Ll Yu-xiang ,LlU Yang,WANG Yu-huiDlNG Yan-fengWANG Shao-huaLlU Zheng-huiLl Ganghua

1 Jiangsu Collaborative Innovation Center for Modern Crop Production/National Engineering and Technology Center for Information Agricultrue/Key Laboratory of Crop Physiology and Ecology in Southern China,Nanjing Agricultural University,Nanjing 210095,P.R.China

2 The Key Laboratory of Oasis Eco-Agriculture of Xinjiang Production Construction Group/Agricultural College,Shihezi University,Shihezi 832003,P.R.China

Abstract Understanding the characteristics of rice productivity is of great importance for achieving high yield formation.However,such traits have not yet been studied for different ages of hydroponically grown long-mat rice seedlings (HLMS),which constitutes a new method of seedling cultivation.Field experiments were conducted to evaluate the effects of seedling age on the growth stage,photosynthesis characteristics,dry matter production,and yield of HLMS.A conventional japonica rice cultivar (Wuyunjing 24) and an indica hybrid rice cultivar (6 Liangyou 9368) were used as test materials.The results showed that the whole phase was shortened by 13-15 days for young seedlings (13-day-old) compared with old seedlings(27-day-old),which occurred because the growth process accelerated with the transplantation of young seedlings.As seedling age increased,the dry matter weight of stems of individual plants and of the population increased at the transplanting stage but decreased at the maturity stage (MS).Compared with that of 27-day-old seedlings,the average ratio of panicle weight to total plant dry weight of 13-day-old seedlings during a 2-year period increased by 3.71% for Wuyunjing 24 and by 3.78% for 6 Liangyou 9368 at the MS.Moreover,as seedling age increased,the leaf area index and photosynthetic potential decreased for both cultivars,and the photosynthetic rate markedly decreased at the heading stage (HS).With the exception of that of Wuyunjing 24 from the jointing stage to the HS in 2014,the crop growth rate was higher for young seedlings than for old seedlings.Grain yield significantly decreased with seedling age,but no significant difference was detected between the 13-and 20-day-old seedlings for either cultivar.Therefore,equilibrious and high biological yield formation,vigorous growth in the late stages,and high photosynthetic production capacity are important characteristics and causes of the efficient and sustainable output of photosynthetic systems and for achieving high yield formation in young transplanted seedlings (13-20-day-old).

Keywords:rice (Oryza sativa L.),seedling age,growth stage,yield,photosynthetic matter production

1.lntroduction

Rice (Oryza sativaL.) is the foremost staple food in Asia,feeds more than 60% of the population in China and constitutes up to 40% of the total national grain production(Wuet al.2013).At present,there is an urgent need to shift rice production toward mechanized farming (Liet al.2016).Mechanical transplantation is an emerging riceplanting method in China and represents an important opportunity for scaled,simplified,commercialized,and modernized development (Rashidet al.2018).Mechanical transplantation has several advantages,such as high and stable yields and high efficiency (Xinget al.2017),and has developed rapidly in China in recent years；the use of mechanical transplantation will guide the future direction of national agricultural production to solve the problems concerning severe shortages in the rural labor force and to achieve full mechanization of rice cultivation.However,traditional rice seedling cultivation methods for mechanical transplantation are time consuming and laborious,and collecting topsoil for the boxes can disrupt soil structure and subsequently reduce crop yields.All of these factors limit the development of mechanical transplantation of rice in China.

The use of hydroponically grown long-mat rice seedlings(HLMS) represents a novel seedling cultivation method that reduces labor costs and improves efficiency (Tasakaet al.1996,1997；Tasaka 1999；Wanget al.1999).This method presents two main features:first,nonwoven cloth is used as the medium instead of soil,and the seedlings are cultivated hydroponically；second,nursery seedling boxes that are 3 to 6 m in length are used rather than traditional small rectangular boxes (0.28 m×0.58 m).The grain yield and economic benefit of HLMS were equivalent or superior to those of traditional nutritive soil methods because of the robust seedling quality,better transplantation quality and higher dry matter accumulation during late growth of the former (Liet al.2016；Leiet al.2017)；in addition,the root entwining force of HLMS is enhanced,resulting in significantly increased flexibility of seedling age and enabling high-yielding and -efficiency rice production (Li 2016).

