The effect of solar radiation change on the maize yield gap from the perspectives of dry matter accumulation and distribution

YANG Yun-shan ,GUO Xiao-xia,LIU Hui-fang,LIU Guang-zhou,LIU Wan-maoMING Bo,XIE Rui-zhi,WANG Ke-ru,HOU Peng,LI Shao-kun

1 The Key Laboratory of Oasis Eco-agriculture,Xinjiang Production and Construction Corps/College of Agronomy,Shihezi University,Shihezi 832003,P.R.China

2 Key Laboratory of Crop Physiology and Ecology,Ministry of Agriculture and Rural Affairs/Institute of Crop Sciences,Chinese Academy of Agricultural Sciences,Beijing 100081,P.R.China

3 College of Agronomy,Ningxia University,Yinchuan 750021,P.R.China

Abstract The uneven distribution of solar radiation is one of the main reasons for the variations in the yield gap between different regions in China and other countries of the world. In this study,different solar radiation levels were created by shading and the yield gaps induced by those levels were analyzed by measuring the aboveground and underground growth of maize.The experiments were conducted in Qitai,Xinjiang,China,in 2018 and 2019. The maize cultivars Xianyu 335 (XY335)and Zhengdan 958 (ZD958) were used with planting density of 12×104 plants ha–1 under either high solar radiation (HSR)or low solar radiation (LSR,70% of HSR). The results showed that variation in the solar radiation resulted in a yield gap and different cultivars behaved differently. The yield gaps of XY335 and ZD958 were 8.9 and 5.8 t ha–1 induced by the decreased total intercepted photosynthetically active radiation (TIPAR) of 323.1 and 403.9 MJ m–2 from emergence to the maturity stage,respectively. The average yield of XY335 was higher than that of ZD958 under HSR,while the average yield of ZD958 was higher than that of XY335 under LSR. The light intercepted by the canopy and the photosynthetic rates both decreased with decreasing solar radiation. The aboveground dry matter decreased by 11.1% at silking and 21%at maturity,and the dry matter of vegetative organs and reproductive organs decreased by 9.8 and 20.9% at silking and by 12.1 and 25.5% at physiological maturity,respectively. Compared to the HSR,the root weights of XY335 and ZD958 decreased by 54.6 and 45.5%,respectively,in the 0–60 cm soil layer under LSR at silking stage. The aboveground and underground growth responses to different solar radiation levels explained the difference in yield gap. Selecting suitable cultivars can increase maize yield and reduce the yield gaps induced by variation of the solar radiation levels in different regions or under climate change.

Keywords:maize,solar radiation,yield gap,cultivar

1.Introduction

Through the expansion of planting area,cultivar replacement and the introduction and intensification of field cultivation management measures,the global grain yield has increased significantly in the past 50 years (Cassman 1999; Foleyet al.2005; Neumannet al.2010). Nevertheless,food security is under threat as natural resources are reduced and food production stagnates due to many factors,especially climate change (Senapati and Semenov 2020). The most effective way to ensure global food security is to improve the yield potential,increase the adaptation of agricultural production to climate change and close the yield gap between current and potential yields (Krandikar and Risbey 2000; Liet al.2014; Senapati and Semenov 2020).

Potential yield refers to the yield obtained by a suitable crop cultivar under good growth conditions,without being restricted by water,fertilizers or the stress of pests and diseases (Yang and Liu 2014; Liuet al.2015). Under optimal management conditions,the yield potential of a particular crop cultivar depends on meteorological factors,such as solar radiation and temperature at a specific location (Van Ittersum and Rabbinge 1997; Liuet al.2016). A previous systematic analysis of global maize,rice,wheat,and soybean yields concluded that if 95% of the world’s planting areas reached climate potential,their yields would increase by 50,40,60,and 20%,respectively (Lickeret al.2010).Due to the limitation of soil,pests,management measures,and farmers’ enthusiasm,the actual yields of crops are far lower than the potential yields of local crops (Liu Z Jet al.2017). The existence of a yield gap between different regions is mainly driven by meteorological factors,especially solar radiation. There are great differences in solar radiation in different regions of China,especially between the eastern and western regions (Yanget al.2011; Houet al.2014).Solar radiation in the western regions is more abundant than that in the eastern regions in China. As a result,there is a great yield gap between the eastern and western regions(Xuet al.2017; Houet al.2018).

