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The Relation Between Rate of Transpiration and Rate of Absorption of Water in Plants

Paul J. Kramer
American Journal of Botany
Vol. 24, No. 1 (Jan., 1937), pp. 10-15
Stable URL: http://www.jstor.org/stable/2436949
Page Count: 6
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The Relation Between Rate of Transpiration and Rate of Absorption of Water in Plants
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Abstract

By the use of Livingston auto-irrigator cones, the rates of absorption and transpiration were simultaneously determined for green ash, Opuntia, yellow poplar, loblolly pine, and sunflower plants growing in soil. In all species except Opuntia transpiration exceeded absorption during the day, but absorption exceeded transpiration during the night. Opuntia differed from the other species in having its maximum rate of transpiration from 6 to 10 p.m., while in the other species the maximum occurred near mid-day. Absorption lagged behind transpiration to about the same extent in all species, indicating that there is no characteristic difference between woody, herbaceous, and succulent species in this respect. Sunflower plants and rooted willow cuttings, growing in a nutrient solution, behaved much like plants rooted in soil, transpiration frequently exceeding absorption during the day, while absorption exceeded transpiration at night. In certain instances where the leaf area was small in proportion to the root system, transpiration did not appreciably exceed absorption. Simultaneous determinations of absorption, transpiration, and leaf moisture content of auto-irrigated sunflower plants indicated that the time of minimum leaf moisture content approximately coincided with the time of maximum transpiration. The time of greatest leaf moisture content came in the middle of the night, several hours before the rate of transpiration began to increase. The results of these experiments emphasize the close relation between absorption and transpiration. The fact that changes in rate of transpiration precede changes in rate of absorption indicates that under ordinary growing conditions, the rate of water intake probably is determined largely by the rate of water loss. The lag of absorption behind transpiration probably results from the fact that the rate of transpiration responds immediately to changes in environmental conditions, but absorption does not increase until the effect of the saturation deficit caused in the leaves by transpiration is transmitted to the roots. At night, closure of stomata checks transpiration abruptly, but since the saturation deficit, which controls absorption, is not immediately satisfied, absorption decreases more slowly than transpiration.

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