between four days and four months generally had photosynthetic rates and leaf conductances similar to those of aerial leaves, indicating maintenance of photosynthetic capacity under water. Daily positive oscillations in glucan content in submerged leaves of P. ovatifolium and C. laurifolia suggest that submerged leaves do not represent a sink for photosynthates produced by aerial leaves.
The effect of a very high CO2 mole fraction (27000–35000 μmol mol−1) on photosynthesis and water relations was studied during the dry and the rainy season in plants of Spatiphylum cannifolium (Dryand.) Schott and Bauhinia multinervia (H.B.K.) DC. growing near natural cold CO2 springs. Xylem water potential in plants of both species was lowered by drought, high CO2 growth‐concentration decreasing it further in S. cannifolium. In plants of both species growing under high CO2 concentration photosynthetic rates measured at a CO2 mole fraction of 1000 μmol mol−1 were higher than in plants growing at ambient CO2 mole fraction and measured at 350 μmol mol−1. The response was the result of a direct effect of CO2 on the photosynthetic machinery. Changes in carboxylation efficiency in response to high CO2 were found during the rainy season, with an increase in S. cannifolium and a decrease in B. multinervia; a significant interaction between growth CO2 concentration and season in B. multinervia resulted from significant effects of both factors. An increase in intrinsic water‐use efficiency due to high CO2 was determined in both species by an increase in photosynthetic rate as well as a decrease in leaf conductance. In high‐CO2 plants of S. cannifolium a 71% decrease in stomatal density and 73% in stomatal index suggested that CO2 affected stomatal initiation, whereas in B. multinervia an 85% decrease in stomatal index and a 72% decrease in stomatal density indicated that CO2 influenced stomatal initiation as well as epidermal cell expansion. Our results indicate that very high CO2 concentrations did not inhibit photosynthesis in these species, and that growth under high CO2 allowed plants to attain carbon balances higher than those of plants growing under low CO2. This was particularly so during the dry season, since the photosynthetic rates at the corresponding ambient concentration were higher in plants nearer the springs, and carboxylation efficiency and some stomatal characteristics of both species apparently acclimated to high CO2, but patterns were not consistent and bore no obvious relationship to photosynthetic capacity.
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