Comparison of the effects of continuous and relieved water stress on nitrogen nutrition of grain sorghum

Abstract

A solution culture experiment was conducted to study the effects of continuous and relieved water stress and of nitrogen (N) supply on growth and N content of grain sorghum cv. Texas 610SR. Polyethylene glycol-6000 was used to impose solution osmotic potentials of -0.1, -0.4, and -1.1 MPa over 9 days beginning on day 21. On day 30, half the plants in the -0.4 and -1.1 MPa treatments were returned to solutions with osmotic potentials of -0.1 MPa (relieved water stress regime), while the other half remained at solution osmotic potentials of -0.4 and -1.1 MPa until harvest (continuous water stress regime). Plants were harvested on day 43. Dry matter yields, total plant N content, mean rate of N uptake per unit root weight, and the number of leaves expanded after imposition of solution osmotic potential treatments were decreased significantly by decreases in N supply and solution osmotic potential in the continuous water stress regime. Decreasing the solution osmotic potential in the continuous water stress regime had no effect on mean N concentrations in expanded leaves or in shoots, but increased N concentrations in roots and in stem plus immature leaves. Plants in the relieved water stress regime produced more dry matter, expanded more leaves after day 21, had higher N contents, N concentrations and mean N uptake rates per unit root weight than plants in the continuous stress regime at equivalent levels of solution osmotic potential. Relationships between leaf N concentrations and relative yield of shoots were non-asymptotic for all three index leaves (youngest emerged blade and the two blades immediately below), and this prevented the calculation of meaningful critical N concentrations in any treatment. The results are discussed in relation to effects of intermittent water stress on rainfed sorghum in the field.