THE RELATION OF LEAF WATER DEFICIT TO SOIL MOISTURE TENSION IN PINUS SYLVESTRIS L.

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Abstract
Summary: Experiments were carried out in which pots with 3–5 year old plants of Pinus sylvestris were allowed to dry from field capacity to soil water tensions of 5 atm. or more, while measurements of soil moisture tension, leaf water deficit and transpiration rate were made daily at sunrise and noon.By fitting multiple regressions it was shown that most of the variation in the leaf water deficit could be accounted for by changes in soil moisture tension and transpiration rate.The conclusion reached in earlier work was confirmed by these experiments, namely that the leaf water deficit at sunrise is the best index of the soil moisture conditions experienced by the plant, since effects of transpiration are negligible at this time. The effect of various factors on the relation between soil moisture tension and leaf water deficit was experimentally examined. It was found that the relation was affected by the age of needles used for the determination, by the time of year, by nitrogen nutrition, and by soil moisture conditions prior to the experimental run. The way in which these factors may operate is discussed.Despite the variability in the relation examined, broad conclusions as to the soil moisture tension experienced by young plants of this species may be drawn from measurements of leaf water deficit made on 1‐year old leaves at sunrise. Thus in soils near field capacity, leaf water deficits rarely exceed 8 per cent. Deficits of 12–14 per cent indicate soil moisture tensions in the 1–5 atm. range, and when the deficits rise above 17 per cent, the soil moisture has probably been reduced to the permanent wilting percentage.