Capability of crop production and translocation are two key factors in crop yield formation.Therefore, studying the characteristics of dry matter and photosynthetic production of rice has great significance in understanding rice yield formation and regulating high yields and stable yields. Regarding the characteristics of production and accumulation of rice photosynthetic substances, researchers have carried out extensive studies on high-yield varieties,high-yield populations and groups that exhibit different production levels and have proposed many valuable theories . Yang et al. reported that a greater material capacity in terms of leaf area during the middle and late stages of development and a relatively high population growth rate resulted in increased material production postanthesis and an increased transport ability of postanthesis substances under ecological conditions in Yunnan Province, China. Compared with high-yield populations, super-high-yield populations exhibited greater dry matter accumulation in the middle stages of development, a greater leaf area index at the HS, a superior population quality, and a greater photosynthetic ability and perfected the coordination of both the output and transport of the stem sheath materials in the late stages of development. Zhang et al. reported that greater material production is an important reason for improving the production potential of super hybrid rice compared with common hybrid rice and conventional rice. Seedling age is one of the key approaches in regulating rice growth and development.
Therefore, it is important to study the effects of seedling age on dry matter production to maximize grain yield from HLMS for different rice cultivars. The objective of this study was to clarify the growth process and dry matter and photosynthetic production characteristics of seedlings of different ages grown as HLMS. The corresponding results will provide a practical reference and lay a theoretical foundation for the large-scale application of HLMS.The dry matter accumulation of the population and its ratio to total dry matter were slightly but not significantly greater for young seedlings than for old seedlings for both cultivars from the TS to the JS in 2014 ; these findings were consistent for Wuyunjing 24 in 2015,mobile grow rack and the difference was significant. The interaction between year, cultivar and seedling age did not affect the dry matter accumulation of the population. The dry matter accumulation of the population and its ratio to total dry matter showed no obvious pattern of differences between the treatments from the JS to the HS. However, the dry matter accumulation of the population was significantly greater for young seedlings than for old seedlings for both cultivars and during both study years from the HS to the MS except for 6 Liangyou 9368 in 2014, and with the exception of 6 Liangyou 9368 in 2014, there was a consistent trend for the ratio to total dry matter. Regarding the harvest index, consistent variation was found: the harvest index decreased with increasing seedling age for Wuyunjing 24 in both years, while no significant difference was detected in 2015, and the opposite trend occurred for 6 Liangyou 9368.The leaf, stem and sheath to total plant dry weight ratios decreased as the rice plants grew;however, the panicle to total plant dry weight ratio increased as the rice plants grew.
There were no significant differences between the different treatments in terms of the leaf to total plant dry weight ratio at the middle and late growth stages, except for 6 Liangyou 9368 in 2015 at the HS. The stem and sheath to total plant dry weight ratios exhibited no obvious pattern of differences between the treatments at the JS, HS and MS, and the panicle to total plant dry weight ratio exhibited no obvious pattern of differences between the treatments at the HS; however, the panicle to total plant dry weight ratio decreased with increasing seedling age and was consistent for both cultivars at the MS. Compared with that of the 27-day-old seedlings, the panicle to total plant dry weight ratio of 13-day-old seedlings of Wuyunjing 24 and 6 Liangyou 9368 increased by 2.33 and 1.79%, respectively, in 2014 and by 5.08 and 5.77% in 2015, but there was no significant difference between the treatments. The interaction between cultivar and seedling age in terms of the panicle-tototal plant dry weight ratio was significant each year.Grain yield significantly decreased with increasing seedling age for both Wuyunjing 24 and 6 Liangyou 9368 . No significant differences in grain yield were detected between the 13- and 20-day-old seedlings at transplanting for either cultivar. Compared with that of 13-day-old seedlings, the grain yield of 27-day-old seedlings decreased by 14.5% in 2014 and by 10.7% in 2015 for Wuyunjing 24, and the corresponding values were 10.5% in 2014 and 9.8% in 2015 for 6 Liangyou 9368. The panicle number significantly decreased with increasing seedling age, except for Wuyunjing 24 in 2014. The spikelets per panicle of Wuyunjing 24 significantly decreased with increasing seedling age, but no significant difference was found for 6 Liangyou 9368.
Moreover, no significant difference was found in the seed setting rate for either cultivar or during either year, except for Wuyunjing 24 in 2014, and no significant difference in the grain weight was found for either cultivar or during either year, except for Wuyunjing 24 in 2015. The interaction between year, cultivar and seedling age did not affect the grain yield. However, the interaction between cultivar and seedling age in terms of spikelets per panicle was significant for each year, but no significant differences were detected in terms of panicle number, grain weight or seed setting rate.Growth process and performance are genetic properties of rice cultivars and are determined mainly by their photonasty, thermoperiodicity and basic vegetation growth phases . These properties are also affected by ST, TS , cultivation methods , and ecological conditions , among other factors. The transplantation of seedlings of different ages is usually accomplished by one of two cultural methods: seeding at different times while transplanting at the same time or seedling at the same time while transplanting at different times. Both of these methods lead to differences in growth characteristics and environmental conditions, which then affect the growth stages. For traditional rice seedling cultivation methods, previous reports have shown that the whole growth phase differed by 19 days when the seedling age at transplanting differed by 18 days , and the WGP differed by 15 days when the seedling age at transplanting differed by 15 days for the cultural method of seeding at different times while transplanting at the same time .
