EFFECT OF POTASSIUM NUTRITION ON CURRENT PHOTOSYNTHESIZED CARBON DISTRIBUTION TO CARBON AND NITROGEN COMPOUNDS AMONG RICE, SOYBEAN, AND SUNFLOWER

Abstract

ABSTRACT Type I crops (wheat and soybean), in which the potassium concentration (Kc) decreases when the nitrogen concentration (Nc) decreases, and type II crop (sunflower), in which Kc remains constant when Nc decreases, were grown in K-sufficient and K-deficient soils in pots. The CO2 exchange rate (Pnet; photosynthetic rate, PR; photorespiratory rate, and DR; dark respiratory rate), the distribution of photosynthesized 14C to organs, and the chemical compounds and free amino acids in fully expanded leaves were investigated. The results obtained can be summarized as follows. 1) Potassium (K) deficiency affected type II crop by causing a decrease in Pnet and an increase in PR and DR, while in type I crops, K deficiency affected the CO2 exchange rate less markedly. 2) Potassium (K) deficiency affected type II crop by causing the 14C distribution ratio to TCA/amino acids to decrease at 0 hours after 14C assimilation, and the 14C distribution ratio to serine to increase at 0.5 hours after 14C assimilation. In type I crops, the 14C distribution ratio among chemical compounds and free amino acids were less affected by K deficiency. 3) When 14CO2 was assimilated for 10 min, the 14C distribution ratio to organs was less affected by K deficiency in type I crops than in type II crop. In type II crop, the 14C distribution ratio to the stem was reduced by K deficiency, an effect that was caused by a low 14C translocation from the assimilated leaf blade. When 14C-sucrose and 14C-serine were introduced directly into the phloem, the translocation rate was not reduced by K deficiency in type II crop. When 14CO2 was assimilated into the leaf at a different leaf position, 14C distribution was not affected in type I crops. In type II crop, however, when 14CO2 was assimilated into the lower leaf, 14C distribution to the roots increased under K deficiency as compared to the control. 4) In type II crop when 14C-sucrose was introduced directly to the phloem, 14C compounds were translocated quickly to the lower organs, especially the roots. When 14C-serine was introduced, a major part of the 14C compounds remained in the assimilated leaf blade. Consequently, in type II crop, it can be assumed that current photosynthate distribution to TCA/amino acids and the distribution of carbon in serine to other amino acids are restricted by K deficiency at first, then, the photosynthetic rate and the translocation of photosynthates from the leaf blade are restricted by K deficiency.