Capability of crop production and translocation are two key factors in crop yield formation (Huanget al.2004).Rice yield formation is essentially the process of dry matter accumulation and distribution (Wuet al.2007).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 (super) high-yield varieties (combination),(super) high-yield populations and groups that exhibit different production levels and have proposed many valuable theories (Ntanos and Koutroubas 2003；Takaiet al.2006；Yanget al.2006；Wuet al.2010；Fukushimaet al.2011；Huanget al.2011；Ansari 2013；Liet al.2013；Liet al.2017).Yanget al.(2006) 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(from the jointing to the heading stages (HS)),a greater leaf area index (LAI) 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 (from heading to the maturity stages (MS)) (Wuet al.2010).Zhanget al.(2009) 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.

2.Materials and methods

2.1.Experimental site and cultivars

All seedlings for 2014 and 2015 were cultured in a glass greenhouse,and field experiments were performed over two rice growing seasons at Danyang Station,Jiangsu Province (31°54′31′′N,119°28′21′′E),China.The topsoil horizon (0-20 cm) at the start of the experiment had the following characteristics:21.02 g kg-1organic matter,1.15 g kg-1total nitrogen,0.48 g kg-1total phosphorus,1.96 g kg-1total potassium,86.40 mg kg-1available nitrogen,13.45 mg kg-1available phosphorus,and 105.45 mg kg-1available potassium.Two widely grown rice cultivars in the Yangtze River basin,a typicaljaponicasuper rice cultivar known as Wuyunjing 24 and anindicahybrid super rice cultivar known as 6 Liangyou 9368,were tested in this study.The 1 000-kernel weight were 28 and 29 g for Wuyunjing 24 and 6 Liangyou 9368,respectively.

2.2.Experimental design and management

In each rice growing season,Wuyunjing 24 and 6 Liangyou 9368 were grown to three different seedling ages (27-,20-and 13-day-old) before transplantation.Both cultivars were sown on 1 June,7 June and 14 June 2014 and on 23 May,30 May,and 6 June 2015.The seeding rates were 600 and 450 g m-2for Wuyunjing 24 and 6 Liangyou 9368,respectively,using presoaked seeds (Liet al.2018a).The nutrient solution for the HLMS was prepared in accordance with the reports by Liet al.(2014,2018b).HLMS were grown in a single nursery bed (0.28 m×4 m) (Liet al.2012) in three replications.The seedlings were then transplanted to a paddy field on 27 June 2014 and 19 June 2015.For transplantation,an Iseki PZ60 speed transplanter was used (PG63DVRF+Long mat (PG6,63) SET,Iseki Co.,Ltd.,Ehime county,Iyo,Japan).The planting density was 30 cm×13.3 cm for Wuyunjing 24 and 30 cm×15 cm for 6 Liangyou 9368.The plot size for the field experiments was 32.4 m2(3.6 m×9 m),and the plots were replicated three times.

The nursery management of the HLMS was conducted in accordance with previously described methods (Liet al.2016).The field management methods included a total nitrogen fertilizer rate of 270 kg ha-1,which was applied as urea (containing 46% N) four times:25% as basal fertilizer,25% at 7 days after transplanting (DAT),25% when the rice had four leaves that have not appeared,and 25% when the rice had two leaves that have not appeared.Phosphate and potassium fertilizers were applied as basal fertilizers at rates of 150 kg ha-1(P2O5) and 300 kg ha-1(K2O),respectively.Water management and pest control were conducted in accordance with local routine management requirements.

2.3.Sampling and measurements

Growth stage:the number of leaves that appeared was recorded for 10 consecutive hills in each plot once every 5 days,and the main growth stages (seeding time (ST),transplanting stage (TS),jointing stage (JS),HS,and MS)were also recorded.

Dry matter accumulation:at the TS,JS,HS,and MS,the tiller (panicle) numbers on 60 hills in each plot were counted,and one intact individual plant was sampled from each of five hills on the basis of the average number of tillers (panicles).The specimen was then heated in an oven for 30 min at 105°C and then dried at 80°C until constant weight.The area of the leaves was measured with an LI-3000 (LI-COR,Lincoln,NE,USA) and expressed as the LAI.The photosynthetic potential and crop growth rate were calculated following the method of Liet al.(2011).

