In the past, considerable efforts have been undertaken to restore Baldcypress (Taxodium distichum) around Lake Pontchartrain in Southeastern Louisiana. Vast amounts of cypress trees have been planted with varying rates of success. Increased salinity is often cited as one reason for the high tree mortality, yet it is not clear whether trees are affected by NaCl toxicity or whether salt-induced reduction of soil water potentials lead to xylem cavitation and high tree mortality. The aim of this study was to investigate the role of hydraulic conductivity, wood density, and xylem cavitation in the response of baldcypress (Taxodium distichum) seedlings to increased soil salinity and drought. 1-yr old, greenhouse-grown seedlings were irrigated daily with a 100 mM (6 ‰) salt solution (salt treatment) or with fresh water (controls). Drought plants were irrigated once per week with fresh water.
Results/Conclusions
Gas exchange rates of stressed plants were reduced by approximately 50% (salt) and 70% (drought), resulting in a 50-60% reduction in diameter growth for both treatments. Stem-specific hydraulic conductivity (KS native) of stressed plants was 33% (salt) and 66% (drought) lower than controls and we observed a strong positive correlation between KS native and gas exchange. In addition, we found a strong relationship between CO2 assimilation rate (A) and the soil-to-leaf hydraulic conductance (kL). The relationship was identical for all treatments, suggesting that our moderate salt stress (as well as drought) did not affect the photosynthetic biochemistry of leaves, but rather reduced A via stomatal closure. Lower KS native of stressed plants was associated with increased wood density and the plant's vulnerability to xylem cavitation. Xylem pressures causing 50% loss of hydraulic conductivity (P50) were -2.88±0.07 MPa (drought), -2.50±0.08 MPa (salt) and -2.01±0.04 MPa (controls). P50s were strongly correlated with wood density (r = -0.71, P < 0.01) and KS native (r = 0.74, P < 0.01). Our findings support the hypothesis that there is a trade-off between a plant's cavitation resistance and its hydraulic efficiency. In addition, our results indicate that stressed plants partitioned their biomass in a way that reduced cavitation vulnerability. Hence, these plants could be better suited to be planted in environments with elevated soil salinity. Whether the observed stress acclimation does indeed translate into reduced seedling mortality under field conditions is currently under investigation.
