Development and performance of wheat roots above shallow saline groundwater

Abstract

Shallow saline groundwater can meet part of a crop’s water requirement by capillary rise (i.e. sub-irrigation) when supplemented by surface irrigation with water of better quality. Proper management of deficit irrigation under these conditions requires better understanding of direct water use by crops from the shallow saline groundwater. This in turn requires information on development and performance of root systems. We therefore studied the development of wheat roots in lysimeters with water-tables maintained at 1 m under a combination of surface and sub-irrigation, each with 3 levels of salinity, by core sampling and with minirhizotrons. Compared to crops grown with adequate rainfall, root systems were shallow, rarely penetrating below 45 cm, and sparse, with maximum root-length densities (RLDs) of 0.5–1.8 cm/cm3 in the top 10 cm of the soil. Downward growth slowed continuously as time passed. Ultimate depths of penetration and RLDs at all depths decreased as the salinities of both surface and sub-irrigation increased. RLDs were inversely proportional to depth rather than decreasing exponentially below the surface. Estimates of RLD from cores exceeded those from minirhizotrons in the top 10 cm but were smaller at greater depth, as expected from the literature. Total root length ranged between 1 and 3 km/m2, around an order of magnitude smaller than for winter wheat in north-west Europe. Grain yields were likewise small, 0.4–1.2 t/ha, but yields per unit root length of 40–50 g/km were comparable to those from high-yielding crops. Roots in the top 10 cm comprised more than half the total root length and were most closely correlated with total water use, which included surface irrigation. Below 10 cm, the correlation was strongest with water extracted from the profile. As demand on global water resources inevitably rises, saline groundwater will increasingly be needed as a component in crop production, particularly in semi-arid parts of the world. The shallow roots found under our experimental conditions indicate a need for frequent irrigation, but the adoption of such a strategy would be likely to restrict uptake from the water-table and at the same time increase non-beneficial evaporation. We present our results as a contribution to finding an optimal irrigation strategy under these conditions.