Photosynthetic rate (Pn):ThePnwas assayed using a portable photosynthesis analyzer (LI-COR 6400,USA) at the HS.

Yields and yield components:rice plants from the replications were harvested during physiological maturity to determine their grain yield parameters (Bahugunaet al.2017).For five hills,plants that had average panicles were sampled.The number of spikelets per panicle and the seed setting rate were individually calculated.The grains were dried at 70°C to a constant weight,and the dry grain weight was subsequently measured.The theoretical grain yield was then adjusted to a moisture content of 13.5%.

2.4.Data calculation and statistical analysis

Data processing and analyses were conducted using Microsoft Excel 2007 Software for Windows.An analysis of variance was performed using SPSS 17.0 statistical software for Windows to test the differences among the treatments.The means of the treatments were compared according to a least significant difference (LSD) test at the 0.05 probability level (P＜0.05).

3.Results

3.1.Growth stage

Compared with those planted with old seedlings,the main growth stages of rice in the paddy fields planted with young seedlings were delayed for both cultivars and years(Table 1).However,this gap diminished as the rice plants grew.In terms of the whole growth period,the old seedlings grew for a longer period than did the young seedlings.The whole growth period of the 27-day-old transplanted seedlings was 13 days longer than that of the 13-day-old seedlings for Wuyunjing 24,and the corresponding value was 15 days for 6 Liangyou 9368.The reduction in the growth period for young seedlings (13-day-old) occurred mainly during the vegetative growth phase (from the ST to the JS,the reduction value was 8-10 days for Wuyunjing 24 and 8-11 days for 6 Liangyou 9368),followed by the vegetative growth and reproductive development phases(from the JS to the HS,the reduction value was 3-4 days for Wuyunjing 24 and 2-3 days for 6 Liangyou 9368),andthe reproductive growth phase exhibited a smaller decrease(from the HS to the MS,the reduction value was 1-3 days for Wuyunjing 24 and 1-2 days for 6 Liangyou 9368) than did the vegetative growth phase.Further analysis showed that the difference in the growth period in the paddy field (from the TS to the harvest) was relatively less for different seedling ages at transplanting and that the gap in the vegetative growth phase was derived mainly from the nursery bed phase (from the ST to the TS).

Table 1 Growth stages of rice affected by different seedling ages under HLMS for Wuyunjing 24 and 6 Liangyou 9368 in 2014 and 20151)

3.2.Number of appeared leaves

The number of appeared leaves for old seedlings was greater than that for young seedlings at the transplanting stage；however,the difference diminished with the growth of rice,and no significant difference was detected at maturity.These results are consistent in the two study years and for both cultivars (Fig.1).In 2014,the number of appeared leaves of Wuyunjing 24 was 1.9,2.9 and 4.1 for the 13-,20-and 27-day-old seedlings at the transplanting stage,and the values were 15.0,14.9 and 15.6 at the MS,respectively.The number of appeared leaves of 6 Liangyou 9368 was 1.8,2.5 and 3.8 for the 13-,20-and 27-day-old seedlings at the transplanting stage,and the values were 14.4,15.0 and 14.8 at the MS,respectively.The trend of the number of appeared leaves in 2015 was similar to that in 2014.

3.3.Dry matter weight per stem (DMWPS) and population during the main stages

At the TS,the DMWPS and dry matter weight of the population (DMWP) increased with increasing seedling age at the transplanting for both cultivars and during both years(Table 2).No significant difference and/or consistent trend in DMWPS and DMWP was observed between the different transplanted seedling ages for either cultivar at the JS or HS.However,the DMWP at the MS significantly decreased with increasing seedling age for both cultivars in both years except Wuyunjing 24 in 2014.In addition,these findings were consistent for the DMWPS,although no significant difference was detected except for 6 Liangyou 9368 in 2015.

3.4.Dry matter accumulation of the population and its ratio to total dry matter

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 (Table 3)；these findings were consistent for Wuyunjing 24 in 2015,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.