As a driving factor for crop growth and development,solar radiation is closely related to plant morphogenesis and yield formation (Demetriades-Shahet al.1994; Bansoulehet al.2009). Maize is a typical C4crop with short sunlight and high light efficiency,and it needs good solar radiation conditions during the whole growth period. Ensuring suitable light time and light intensity are beneficial for promoting photosynthesis and yield formation (Liet al.2002). In the absence of other limiting factors in the environment,maize yield is closely related to the intercepted photosynthetically active solar radiation,and a simple linear relationship exists between them (Muchowet al.1990; Braconnier 1998;Hammadet al.2016). With the decrease of intercepted solar radiation,the yield decreases (Muchowet al.1990; Hammadet al.2016). At the stage of maize reproductive growth,a decrease in solar radiation will restrict the establishment of reproductive organs,and lead to declines in leaf area index,photosynthetic rate and aboveground and underground biomass (Mbewe and Hunter 1986; Zhanget al.2007; Cuiet al.2013; Renet al.2016; Gaoet al.2017). At the same time,the rate of grain filling is decreased,the durations of the grain filling peak and active filling days are shortened,the effective filling period of maize is limited,and the grain size and weight are decreased,eventually leading to the yield decrease (Andrade and Ferreiro 1996; Chenet al.2014; Yanget al.2016; Gaoet al.2018).

Agriculture is a human activity that is intimately linked to climate,and farmers are always faced with an array of changing contexts. A major concern in understanding the impacts of climate change is the extent to what agriculture will be affected and its adaptation to climate change (such as solar radiation) (Krandikar and Risbey 2000). In recent years,solar radiation intensity has been declining globally as a result of climate change (Stanhill and Cohen 2001; Cheet al.2005; Abakumovaet al.2008; Liuet al.2012). Solar radiation in China has also shown a decreasing trend due to environmental pollution (Shiet al.2008; Qiet al.2019;Zhang Qet al.2019; Chen and Pang 2020). At the same time,due to differences in economic development between regions,the environmental situation varies as well as the degree of declination in solar radiation among regions (Liuet al.2015; Liu Set al.2019; Zhaoet al.2020). Previous studies on yield gap have focused on the effects of field management systems on crop yield,such as the effects of weed and pest stress (Hochman and Horan 2018),water management (Mohammadi-Ahmadmahmoudiet al.2020)and fertilization (Rhebergenet al.2020). The effect of solar radiation on the yield gap is mostly simulated by models,which have indicated that the yield gap decreases due to the decrease in simulated yield potential after a solar radiation decline and the actual yield increase under optimized cultivation management measures (Liuet al.2012; Taoet al.2015; Zhaoet al.2015). However,few studies have focused on the effect of solar radiation on the yield gap through field experiments. In addition,the previous studies mainly focused on the effect of shading on maize growth,but did not incorporate the quantification of radiation. In this study in the region with the best solar radiation resources in China,we used shading to mimic the low solar radiation conditions in other regions of China,and explored the yield gap caused by the experimental decrease of solar radiation and its internal physiological mechanisms. The results will provide a theoretical basis for reducing the yield gap between regions and under the conditions of solar radiation declines caused by climate change.

2.Materials and methods

2.1.Experimental design

The experiment was conducted in Qitai (Xinjiang Uygur Autonomous Region,China,43°49´27´´N,89°48´22´´E)in 2018 and 2019. Meteorological data for the 2018 and 2019 maize growing seasons were obtained from a“Watch Dog” Data Logger (Spectrum Technologies,Inc.,USA) located in the experimental field. The two-year meteorological data are summarized in Table 1. The effect of solar radiation on yield gap was studied by using single factor experiments involving two widely planted high-yield maize cultivars,namely Xianyu 335 (XY335) and Zhengdan 958(ZD958). The planting density was 12×104plants ha–1.Solar radiation intensity was 70% of natural solar radiation after shading (low solar radiation,LSR),and that level was compared with natural solar radiation (high solar radiation,HSR). Shading was imposed from the three-leaf stage until maturity. The experimental plots were 11 m×10 m in size,and adjacent plots were separated by a walkway with a width of 1 m. Shading nets of different shading levels were designed and fabricated. A detachable shade shed was built using a shade net,the top of which was positioned about 1.5 m away from the maize canopy,which ensured that the microclimatic conditions (other than the solar radiation) in the shade shed were consistent with the unshaded portions of the field (Yanget al.2019). A photo of the field experiment is shown in Fig.1.

