The Relationship Between Transpiration Rate, Water Potential, and Resistances to Water Movement in Sunflower (Helianthus annuusL.)

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Abstract
The effects of transpiration rate on the vertical gradients of leaf and stem xylem water potential (Ψ and ψ) were examined using hydroponic sunflower plants. Transpiration was varied by stepwise alterations of environmental conditions. The gradients of Ψ and Ψ were relatively small (2.3 and 0.8 × 105 Pa m−1) when transpiration rates approached zero, but increased sharply to 5.4 and 2.3 × 105 Pa m−1 as transpiration increased. However, the gradients were independent of transpiration rates above 0.4 gΨ dm−2 h−1 owing to variability of the plant resistance. The gradients of ΨI were usually less than half those of ΨI. ψ1 in individual leaves remained constant over a wide range of transpiration rates (0.4—2.4 g dm−2 h−1) and each leaf possessed a characteristic plateau value related to its elevation. ΨI responded similarly but was approximately 2.0 × 105 Pa higher than ΨI at the same elevation. Identical results were obtained regardless of the procedure employed to vary transpiration. The drop in water potential between stem and leaf implies that the leaf resistance is appreciable. This was confirmed using rapidly transpiring excised leaves freely supplied with water. ΨI increased by 2.0–2.5 × 105 Pa following removal of the root resistance but remained 2 × 105 Pa lower than similar excised leaves in darkness. Furthermore, ΨI in excised leaves remained constant over a wide range of transporting rates, demonstrating that the leaf resistance is also variable. The results are discussed in relation to previous reports.