PYRUVATE-KINASE ACTIVITY AS A BIOINDICATOR OF CATIONS IN GRAFTED WATERMELON PLANTS

Three cultivars of Cucurbita pepo L. were used as rootstocks (Brava, S hintoza and Kamel), and as scion part, four cultivars of Citrullus lan atus (Trumb) Mansfield (Sugar King, Early Star, Dulce Maravilla, and S ugar Bell) were grafted and subjected to the determination of calcium (Ca), potassium (K), sodium (Na), and magnesium (Mg), and pyruvate kin ase activity in leaves. Control plants (not grafted) gave the highest level of total K with respect to the other cultivars. The cross Brava x Sugar Bell showed the higher K level, while the lowest was found wit h the cross Brava x Sugar King. Total sodium (Na) content did not show differences within the experimented crosses as 0.77 mg Na/g d.w. was obtained for all cultivars, while control plants gave levels of Na fiv e times higher (3.49 mg/g d.w.). The cultivar Sugar King had the highe st Ca integration capacity, whereas Dulce Maravilla showed a decrease, both with respect to the control plants. The results of soluble Mg we re lower than those obtained in the control plants, the higher concent ration of this cation was observed in Kamel x Dulce Maravilla, while S hintoza x Dulce Maravilla and Sugar Bell had the lower level of Mg wit h respect to the control. The endogenous activity of pyruvate kinase ( PK) was higher in the grafted plants than the control plants. The essa ys in the presence of Mg or K induced an increase in the PK activity o ver the endogenous one. The results of the PK essay in the presence of Ca were higher with respect to the previous essays. It seems that gra fted plants have a physiological deficiency, especially for Mg and K, whereas Ca had excessive concentrations, but some crosses showed a phy siological demand for active Ca.