The influences of temperature and photoperiod on osmotic potential (ψπ) at full turgor (ψπSAT) of container-grown Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings, which had completed one growing season, were examined in four experiments. Osmotic potential was measured cryoscopically on expressed sap and also by the pressure volume curve method in experiment 4. In experiment 1, started in January when ψπSAT was low, seedlings exposed to 25 °C for 5 weeks showed an increase in ψπSAT of 0.48 MPa compared with seedlings held at 8 °C, which showed little increase. There were no differences in ψπSAT between 8- and 16-h photoperiods. In experiment 2, started in September when ψπSAT was relatively high, seedlings of interior origin showed decreases in ψπSAT at 8 °C under an 8-h photoperiod (0.55 MPa) and at 25 °C under a 16-h photoperiod (0.69 MPa). Relatively little decrease in ψπSAT occurred in interior seedlings at 8 °C under a 16-h photoperiod. Coastal seedlings showed no decrease in ψπSAT in this experiment. In experiment 3, seedlings were exposed to 1 and 8 °C under an 8-h photoperiod, but at lower irradiance levels (85 μE•m−2•s−1) than those used in the other experiments (340 μE•m−2•s−1). Starting at values of about −1.9 MPa in October, ψπSAT decreased, on average, for coastal and interior seedlings by 0.76 MPa at 1 °C and 0.42 MPa at 8 °C over 6 weeks. Values for coastal seedlings decreased less (0.36 MPa) than those for interior seedlings (0.83 MPa). In experiment 4, seedlings were maintained under ambient conditions outdoors (mean temperatures of 5 °C day and 3 °C night), or at 12 or 25 °C under a 10-h photoperiod, in January and February. After 5 weeks of treatment, ψπSAT at 25 °C was 0.50 MPa, measured cryoscopically, or 0.80 MPa, measured by the pressure-volume method, higher than for seedlings under ambient conditions. At the turgor loss point (ψπSAT), ψπ was 1.36 MPa higher at 25 °C than at ambient temperature. The increase in ψπSAT at the beginning of the season and the reduction of ψπSAT at the end of the season were therefore dependent on temperature, but an interaction of photoperiod with temperature was observed in September. Index of injury values, calculated from leakage of electrolytes from drought-stressed needles and stems, showed a linear relationship to moisture stress between − 10 and − 2 MPa xylem water potential. In experiment 4, needle and stem index of injury values increased with increase in temperature, as did ψπSAT and ψπTLP values, tending to confirm that information about drought tolerance can be obtained by either method.
Accuracy of the pressure-volume curve method compared to quantitative lung CT scan to assess the recruitable lung in patients with acute respiratory failure
