ObjectivesNitrogen (N) and potassium (K) play important roles in rice growth, development and yield formation. K deficiency stress and unbalanced fertilization of N and K in paddy fields have been the most important factors restricting rice grain yields. A field trial was carried out to explore the effects of K deficiency stress on rice growth, leaf nutrients and physiological status under different N rates, and further to clarify the physiological mechanism of leaf color dark green due to potassium deficiency.

MethodsA completely random two-factor field trial was conducted. The first factor was N application rates:0, 90 kg/hm² (low N) and 180 kg/hm² (normal N); the second factor was K₂O application rates of 0 and 120 kg/hm² (normal K). The dry matter, leaf area index, contents of N, K and Mg of leaves, and chlorophyll (leaf color value) of leaves, leaf water content, leaf soluble sugar content, specific leaf weight, and leaf SPAD value at the tillering and panicle initiation stages were measured.

Results1) Without N application, the rice dry matter and leaf area index were not significantly affected by K deficiency at tillering and initial panicle stages, but with N application, the effect became significant, and the decrease became larger with the higher N application, . The interaction of N and K fertilization was significant on the dry matter and leaf area index in all the growth stages. 2) Without N application, neither the contents of N, chlorophyll and soluble sugar in leaves, nor leaf weight and leaf SPAD values were significantly affected by K deficiency at the tillering and panicle initiation stages, on the contrary, these index values were increased significantly under the N application condition, and the leaf N and chlorophyll contents in treatment of N 90 kg/hm² plus zero K were similar to those under the combination of N 180 kg/hm² plus K₂O 120 kg/hm². No matter with or without N application, K deficiency significantly reduced the leaf K content while increased leaf Mg content at the tillering and panicle initiation stages. 3) There was a significant (P < 0.01) and positive relationship between the specific leaf weight and the leaf soluble sugar content.

ConclusionsThe responses of leaf nutritional and physiological status to K deficiency are affected significantly by N application rates. The K deficiency stress results in high soluble sugar content and leads to the increased unit leaf weight in case of N application. However, K deficiency stress significantly increases leaf N contents and leaf chlorophyll contents and weight of unit leaf area under the condition of N application, which is the most important cause of the dark green leaf and this phenomenon might be detrimental to plant nitrogen nutrition diagnosis at the early growth stage of rice.