EFFECT OF SOIL TEMPERATURE ON TRANSPIRATION IN HELIANTHUS ANNUUS

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Abstract
The effect of soil temps. in the range 34[degree]-100[degree] F on transpiration of H. annuus phytometers 6-8 wks. old was measured by using 5 sets of 4 or 9 plants each, run simultaneously in water baths of different temp. Transpiration varied little with soil temp. between 55[degree] and 100[degree], but fell rapidly below 55[degree], was reduced to half at 38[degree], and approached zero at 32[degree]. Strips of lower epidermis of the leaves showed that the stomata do not begin to close until the soil temp. is lowered to about 40[degree], but are nearly completely closed at 37[degree] F. The plants usually start to wilt at about 40[degree] and at 34[degree] were wilted so that the crowns were bent nearly right angles to the main stems. The plants recovered rapidly when the soil temp. was raised.