Systematical regulation involved in heterogeneous photosynthetic characteristics of individual leaf in pima cotton

ZHANG Yu-jie,HAN Ji-mei,LEI Zhang-ying,MENG Hao-feng,ZHANG Wang-feng,ZHANG Ya-li

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

Abstract Light heterogeneity leads to anatomically and physiologically heterogeneous features in leaves.However,little attention has been paid to the effects of nonuniform illumination on the anatomical and photosynthetic performance on both sides along the leaf main vein.This study explored such effects by combining in situ determination in the field with shading simulation in the phytotron,on pima cotton that has cupping leaves.Photosynthetic characteristics and morphological structures were measured in the field on both sides along the main vein of eastward,westward,southward,and northward leaves.The results showed that the difference in photosynthetic capacity between the two sides along the main vein in different directions was closely related to the daily photo irridiance (DPI).This result indicates that the photosynthetic heterogeneity between the two sides is related to their intercepted light energy.The conclusion was further verified by the shading simulation experiments.Photosynthetic capacity and leaf thickness of the unshaded sides of leaves in the half-shaded treatment decreased,compared to those in the unshaded treatment.Therefore,it is conjectured that the development of photosynthetic characteristics on one side is systematically regulated by that on the other side.The study provides theoretical guidance on accessing the feasibility of sampling and directional planting.

Keywords:photosynthetic heterogeneity,morphological traits,leaf mass per area,shade,systemic regulation

1.Introduction

Leaves are the main photosynthetic organs of plant and are highly sensitive to the environmental variations and highly plastic (Chitwood and Sinha 2016;Mathuret al.2018).Many studies have shown that the structural and functional diversity can be observed within an individual leaf,such as rice (Oryza sativaL.) (Xionget al.2015;Yuanet al.2015),tobacco (Nicotiana tabacumL.)(Nardiniet al.2008),Viola baoshanensis&V.yedoensis(Denget al.2007),andAlocasia macrorrhizaL.(Liet al.2013).These studies reported that photosynthetic performance varies greatly from the tip to the base of the leaf (Denget al.2007;Nardiniet al.2008;Liet al.2013;Xionget al.2015;Yuanet al.2015).This longitudinal heterogeneity of leaf may be due to the fact that the leafbase is constantly generated,leading to the leaf-base tissue being formed later than the tip or the margin of leaf (Strainet al.2006;Denget al.2007).In addition,there are differences in light energy intercepted by the tip and the leaf base (Meinzer and Saliendra 1997;Xionget al.2015).Songet al.(2013) suggested that the upper portion of leaf blade may shade the basal portion of leaf blade.Because under natural conditions,not all parts of the plant are exposed to the same light environment,plants are able to rapidly transmit signals from one part to another to optimize the overall photosynthetic activity of the plant (Devireddyet al.2018).This signal from one part to another is thought to be a systematic regulation signal (Jianget al.2011;Liet al.2015),which plays a vital role in the acclimation and survival of plants during fluctuating ambient light environment.Since light is the energy source of leaf photosynthesis,changes of light environment have significant influence on the morphology,anatomical structure and physiological function of leaves (Hanbaet al.2002;Kalveet al.2014;Mathuret al.2018;Slotet al.2019).

Studies on leaf photosynthetic heterogeneity mainly focused on the heterogeneity along the leaf developing direction (Liet al.2013;Xionget al.2015),but heterogeneity in morphological structures and photosynthetic characteristics in the same development age within individual leaves has not been reported.Zhanget al.(2018) preliminarily indicated that one side along the main vein (OSAMV)within an individual leaf was shaded by the other side,such as a cupped leaf,leading to the net photosynthetic heterogeneity between two sides along the main vein(TSAMV).Leaf cupping occurs when the leaf forms a dihedral angle with the main vein as the arris (i.e.,the ratio of leaf area to leaf shadow is more than 1 when the leaf receives direct sunlight).This may affect light interception on TSAMV within individual leaves,which will lead to the heterogeneity of morphological structure and physiological functions.Devireddyet al.(2018) demonstrated that an individual leaf of low-light-adaptedArabodopsis thalianaplants in light stress results in a coordinated stomatal closure in untreated leaves in response to light stress by rapid systemic regulation.Given the systemic light regulation in local to overall leaves within a canopy,it is likely that the systemic regulation also occurs between TSAMV within individual leaves.

In this study,pima cotton (Gossypium barbadenseL.)was used as test materials,as its leaf is obviously cupped.The test was carried out by the method of combiningin situdetermination in the field and shading simulation in the phytotron.The objects of this study were to:1) determine how leaf cupping affects its own light environment and whether the light environment influences leaf development;2) verify the relationship between captured light,morphological structures,and photosynthetic physiological characteristics;and 3) discuss the possible systemic regulation between the TSAMV within individual leaves.

