Do pigeon pea and sesbania supply groundwater to intercropped maize through hydraulic lift?—Hydrogen stable isotope investigation of xylem waters

Abstract

Hydraulic lift is a movement of water between soil layers contrasting in water potential through plant root systems. We expected that an introduction of deep-rooted plants into intercropping systems could provide an opportunity for the associated crops to utilize water from deep in the soil layers through hydraulic lift. A stable hydrogen isotope (deuterium) was used to investigate this possibility in a field experiment conducted at the Zambia National Irrigation Research Station. Maize ( Zea mays) plants were grown adjacent to the deep rooting legumes, pigeon pea ( Cajanus cajan) and sesbania ( Sesbania sesban), whose roots accessed groundwater having the deuterium/hydrogen isotope ratios (δD) different from those of rainfall. We hypothesized that the δD values of xylem waters from shallow-rooted maize would become closer to those of groundwater when hydraulic lift occurred, and this would be affected by the distance between the maize and the legumes. The maize near pigeon pea showed δD values closer to those of groundwater while the maize at a distance did not. However, we could not detect such a distance-dependence in maize plants grown adjacent to sesbania. Based on the field trial, a glasshouse experiment was implemented for the pigeon pea–maize association. The plants were grown in a split-root container with an air gap between the two soil layers. The pigeon pea roots were allowed to develop in the bottom soil injected with D 2O while the maize roots were confined in the top soil. Immediately after the injection, a higher D concentration above the level of natural occurrence was detected in xylem waters from each of the maize plants, confirming that pigeon pea lifted water from the bottom and transferred it to the maize. Moreover, the water supply from the pigeon pea to the maize was enhanced by the shading treatment on the pigeon pea, implying the possibility of regulating the function of the hydraulic lift. It is concluded that at least pigeon pea can supply water from deeper soil layers to the associating maize plants through hydraulic lift, and that such a sprinkler-like function (pumping up and transferring water to surrounding plants) may be regulated by agricultural practices that suppress transpiration rate.