| Rice is one of the most important crops in China. Continuously increasing the rice yield of per unit area also has been one of the main tasks and objectives of grain production in China. Nitrogen (N), the key factor affecting the yield of rice, constitutes an important part of rice production. Rice root is both the main organ of moisture and nutrient absorption and it plays an important role in the synthesis of hormones, organic acids and amino acids. Its morphological and physiological characteristics have a close relationship with the growth and development of the aboveground parts, yield and quality formation. There are many researches on the effects of N on rice yield and growth and development of aboveground parts, however, the effects of N on root morphological and physiological traits and their relationship with grain yield have not been clearly understood. With indica rice Liangyoupeijiu (LYPJ), Yangdao 6 (YD 6) and Japonica rice Ningjingl (NJ 1), Yangjing4227 (YJ 4227) as materials and cultivated in pots in this study, the effects of N rates and root removal at main growth stages on morphological and physiological traits of roots and grain yield were investigated. The main results were as follows:1. There were significant differences in the responses of grain yield to N rates among rice cultivars. Grain yields of YD 6 and YJ 4227 increased rapidly with N rates under low or middle N rates, and then decreased with increase in N rates, while grain yields of LYPJ and NJ 1 increased with the increase in N rates in the experiment. According to the curvilinear equation of grain yields and N rates, the highest yields among the four cultivars were nearly the same (90.9～92.8 g pot-1), however, the N rates to achieve such yields for the four cultivars were not nearly (1.8-2.6 gpot-1). Partial factors productivity of applied N of LYPJ and NJ 1 were 37.0 g g-1 and 35.9 g g-1, which were significantly lower than those of YD 6 and YJ 4227 (46.6 g g-1 and 50.0 g g-1), indicating that YD 6 and YJ 4227 were sensitive while LYPJ and NJ 1 were tolerant to N rates.2. Fertilizer tolerant cultivars (LYPJ and NJ 1) had a higer root-shoot ratio and root biomass (root weight, root length, root surface area, etc) increased with N rates, but their root physiological traits (root oxidation activity, main hormone contents) reduced under high N rate, while it had opposite results in fertilizer sensitive cultivars (YD 6 ang YJ 4227). The correlation analysis of rice root morpho-physiological characteristics and yield showed that main morphological indices of roots were very significantly and positively correlated with grain yield, and no such correlation could be found between physiological traits of roots and yield for fertilizer tolerant rice cultivars at heading stage. Ror N sensitive rice cultivars, the results were reversed. According to the above results, higher root biomass and higher root physiological activity might be the main reason for the yield increase in fertilizer tolerant and fertilizer sensitive cultivars, respectively. And fertilizer tolerant cultivars might have root "redundant growth".3. No significant differences in grain yields were observed between 1/6 root removal at mid-tillering and panicle initiation stages and their controls (without root removal) in fertilizer tolerant cultivars (LYPJ and NJ 1), however, the grain yields in fertilizer sensitive cultivars (YD 6 and YJ 4227) were significantly decreased. Root oxidation activity, cytokinin (zeatin+zeatin riboside) content per pot in rice roots of LYPJ and NJ 1 were not significantly decreased, while these indices significantly decreased in fertilizer sensitive cultivars (YD 6 and YJ 4227), when compared with their controls under the treatment of 1/6 root removal. Grain yields were all significantly decreased under 1/4 root removal, but the decreasing extents of LYPJ and NJ 1 were significantly lower than those of YD 6 and YJ 4227. These results showed that appropriate removal of rice roots in LYPJ and NJ 1 have no significant effects on grain yield, which further indicated that there are redundant root growth in some fertilizer tolerant cultivars. |
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