2.Materials and methods

2.1.Plant materials and experimental design

This study was performed simultaneously with a research of photosynthetic heterogeneity in different parts of a single leaf induced by leaf cupping in pima cotton by Zhanget al.(2018).Daily photo irradiance (DPI),chlorophyll(Chl) content and leaf area in the field experiment were all taken from Zhanget al.(2018).The field experiment was conducted at the experimental station of College of Agronomy,Shihezi University,Xinjiang,China (45°19′N,86°03′E) in 2016.Pima cotton (Gossypium barbadenseL.Xinhai 25) seeds were sown on 23 April.The plot size was 60 m2with three replications and planting density of 1.8×105ha-1.The plots were fertilized deeply before sowing with 1 500 kg ha-1organic fertilizers,240 N ha-1(urea),and 150 kg P2O5ha-1((NH4)3PO4).The plots were well irrigated,with a total irrigation amount of 6 000 m3ha-1for 12 times until the end of August.An additional 260 kg N ha-1(urea)was applied by drip irrigation during the growing seasons.Mepiquat chloride (N,N-dimethylpiperdinim chloride) was applied six times during the growing season (300 g ha-1) to regulate cotton growth.Weeds and pests were controlled in the field using standard management practices.At the onset of bolling,penultimate fully expanded leaves on the main stem of the cotton were selected for experiment.At least three leaves were measured in each of the four directions with the leaf tip pointing to the east (E),west (W),south (S),and north (N).Pima cotton leaves are highly cupped and cracked into three parts.In this study,the left and right sides along the main vein in the middle of trilobate leaf were used for experiment.Keeping the tips of the middle of trilobate leaf perpendicular to tester,the left and right sides of the leaf was divided with the main vein as the dividing line.

The shade simulation test was a pot experiment conducted at the overhead lighting phytotron in 2016.Plants of pima cotton (Xinhai 25) were grown in plastic pots(19 cm in diameter,28 cm in height) filled with substrate(Vsoil:Vvermiculite:Vpearlite=1:1:1).The light intensity,photoperiod(day/night),and growth temperature (day/night) in the phytotron were 1 000 μmol m-2s-1,14 h/10 h,and (28±2)°C/(25±2)°C,respectively.Five or six seeds were sown in each pot.When the first true leaf was fully expanded,the unwanted seedlings were gently pulled out and the healthiest one was kept in each pot.The seedlings were watered every second day and the relative soil humidity was maintained at 40%.When the third true leaf emerged,the seedlings were watered with the nutrient solution every four days;when four leaves were present,the seedlings were then divided into three groups for different shaded treatments;the topmost expanded young leaf on the main stem of each plant was tagged.The composition of the nutrient solution was 5 mmol L-1KNO3,5 mmol L-1Ca (NO3)2·4H2O,2 mmol L-1MgSO4·7H2O,1 mmol L-1NH4H2PO4,47 μmol L-1H2BO3,6 μmol L-1MnCl2·4H2O,1 μmol L-1ZnSO4·7H2O,0.3 μmol L-1CuSO4·5H2O,0.4 μmol L-1H2MoO4,and 0.05 mmol L-1EDTA-Fe.Three treatments were used:UL (unshaded leaf),IL (individual-shaded leaf) and HL (half-shaded leaf)(Fig.1).The tagged leaves were kept flat with transparent filament,and the shaded leaves were covered with one-layer commercial black plastic shading net (about 60% shaded).The position and direction of pots were adjusted randomly every day to ensure homogeneous illumination during the experiment.When the tagged leaves grew into the third leaves from the top,the cotton topping was conducted and then the tagged leaves were used to do measurements after shading for one month.

Fig.1 Shading treatment of simulation experiment.UL,unshaded leaf;HL,half-shaded leaf;IL,individual-shaded leaf.

2.2.Diurnal variation of solar radiation

At first,a pima cotton leaf model was established based on the mean of foliar angle and the mean of leaf inclination angle in all directions.Then the photosynthetically active photon flux density (PPFD) incident to the surface of the leaves was determined on both sides along the main vein within individual leaves in different orientations using the leaf model with a fixed angle.The diurnal variation curves of PPFD were measured using a portable saturation-pulse fluorometer PAM-2100 equipped with a 2030-B microquantum/Ni/NiCr-thermocouple sensor (PAM-2100,Walz,Effeltrich,Germany) every hour from 08:00 to 20:00 of the local solar time.DPI was calculated by integrating the PPFD-curves.

