Dependence of calcium uptake on water absorption and respiration in roots of tomato plants (Lycopersicon esculentum Mill.)

Root xylem sap exuded from the decapitated stem stumps of tomato plants (Lycopersicon esculentum Mill.) was collected in the daytime and the nighttime, and dependence of calcium uptake on water absorption and respiration in the tomato roots is examined by analyzing exudation rate (J_W; μLs^-1), calcium concentration ([Ca²⁺]; mmol L^-1) in the xylem sap and root calcium uptake rate (J(Ca); nmol s^-1) in the root exudation. J_W from the stem slump was about one third of transpiration rate from the whole shoot before decapitation and was kept two to five times higher in the daytime than in the nighttime. The xylem sap [Ca²⁺] was about 3 to 4.5 times higher than that of the nutrient solution in the hydroponic pot, and the xylem sap [Ca²⁺] in the daytime was 1.5 times higher than that in the nighttime. Consequently, J(Ca) in the daytime was 3 to 5.5 times higher than that in the nighttime. The daytime higher J(Ca) was depressed in the restrictedly water absorbing plant, and the restriction of root respiration in the daytime lowered the xylem sap [Ca²⁺], which resulted in significant depression in J(Ca). Furthermore, the lower J(Ca) in the nighttime was not enhanced by activating the nighttime root respiration. Thus, the daytime root calcium uptake was regulated by both of the water absorption and the respiration in roots, and the nighttime root respiration was not a rate-limiting factor for the lower calcium uptake in the nighttime. The observed dependence of calcium uptake on water absorption and respiration in roots was explained in relation to the important role of the root endodermis.