| 【Object】Sufficient solar radiation is particularly important for high and steady yields,especially in maize（Zea mays L.）,which is a typical C4 plant.In production,there are huge regional disparities of solar radiation resources in different regions across the world.In addition,in some regions,under the influence of overcasting and rainy weather,maize is often suffered low-light stress during its critical growth stages.In this study,various solar radiation conditions were achieved by simulating solar radiation stresses in different regions with the combination of different shading levels and different planting densities,and then,the quantitative effects of the interaction among genotype,planting density,and solar radiation on maize production were analyzed,so as to explore systematically,in terms of such aspects as yield,photosynthetic material production,ear traits and lodging,the optimal matching between growth and development of maize cultivars and their planting densities under the condition of different solar radiation,thus,giving a reference for maize’s breeding and selection towards high and stable yield under the coming adverse climate conditions caused by global climate change.【Methods】Field experiments were conducted at Qitai Farm（Qitai,Xinjiang Uygur Autonomous Region,China）and Ningxia University Farm（Yinchuan,Ningxia Hui Autonomous Region,China）,respectively,both of which are located in the higher solar radiation area of China.A split block design was conducted with cultivars as the principal factor,planting density as the subplot factor,and shading level as the secondary subplot factor.Three maize cultivars widely grown in China,Xianyu 335（XY335）,Zhengdan 958（ZD958）,and Denghai 618（DH618）,were planted at five densities（4.5×104（D1）,7.5×104（D2）,9×104（D3）,10.5×104（D4）,and 12×104（D5）plants/ha）.From the silking stage to maturity and from the three-leaf stage to maturity,four shading treatments（85%（S1）,70%（S2）,and 50%（S3）natural solar and no shading（CK））were designed to create different solar conditions,respectively.This study was focused on the effect of solar radiation on the growth and yield of maize whereby,clarifying the internal mechanism and quantitative relation of the interactive effects of genotype,planting density,and solar radiation on the growth of different maize cultivars.【Results】（1）The yield of different maize cultivars decreased with the reduction of solar radiation,and the yield under different solar radiation treatments was as follows:CK>S1>S2>S3.As illustrated by the comparison among cultivars,XY335 has a higher yield than ZD958 within the same range of solar radiation.In contrast,ZD958 requires more solar radiation within the same yield range,while XY335 could yield results similar to ZD958 under lower solar radiation conditions and is more efficient in utilizing weak solar than ZD958.Apart from that,maize yield responds differently to planting density within different solar radiation ranges.To be specific,when the solar radiation after silking was less than 498.9 MJ/m2,D2 planting density was more suitable,and when the ranges of which from 498.9 to 577.7 MJ/m2,the appropriate planting density gradually increased from low to high as the solar radiation gradually heightens;and when which was greater than 577.7 MJ/m2,the planting density was D5,maize utilized solar resources more sufficiently and yields higher.To put it another way,under the condition of less than 498.9 MJ/m2 of solar radiation after flowering,if the planting density is higher than D2,the yield will decrease.In a solar radiation environment with the solar radiation after flowering higher than 577.7 MJ/m2,if the planting density is lower than D5,the abundant solar radiation resources cannot be adequately utilized.Aside from that,as revealed by the analysis of the relative change correlation between solar radiation and yield among different treatments,when the relative change rate of solar radiation exceeds 1%,the yield will be changed by 1.16%.（2）Low-solar stress inhibits the formation of maize ear number,kernel number per unit area（KN）,and kernel weight（KW）production.The direct path coefficient of each ear trait ranged from large to small was KN>KW>ear density>number of kernels per row>bald tip length>grain abortion rate.Therefore,KN is the key factor that directly affects yield.The indirect path coefficient showed that the effects of ear density and KW on yield were caused primarily by the indirect KN effect.The indirect number of kernels per row and grain abortion rate path coefficients through KN were 0.28 and-0.29,respectively,indicating that the number of kernels per row and grain abortion rate greatly influence yield through KN.The results showed that,compared with CK,the number of grains per row of S1,S2,and S3 decreased by 7.2%,17.2%,and36.8%and bald tip lengths were 1.4-,1.6-,and 2.7-times higher,respectively.Meanwhile,the grain abortion rates under CK,S1,S2,and S3 were 25.4%,30.7%,37.9%,and 54.2%,respectively.As planting density increased from D2 to D5 under low-solar stress,the kernel rows per ear reduction rate increased from 15.9%to 24.5%and those of the CK bald tip lengths decreased from 2.2 times to 1.3 times,and the average grain abortion rate increased from 30.9%to 43.3%,respectively.When the grain abortion rate increased by 10%,the KN decreased by 576 kernels/m2,and when the average bald tip length increased by 1 cm,the KN decreased by 205 kernels/m2.For each decrease of 100 MJ/m2 in solar radiation,the KN decreased by 803.2kernels/m2.Shading could significantly increase the earless plant rate and decrease the ear number,and the greatest ear number decreases and the earless plant rate increases at the highest shading level.The results showed that,on average,under S1,S2,and S3,ear numbers decreased by 3.4%,4.0%,and 29.2%,respectively.Compared with the CK,the earless plant rate was 0.5%,1.6%,and 18.5%,respectively,higher than CK,which were 1.8,3.5,and 30.6 times of CK.Compared with different planting densities,it was found that the ear number decrease and the earless plant rate increase were both less at D2 than at D5 after shading.In