Water acquisition from rainfall and groundwater by legume crops developing deep rooting systems determined with stable hydrogen isotope compositions of xylem waters

Abstract

The deuterium/hydrogen isotope ratios (δ D) of xylem waters from pigeon pea ( Cajanus cajan) and sesbania ( Sesbania sesban) plants grown in semi-arid Zambia were investigated to know the seasonal variation in water sources for those crops. The study was conducted at a field site in the Zambia National Irrigation Research Station from November 2000 until April 2001. We measured the δ D values of xylem waters exuded by both of the legumes, in addition to those of rainfall and groundwater. This was done by use of an isotope ratio mass spectrometer. While the δ D values of rainfall fluctuated considerably, those of groundwater were almost constant throughout the experimental period. The δ D values obtained from the xylem waters from both species also fluctuated, suggesting the influence of water from rainfall. The tap roots of both of the plants reached at least 2 m below the soil surface, where groundwater was available. However, xylem waters from sesbania always showed δ D values closer to those of the groundwater, as compared to values measured in pigeon pea. This implies that sesbania has a greater dependency on groundwater for its water supply than pigeon pea. During the dry season, only pigeon pea exuded xylem sap despite its smaller dependency on groundwater. Therefore, it was hypothesized that pigeon pea could more actively absorb water from drying surface layers, while sesbania was less able. This water from surface layers would result in a diluted groundwater composition in xylem waters of pigeon pea. We have shown that δ D analysis for xylem waters can aid in the understanding of the function of deep rooting systems of crops grown in fields.