Accessing the performance of the chamber method to estimate the isotopic composition of transpiration of the entire plants

The isotope composition of transpired vapor (T) is a crucial parameter when quantifying the ratio of plant transpiration to evapotranspiration using the isotope method. The chamber method based on the basic gas exchange principle is considered an effective measure to quantify T. By measuring the vapor concentration in the chamber (Cm) and in the background (Cb), the isotope composition of vapor in the chamber (m) and in the background (b), T was able to calculate at a leaf scale. However, this method is rarely applied to the entire plant, and its performance at the whole plant scale still needs to be examined. To assess the performance of the chamber method on the whole plant, we conduct a series of tests on maize plants in the field. We used xylem water isotopic compositions as benchmarks and compared chamber method derived T with xylem water isotopic compositions. The results showed that the deviation of T was significantly larger in May, July, and August than in June and September. The results suggested that the chamber method was able to calculate T of the entire plant. The significant deviation of T in May might be due to low transpiration rate at that month, while the reason for the substantial deviation in T in July and August might be due to the unexpected condensation. The deuterium excess of m might be useful to indicate the occurrence of condensation.