| Lodging is one of the primary factors to limit the rice productions. The succed of first Green Revolution’dwarf breeding’increased the rice lodging resistance, and improved the grain yield significantly. However, with the rapidly growth of the world population, the needs of food is growing. And now, cultivates of "super rice" further improved the grain yield by increasing the plant height and the biomass, therefore the lodging is becoming an important factor limiting rice yields. Field experiments were conducted at the following two eco-sites:Taoyuan (a special high-yield eco-site) in Yunnan province and Danyang (representative eco-site of the middle and lower Yangtze) in Jiangsu province, China in 2011 and 2012. A super indica hybrid rice called ’Y Liangyou 2’ was grown at each eco-site under different N levels (0 N kg/ha,150 N kg/ha,300 N kg/ha,450 N kg/ha at Taoyuan and 0 N kg/ha,150 N kg/ha,300 N kg/ha at Danyang). Nitrogen applications were applied as urea four times (basal fertilizer was applied 1day before transplanting, tillering fertiliser was applied 7 days after transplanting and topdressing fertilizer was applied at the panicle initiation stage and when the 2nd leaf from top was fully extended) using 25%:25 %:25%:25% and 40%:40%:10%:10% at each stage, respectively. In this experiment, we investigated the differences of grain yield, yield components and compared the rice lodging resistance, surveyed at the 20 days after heading, from the mechanics, the morphology, the physiology and the anatomy at the two eco-sites. Also, we contrasted the responses to nitrogen of the characteristics related to lodging. The primarily results were listed as follow:(1) The grain yield at special high-yielding eco-site was significantly greater than that at representative eco-site of the middle and lower Yangtze. The grain production at Taoyuan mainly ranged from 10 t hm"2 to 1810 t hm-2 and at Danyang from 10 t hm-2 to 13 t hm-2. The average yield over 2011 and 2012 was significantly 49.5% more than yield of Danyang, It was more sensitive that the response to nitrogen of grain yield and yield components at Taoyuan eco-site. Compared to nitrogen-free treatment, the increasing of yield at higher nitrogen application could reach up to 72.6% at Taoyuan, and the increasing at Danyang only reached to 24.3%. The superiority of grain yield-gained primarily reflected on the spikelets per square meter and the panicles per square meter. As raised the input of nitrogen rates, the spikelets per square meter, the panicles per square meter and grain yield increased significantly at Taoyuan eco-site, and at Danyang eco-site, nitrogen mainly affected the panicles per square meter significantly, then the grain yield increased slightly. With the delaying of nitrogen applied, the spikelets per square meter increased significantly, which was beneficial for rising the grain yield.(2) From the mechanic point, the rice lodging index at the special high-yielding eco-site was significantly lower than the index at the representative eco-site of the middle and lower Yangtze. Compared to Danyang, the average lodging index of the two study years at Taoyuan decreased by 21.1%, resulting from the decreasing of bending moment of whole plant (about 31.9%), which caused by the shorter length from broken point to panicle and the lighter fresh weight from broken point to panicle top, and from the decreasing of bending moment at breaking (approximately 11.7%), which caused by the smaller section modulus in despite of the higher bending stress. Moreover, the decreasing magnitude of bending moment of whole plant was higher than the magnitude of bending moment at breaking, thus the lodging index at Taoyuan eco-site decreased. Nitrogen could impact significantly on the mechanic characteristics at the special high-yielding eco-site. With increased the nitrogen rates, at Taoyuan, the length and fresh weight from broken point to panicle top raised significantly, the bending strength and breaking load decreased significantly, and at Danyang eco-site, the bending moment of whole plant increased and bending moment at breaking decreased slightly. With the delaying of nitrogen applied, the bending stress decreased significantly, and then reduced the basal stems bending strength.(3) From the morphology point, the bending moment of whole plant at the special high-yielding eco-site was significantly lower than that at the representative eco-site of the middle and lower Yangtze. The decreasing of bending moment of whole plant primarily resulted from the decreasing of the length from broken point to panicle top, which caused by the shorter panicle height, flag leaf height and panicle-neck height, and from the decreasing of the fresh weight from broken point to panicle top, which caused by the smaller the height of center of gravity, leaf length and width from flag leaf to 2nd leaf from the top and the dry weight of top-three leaves. The decreasing of bending moment at breaking primarily resulted from the lower basal culm diameter, wall thickness and the dry weight per cm of basal culms. The morphology characteristics were influenced significantly by nitrogen. With increased the nitrogen rates, at Taoyuan, plant height, height of center of gravity, length and width from flag leaf to 2nd from panicle top, the dry weight of top-three leaves raised significantly, and the dry weight per cm of basal culms and leaf sheaths decreased significantly. At Danyang eco-site, nitrogen only increased significantly the plant height, length and width from flag leaf to 2nd from panicle top and the dry weight of top-three leaves. With the delaying of nitrogen applied, it was beneficial for improving the basal culms diameter and wall thickness, and these trends were consistent at the two eco-sites.