Effects Of Sowing Date And Density On Yield And Quality Of Direct-seeded Ratoon Rice

Direct-seeded ratoon rice is a simplified rice production that effectively enhances farmers’enthusiasm for grain production and is of great significance for ensuring food security.The development of direct-seeded ratoon rice is limited by the availability of temperature and light resources,which poses a risk of in safe and uniform heading during the ratoon season.The sowing date and sowing density are important because that sowing date and density affect seedling growth and population quality of direct-seeded ratoon rice.In actual production,farmers often adopt high sowing densities and early sowing for ensureing sufficient panicles in the main season and early harvest of main season,and elonging growth period in ratoon season.However,early sowing is prone to low temperatures during late spring reason,and high sowing densities may also result in lodging and more production costs.On the other hand,the effects of sowing dates and densities on rice quality are not clear yet in direct-seeded ratoon rice.This study was conducted using a two-factor three-level design,three sowing dates consisted of sowing date I（T1,sowing on5th April）,sowing date II(T2,sowing on 10th April,and sowing date III（T3,sowing on15th April）,and three planting densities conclude low density(D1,sowing seed amount of30 kg ha^-1),medium density(D2,sowing seed amount of 37.5 kg ha^-1),and high density(D3,sowing seed amount of 45 kg ha^-1).Additionally,a conventional transplanting ratoon rice was as control treatment（CK）.The research objectives were:1)to explore the growth and development and yield formation of direct-seeded ratoon rice under different sowing dates and densities,2)to evaluate the impact of different treatments on the lodging and rice quality of ratoon rice,therefore clarify the optimal sowing date and optimal sowing density for direct-seeded ratoon rice in the east-southern Hubei.The main results are as follows:（1）In this study,the main season yield of transplanting ratoon rice was 8.74 t ha^-1,and the main season yield of direct-seeded ratoon rice was 6.31-9.21 t ha^-1;the ratoon season yield of transplanting ratoon rice was 5.68 t ha^-1,and the ratoon season yield of direct-seeded ratoon rice was 4.49-5.32 t ha^-1.The main season yield of direct-seeded ratoon rice was the highest at middle sowing density under sowing date I condition,and increased by5.4%compared with the main season yield of transplanting ratoon rice;the ratoon season yield of direct-seeded ratoon rice was the highest at high sowing density under sowing date II condition,decreased by 6.3%compared with the ratoon season yield of transplanting ratoon rice.Compared with transplanting ratoon rice,the increased panicles per m² in the main season of direct-seeded ratoon rice is conducive to the increased yield of the main season,the decreased seed setting rate in the ratoon season resulted in decrease in yield of the ratoon season.Compared with sowing date I,the number of panicles per m²in the main season averagely decreased by 2.7%and 10.9%under sowing date II and sowing date III,and seed setting rate averagely decreased by 2.1%and 11.3%,and the yield averagely decreased by 9%and 23.7%,respectively.In the ratoon season,compared with sowing date I,the number of panicles per m² and seed setting rate averagely increased by 9.1%and4.1%,respectively,resulting in an increase of 2.4%in yield of the ratoon season,the number of panicles per m² and seed setting rate averagely decreased by 8.6%and 3.3%,respectively,resulting in a 6.6%decrease in yield of the ratoon season under the sowing date III.The effects of different density treatments on the main season yield under different sowing dates were different,the main season yield was in the order of medium density treatment>low density treatment>high density treatment under sowing dates I,while the main season yield was in the order of high density>medium density>low density under sowing date II and sowing date III;the ratoon season yield of direct-seeded ratoon rice increased with increase in sowing density under different sowing dates.（2）The main season and ratoon season yield are affected by the translocation of pre-heading accumulated dry matter to grain and post-heading dry matter production.Compared with transplanting ratoon rice,direct-seeded ratoon rice increased the average translocation of pre-heading accumulated dry matter to grain and contribution percentage of pre-heading dry matter translocation to grain yield by 94.3%and 94.4%under sowing date I,respectively,but decreased the post-heading dry matter production and contribution percentage of post-heading dry matter production to grain yield by 10.7%and 11.4%,respectively.Compared with sowing date II and sowing date III,sowing date I had the highest of the translocation of pre-heading accumulated dry matter to grain and contribution percentage of pre-heading dry matter translocation to grain yield under three density treatments.Under the same sowing date,there was no significant differences in the translocation of pre-heading accumulated dry matter to grain and post-heading dry matter production between direct-seeded ratoon rice and transplanting ratoon rice.Compared with direct-seeded ratoon rice in the ratoon season,transplanting ratoon rice had higher translocation of pre-heading accumulated dry matter to grain and contribution percentage of post-heading dry matter production to grain yield;compared with sowing date I,sowing date II averagely decreased the average translocation of pre-heading accumulated dry matter to grain by 25.4%under three density treatments,while increased the average post-flowering dry matter transport by 34.3%.The average post-flowering dry matter transport of each density treatment increased by 3.6%,but the average translocation of pre-heading accumulated dry matter to grain of each density treatment decreased by 25.4%under sowing date III in comparison with those under the sowing date I.These results indicated that early sowing can promote the translocation of pre-heading accumulated dry matter to grain in the main season and ratoon season of direct-seeded ratoon rice,thus further increase the grain yield.