Previous studies found that shading did not affect the quality of incident light in the maize canopy (Yanget al.2019). The amount of the photosynthetically active radiation reaching the top and bottom of the canopy were measured with a Sunscan (SUNSCAN,Delta-T,UK). The total intercepted photosynthetically active radiation (TIPAR)from the period of emergence to the maturity stage was calculated according to the following formula:

where A is photosynthetically active radiation (PAR)above the canopy,B is transmitted photosynthetically active radiation at the bottom of the canopy and C is total accumulated photosynthetically active radiation from the period of emergence to the maturity stage. PAR was measured with a diagonal orientation every 30 cm above the ground (Liu Get al.2019).The fraction of intercepted PAR was measured on clear days during the silking stage,using a line quantum sensor (SunScan,Delta-T) in 2018 and 2019. The three ear leaves were chosen at the silking stage for photosynthesis measurements in 2018 and 2019.The measurements of gas exchange were carried out on clear days at 12:00–13:00 using a programmable,openflow gas exchange portable system (LI-6400,Li-Cor Inc.,Lincoln,NE,USA) operated at a 500-μmol s–1flow rate,and the CO2concentration of the reference chamber (CO2-R)was set at 400 μmol mol−1. Light inducements were done by keeping the leaves in the chamber under PAR=2 000 μmol m−2s−1until parameter readings were stable (Sofoet al.2009; Zhang Yet al.2019).

Sufficient water was applied to prevent water stress.On the first day after sowing,all experimental plots were irrigated (15 mm),and starting from 60 d after sowing,single water applications of 58 mm were delivered at 9–10 d intervals throughout the growing season for a total of nine applications; the total irrigation amount was about 540 mm(Zhanget al.2017). Base fertilizers were applied as 150 kg N ha–1from urea,225 kg P2O5ha–1(super phosphate) and 75 kg K2O ha–1(potassium sulfate) prior to sowing. An additional 300 kg N ha–1was applied during the growing season to ensure a non-limiting supply of nutrients. Weeds,diseases and pests were controlled manually in the plots to eliminate confounding effects.

2.2.Sampling and measurement

Dry matterAt the silking and physiological maturing stages,three consecutive plants were measured in the center row ofeach plot. All organs were cut off in their natural state within the canopy. The organs were separated into the stem,leaf,sheath,tassel,ear,and bract,and dried to constant weight at 85°C (Liu G Zet al.2017).

Table 1 Mean daily maximum temperature (Tmax),minimum temperature (Tmin),diurnal temperature variation (Td),solar radiation,relative humidity (RH),and precipitation (Pre) during the maize growing season at Qitai Farm,Xinjiang,China in 2018 and 2019

Fig.1 Maize fields under natural and 70% natural solar radiation levels.

RootsIn each plot,the roots of three adjacent plants from the same inside row were removed from the soil by digging at the silking stage. Each root system was excavated with a certain volume of soil,depending on the average area that the plant occupied,at a depth of 60 cm; and this sample was then divided into three layers of 20 cm each. The soil from each of these three layers was kept in a separate numbered nylon bag for subsequent manual root washing and picking,and after washing and picking,the roots were scanned with a scanner (Epson V800,Indonesia) and analyzed by an analysis program (WinRhizo Pro Vision 5.0,Canada) to obtain the root length. Finally,the roots were dried at 85°C to a constant weight to determine the root dry weight.

YieldAt physiological maturity,an area of three central rows and 5 m long was harvested manually from the center of each plot and grain weight was measured. A portable moisture meter (PM8188,Kett Electric Laboratory,Tokyo,Japan) was used to determine the moisture content of the grains. Grain yield was then determined at 14% grain moisture content.

Statistical analysisStatistical calculations were performed using the Microsoft Excel 2010 Software,plots were generated with SigmaPlot,and statistical analyses were performed using the SPSS 21.0 Software (IBM,Armonk,NY,USA).

