| Potassium is one of the major three important essential elements for plant growth. The requirement for K supply increases with the increasing of crop yield and quality. However, potassium deficiency in soils is widespread in China, especially in the paddy soils, where low K has become the major limiting factor for obtaining high yield. China is short of K fertilizer resources, so it is importance to understand the mechanisms of plant adaptation to low K in soils. There existed large genotypic differences in K efficiency in lowland rice (Oryza sativa L.). The major objectives of this study were to investigate the characteristics of root morphology, K absorption kinetics, and interaction of K with other nutrients in the K-efiicient plant genotype, to find out the plant trait for screening and identifying K-efficient rice genotypes, and understand the characteristics of rhizosphere processes of K in the paddy soils. The major results obtained were su mmarized as follows:1. Under low K, root fresh weight and volume decreased, and more severe reduction for K inefficient genotypes was noted than for efficient ones. Under moderate K deficiency, root growth was stimulated and the stimulation was more obvious in the efficient genotypes than in the in-efficient ones. However, the root total absorbing surface area, root active absorbing surface area and root specific absorbing surface area were reduced by K deficiency more severely in the efficient rice genotypes than in the in-efficient ones. The Km values of K uptake by root of the efficient genotypes were higher while Cmin and (3 were lower than those of the inefficient ones, whereas minimal differences were observed for the Imax values among different rice genotypes. The results indicated the K efficiency was relative to the root affinity for K+, the minimal K+ concentration in the culture medium (Cmjn), but not to the maximum K+ uptake rate.2. The nitrogen concentration and the N/K ratio in the plant of rice cultured in K deficiency soil was higher than that in K sufficient soil, but the increase was greater in the K-efficient genotype than in the in-efficient one. The sodium concentration, accumulation and the Na/K ratio in rice cultured in K deficient soil was also higher than that in K sufficient soil, but the K-efficient genotypes showed greater increased in Na concentration and Na/K ratios than the K-inefficient ones under low K. Phosphorus and magnesium concentration and accumulation seemed not relative to K application and K efficiency of rice genotypes.3. Under field condition, genotypic differences in K efficiency (dry matter yield or grain yield (kg)/K taken up (kg) were observed, and the genotypic differences were comparable to those in solution culture experiments. The K-efficient genotypes showed higher relative K accumulation (RKA) under low K than the K-inefficient ones. Potassium efficiency in rice genotypes was closed associated with different agronomicVmorphological and physiological traits. Among all the plant traits, plant K concentration (KC) and plant K accumulation (KA) were found to be the most closely correlated parameters, followed by plant dry weight (DW), root number (RN), shoot height (SH), sheath length (SL), highest leaf length (HLL) and root/shoot ratio (R/S), and the longest root length (RL), leaf age (LA) and appearance rate of leaf were less correlated to K efficiency.4. It was found that the plant traits obtained under low K only could not better indicating or identifying K-efficient genotypes, if the relative whole-plant K accumulation (RWKA) was used as the standard screening index. Based on correlation analysis and variability analysis, relative root dry weight (RRW), relative shoot dry weight (RSW) and whole plant dry weight (RWW) under low K to sufficient K conditions were noted as the suitable plant parameters for large volume screening and conveniently identifying K-efficient rice genotypes in solution culture experiment.5. An inexpensive rhizobox technique was improved to study...
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