The corresponding values were 5–8, 8 and 6–12 days when the seedling age at transplanting differed by 14, 15 and 20 days, respectively, for the cultural method of seedling at the same time while transplanting at different times . In this research, the WGP differed by 13–15 days when the seedling age at transplanting differed by 14 days under the method involving HLMS. The above results showed that the difference during the MS was much smaller than that during other growth stages, although the seedling age at transplanting was different. Further analysis revealed that the growth process was accelerated with the transplantation of young seedlings and that the whole phase was shortened. The acceleration occurred mainly during the vegetative growth phase , and there was little difference in the reproductive growth phase . A previous report suggested that a longer growth phase increases grain yields when rice plants head and mature normally . Nevertheless, Wang et al. reported that the grain yield increased with the extension of the growth phase has a limit. In the present study, the grain yield did not increase significantly for the old seedlings and even declined, although their growth phase was longer than that of the young seedlings. In addition, the results showed that the optimum seedling age for maximizing grain yield from HLMS not only depends on the length of the growth period but also may be associated with seedling quality, transplantation shock, etc. These results were consistent with those of the manually transplanted rice reported by Lampayan et al. .
Rice grain yield is the product of total dry matter accumulation and the harvest index, whereas yield formation is the result of individual plant and population dry matter accumulation,ebb and flow table distribution, translocation, and transformation . The transplantation of seedlings of different ages leads to differences in the uptake and utilization of heat, light and resources by rice plants and further affects the growth of individual rice plants and the population. Many studies have consistently shown that grain yield is closely related to total dry matter accumulation at the MS and to dry mater accumulation from the HS to the MS and that there is no significant relationship with dry matter accumulation before the JS or the harvest index under conditions of high yield . Previous reports suggested that the LAI at the HS and Pn decreased with increasing seedling age at transplanting, resulting in decreased total dry matter accumulation . The DMWPS and DMWP are major signs of individual rice plant growth and population quality, and increasing the potential of an individual plant to accumulate dry matter is beneficial for improving population dry matter accumulation . Su et al. suggested that a higher DMWPS leads to a slow reduction in leaf area, increased photosynthetic potential and net assimilation rates and, ultimately, high grain yields. In this research, the grain yield was significantly positively correlated with total dry matter accumulation at the MS and with the dry mater accumulation from the HS to the MS , and the grain yield was not significantly correlated with the harvest index for different seedling ages at transplanting for HLMS; these results were similar to those reported by Wu et al. . Grain yield was negatively correlated with dry mater accumulation from transplanting to the JS and was quadratically related to dry matter accumulation at the HS; these results were similar to those reported by Gong et al. . Therefore, dry mater accumulation at early growth stages should be controlled properly, and it proportionally increased during the middle growth stage and increased the dry matter accumulation during the late growth stage drastically.
The lower LAI at the HS and Pn for the old seedlings may explain the lower dry matter accumulation for the HLMS; these results are similar to those of Zhang and Gong and Zhu et al. . The DMWP is determined by both the DMWPS and the stems and tillers of the population . In the present study, the DMWPS from transplanting young seedlings at the MS was greater than that from transplanting old seedlings, and there were more stems and tillers in the population of young seedlings than in that of old seedlings, which is mainly due to the robust seedling quality, better mechanical transplantation quality, lower transplantation shock and rapid tiller emergence of young seedlings transplanted compared with old transplanted seedlings . A portion of the grain-filling material of rice comes from the photosynthetic products after heading, and the other portion comes from the redistribution of stored materials from the leaves, stems and sheaths . Compared with low-yielding rice, high-yielding rice generally has greater amounts of stored photosynthetic products in the leaves, stems and sheaths at the early and middle stages of growth and has greater proportions of photosynthetic products distributed to the panicle, and greater amounts of materials stored in the leaves, stems and sheaths of the latter can be transported to the panicle . Li et al. reported that the dry matter exportation and exportation rates of the leaves, culms and sheathes per shoot of 25-day-old seedlings were significantly greater than those of 40-day-old seedlings. In this study, the leaf-to-total plant dry weight ratio did not clearly differ at the JS, decreased slightly at the HS, and increased slightly at the MS with increasing seedling age. In addition, the panicle-to-total plant dry weight ratio at the MS decreased with increasing seedling age. All of these results showed that the young transplanted seedlings had better Pn s and population support systems, proper allocation of dry matter to the different organs and a high panicle-to-total dry matter ratio at the MS. In addition, the transplantation of young seedlings results in a strong growth advantage, which may be related to the absence of premature senescence, strong root activity, a long duration of photosynthetic function of the leaves, high physiological and biochemical activity, etc.; all these factors need to be further studied.