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Sap Flow in a Dicotyledon (Sunflower) and a Monocotyledon (Sorghum) by the Heat-Balance Method

Dep. of Agronomy, Throckmorton Hall, Kansas State Univ., Manhattan, KS 66506

^* Corresponding author (Email: mbk{at}ksuvm.ksu.edu).

The heat-balance method is widely used to monitor sap flow in stems of herbaceous plants. Measurements of sap flow made on monocotyledons, with scattered vascular bundles in their stems, may not be as accurate as measurements on dicotyledons, with annular vascular systems located next to the heater of the gauge. Therefore, we measured the accuracy of sap flow measurements made by the heat-balance method for sunflower (Helianthus annuus L. ‘Hysun 354’, a dicotyledon with C₃ photosynthesis, and sorghum [Sorghum bicolor (L.) Moench, Funk's hybrid G522DR], a monocotyledon with C₄ photosynthesis. Plants were grown in pots with a commercial potting medium under greenhouse conditions. Measurements were made during a cycle when the plants were well watered, drought stressed, or rewatered. A potted plant was placed on a scale and sap flow (SF) values (units of g h^–1) were compared with weight lost during the same period of time (WL, g h^–1). Accuracy of the sap flow measurements was determined by calculating error, defined as (SF – WL)/WL. The gauge had good accuracy (error = 8.8%) only with watered sunflower, which had high flow rates (maximum rate = 64.7 g h^–1). (This sap flow rate, nevertheless, is less than those for woody plants under field conditions, which are often >100 g h^–1.) Sap flow rates in watered sorghum were less than those in watered sunflower (maximum sap flow rate = 18.9 g h^–1), and error for watered sorghum was high (72.6%). The gauge had good accuracy when sap flow rates were 20 g h^–1 or more. Despite gauge errors, gauge measurements usually paralleled scale measurements and rapidly monitored changes in sap flow rate. The data suggest that sap flow rate is more important than stem anatomy in determining the accuracy of the gauge.

Received for publication September 24, 1994.