(4) From the physiology point, there was no significant difference in the percentage of the basal culm carbohydrates between the two eco-sites at 20 days after heading, but the proportion of structural carbohydrates, which was primarily responsible for the stems stiffness and strength, in the basal culms at Taoyuan eco-site was higher than that of Danyang. Despite of the basal culm diameter, wall thickness and the culm plumpness were significantly lower than that of Danyang, the higher bending stress and the, greater dry weight per cm of basal leaf sheaths, at a certain extent, increased the basal stems bending strength, which reduced the gap on the breaking resistance of the two eco-sites. At Taoyuan eco-site, there were positive correlation between bending stress and carbohydrates, e.g. soluble sugars, starch, cellulose and lignin, and the soluble sugars and starch contents were significantly lower than the contents of Danyang, but higher significantly in the cellulose and lignin contents than that of Danyang. Moreover, the contribution to bending stress of cellulose and lignin were greater than the nonstructural carbohydrates (e.g. soluble sugars and starch) at Taoyuan eco-site. Nitrogen can impact the basal stems quality obviously at Taoyuan eco-site. With increased the N input, at Taoyuan, the dry weight per cm of soluble sugars, starch, cellulose and lignin decreased significantly, which led to a weaker stems quality, and at Danyang, all of the four carbohydrates decreased slightly. With the delaying of nitrogen applied, the soluble sugars content decreased, but the starch increased at the low N rates and decreased at the higher N applications, these trends were consistent at the two eco-sites. However, there were some differences in cellulose and lignin content, delaying the N applications reduced significantly the dry weight per cm of cellulose and lignin, but these increased at Danyang slightly, these mainly contributed to the dry weight of basal stems.(5)From the physiology point, we compared the differences of rice lodging resistance and the responses to nitrogen at two eco-sites. At the nitrogen-free treatment, there was no significantly difference in the numbers of primarily vascular bundle (PVB) and secondary vascular bundle (SVB). At the higher nitrogen applications, compared to Danyang, the numbers of PVB and SVB at Taoyuan were significant lower, however, the intensity of PVB increased at the high N rate. Moreover, the distance between the PVB and SVB to the periphery at Taoyuan eco-site were nearer, which could increase stem stiffness and the outer fibers endure maximal forces. The numbers of sclerenchymatous cell layer of the PVB and SVB were greater than that of Danyang as well as the numbers of sclerenchymatous cell layer and thickness of mechanic tissue, which led to deeper lignifications. In addition, the area and perimeter of the air cavity decreased significantly compared with Danyang eco-site. As raised the N input, at Taoyuan, the numbers of PVB and SVB increased significantly, it was consistent at the two eco-sites. The radial direction length, circumferential direction length, the numbers of sclerenchymatous cell layer of PVB and SVB increased significantly at Taoyuan, which strengthened the cellulose and lignifications. However, at Danyang, all of these varied slightly and cellulose and lignifications decreased.Summarily, the lodging index of high-yielding population at Taoyuan eco-site was lower than that at Danyang eco-site. The reasons were that:① compared with Danyang, the shorter basal internodes length reduced the plant height. The decreasing of the upper leaf length, width and dry weight, which let the plant be erecter and shorter, decreased the height of the center of gravity. Both the two parts shorten the bending moment focused on the basal stems of the whole plant, and the magnitude of the decreasing of bending moment of whole plant was more than the bending moment at breaking. ② despite of the weaker culm diameter and wall thickness, the greater numbers of sclerenchymatous cell layer and thickness of the PVB, SVB and mechanic tissue strengthened lignifications. The higher proportion of structural carbohydrates in the basal culm and the greater plumpness of basal leaf sheaths enhanced the basal stems strength and stiffness, which narrowed the gap in the breaking strength related to Danyang. It was more sensitive that the characteristics lodging-related responded to nitrogen in the special high-yielding eco-site. As raised the N input, at Taoyuan, the length of basal internodes, the length, width and dry weight of top-three leaves increased significantly, and the carbohydrate contents per cm in basal stems decreased significantly. At Danyang, there was no obviously effect except the basal culm morphological characteristics. With delaying the N applied, it could be helpful for improving the grain yield, culm diameter and wall thickness, and shortened the basal internodes elongated length. However, to a certain extent, it reduced the breaking resistance of basal stems, and even decreased the longing index. |
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