2.3.Gas exchange

Gas exchange was measured using an open infrared gasexchange analyzer system (Li-6400;Li-Cor,Inc.,Lincoln,NE,USA) equipped with a blue-red LED light source (Li-6400-02).CO2concentration in the Li-6400 leaf chamber was set by a CO2cylinder and maintained constantly at 400 μmol CO2mol-1.Light-response curves were performedin the middle section of each side of leaves under the following PPFD values:2 000,1 800,1 500,1 200,1 000,800,500,300,200,150,100,50,and 0 μmol m-2s-1in the field experiment and 1 500,1 200,1 000,800,500,300,200,150,100,50,and 0 μmol m-2s-1in the shade simulation test.Leaf temperature and air flow were set to 30°C and 300 μmol s-1,respectively,both in the shade simulation and field test.Estimation of the maximum net photosynthetic rate (Pmax),apparent quantum yield (AQY),and light saturation point(LSP) were made by fitting a maximum likelihood regression below and above the inflection of the net photosynthetic rate-PPFD response using ‘Photosynthetic Assistant’ (Li-Cor,Inc.).

2.4.Leaf area,the mass per area and Chl content

For the measurement of the leaf area (LA),the leaf and sheet metal (1 cm2) were placed simultaneously on the whiteboard and photographed vertically with high resolution camera (EOS 750D,Canon,China).Then the image was measured with Photoshop CS5,and the area of both sides along the main vein within individual leaves was calculated by the pixel number of the leaf.

Leaf mass per unit area (LMA) was calculated by dry weight/leaf area.Five leaf discs (diameter=0.85 cm) were punched from one side of the leaf,heated to de-enzyme at 105°C for 30 min and dried at 80°C in an oven until a constant weight (dry weight) was reached.

The Chl content of leaves was determined in eight fresh leaf discs (diameter=0.68 cm) from one side of leaf.Discs were extracted in 80% (v/v) acetone to white at room temperature in the dark.The absorbance of an extract was measured with a UV-2041 spectroscopy (Shimadzu,Japan)at 470,645 and 663 nm,and the Chl content was calculated according to Lichtenthaler (1987).

2.5.Anatomical traits

Leaf samples (2 mm×10 mm) without major veins were cut from the middle part in one side of the leaf with a razor blade.The samples were fixed in fixative FAA(Vformalin:Vvsaceticacid:V70%ethanol=1:1:18) at 4°C.The fixed samples were dehydrated in 50,70,80,95,and 100%ethanol,the samples were infiltrated and embedded in wax.Then semi-thin leaf cross-sections of 5 μm were sliced with a microtome (RM2235,Leica,Germany).Next,dewaxing was performed with xylene and alcohol gradient(100,95 and 80% ethanol in order),and stained with 1%Safranin and 1% Fast Green.Finally,the tissue sections were mounted using neutral balsam (Solarbio,China) as the mounting agent for photographic observation.Pictures of the sections were taken with a light microscopy fitted with a digital camera (Olympus BX51,Japan).The thickness of leaf (LT),palisade tissue (PT) and spongy tissue (ST)were obtained using Image J (National Institute of Health,Bethesda,MD,USA).The leaf density (LD) was calculated by dividing the mass per area by LT.

2.6.Statistical analysis

Data were compared with one-way analysis of variance using SPSS (version 18.0) at the level of 0.05.Correlation of linear regressions between parameters and daily photo irradiance (DPI)were calculated using SigmaPlot (version 12.0).

3.Results

3.1.Light energy intercepted within individual leaves

The results of DPI indicated that there was a large difference between TSAMV facing east and west (E-L and E-R,W-L and W-R),reaching 63 and 62%,respectively,while the differences in the southward and northward leaves (S-L and S-R,N-L and N-R) were only 8 and 13%,respectively(Appendix A).Meanwhile,the interception of DPI was the highest on the southward leaves,less on the eastward and westward leaves,lowest on the northward leaves,and there was no difference between the eastward and westward leaves (Appendix A).

3.2.Morphological traits and photosynthetic characteristics in field

Pmax,light saturation point,LMA,LA,and LD were higher,but the Chl contents were significantly lower,in E-R and W-L than in E-L and W-R,respectively;but no difference between TSAMV within individual leaves were observed in southward and northward leaves (Fig.2-A and B).Meanwhile,Pmax,light saturation point,LMA,LA,and LD were higher in leaves in south direction than in those in north.The Chl T was lower in south direction than in north.There were no regular changes in LT between the TSAMV in all directions (Fig.2-B).However,longitudinal scanning of the eastward leaf demonstrated that the thickness of the right sides along the main vein within individual leaves was significantly higher than that of the left,reaching 15%(Appendix B).

