Water-Extraction by Split-Roots of Sesbania and Pigeon Pea Exposed to Spatially Heterogeneous Distribution of Soil Water

Abstract

Previous studies have suggested that the deep roots of sesbania (Sesbania sesban) function less efficiently in water acquisition than those of pigeon pea (Cajanus cajan) despite similar rooting depths. To investigate this phenomenon, both species were grown in a vertically split-root system. The top soil was watered at two-day intervals and the bottom soil was kept wet. Fifty-seven days after sowing, the watering to the top soil was withheld and the water uptake was monitored in both the layers. At any given rate of transpiration, the water influx rate per unit root surface (WIR/RS) was higher in the top soil than in the bottom soil in sesbania, despite the greater availability of water in the latter. By contrast, in pigeon pea, the WIR/RS was higher in the bottom soil than in the top soil. In sesbania, aerenchyma tissue was observed only in the cortex of the roots in the bottom soil. On the other hand, aerenchyma tissue was scarcely observed in pigeon pea roots, suggesting that the presence of aerenchyma tissue led to the reduced WIR/RS of sesbania roots in the bottom soil. Thermal image analysis showed that the stomata of sesbania leaves did not respond to water shortage. Instead, the sesbania leaves were shed in order to avoid desiccation, further reducing the potential to extract water. We therefore conclude that the water-extraction ability of deep roots was lower in sesbania than in pigeon pea as a result of aerenchyma formation and leaf shedding.