Characteristics Of Photosynthesis,Carbon And Nitrogen Transportation In Different Harvest Index Rapeseed (Brassica Napus L.) Genotypes Grown Under Different Planting Density

A global challenge to high productivity and sustainable agriculture has been created as the world population rises and the amount of land suitable for farming decreases.Rapeseed is the world’s 3rd most important oil crop,and it offers both edible oil and raw materials for bio-energy uses.Due to a lack of rural manpower,mechanical harvesting of rapeseed has become a significant development trend in the Yangtze River Basin and has supplanted the direct-seeding method of simplified agriculture.Sichuan is one of the main producing region of winter rapeseed in China,and direct seeding cultivation of rapeseed is also an important way to promote the production of rapeseed in this region.Previous studies have shown that under optimum nitrogen application rate,increasing the planting density does not necessarily increase the seed yield of direct-seeding rapeseed.It may be due to mutual shading and intensified competition among plants,which is not conducive to photosynthesis and material transportation,thereby reducing the productivity of a single plant and resulting in a population yield that does not increase with plant density.Harvest index refers to the ratio of economic output（grain）to biological output when a crop is harvested.Its physiological nature reflects the distribution ratio of crop assimilates on grain and vegetative organs.A high harvest index means a higher ability to convert photosynthetic products into economic products.So,under dense planting conditions,selecting varieties with higher harvest index may be an important way to improve the yield of direct seeding rapeseed.However,there is lack of relevant research on the photosynthetic characteristics,carbon and nitrogen transport characteristics of different harvest index rapeseed varieties,as well as their response to increased planting density.For that reason,we conducted three different field experiments from 2019 to 2022.First experiment was arranged for screening of different rapeseed varieties on basis of harvest index.The second experiment used a two-factor split plot design.Based on the variety screening results from the previous year,four hybrid rapeseed varieties with two different harvest index types were selected（Type 1,relatively low harvest index:Za 1945 and Zhongyouza 19;Type 2,relatively high harvest index:Jiayou1hao and Nanyou 6211）,with two planting densities（traditional direct seeding planting density（LPD）=150000 plants/ha and High planting density（HPD）=300000 plants/ha）.The third experiment used traditional direct seeding planting density（LPD）,and black net was used at flowering and pod stage to create a similar condition of mutual shading of plant individuals during the flowering and pod development stages under dense planting conditions.In this experiment,two hybrid rapeseed varieties(Chuannong（relatively low harvest index）and Zhongyouza（relatively high harvest index）were selected,and three shading treatments;S0=control（ambient light）,S1=shade at flowering stage and S2=shade at pod development stage were used.Source sink characteristics,photosynthetic behavior,dry matter accumulation and distribution,carbon and nitrogen transportation,and yield were assessed under different planting densities and shade simulations experiment by using stable isotope(¹³C,¹⁵N)labeling technology.The main research findings are as follows:1.High planting density and shading significantly reduces the photosynthetic area,chlorophyll content,photosynthetic enzyme activity,net photosynthetic rate,and carbohydrate content of the tested rapeseed varieties.Compared to LPD,HPD increase the photosynthetic area（LAI and PAI）of all genotypes.Compared with LPD,the PAI of Za1945,Zhongyouza19,Jiayou1hao and Nanyou6211 was increased by 9.64,9.76,13.71and 11.26%under HPD,respectively.In case of sink,HPD increase number of pods in type 1（low harvest index）and type 2（high harvest index）genotypes by 50.51 and 54.75%while number of seeds increased by 34.42 and 56.09%,respectively.Compared with LPD treatment,the chlorophyll content and photosynthetic rate of leaf and pod were declined under HPD treatment.Furthermore,HPD decreased the Rubp,SS,AGpase activities and carbohydrates content of leaves and pods.The LAI and PAI of both genotypes decreased significantly under S1 and S2 shading treatments.The shade stress at both growth stages significantly decreased both tested genotypes’leaf and pod enzymatic activity.The shading（S1 and S2）treatment considerably reduced the sucrose,reducing sugar and soluble sugar contents of both tested genotypes’stem and pod shell.2.High planting density and shading decreases the transportation and distribution of ¹³C and ¹⁵N labeled isotopes in the tested rapeseed varieties.HPD significantly reduced theδ¹³C,¹³C excess and ¹³C organ（%）in different types of varieties.There was a declining trend in ¹⁵N accumulation in pod shells and seeds,with the order being S0>S1>S2.3.High planting density and shading significantly declined the accumulation and transportation of dry matter,oil content and grain yield of all rapeseed varieties.Compared with LPD,HPD significantly reduces the dry matter per plant of each genotype but increase the dry matter per hectare due to high plant population.Moreover,HPD declined the harvest index of Za1945,Zhongyouza19,Jiayou1hao,and Nanyou6211 genotypes by5.88,10.21,2.85 and 3.59%.Compared with the LPD,the yield per hectare increased by6.41,5.05,15.14 and 17.46%under HPD,respectively.S1 and S2 treatment significantly decreased the harvest index,compared with S0.In case of yield,the S1 and S2 declined the yield of Chuannong by 13.31 and 50.03%and this reduction was 11.06 and 37.01%in Zhongyouza,respectively.4.Under dense planting conditions,high harvest index varieties showed higher values of source-sink parameters,photosynthetic characteristics,photosynthetic enzyme activity and carbohydrate content,resulting in relatively stronger photosynthetic capacity;Meanwhile,higher harvest index varieties exhibited higher dry matter transportation and contribution to grain yield.5.Under dense planting conditions,δ¹³C decreases was relatively low,and the proportion of ¹³C atoms in organs of high harvest index genotypes was relatively high.During the flowering period,the proportion of ¹³C organs（stems,buds）allocation was higher in high harvest index varieties.In case of ¹⁵N label in shade experiment,higher content of ¹⁵N of seeds was observed in Zhongyouza variety（relative higher harvest index）as compared to Chuannong variety（relative lower harvest index）.In summary,increasing planting density has adverse effects on the chlorophyll content,photosynthetic enzyme（Ru BP,SS,AGpase）activity,and net photosynthetic rate（Pn）of rapeseed plant organs,reducing the transport capacity of photosynthetic products.However,under densification conditions,compared with varieties with lower harvest indices,varieties with higher harvest indices have a relatively larger population of photosynthetic source pools,higher photosynthetic enzyme activity,and higher carbohydrate content,thus possessing relatively strong photosynthetic capacity;Varieties with higher harvest indices also have higher dry matter transport capacity and their contribution to grain yield.Therefore,selecting varieties with higher harvest index under dense planting conditions is an important strategy to improve the yield of direct seeding rapeseed.In addition,shade stress for a specific period of time affect the photosynthetic behavior and carbohydrates content of leaves and pod wall which reduces the yield and quality characteristics of rapeseed.Our study reveals the physiological mechanism of the effects of low light stress or mutual shading of individual plants on the photosynthetic capacity of rapeseed during the critical growth period.We also found that the photosynthetic capacity of rapeseed pod wall had a greater impact on yield and seed quality than leaves.Future researches should focus on the genotype selection,cultivation management under high planting/low light conditions,improve photosynthetic efficiency of pod wall and promote photo-assimilates translocation to the grain.