| Direct-seeded and double-season rice（DDR）,a method to reduce labor and resource inputs,could increase farmers’willingness to adopt double-season rice and ensure the national food security in the context of declining area of double-season rice due to labor shortage and resource scarcity.Specifically,the early-and late-season rice in central China needs varieties with approximately 95 days of total growth duration so that both crops could reach maturity under DDR.Meanwhile,lodging-resistant rice variety is of great importance in improving crop yield and yield stability in DDR since lodging is a major constraint to high-yielding in direct-seeded rice.However,there is limited information on agronomic traits,lodging resistance and its physiological mechanism in DDR in central China.Although nitrogen（N）and seeding rates are important factors affecting grain yield and N use efficiency of direct-seeded rice,limited information is available on the grain yield and N use efficiency of the ultrashort-duration rice variety grown under different N and seeding rates in DDR.Therefore,the objectives of this study were to（1）identify the genotypic differences in agronomic and lodging-related traits among ultrashort-duration genotypes under DDR;and（2）determine the effects of N and seeding rates on grain yield and N use efficiency;and（3）elucidate the physiological mechanisms of the seasonal difference in the response of yield formation and N use efficiency to N and seeding rates under DDR.This research was conducted from 2017 to 2019 at Wuxue and Qichun Counties,Hubei Province,China,during eight seasons.The first field experiment was conducted in 2017and 2018 at Wuxue County,Hubei Province,China with two short rice varieties（Xiangzaoxian6 and Zaozhenzhu）and two tall rice varieties（Zaoxian615 and Zaoxian240）grown under two N rates(LN,60 kg N ha-1-90 kg N ha-1;HN,120 kg N ha-1-200 kg N ha-1).A subsequent field experiment was conducted in 2018 at Wuxue County and 2019 at Qichun County with an ultrashort-duration variety（Xiangzaoxian6）grown under four N rate(N0,0 kg N ha-1;LN,70 kg N ha-1-100 kg N ha-1;MN,120 kg N ha-1-150 kg N ha-1;HN,170 kg N ha-1-200 kg N ha-1)and three seeding rate(LD,4 g m-2-6 g m-2;MD,7 g m-2-9 g m-2;HD,10 g m-2-12 g m-2)treatments.The results of this study are as follows:（1）The field experiment conducted in 2017 and 2018 showed that rice yield ranged from 4.59 t ha-1 to 7.61 t ha-1 with total growth duration ranging from 85 d to 97 d.Varietal effect on grain yield was significant in the late season but not in the early season.Among four varieties,the highest grain yield was observed in Xiangzaoxian6 and Zaoxian615 in the late season of 2017 and Zaozhenzhu and Zaoxian615 in the late season of 2018.There was an insignificant difference between tall and short varieties in yield and aboveground total dry weight（TDW）.As comparted with tall varieties,the negative effects of lowering plant height on TDW could be compensated for by higher tiller number,leaf area index（LAI）and harvest index（HI）of short varieties with large number of panicles.（2）The lodging index of the four varieties ranged from 38.4 to 81.3 across the two N rates and the four seasons.Breaking resistance did not affect lodging index significantly.Genotypic difference in lodging index was mainly explained by the bending moment,which was closely related to plant height in 2017 and 2018.Compared with tall varieties,plant height of short varieties was shortened from 95.4 cm to 80.5 cm,decreasing bending moment by 32.0%and lodging index by 22.4%without grain yield penalty.Therefore,reducing plant height was effective to improve the lodging resistance of ultrashort-duration varieties grown under DDR.（3）In the case of the subsequent field experiment performed in 2018 and 2019,the grain yield and total growth duration of the ultrashort-duration varieties across all treatments ranged from 4.10 t ha-1 to 8.23 t ha-1 and 85 d to 97 d,respectively.While seeding rates had minor and inconsistent effect on grain yield,N rates significantly increased grain yield in both early-and late-growing seasons.As compared with N0,the increase in grain yields from applied N was mainly explained by higher TDW rather than HI.Higher TDW from N application was mainly due to increased light interception percentage and intercepted radiation,which was associated with improved LAI.On the basis of yield formation and mechanism of the effect of N rates on grain yield,optimum N rates was 100kg N ha-1-150 kg N ha-1 in the early season and 70 kg N ha-1-120 kg N ha-1 in the late season for the ultrashort-duration variety to achieve high-yielding under DDR in central China.（4）Remarkably higher N response was achieved in the early season compared with that of the late-season rice in 2018 and 2019.The difference in N response between two seasons was mainly due to the difference in grain yield under N0 treatment.Grain yield under treatment without N application was 2.30 t ha-1lower in the early than in the late season averaged across years and seeding rates.The differences in N response between the two seasons were mainly due to the differences in indigenous soil N and N uptake capacity.There was an obvious seasonal difference in total N uptake,which was 30.7 kg ha-1 lower in the early than in the late season.Additionally,partial factor productivity of applied N was significantly higher in the late season than the early season,and the reverse was true for agronomic N use efficiency.Furthermore,there were no consistent differences in recovery N use efficiency and internal N use efficiency between two seasons.In conclusion,ultrashort-duration varieties with low plant height and large number of panicles are of great importance in achieving high-yielding and efficient N use under moderate N input in DDR.This study emphasizes that understanding the seasonal differences in indigenous soil N and N response would benefit to optimize N management,which is critical to increasing grain yield and resource use efficiency of ultrashort-duration varieties with the goal of achieving the sustainable development of DDR. |
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