3.Results

3.1.Yield gap of maize induced by solar radiation change

The difference in solar radiation level caused a variation in maize yield,which was HSR>LSR. There were significant differences in yield between the two different years,and the average yield in 2019 was higher than that in 2018.The yield gap between HSR and LSR in 2018 was greater than the gap in 2019,and the TIPAR showed the same trend. TIPAR of XY335 and ZD958 decreased by 323.1 and 403.9 MJ m–2,respectively,and the average yield gaps were 8.9 and 5.8 t ha–1(by averaging 2018 and 2019 data).In other words,the reductions of average TIPAR were 26.3 and 33.5%,and the average yield reductions were 41.8 and 30.6%,respectively. The average yield of XY335 (21.4 t ha–1) was higher than that of ZD958 (18.8 t ha–1) under HSR,so the yield gap between the two cultivars was 2.6 t ha–1under HSR. The average yields of XY335 and ZD958 under LSR were 12.5 and 13.0 t ha–1,respectively,so the yield gap between the two cultivars was 0.5 t ha–1,which was far less than the yield gap under HSR. These results indicated that XY335 was more sensitive to changes in solar radiation,while ZD958 had better adaptability to the decreased solar radiation (Fig.2).

3.2.Effect of solar radiation change on aboveground dry matter accumulation and distribution in maize

At silking and maturity stages,the amounts of dry matter in each organ decreased as solar radiation decreased,and the decreases in XY335 were greater than those in ZD958. The aboveground dry matter of XY335 and ZD958 decreased by 13.1 and 9.1% at silking and by 22.4 and 19.6% at maturity,respectively (by averaging 2018 and 2019 data). The proportions of stem,leaf and sheath dry matter increased with decreasing solar radiation,and the proportions of ear and tassel dry matter decreased with decreasing solar radiation at silking and maturity stages.The dry matter proportions of stem,leaf and sheath were greater than those of ear and tassel.

At the silking stages in 2018 and 2019,the stem,leaf,sheath,ear,and tassel dry matter weight proportions of XY335 were 43.3,31.1,17.3,5.5,and 2.9% under HSR,respectively,while they were 43.4,32.2,16.9,5.0,and 2.6% under LSR during the two years. The stem,leaf,sheath,ear,and tassel dry matter weight proportions of ZD958 were 37.5,32.7,17.5,6.2,and 6.4% under HSR,respectively,while they were 38.8,34,16.5,5.1,and 5.7%under LSR (Fig.3-A and B).The overall reductions of the dry matter of vegetative organs and reproductive organs were 9.8 and 20.9%.

Fig.2 Maize yield and the total intercepted photosynthetically active radiation (TIPAR) from the period of emergence to the maturity stage of Xianyu (XY335) and Zhengdan 958 (ZD958) under high solar radiation (HSR) and low solar radiation (LSR) in 2018 and 2019. Error bars represent the standard deviations of three replicates. Means with different letters are significantly different at the level of 0.05.

At the maturity stages in 2018 and 2019,the dry matter weight proportions of stem,leaf,sheath,ear,bract,and tassel of XY335 were 18.7,11,5.4,59.7,4.75,and 0.5%under HSR,respectively,while they were 20,11.7,5.9,57.9,4.2,and 0.5% under LSR. The dry matter weight proportions of stem,leaf,sheath,ear,bract,and tassel of ZD958 were 15.6,12,5.6,61.6,4.4,and 1% under HSR,respectively,while they were 17.3,14.9,6.1,56.3,4.2,and 1.3% under LSR. In 2018 and 2019,the proportions of vegetative organs (stem+leaf+sheath) of XY335 were 35.1 and 37.5%under HSR and LSR,respectively,and they were 33.2 and 38.3% for ZD958. The proportions of reproductive organs(ear+bract+tassel) of XY335 were 64.9 and 62.5% under HSR and LSR,respectively,and they were 66.9 and 61.8%for ZD958. Compared to HSR,the proportions of vegetative organs of XY335 and ZD958 were decreased by 17 and 7.1%under LSR,respectively,in 2018 and 2019. The proportions of reproductive organs of XY335 and ZD958 were decreased by 25.5 and 25.5% under LSR,respectively,in 2018 and 2019.The reductions of the dry matter of vegetative organs and reproductive organs were 12.1 and 25.5%. Comparing the data of the two cultivars under HSR,the decrease in XY335 was greater than that in ZD958. The reductions of the dry matter of vegetative organs and reproductive organs in 2018 were greater than that in 2019 (Fig.3-C and D).