Fig.2 Changes of photosynthetic (A) and morphological (B) traits of the two sides along the main vein of pima cotton leaves in different orientations in the field.Pmax,the maximum net photosynthetic rate;LSP,light saturation point;AQY,apparent quantum yield;Chl T,total chlorophyll content;LMA,leaf mass per unit area;LA,leaf area;LD,leaf density;LT,leaf thickness.E,eastern leaf;W,western leaf;S,southern leaf;N,northern leaf.L,the left side along the main leaf vein;R,the right side along the main leaf vein.All values except LMA and LD (n=5) are means of three replicates±SE.All indexes were compared separately,and different letters (a-e) mean significant differences at the 0.05 level with the same trait.

3.3.Relationship between the light energy intercepted by leaves and their morphological traits and photosynthetic characteristics

Pmax,light saturation point,the ratio of Chla/b,LMA,LA,and LD were positively correlated with the local light environment(Fig.3),the contents of total Chl and carotenoid showed negative correlation (Fig.3-B),while no correlation existed between apparent quantum efficiency and LT with the local light environment of leaves (Fig.3-A and C).

Fig.3 Relationship between photosynthetic (A),pigment content (B) and morphological (C) traits with daily photo irradiance (DPI) in the two sides along the main vein of pima cotton leaves in different orientations in the field.Pmax,the maximum net photosynthetic rate (r2=0.76,P＜0.01);LSP,light saturation point (r2=0.65,P＜0.05);AQY,apparent quantum yield(r2=0.22,P=0.24);Chl T,the content of total chlorophyll (r2=0.89,P＜0.001);Chl a/b, the ratio of the content of chlorophyll a to chlorophyll b (r2=0.68,P＜0.05); Car,the content of carotenoid(r2=0.63,P＜0.05);LMA,leaf mass per unit area (r2=0.86,P＜0.01);LA,leaf area (r2=0.81,P＜0.01);LD,leaf density(r2=0.57,P＜0.05);LT,leaf thickness (r2=0.11,P=0.42).Each trait is represented by a regression line.Solid lines and dotted lines indicate significant and insignificant relationships,respectively.

3.4.Morphological traits and photosynthetic characteristics in simulating lab test

Photosynthetic capacity,Chl content,anatomy and morphological traits between TSAMV within individual leaves in the UL and IL treatments showed no significant changes.By contrast,the HL treatment caused a marked heterogeneity in measured indexes.In the HL treatment,shading OSAMV decreasedPmax,light saturation point,LT,PT,ST,LA,and LMA,but increased Chl content,compared with the unshaded part.In addition,in the HL treatment,shading OSAMV within individual leaves decreasedPmax,light saturation point,Chl content,LT,and PT,but increased LMA and LD in the unshaded neighbors,compared to the UL treatment.This observation indicated that the photosynthetic capacity,Chl content,anatomical and morphological traits of the unshaded were affected by the light of the shaded neighbors.Exposing OSAMV,Chl content was lower in the shaded sides of the HL treatment when compared with the IL treatment,where LT,ST and LMA were relatively higher.It implied that Chl content,LT,and LMA were affected by the light environment of the sun-exposed sides along the main vein within individual leaves (Fig.4).

Fig.4 Changes of photosynthetic (A),anatomical (B) and morphological (C) traits between two sides along the main vein of pima cotton leaves in different orientations in shade simulation treatments.Pmax,the maximum net photosynthetic rate;LSP,light saturation point;AQY,apparent quantum efficiency;Chl T,total chlorophyll content;LT,leaf thickness;PT,palisade tissue thickness;ST,spongy tissue thickness;LMA,leaf mass per area;LA,leaf area;LD,leaf density.HL,half leaf shaded;IL,individual leaf shaded;UL,unshaded leaf.L,the left side along the main leaf vein;R,the right side along the main leaf vein.Diagonal texture indicates shading with shading net.Values are means of three replicates±SE.All indexes were compared separately,and different letters(a-d) mean significant differences at the 0.05 level with the same trait.

4.Discussion

4.1.Photosynthetic and morphological heterogeneity within an individual leaf caused by light heterogeneity

Zhanget al.(2018) have shown that the light energy captured by the TSAMV within an individual leaf differed significantly in the eastward and westward leaves,which is likely to be related to a weak heliotropism and leaf cupping in pima cotton and the changing solar zenith and azimuth angles.Due to the self-shading or mutual shading from TSAMV in a cupped leaf,the light energy is not symmetrically received by TSAMV,resulting in a high heterogeneity in leaf characteristics on opposite sides along the main vein.However,the light interception between TSAMV of the southward leaves is relative high and uniform,and the direction of seeds entering the soil can be controlled artificially to improve the plants’ light energy efficiency in production.

