Mechanism Involved In Carbon and Nitrogen Metabolism Underlying the Genetic Variation In the Tolerance to Potassium Deficiency In Cotton (Gossypium hirsutum L.) Seedlings

In this study, two cotton (Gossypium hirsutum L.) cultivars, ‘Liaomian18’, non- insect-resistant and potassium (K)-efficient, and ‘NuCOTN99^B’, insect-resistant and potassium (K)-deficient, were used to investigate the mechanism conferring K efficiency in cotton seedlings with respect to carbon (C) and nitrogen (N) metabolism. Experiment was conducted hydroponically in growth chamber under two K regimes: K-sufficiency with 2.5 mM (K2), and K-deficiency with 0.03 mM (K1). The results indicated that K-deficiency significantly decreased biomass production in cotton seedlings; and ‘NuCOTN99^B’ was impacted much more than ‘Liaomian18’, i.e. its tolerance to K-deficiency was lower when compared to ‘Liaomian18’. The concentration of soluble sugar in the youngest fully-expand leaf was increased under K-deficiency because of its weaker ability to synthesize carbohydrate. Furthermore, ‘NuCOTN99^B’ had much more soluble sugar in leaf than ‘Liaomian18’, suggesting its lower capacity in carbonhydrate formation under K-deficiency. Affected carbon metabolism profiles by K-deficiency were also characterized by decreased leaf chlorophyll, RuBPcase activity and RuBP regeneration and leaf photosynthetic capacity. Again, ‘NuCOTN99^B’ was impaired more than ‘Liaomian18’. Although photoinhibition in photosystem II (PSII) occurred under K-deficiency, indicated by significantly decreased maximum photochemical efficiency (Fv/Fm), photochemical quantum yield of photosystem, photochemical quenching (q^p), probably it was not related to the difference in the tolerance to K-deficiency between cultivars, because there were no differences in these traits between ‘NuCOTN99^B’ and ‘Liaomian18’. With respect to nitrogen metabolism under K-deficiency, the concentration of free amino acids in cotton leaf was enhanced, and protein concentration and nitrate reductase (NR) activity were reduced, displaying the impaired capacity of leaf to form protein. However, ‘NuCOTN99^B’ had higher protein concentration and NR activity than ‘Liaomian18’, which suggested its stronger nitrogen metabolism. Therefore, it was concluded that the higher tolerance to K-deficiency of ‘Liaomian18’ compared to ‘NuCOTN99^B’ likely resulted from its relative stronger carbon metabolism.