3.3.Distribution of light in the canopy

Light transmission increased with increasing plant height under both treatments in 2018 and 2019 (Fig.4).The distributions of light for XY335 under HSR and LSR matched logarithmic functions,which werey=53.785ln(x)+75.767 andy=55.598ln(x)+23.094,in 2018,andy=67.299ln(x)+71.286 andy=82.481ln(x)–21.238,respectively,in 2019. The distributions of light for ZD958 under HSR and LSR matched logarithmic functions ofy=67.720ln(x)–9.187 andy=42.583ln(x)+26.297,in 2018,andy=54.737ln(x)+17.798 andy=53.781ln(x)+17.987,respectively,in 2019. The averages of photosynthetically active radiation transmission of HSR and LSR for XY335 were 21.3 and 36.3%,and 23.9 and 31.5% for ZD958,respectively,in 2018. The averages photosynthetically active radiation transmission of HSR and LSR for XY335 were 20.6 and 31.1%,and those of ZD958 were 30.3 and 32.3%,respectively,in 2019. These results indicated that the average light interception by the canopy of the population decreased when the solar radiation decreased. The average photosynthetically active radiation transmission gaps of HSR and LSR for XY335 were 15.0 and 10.5% in 2018 and 2019,respectively,while they were 7.6 and 2.0% for ZD958. These results indicated that the photosynthetically active radiation transmission gaps of the two treatments in 2018 were greater than those in 2019,and they were greater for XY335 than for ZD958.

Fig.3 Dry matter accumulation and distribution among various organs of Xianyu 335 (XY335) and Zhengdan 958 (ZD958) under high solar radiation (HSR) and low solar radiation (LSR) in 2018 and 2019 at the silking and physiological maturity stages. Error bars represent the standard deviations of three replicates. Means with different letters are significantly different at the level of 0.05.

3.4.Photosynthetic rate of maize induced by solar radiation change

The photosynthetic rates of XY335 and ZD958 decreased with decreasing solar radiation at the silking stage. In 2018,the photosynthetic rates of the XY335 and ZD958 cultivars were 34.5 and 36.9 μmol m−2s−1under HSR,and 30.1 and 30.7 μmol m−2s−1under LSR,respectively. Compared to the HSR,the photosynthetic rates of XY335 and ZD958 decreased by 12.6 and 16.9%,respectively,under the LSR.In 2019,the photosynthetic rates of XY335 and ZD958 were 38.6 and 35.9 μmol m−2s−1under HSR,and 28.4 and 34.6 μmol m−2s−1under LSR,respectively. Compared to the HSR,the photosynthetic rates of XY335 and ZD958 decreased by 26.9 and 3.7% under the LSR (Fig.5).

3.5.Distribution of roots

The root dry weight and root length of the 0–60 cm soil layer decreased with decreasing solar radiation. In 2018 and 2019,the average dry weights of XY335 and ZD958 decreased by 54.6 and 45.5% under decreased solar radiation. The significant difference in the effect of solar radiation on the root dry weight was mainly found in the 0–20 cm soil layer. In 2018 and 2019,the average root dry weights for XY335 and ZD958 in the 0–20 cm soil layer under LSR were 55.4 and 45.9% lower than those under HSR,respectively (Fig.6-A–D). The root length of the 0–20 cm soil layer decreased more than that of the 40–60 cm soil layer.In 2018 and 2019,the root lengths of the two cultivars in the 0–20 cm soil layer decreased significantly under reduced solar radiation,and the average root lengths for XY335 and ZD958 in the 0–20 cm soil layer under LSR were 50.9 and 46.3% lower than HSR,respectively (Fig.6-E–H). Under the different solar radiation conditions,the root dry weight in 2018 was greater than that in 2019,but the difference was not significant. However,the root length in 2019 was significantly greater than that in 2018. Root dry weights and root lengths of the two cultivars were significantly different,and both were greater in XY335 than in ZD958,especially under HSR.

