Abstract:

Stable hydrogen isotope composition can be used to trace water source and use in plants. In this experiment, deuterium ratio in water was used to study seasonal variation in water use of Ziziphus jujuba in different environmental water regimes. Results showed that δD value of water in deep soil layers remained steady while δD value in upper soil layers had a significant variation over seasons because deuterium isotope was fractionated during evaporation. Deuterium isotope was enriched and δD value increased in the shallow soil. There was a significantly negative correlation (R²=0.69,P<0.01) between the δD value and water content in the shallow soil layers in dry season. δD value of branch xylem water was significantly and positively correlated (R²=0.75,P<0.01) with the water content in shallow soil but not with that in deep soil. The results indicated δD value of branch xylem water was heavily influenced by water in shallow soil. Precipitation increased water content of shallow soil layers in wet season, which promoted new root growth in the shallow soil. New root length in the shallow soil increased by 621.46% in wet season than that in dry season. Most of the new roots were fine roots and the average diameter was 0.16 mm. The increased fine root was able to enhance water uptake from the shallow soil. Over 50% of the xylem water of Z. jujuba branch were extracted from the deep soil layers, especially in dry season. However, the contribution of shallow soil water significantly affected the water balance and stress degree in Z. jujuba. Data analyses showed δD value of water in the branch xylem was strongly correlated with branch embolism, predawn leaf water potential and photosynthesis rate. The results suggested that δD value of xylem water could be used as an indicator of the plant water status. Quantification of drought stress of plants with δD value would provide a credible reference in studying water use efficiency of plants.

Key words: Ziziphus jujuba, deuterium isotope, water source, branch embolism, transpiration