Fig.1 Number of appeared leaves of rice affected by different seedling ages under hydroponically grown long-mat rice seedlings(HLMS) for Wuyunjing 24 and 6 Liangyou 9368 in 2014 and 2015.DAT,days after transplanting.Error bars above means denote standard error of the three replicates.

Table 2 Dry matter weight per stem and population affected by different seedling ages under hydroponically grown long-mat rice seedlings (HLMS) for Wuyunjing 24 and 6 Liangyou 9368 in 2014 and 20151)

Table 3 Period dry matter accumulation of population and its ratio to total dry matter in main growth period affected by different seedling ages under hydroponically grown long-mat rice seedlings (HLMS) for Wuyunjing 24 and 6 Liangyou 9368 in 2014 and 20151)

3.5.Dry weight ratios of leaves,stems and sheaths and of panicles to the total plant dry weight ratio

The leaf,stem and sheath to total plant dry weight ratios decreased as the rice plants grew (from the JS to the HS and from the HS to the MS) (Table 4)；however,the panicle to total plant dry weight ratio increased as the rice plants grew (from the HS to the MS).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.

3.6.Photosynthetic rate (Pn)

ThePnin both cultivars decreased with increasing seedling age for both years at the HS (Fig.2).No significant difference inPnwas detected between 13-and 20-day-old 6 Liangyou 9368 seedlings.Compared with that of 13-dayold seedlings,thePnof 27-day-old seedlings decreased by 18.5% in 2014 and by 16.9% in 2015 for Wuyunjing 24,and the corresponding values were 8.9% in 2014 and 16.7% in 2015 for 6 Liangyou 9368.

3.7.Leaf area index (LAl),photosynthetic potential and crop growth rate

The LAI decreased with increasing seedling age for bothcultivars and during both years (Table 5),and there was a significant difference in LAI except for 6 Liangyou 9368 in 2014 and Wuyunjing 24 in 2015 at the JS and for Wuyunjing 24 in 2014 at the HS.The interaction between year,cultivar and seedling age did not affect the LAI.The photosynthetic potential consistently tended to decrease with increasing seedling age for both cultivars and during both years,but no significant difference was found exceptfor 6 Liangyou 9368 in 2015 from the TS to the JS and in 2014 from the JS to the HS.The crop growth rate from the TS to the JS decreased significantly with increasing seedling age for both cultivars and during both years except for 6 Liangyou 9368 in 2015,and there was no obvious pattern of differences in crop growth rate between the treatments from the JS to the HS.The crop growth rate from the HS to the MS exhibited a similar pattern of differences；the crop growth rate for young seedlings was significantly greater than that for old seedlings except for 6 Liangyou 9368 in 2015.

Table 4 Dry weight ratio of leaf,stem and sheath,panicle to total plant and its variation at middle and late growth stages affected by different seedling ages under hydroponically grown long-mat rice seedlings (HLMS) for Wuyunjing 24 and 6 Liangyou 9368 in 2014 and 20151)

Fig.2 Photosynthetic rate (Pn) at the heading stage affected by different seedling ages under hydroponically grown long-mat rice seedlings (HLMS) for Wuyunjing 24 and 6 Liangyou 9368 in 2014 and 2015.Error bars above means denote standard error of three replicates.Different letters above error bars indicate that treatments are significantly different at the 0.05 probability level.

Table 5 Leaf area index,photosynthetic potential and crop growth rate affected by different seedling ages under hydroponically grown long-mat rice seedlings (HLMS) for Wuyunjing 24 and 6 Liangyou9368 in 2014 and 20151)

3.8.Grain yield and its components

Grain yield significantly decreased with increasing seedling age for both Wuyunjing 24 and 6 Liangyou 9368 (Table 6).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.

4.Discussion

4.1.Effects of seedling age on the growth process of HLMS

Growth process and performance are genetic properties of rice cultivars and are determined mainly by their photonasty,thermoperiodicity and basic vegetation growth phases (Weiet al.2012).These properties are also affected by ST,TS(Huanget al.2009),cultivation methods (Chenget al.2010),and ecological conditions (Liet al.2011),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 (WGP) differed by 19 days when the seedling age at transplanting differed by 18 days (Qi 2015),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 (Wuet al.2007).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 (Jia 2012；Chenet al.2015；Lampayanet al.2015).