4.Discussion

4.1.Effect of solar radiation change on maize yield gap

Recent studies have shown that solar radiation and temperature resources are significantly related to maize grain yield,and the negative impact of decreased solar radiation on increased yield is greater than the effect of temperature (Yanget al.2018; Wanget al.2019). Variations in resources and environment result in yield gaps among maize growing regions (Lobellet al.2009). Due to the special geographical environment of China,there are varying degrees of regional differences in solar radiation and temperature resources,resulting in the existence of yield gaps between different regions (Yanget al.2011; Houet al.2012; Xuet al.2017). The results of this study showed that when the level of solar radiation was decreased,the effective solar radiation interception during the growth period of the maize decreased,and the photosynthetic rates of XY335 and ZD958 at the silking stage also decreased with decreasing solar radiation,which led to the decreases in maize yield and caused the yield gaps. The difference in yield gap between different years was mainly due to the difference in TIPAR in the two years.

Fig.4 Distribution of photosynthetically active radiation transmission every 30 cm in the canopy of Xianyu 335 (XY335) and Zhengdan 958(ZD958) under high solar radiation (HSR) and low solar radiation (LSR) in 2018 and 2019. ＊＊,significant at the level of P<0.01.

Fig.5 Effects of photosynthetic rate at the silking stage in leaves under high solar radiation (HSR) and low solar radiation (LSR)in 2018 and 2019. XY335,Xianyu 335; ZD958,Zhengdan 958. Error bars represent the standard deviations of three replicates.Means with different letters are significantly different at the level of 0.05.

Canopy light distribution is important for crop photosynthesis,which is a key factor for crop production(Guet al.2014; Kromdijk and Long 2016). Dry matter production is the physical basis of maize grain yield (Houet al.2020),and photosynthesis is the basis of biomass formation. In this study,light transmission in the canopy increased with increasing plant height and the average amount of light intercepted by the canopy decreased when the solar radiation was reduced,which indicated that under the low solar radiation,the light captured by the canopy was weak. The photosynthetic rates at the silking stage decreased under reduced solar radiation,as did the amounts of dry matter of each organ and dry matter accumulation,thus resulting in yield gaps (Zhuet al.2016; Guet al.2017;Khanet al.2017; Yaoet al.2017; Liu Get al.2020).

4.2.The aboveground and underground growth responses to different solar radiation levels

The root draws water and nutrients from the soil to supply the growth of the ground of the plant,which is the key determinant of the plant growth potential and closely related to the yield formation. Root growth is closely related to solar radiation,maize root growth under high solar radiation is better than that under low solar radiation,and the responses of roots of different genotypes to solar radiation are different,resulting in varying yield gaps (Sonet al.1988; Amos and Walters 2006; Niuet al.2019). The results of this study confirmed this view. With the reduction of solar radiation,the root dry weight and root length in each soil layer decreased.The 0–20 cm soil layer had a larger rate of decrease,and the decrease rate for the two cultivars was XY335>ZD958(Fig.6). Root growth of XY335 was greatly affected by solar radiation,which might result in a larger yield gap. In this study,the total root lengths of the two cultivars in the LSR treatments in 2019 were found to be significantly higher than those in 2018. Therefore,the absorption and utilization of water and nutrients might increase,which could increase the yield and reduce the yield gap between HSR and LSR in 2019 (Fig.6). In addition,the photosynthetically active radiation transmission gaps between HSR and LSR in 2018 were greater than that in 2019,and the reduction of aboveground dry matter accumulation in 2018 was greater than that in 2019; all of which might have been the reason for the greater yield gap in 2018.

The aboveground and underground components of maize complement each other to form the overall functional system of the crop,which is self-adaptive and -adjusting to the external environment. When the solar radiation decreases,the plant supplies photosynthates for nearby organ growth,so the aboveground growth is better while underground growth is severely limited (Casper and Jackson 1997; Chenet al.2002; Miralleset al.2011). This mechanism is consistent with the results of the present study.At silking,the root weights of XY335 and ZD958 under LSR were decreased by 54.6 and 45.5%,respectively,and the aboveground dry matter decreased by 13.1 and 9.1%,which meant that the reduced level of solar radiation had a greater impact on the underground root growth. Meanwhile,compared to the HSR,with the decrease of the radiation level,the dry matter weight of each organ on the ground decreased,and the reductions of the dry matter of vegetative organs and reproductive organs were 9.8 and 20.9% at silking stage,and 12.1 and 25.5% at the physiological maturity stage,respectively,so the dry matter of reproductive organs was greater than that of vegetative organs. This pattern was consistent with previous studies which indicated that under LSR,the development of maize ears (a reproductive organ) was limited,and the growth of tassel and ears was unbalanced,which would lead to an increase in male and female separation during the flowering period and thus decrease grain yield (Cuiet al.2015; Yanget al.2016).

