Seasonal changes in the water balance of Douglas-fir (Pseudotsuga menziesii) seedlings grown under different light intensities

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Abstract
The water relations of germinant seedlings of Douglas-fir (P. menziesii (Mirb.) Franco) grown outdoors under 9, 44 and 100% full light were analyzed during the summer, autumn and winter seasons. Empirical regression models, based on the relationship plant water potential = f(soil water potential, leaf conductance), were determined for selected light treatment and season combinations. Plant water potential is higher during active summer shoot growth than after elongation growth has ceased in the autumn owing to earlier stomatal closure in response to soil drought in summer compared with a more abrupt closure at a lower plant water potential in the autumn. For all light treatments, leaf conductance declined in the winter independent of plant water potential and simultaneously with the onset of subfreezing air temperatures. During the 1st winter water potential of seedlings was higher than during the previous summer or autumn over a wide range of equivalent soil water potentials. Seedlings grown under low light intensity are less drought resistant and have lower plant water potential than those grown under full light regardless of soil moisture status.

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