CARBOHYDRATE STATUS IN ROOTS OF 2 SOYBEAN CULTIVARS - A POSSIBLE PARAMETER TO EXPLAIN DIFFERENT EFFICIENCIES CONCERNING PHOSPHATE-UPTAKE

The carbohydrate levels in the roots of two soybean cultivars (Century, Woodworth) were determined during the early vegetative growth stage. Based on Michaelis-Menten kinetics Century was characterized as being more efficient at phosphate uptake when compared to Woodworth. The experiments were carried out at normal nutrition (+P treatment) as well as under a condition of moderate phosphate stress (-P treatment). The stress situation was created by withdrawal of phosphate from the nutrient solution for five or nine days. Compared to the +P-treatment the deprivation of phosphate led to a reduction of more than 50% in the phosphate content of the plant tissue. The carbohydrates in the roots exhibited differences between both cultivars with Century generally having higher sucrose levels. During the morning the +P treatment of Century exhibited ≈ 10 mg g-1 and in the afternoon ≈ 16 mg g-1 sucrose (plant dry weight basis). This compares to ≈ 8 mg g-1 sucrose (morning) and ≈ 13 mg g-1 sucrose (afternoon) for Woodworth. Phosphate stress increased the levels of hexoses (glucose, fructose) in both varieties significantly. The sucrose levels remained higher for the Century variety even during phosphate stress. We conclude that the shoot of the Century variety supplies its root with an increased amount of energy in form of carbohydrates compared to the Woodworth variety. This may enable the plant to take up phosphate more efficiently. It is also remarkable that the Century variety reduces the pH of the nutrient medium more rapidly. This active proton deposition may create a high membrane potential which is known to be necessary for phosphate uptake. © 1990 Kluwer Academic Publishers.