EFFECT OF SOIL TEMPERATURE ON TRANSPIRATION IN <i>HELIANTHUS ANNUUS</i>

Abstract

In the researches upon the adaptation of transplants to alpine and dune habitats, it has been essential to develop field methods for analyzing the factor complex. Chief among these are control batteries of phytometers in which one major factor is maintained in a series of intensities or levels while the others are held practically uniform, except for the daily march. Water content, light, wind, and soil temperature in particular permit such manipulation, and all are under investigation in the series of transplant gardens at Pike's Peak and at Santa Barbara. In both the alpine climax and the xerosere of litoral dunes, instrumentation has shown that wind and soil temperature are relatively extreme, and much attention has in consequence been directed to them. The first experiments in this field were conducted by Whitfield (12, 13) at the Alpine Laboratory in 1930-1931 by means of standardized phytometers of Helianthus annuus, and the native Mertensia sibirica, which is much utilized in the transplant project. He demonstrated that a reduction of soil temperature from 113° to 51° F. lowered transpiration very little, while a decrease from 51° to 34° F. exerted a pronounced influence. Records of soil temperature during the growing season at 12,000 feet in the alpine tundra of Pike's Peak indicated a range from 35° to 48° F., with an average of 44° F. for the four years of observation (14). A physiological test indoors of the relation between temperature and absorption was long ago made by Sachs (7), who placed a well-watered tobacco plant in a warm room and surrounded the pot with ice. After a brief period the plant began to wilt, but when the ice was removed and the soil heated, it recovered without the addition of water. By means of potometers, Vesque (10) found that within the limits of 10° to 15° C. the absorption by roots increased rapidly as the temperature rises. Kozarow (3) employed the same method to show that a fall from 20° to 0° C. retarded the rate of water supply by 25-30 per cent. Later investigations by Delf (2), Stiles and J^rgensen (9), and by Weber and Hohenegger (11) agree in demonstrating that the rate of absorption by a plant cell always increases with a rise of temperature, at least up to 30° C. Such responses in absorption are naturally reflected in transpiration, although Stahl (8) has discovered that temperatures near 0° C. may still permit a slight loss of water. 619