Abstract
To investigate the beneficial effects of sodium related to available potassium levels, three kinds of leafy vegetables-Beta vulgaris, Spinacia oleracea, Amaranthus mangostanus-were pot-cultured with Masa soil with low potassium availability. As to the application rates of potassium (KCl) and sodium (NaCl), seven treatments were set up i. e., K (control ; 1.0 g K_20 pot^<-1>=21.2 mmol K pot^<-1>), 1/2K, 1/5K, -K, 1/2K1/2Na (K=Na=10.6 mmol pot^<-1>), 1/5K4/5Na and -K+Na (Na=21.2 mmol pot^<-1>). Experiments were carried out in triplicate in a hot-house. The results obtained were as follows. 1) The effect of sodium was most remarkable on Beta vulgaris. The growth of -K+Na plants was quite normal and better than that of the control (K) plants, while -K plants showed potassium deficiency symptoms and growth was decreased to half that of the control plants. A similar but lesser effect was obtained for Spinacia oleracea. Differently from these two plants, sodium did not have any effect on Amaranthus mangostanus except for -K+Na treatment. All the plants receiving the -K treatment stopped growth soon after germination and then wilted. However, all the plants receiving the -K+Na treatment continued growth without any abnormal symptoms, though the fresh weight at harvest time was about one-fifth of the control plants. 2) The differences in the effect of sodium were mainly attributed to the difference in the rate of sodium uptake by the plants. In Beta vulgaris and Spinacia oleracea, the decrease in potassium content was compensated by an increase in sodium uptake. As the result, the concentrations of univalent alkali cations (K+Na) in the plants did not decrease. The beneficial effect of sodium in this case seemed to be mainly due to the substitutional effect of sodium for potassium in producing osmotic pressure in the apoplast of the plants. This might contribute to keeping water content in the tissue and increasing the fresh weight. The rate of sodium uptake by Amaranthus mangostanus was very low even in the potassium-deficient condition, so the effect of sodium on -K+Na plants is believed to be different from the case of the former two plants. The cause of this effect should be investigated further.