（3）There was an insignificant difference in the content of non-structural carbohydrate between different densities at mature stage of the main season and heading stage of the ratoon season at the same sowing date.As compared with the sowing dates I and II,the stubble soluble sucrose content averagely increased significantly by 84.5%and 80.7%at heading stage of the main season across different densities under sowing date III,the highest number of ratoon buds averagely increased by 15.4%and 10.1%.Compared with sowing date I and sowing date II,the percentage of productive tiller of ratoon buds averagely decreased by 11.5%and 8.9%under the sowing date III due to low temperature stress（the lowest temperature<15°C 16 d）from mature stage of the main season to heading stage of the ratoon season.The correlation analysis showed that the stubble soluble sucrose accumulation of the main season mature stage was positively correlated with the highest number of ratoon buds per m².In conclusion,direct-seeded ratoon rice with timely and early sowing may increase ratoon season yield under cold conditions by increasing the percentage of productive tillers and the number of panicles per m² in the ratoon season.（4）Compared with transplanting ratoon rice,direct-seeded ratoon rice significantly decreased the internode diameter,culm wall thickness,the internode length of the second inverted node,the internode diameter,culm wall thickness of the third inverted node and the lodging index of the third inverted node.However,different sowing date and density treatments had no significant effect on lodging resistance and related traits in direct-seeded ratooning rice.The results showed that compared with transplanting ratoon rice,direct-seeded ratoon rice had lower lodging resistance and higher lodging risk.（5）Compared with transplanting ratoon rice,direct-seeded ratoon rice had lower processing qualities and appearance qualities in the main season,and higher chalky grain rate and chalkiness in the ratoon season.Compared with sowing date I,sowing date II averagely decreased the yield of milled rice by 2.9%in the main season,and significantly increased the chalkiness and the chalky grain rate by 35.6%and 21.4%,respectively.In comparison with sowing date I,sowing date III averagely decreased the yield of milled rice by 27.8%in the main season,and averagely reduced the chalkiness and the chalky grain rate by 10%and 1.1%,respectively.There was an insignificant difference in head rice rate,chalkiness and chalky grain rate among different sowing dates of direct-seeded ratoon rice in the ratoon season.Compared with sowing date I and sowing date II,sowing date III averagely increased the chalkiness by 9.7%and 29.5%respectively,and the chalky grain rate increased by 14.1%and 19.5%respectively.Compared with low sowing density treatment,medium sowing density decreased the chalkiness of the main season rice by 32.3%and 15.4%under sowing date I and sowing date III,respectively,and high sowing density significantly decreased the chalkiness of the main season rice by 28.4%under sowing date I.Different sowing densities and different sowing dates had no effects on the brown rice rate,head rice rate,chalky grain rate,grain length and length-width ratio in the main and ratoon season.Therefore,early sowing can increase the yield of milled rice in main season and reduce chalkiness and chalky grain rate in the ratoon season of direct-seeded ratoon rice.In this study,the yield of the ratoon season was the highest under the sowing date II treatment,however,the main season yield and annual yield were lower than those under the sowing date I treatment,and the rice appearance quality of the main season was poorer.The performance of yield and quality of the ratoon season under the sowing date I treatment were poorer than those in the sowing date II treatment,but yield in the main season and annual yield were the highest under sowing date I,and the appearance quality of the main season rice under sowing date I was the best among the three sowing date treatment.Under the sowing date I treatment,the middle density treatment had the highest annual yield among the three sowing date treatments,and was lower than that of the conventional transplanting treatment,and the appearance quality of the main season rice was the best.The quality of the ratoon season was comparable to that of the low-density and high-density treatments.Therefore,the suitable sowing date and seeding rate for high-yield and high-quality direct-seeded ratoon rice in the central region of China are April 5th and 37.5 kg ha^-1,respectively.Compared with the transplanting ratoon rice,the yields of the main season and ratoon season,and annual yield decreased in different sowing date treatments,but sowing on 5th April with a seeding density of 37.5 kg ha^-1 may achieve the yield level of conventional transplanting ratoon rice.However,this study showed that compared with transplanting ratoon rice,the processing and appearance quality of the main and ratoon seasons of direct-seeded ratoon rice significantly decreased,and the risk of lodging was higher than that of transplanting regenerated rice.Therefore,in the production of direct-seeded ratoon rice,sowing date and sowing density should be optimized for high and stable yield.