Pigeon Pea- Hydraulic Lift

Pigeonpea (Cajanus cajan) is a multipurpose, tropical semi-perennial legume suitable for production in different agro-ecologies (Nam et al., 1993; Snapp et al., 2003). The plant generally has deep rooting system with extensive development in the upper 60 cm of the soil profile. This helps with its soil moisture extraction pattern and adaptation to rain-fed production environments (Natarajan and Willey, 1980; Chauhan, 1993). Studies have demonstrated that differences in rooting depths and root length densities in pigeonpea have implications for soil water extraction (Subbarao et al., 2000; Matsunaga et al, 1994). The long and medium duration cultivars have deep rooting systems which makes them well adapted to environments with varying soil moisture conditions. However, the extra-short duration is highly susceptible to drought stress, especially during flowering and pod-filling, due to its shallow rooting systems (Nam et al., 1993). The drought resistance in pigeonpea is also attributed to its osmotic adjustment, helping the plant to accumulate solutes within the plant tissue when there is a lower soil potential (WP_s) thereby reducing the effects of water deficits (Flower and Ludlow, 1986; Ludlow and Muchow, 1990; Subbarao et al, 1995).

Water deficit negatively impacts plant performance, distribution and abundance (Dawson, 1993) and this has been implicated as the most important factor limiting carbon fixation, plant growth and global net primary production (Schulze, 1986). Therefore strategies such as hydraulic lift are needed by plants to cope with water deficit. The HL process has been described as the movement of water from the roots to the soil due to the difference between the root xylem potential and the surrounding soil potential when transpiration rates are low. This process is nocturnal, and the lifted water is reabsorbed by the roots the following day as part of the daily transpiration (Caldwell et al., 1998). Much of the work on HL has been documented mostly in the field with shrubs and trees (Caldwell and Richards, 1989; Dawson, 1993; Richards and Caldwell, 1987; Wan, et al., 1993) and rangeland grasses (Caldwell, 1990). A number of studies, however, have indicated that HL does occur in herbaceous species as well (Liste and White, 2008; Sekiya and Yano, 2004; Wan, et al., 2000; Xu and Bland, 1993).

HL may be beneficial to the plant conducting it, and also as an important source of water for the neighboring plant. The hydraulically lifted water utilized by neighboring plants has positive influence on their water use patterns and growth (Dawson, 1993). Some other implications of hydraulic lift include nutrient acquisition, biogeochemical nutrient cycling processes, and root growth and persistence (Caldwell et al., 1998). For example, an agricultural field trial by Sekiya and Yano (2004) showed that the introduction of a deep-rooted crop into the system allowed maize to utilize otherwise unavailable water sources deep in the soil layers. The cropping systems were maize (Zea mays)-pigeonpea and maize-sesbania (Sesbania sesban) intercrops. HL was only detected in pigeonpea though to date, this is the only report that indicates HL in pigeonpea. Princess Adjei-Frimpong is investigating how the hydraulic lift function in pigeonpea will limit water competition in a sorghum-pigeonpea cropping system and enhance the overall water-use efficiency of the system.

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