Nitrate uptake, efflux, and in vivo reduction by Pima and Acala cotton cultivars

Nutrient assimilation by seedlings is an early indicator of subsequent plant growth and development. This study was conducted to compare NO3- assimilation (net uptake, efflux and in vivo reduction) by 8-d-old seedlings of three Pima (S-7, Oro Blanco, Conquistador) (Gossypium barbadense L.) and four Acala (Maxxa, GC-510, Royal, SJ-2) (Gossypium hirsutum L.) cotton cultivars. The seedlings were grown hydroponically in a growth chamber and induced with 0.05, 0.1, or 1.0 mM NO3- for 24 h. Net NO3- uptake rates were determined by following its depletion from uptake solutions that initially contained the same concentration of NO3- as the induction solutions. Efflux was measured by following NO3- accumulation in an external solution. At 0.05 mM NO3-, net uptake rates for the Acala cultivars were about 40% lower than those for Pima cultivars. These differences disappear as the NO3- concentration in the growth medium was increased to 1.0 mM. In contrast, efflux from Acala roots was 40 to 50% lower than that from Pima roots at all NO3- levels. Although NO3- concentration of roots increased about three fold as the NO3- level in the induction medium increased, efflux increased only 20 to 30%. In vivo NO3- reduction by Pima and Acala cultivars was similar when the seedlings were supplied with 1.0 mM NO3-. However, at 0.05 mM NO3- the Pima cultivars reduced NO3- more effectively than did the Acala cultivars. The results indicate that Pima cultivars are better able to utilize available NO3- at lower concentrations even though they efflux greater amounts of NO3-