addition,there was a significant positive correlation between solar radiation and KN,KW,and grain growth rate.With each increase of 100 MJ/m2 in solar radiation,the average grain-filling rate（Gave）,maximum grain-filling rate（Gmax）,and the kernel weight at the time of maximum grain-filling rate（Wmax）increased by 0.073[mg/（kernel·d）],0.23[mg/（kernel·d）],and 0.24 mg/kernel,and the time of maximum grain-filling rate（Tmax）delayed by 0.91 d.Through the analysis of the relative value between Gave under different solar radiation,it was found that with each change of 1%in the solar radiation,Gave was changed by 0.17%.When the solar radiation was more than 674.3 MJ/m2,KW was likely to stabilize at 36.2 g/hundred kernels and the growth rate of 100-grain weight was 5.82[g/（100 MJ/m2）].Under weak solar conditions,Maize cultivar DH618 has better grain-filling ability and stronger tolerance to high planting density than XY335 and ZD958.（3）The vegetative organ dry matter translocation（DMT）and its contribution to grain were increased with the increase of shading degree and planting density.The dry matter assimilation amount after silking（AADMAS）was increased by enhancing solar radiation and raising planting density.For XY335 and ZD958,when solar radiation was less than 580.9 MJ/m2 and 663.6 MJ/m2,respectively,the increase in the AADMAS was primarily related to solar radiation amounts;and when solar radiation was higher than those amounts,an increase in the AADMAS was primarily related to planting density.Although the pre-silking storage material would be transferred to the grain after the decrease of solar radiation,maize yield is primarily determined by photosynthate accumulation and its distribution,and the contributions of vegetative organs to grain could hardly compensate for the yield loss caused by insufficient solar radiation.Shading could significantly reduce the leaf photosynthetic rate of ear position（Pn）and leaf area duration（LAD）,which consequently reduce the amount of dry matter assimilated and thus lower maize yield.When the Pn increased by 1μmol CO2/m2/s,DMT decreased by 0.19 t/ha and AADMAS increased by 0.68 t/ha.Also,when the decreases in LAD increased by 1 m2 day,DMT decreased by 0.15 t/ha and AADMAS increased by 0.31 t/ha.The senescence rate of maize leaves accelerated significantly after shading,and the senescence rate of XY335 and ZD958 at S1,S2,and S3 were 2.54%,15.47%and 26.69%,and 17.52%,33.64%and 68.93%lower than CK,respectively.Therefore,the senescence rate of ZD958 leaves is faster than that of XY335 leaves,indicating that XY335 has stronger adaptability to weak light than ZD958 and could maintain higher photosynthetic capacity and anti-aging ability.（4）The stalk bending strength（SBS）,rind penetration strength（RPS）,vertical root pulling resistance（VRPR）,dry weight per unit length（DWUL）,the cellulose and lignin content,and grain yield,all of them were significantly negatively correlated with the lodging percentage and significantly decreased with decreasing solar radiation,respectively.Quantitative analysis showed that for each 100 MJ/m2 reduction in solar radiation,the SBS,RPS,and VRPR were decreased by 6.67 N,7.5 N,and 159.4 N,respectively,while DWUL and the content of cellulose and lignin of maize stalk were significantly decreased with decreasing solar radiation,by 88 mg/cm,30 mg/g and 6 mg/g,respectively,and the lodging percentage increased by17%.The variation of stem mechanical properties of XY335 with solar radiation is greater than that of ZD958,indicating that the stalk strength of XY335 is more sensitive than ZD958 to solar radiation decline.However,SBS,RPS,DWUL,and the cellulose and lignin content of XY335 are higher than ZD958 and the lodging percentage of XY335 is lower than ZD958,demonstrating that XY335 has a stronger stalk strength resistant to lodging.【Conclusion】（1）When the amount of solar radiation after silking was below 498.9 MJ/m2,D2 planting density was suitable.When the solar radiation after anthesis was in the range of 498.9-577.7 MJ/m2,the suitable planting density gradually increased from low density to high density with the increase of solar radiation.When the amount of solar radiation after silking was greater than 577.7 MJ/m2,and the planting density was D5,the maize utilized the solar radiation resources more fully and the yield was higher.In other words,under the condition that the post-silking solar radiation was less than 498.9 MJ/m2,if the planting density was higher than D2,the yield would decrease.In an environment that with solar radiation after silking higher than 577.7 MJ/m2,if the planting density was lower than D5,the abundant solar radiation resources would not be fully utilized.（2）DH618 and XY335 both have characteristics such as a long green leaf period,high photosynthetic efficiency,strong grain-filling and lodging resistance,and high grain yield,making them highly tolerant to low solar radiation environments.Planting DH618 and XY335 in areas with insufficient solar radiation is more beneficial for increasing maize yield.（3）Under low solar radiation conditions,KN is the most direct intrinsic factor determining maize yield,compared to ear number and grain weight.Insufficient solar radiation leads to a dramatic increase in seed sterility rate and longer barren tip lengths,reducing the number of grains per unit area.（4）Under insufficient solar radiation conditions,the transfer and contribution of nutrients from vegetative organs to grains are not enough to compensate for yield loss caused by reduced solar radiation,while the accumulation and direct allocation of photosynthetic products are the main sources of grain yield.（5）DWUL and the content of cellulose and lignin significantly affect SBS and RPS,causing a decrease in stem mechanical strength and grain yield,and an increase in lodging rate.The quantitative relationship between solar radiation and the lodging resistance of maize was further clarified by analysis,which provided a theoretical basis for improving the lodging resistance of maize cultivars under different solar radiation conditions. |
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