The Nature and Location of Variable Hydraulic Resistance inHelianthus annuusL. (Sunflower)

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Abstract
Koide, R. 1985. The nature and location of variable hydraulic resistance in Helianthus annuus L. (sunflower).—J. exp. Bot. 36: 1430–1440. Hydraulic resistances for whole sunflower plants (Helianthus annuus L.) and sunflower leaves, stems, petioles and roots were measured. Whole plant hydraulic resistance was shown to decline with an increase in transpiration. Leaf hydraulic resistance was shown, with one technique employing transpiring leaves, to vary with transpiration and with another technique, employing pressure-induced flow in leaves, to be constant over a wide range of transpiration. Stem and petiole hydraulic resistances were constant over a wide range of exudation. Pressure-induced flow through root systems was shown to be an inappropriate method for characterizing their hydraulic properties because flow may occur through unnatural paths. The technique employing measured transpiration rates and water potentials of non-growing leaves and soil is suggested to be better. The evidence presented in this study suggests that the hydraulic resistance of the transpiration stream does vary and that the site of variability is the root