COMPARATIVE MEASUREMENT OF STEM-FLOW AND TRANSPIRATION IN COTTON

Whole-plant transpiration (T) measurements have many applications, but appropriate methods have remained somewhat elusive. A new method using a constant power heat balance gauge, wherein the xylem mass flow rate is calculated from a balance of heat into and out of a stem, has been shown to provide accurate stem flow measurements. To evaluate the applicability of this promising method to field experiments, cotton (Gossypium hirsutum L. "GP 3774") stem flow measurements were compared with T measured from a weighing lysimeter. Initially to confirm method accuracy, stem flow values were compared in the glasshouse with T values determined by mass measurements of a potted plant. The root mean square error (RMSE) between the daylight losses from both (n = 16) was 8.6% of the mean measured T values. In the field, hourly stem flow and lysimeter T values were also similar, but there was a large variation in stem flow values among the different plants. To account for differences in plant size between the plants with gauges and all lysimeter plants, stem flow values were adjusted using a stem area ratio factor, which adjusted values, on the average for the season, by 25%. Before adjustment, daylight stem flow totals were consistently greater than lysimeter T values. After adjustment, the means differed by only 9%, and the RMSE was reduced from 129 to 69 g plant-1 d-1. The coefficient of variation of daylight stem flow totals increased throughout the season. In the glasshouse, method accuracy was comparable (errors < +/- 10%) to what has been previously determined. In the field, determining method accuracy was confounded by plant-to-plant variability and, possibly, by errors, unique to the gauge design used in this study, at high flow rates. Thus, this method can provide accurate flow measurements from individual herbaceous plants and is a valuable technique for many applications.