Morphology And Physiology Of Rice Roots In Relation To Yield Formation And Cultivation Regulation Techniques

Roots are an integral part of plant organs and involved in acquisition of nutrients and water, synthesis of plant hormones, organic acids and amino acids, and anchorage of plants. Root morphology and physiology play an important role in the growth and development of aboveground plants. As a "hidden half", however, it remains much yet to be understood the relationship between root morphological and physiological traits and the yield formation in rice, relative to the aboveground growth and its relation to grain yield. Using typical rice cultivars as materials, this study investigated changes in morphological and physiological traits of roots and their relationships with grain yield during the evolution of indica and japonica rice cultivars, changes in morphological and physiological traits of roots in super rice at different growth stages, roles of root-produced hormones in regulating grain filling of superior and inferior spikelets, and the cultivation techniques to increase yield by regulating morphology and physiology of rice roots. The main results are as follows:1. Changes in morphological and physiological traits of roots and their relationships with grain yield during the evolution of indica and japonica rice cultivarsMorphological and physiological traits of roots and their relationships with grain yield formation for the mid-season indica and japonica rice cultivars in Jiangsu Province showed a similar changing pattern during the evolution of rice cultivars. The root dry weight, root weight density, root length, root length density and root diameter at main growth stages were increased with the evolution of the cultivars. Compared with other cultivars, the root-shoot ratio of super rice cultivars was increased significantly at the panicle initiation stage, but showed no significant differences among the types of cultivars at latter growth stages. The root oxidation activity, leaf photosynthetic rate, total absorbing surface area and active absorbing surface area of root, and the content of cytokinins (zeatin+zeatin riboside) in root bleeding were increased or increased significantly with the evolution of the cultivars during the early and middle stages. Grain yield were increased gradually with the evolution of the cultivars. Increase in grain yield was attributed mainly to the increase in total number of spikelets, which resulted mainly from a large panicle. Regression analysis showed that the root dry weight, root length, root diameter, root oxidation activity, total absorbing surface area and active absorbing surface area of root very significantly correlated with grain yield. The numbers of spikelets, grain weight, and yield were associated with components and concentration of organic acids and amino acids in root exudates/bleeding. The number of mitochondrion, golgi body, ribosome and starch body in root-tip cells increased significantly with the evolution of the cultivars at the tillering stage. The number of mitochondrion and golgi body were significantly or very significantly and positively correlated with root dry weight, root oxidation power, seedling dry weight, and tiller number of rice at the tillering stage. The results suggest that the improved root and shoot growth increases grain yield of the modern cultivars, especially that of super rice cultivars.2. Changes in morphological and physiological traits of roots and their relationships with shoot growth in super riceRoot, shoot dry weight, and root length density was significantly greater in super rice varieties than in check ones throughout the growth season. Root oxidation activity (ROA) and root zeatin (Z) zeatin riboside (ZR) content, in per plant basis, were significantly greater in super rice than check varieties before and at heading time. However, both ROA and root Z+ZR content were significantly lower in super rice than in check varieties at the mid and late grain filling stages. Grain yield of the super rice varieties, on average, was10.2t ha-1and11.4t ha-1, and was13%and21%higher than that of check varieties, respectively. The high grain yield was mainly due to a larger sink size (total number of spikelets) as a result of a larger panicle. The percentage of filled grains of the super rice varieties, on average, was72.9%and79.0%, lower than that of the check varieties. The mean ROA and root Z+ZR content during the grain filling period significantly correlated with the percentage of filled grains. The results suggest that an improved root and shoot growth, as showing a larger root and shoot biomass and greater root length density during the whole growing season and higher ROA and root Z+ZR content per plant at early and mid growth stages, contributes to the large sink size and high grain yield in the super rice varieties. The results also suggest the yield of super rice varieties could be further increased by an increase in filled grains through enhancing root activity during grain filling.3. Hormones in the grains and roots in relation to post-anthesis development of superior and inferior spikelets in japonicalindica hybrid riceCompared with inferior spikelets of indicalindica hybrid rice and superior spikelets, the inferior spikelets of japonicalindica(J/I) hybrids exhibited low rate of endosperm cell division and slow grain filling. During the early grain filling period, the inferior spikelets of J/I hybrids had more gibberellins (GAs), but less Z+ZR, indole-3-acetic acid (IAA), and abscisic acid (ABA). Rates of endosperm cell division and grain filling were positively and significantly or very significantly correlated with Z+ZR and ABA concentrations in both grains and roots or IAA in grains, whereas not significantly correlated with GAs either in grains or roots or IAA in roots. Applications of kinetin, IAA, or ABA to panicles, or kinetin and ABA to roots, enhanced cell division and grain filling in the inferior spikelets. Results suggest that low concentrations of Z+ZR and ABA in both grains and roots, and IAA in grains may result in the poor filling of inferior spikelets in the J/I hybrid rice.4. Effects of irrigation regimes on morphology and physiology of roots, yield, and quality of rice4.1Effects of alternate wetting and drying during the grain filling stage on morphology and physiology of roots and yield of riceThree irrigation regimes, alternate wetting and moderate soil drying (WMD), alternate wetting and severe soil drying (WSD), and conventional irrigation (CI, continuous flooding), were imposed from6d after heading to maturity. No significant differences in grain filling rate, grain weight, and cytokinin contents were observed for the earlier-flowering superior spikelets among the three irrigation regimes. Compared with the inferior spikelets in the CI, grain filling rate and grain weight were significantly increased in the WMD, whereas significantly reduced in the WSD. Cytokinin contents in inferior spikelets and the flag leaves in the WMD at the soil drying time were comparable with those in the CI, but they were significantly increased when plants were re-watered. The WSD significantly reduced cytokinin contents in inferior spikelets and the flag leaves either during the soil drying or during the re-watering period. Cytokinin contents in roots showed no significant difference between WMD and CI regimes. The WSD significantly increased trans-zeatin-type cytokinins, whereas reduced isopentenyladenine-type cytokinins, in roots. Grain filling rate and grain weight of inferior spikelets were significantly or very significantly correlated with cytokinin contents in these spikelets. The results suggested that a post-anthesis WMD holds great promise to improve grain filling of inferior spikelets through elevating cytokinin levels in inferior spikelets and the flag leaves of rice.4.2Effects of alternate wetting and drying during the grain filling stage on rice qualityRoot oxidation activity, the photosynthetic rate of the flag leaf, and activities of key enzymes in sucrose-to-starch conversion in grains at the late grain filling stage were significantly increased under the WMD, whereas they were significantly reduced under the WSD. The grain yield was increased by9.3-9.5%under the WMD, while it was reduced by7.5-7.8%under the WSD, when compared with that under the CI. The WMD significantly improved milling, appearance, and eating qualities, while WSD decreased these qualities. The results suggested that alternate wetting and moderate soil drying regime during the grain filling period holds great promise to both increase yield quantity and quality and also could save precious fresh water resources.4.3Effects of alternate wetting and drying during the whole growing season on morphology and physiology of roots and yield of riceThree irrigation regimes, WMD, WSD, and CI, were conducted during the whole growing season of rice. Compared with the CI, the WMD significantly increased, whereas the WSD reduced, root oxidation activity, cytokinin concentrations in roots, leaves, and grains, leaf photosynthetic rate, activities of key enzymes involved in sucrose-to-starch conversion in grains, and grain yield of rice. Both WMD and WSD increased water use efficiency (WUE). The results suggested that a moderate wetting and drying regime can enhance root growth which benefits other physiological processes and result in higher grain yield and WUE.5. Effects of nutrient management and cultivation patterns on morphology and physiology of roots and yield of rice5.1Site-specific nitrogen managementTwo nitrogen (N) managements, site-specific N management (SSNM, N application according to soil fertility, leaf color and growth status, and cultivar types) and farmers’fertilizer practice (FFP), were designed. The root dry weight in the FFP was higher than that in the SSNM before heading, whereas it was lower at and after heading. SSNM significantly increased the root-shoot ratio during the whole growing season. Compared with that under FFP, SSNM increased ROA at and after panicle initiation (especially ROA per stem). SSNM increased the photosynthetic rate of the flag leaves after heading. Compared with FFP, SSNM increased grain yield by7.2-9.4%and N use efficiency expressed as N partial-factor productivity (grain yield/amount of N applied) by38-46%. The enhancement of root growth at the latter growing stage would account for the increase in grain yield and N use efficiency under SSNM.5.2Application of rapeseed cake combined with inorganic nitrogen fertilizerFour treatments, no N application (T1), application of inorganic N fertilizer (T2), application of rapeseed cake (T3) and application of rapeseed cake combined with inorganic nitrogen fertilizer (T4), were conducted. The order of grain yield was T4> T2>T3>T1, the differences among four treatments were significant. The T4treatment significantly increased panicles, percentage of filled grains,1000-grain weight, and harvest index. It also significantly increased effective leaf area index and high effective leaf area index at the heading time, SPAD (soil-plant analysis department) values and photosynthetic rate during the grain filling period. Root dry weight, root length, root oxidation activity, amount of root bleeding, the total absorbing surface area and active absorbing surface area at different growth stages were greater under the T4treatment than those under any other treatments. The results suggest that application of rapeseed cake combined with inorganic N fertilizer can improve root growth, and consequently enhance shoot growth and increase grain yield.5.3High-yielding and high efficiency cultivation (HYHE)Two cultivation patterns, the high-yielding and high efficiency cultivation (HYHE, a cultivation system with SSNM and WMD as key techniques) and the local high-yielding cultivation (LHY), were practiced. Compared with LHY, the HYHE increased the biomass of root and shoot, root cytokinin concentration and root oxidation activity, and increased grain yield by31%. The HYHE also significantly increased water use efficiency and N use efficiency. The results suggest that root and shoot growth could be improved, and high grain yield high efficiency of water and N utilization could be achieved through integrating and optimizing cultivation techniques in rice production.This study would provide novel insights to understand the changes in morphological and physiological traits of roots and their relationships with grain yield during the evolution of indica and japonica rice cultivars, morphological and physiological index of roots for high-yielding, morphological and physiological traits of roots and their relationships with shoot growth in super rice, hormonal levels in roots and grains and their regulatory roles in grain filling of superior and inferior spikelets, and cultivation techniques to regulate morphology and physiology of roots, and which would have great significance in the breeding of root system and the cultivation for high grain yield, good quality, and high efficiency.