Table 6 Grain yield and its components affected by different seedling ages under hydroponically grown long-mat rice seedlings(HLMS) for Wuyunjing 24 and 6 Liangyou 9368 in 2014 and 2015

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 (from ST to the JS),and there was little difference in the reproductive growth phase (from the HS to the MS).A previous report suggested that a longer growth phase increases grain yields when rice plants head and mature normally (Wang 2006).Nevertheless,Wanget al.(1995) 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 Lampayanet al.(2015).

4.2.Effects of seedling age on the dry matter production and distribution of HLMS

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,distribution,translocation,and transformation (Huoet al.2012；Huanget al.2019).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 (Yinget al.1998；Linget al.2000；Lianget al.2017).Previous reports suggested that the LAI at the HS andPndecreased with increasing seedling age at transplanting,resulting in decreased total dry matter accumulation (Maet al.2011).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 (Liet al.2011).Suet al.(1993)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(r=0.786＊＊) and with the dry mater accumulation from the HS to the MS (r=0.652＊＊),and the grain yield was not significantly correlated with the harvest index (r=-0.229)for different seedling ages at transplanting for HLMS；these results were similar to those reported by Wuet al.(2007).Grain yield was negatively correlated with dry mater accumulation from transplanting to the JS (r=-0.325) and was quadratically related to dry matter accumulation at the HS；these results were similar to those reported by Gonget al.(2014).Therefore,dry mater accumulation at early growth stages (from the TS to the JS) should be controlled properly,and it proportionally increased during the middle growth stage (from the JS to the HS) and increased the dry matter accumulation during the late growth stage (from the HS to the MS) drastically.The lower LAI at the HS andPnfor the old seedlings may explain the lower dry matter accumulation for the HLMS；these results are similar to those of Zhang and Gong (2014) and Zhuet al.(2019).The DMWP is determined by both the DMWPS and the stems and tillers of the population (Liet al.2011).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 (unpublished).

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 (Laenoiet al.2018；Zhanget al.2018).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(Liu and Yu 1998).Liet al.(2017) 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 betterPns 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.

4.3.Effects of seedling age on the photosynthesis production of HLMS

Photosynthetic production is the source of rice grain yield formation (Wuet al.2018).Leaf area,photosynthetic potential and crop growth rate are important indexes for representing the photosynthetic capacity of a population.Wuet al.(2007) suggested that the leaf area,photosynthetic potential and population growth rate of superhybridizedjaponicarice were significantly greater than those of Shanyou 63 control rice at the middle and late stages.Huoet al.(2012) reported that,with a delayed sowing date,the LAI significantly decreased at the JS,HS and dough stage and slightly decreased at the MS.The photosynthetic potential significantly or extremely significantly declined from sowing until the JS and from the JS to the HS.The crop growth rate markedly increased from sowing until the JS,and no significant difference was observed from the JS to the HS,whereas the rate decreased significantly from the HS to the MS.The net assimilation rate noticeably increased from sowing until the JS and from the JS to the HS but noticeably decreased from the HS to the MS.However,few reports exist on the effects of seedling age at transplanting on the photosynthetic production characteristics of mechanically transplanted HLMS.In this research,with increasing seedling age,the LAI decreased for both cultivars at the HS and MS,the photosynthetic potential also decreased from the TS to the JS and from the JS to the HS,and the crop growth rate decreased from the TS to the JS,except for 6 Liangyou 9368 in 2015.No obvious pattern of differences in crop growth rate was observed from the JS to the HS,but the rate was greater for the young seedlings than for the old seedlings from the HS to the MS.With increasing seedling age,thePnmarkedly decreased at the HS,but there was no significant difference inPnbetween the 13-and 20-day-old 6 Liangyou 9368 seedlings.

5.Conclusion

The growth process accelerated with the transplantation of young seedlings compared with old seedlings,and the WGP was shortened (13-15 days)；the acceleration occurred mainly during the vegetative growth phase (from seeding to the JS).Compared with the transplantation of old seedlings(27-day-old),the transplantation of young seedlings(13-20-day-old) under the system of HLMS resulted in appropriate dry matter accumulation,high photosynthetic production capacity and an efficient and sustainable output of the photosynthetic systems,resulting in high yields.