4.3.Yield gap between different regions induced by solar radiation

Previous studies have indicated that adequate light conditions can increase grain yield (Liet al.2010; Chenet al.2014). The east-west distance spanning the maize ecological regions in China is large and this results in a significant difference in solar radiation,especially between the west and east regions,and leads to the differences in planting densities and final grain yields (Li and Wang 2008;Xuet al.2017; Yanget al.2019; Houet al.2020). In this study,shading was adopted to mimic regions of low solar radiation conditions,especially the eastern region of China.One of the most effective measures for maximizing maize yield has been adjusting the plant density to different climatic conditions (Xuet al.2017). In this study,the variation of solar radiation that was tested resulted in yield gaps ranging from 5.8 to 8.9 t ha–1,which were smaller than the actual conditions in the field. This difference might be because the higher yield gap between the western and eastern regions is not only related to solar radiation change,but also closely related to the actual planting density (Xuet al.2017; Houet al.2020). The planting density in the western region is higher than that in the eastern region (Xuet al.2017; Zhang Det al.2019).

4.4.Reducing yield gap by using suitable maize cultivars

Other factors,such as cultivars,weeds,plant diseases,water management (Mohammadi-Ahmadmahmoudiet al.2020),fertilization (Rhebergenet al.2020),and social economy (Yang and Liu 2014),also affect the yield gaps between regions (Liu Wet al.2020). In addition,abiotic factors can constrain yield as well,such as climate change(Liuet al.2012,2016),temperature and rainfall (Xuet al.2017; Abdulaiet al.2020). Considering these factors,selecting the most appropriate cultivars is more important for reducing the regional yield gaps (Andreaet al.2018). In this study,different cultivars responded differently to changes in solar radiation. The difference in yield of XY335 caused by the reduced solar radiation was greater than for ZD958,indicating that XY335 was more sensitive to changes in solar radiation. However,the average yield of XY335 was higher than that of ZD958 under HSR (Fig.1). In this respect,XY335 can be used to improve production potential in areas with abundant light resources. Previous studies on light response curves of different cultivars at the silking and grain filling stages have indicated that XY335 has a greater ability to utilize strong light and ZD958 has a greater ability to utilize weak light under the same planting density in the same area (Liu 2016). In this study,the average yield of ZD958 was higher than that of XY335 under LSR,indicating that ZD958 had better adaptability to LSR.Therefore,cultivars similar to XY335 should be selected in areas with high solar radiation resources,and on the contrary,cultivars similar to ZD958 should be selected in areas with low solar radiation resources to increase grain yields and reduce yield gaps.

5.Conclusion

In this study,we found that yield gaps could be induced by solar radiation change and different gaps were found between cultivars XY335 and ZD958. Under HSR,grain yield of XY335 was higher than that of ZD958,while the opposite trend was found under LSR. This indicated that XY335 could be used in areas with abundant light resources and ZD958 should be used under low solar radiation conditions to increase grain yields and reduce the yield gap.The light intercepted by the canopy,photosynthetic rates,the amounts of dry matter of different organs,and root dry weight and length all decreased at reduced solar radiation levels,and the decreases in XY335 were greater than those in ZD958,resulting in different yield gaps. The reduced level of solar radiation had a greater impact on the underground root growth than aboveground growth,and it had a greater impact on reproductive organ growth than vegetative organ growth,which explained the difference in the yield gaps.Selecting suitable cultivars can increase yields and reduce yield gaps that are driven by variations of solar radiation in different regions or under climate change.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China(2016YFD0300110,2016YFD0300101),the National Natural Science Foundation of China (31871558) and the National Basic Research Program of China (973 Program,2015CB150401).