In the field,it was observed that there was a significant difference inPmaxbetween the TSAMV within individual leaves in the eastward and westward leaves (Fig.2-A),suggesting that the individual eastward and westward cupped leaves had heterogeneity in the ability to utilize the high light.But they had consistent ability to utilize the low light,verified by a similar apparent quantum yield from the TSAMV in leaves of different directions (Fig.2-A).The photosynthetic capacityPmaxand light saturation point were higher in the high-irradiance sides of leaves than those in low-irradiate sides,supporting previous findings that the photosynthetic capacity of high light/top canopy leaves is higher than that of the low light/bottom canopy ones(Gonzalez-Real and Baille 2000;Evans and Poorter 2001;Scartazzaet al.2016;Slotet al.2019).The heterogeneity of Chl content within individual leaves between east and west directions may be related to the phototropism of leaves and the distribution of Chl within the leaves (Xionget al.2015;Mathuret al.2018).

Leaf morphology and anatomy are highly plastic and readily affected by light environment.LMA is an important morphological indicator of plant strategies and is often tightly related to photosynthetic capacity (Poorteret al.2009).Our results showed that the higher irradiance sides along the main vein within the eastward and westward leaves had a considerably higher photosynthetic capacity and LMA than the lower irradiance sides (Fig.2-A).Poorteret al.(2009)reviewed that an increase in light irradiance induces a remarkable growth in LMA in almost all tests.Higher LMA and LD in the leaf sides under high irradiance is beneficial in attenuating light transmission in the mesopyll tissue.Accordingly,the leaf sides under high irradiance are thicker and larger,with higher LMA,than those under low irradiance(Fig.2-D;Appendix B).In this study,the longitudinal scanning of eastward leaf of pima cotton showed that the higher irradiance sides along the main vein within individual leaves were thicker (Appendix B).However,LT changed irregularly under the local heterogeneous light environment(Figs.2-B and 3-C),which may be caused by unevenness of leaf surface.Therefore,this study suggests that the morphological heterogeneity exists not only in different directions,but also in the TSAMV within an individual leaf,even though they are in the same process.Heterogeneity within individual leaves should be considered when studying the light environment of pima cotton leaves or other cupped leaves.This study could provide a theoretical direction for accessing the feasibility of sampling.

4.2.The relationship between the leaf structural and functional heterogeneity and local light environment

Pima cotton leaves show phenotypic plasticity,responding with DPI-induced morphological and photosynthetic changes.In this study,variation in LA,LMA and LD arose mainly from variations in DPI,and all of them increased with increasing DPI (Fig.3-C).It may be an accommodative strategy of leaves in response to the heterogeneous light environment:decreasing the shade on the high irradiance leaves by reducing the area of the low irradiance leaves would enhance light capture,thus promoting the efficiency of light energy utilization in plants.It is generally acknowledged that light availability has a strong positive effect on the LT (Onodaet al.2008),and that effect progressively increases during further development (Kalveet al.2014).Although this correlation was not observed between LT and intercepted light in this study,the longitudinal section scan showed that LT was thicker in the high irradiance sides,which is consistent with previous studies (Kalveet al.2014).In some cases,variation in LMA is mainly caused by LD or (and) thickness,which can be calculated by the product of LD and thickness(Witkowski and Lamont 1991).In our study,LD was higher in the high irradiance sides along the main vein within the eastward and westward leaves when compared with the low irradiance sides,whereas the LT had no difference.Simultaneously,the high irradiance sides along the main vein within individual leaves had a considerably higher LMA(Fig.2-B).The results imply that LMA mainly relies on LD whereas its relationship with LT was feeblish.

In addition to leaf anatomy,light absorption gradient within a leaf depends strongly on leaf Chl content (Mandet al.2013).Similarly,one of the leaf characteristics most affected by shading,or by prolonged reductions in incident light,is pigment content (Larbiet al.2015).This study has identified a negative linear correlation between Chl content and DPI of different portions of leaves (Fig.3-B).To achieve optimal photosynthetic productivity,the lower irradiance sides along the main vein within individual leaves must promote the synthesis of Chl to make full use of the finite light energy,while the higher irradiance sides can reduce the content of Chl to avoid the photooxidative damage of high light.An excessive absorption of irradiance by Chl can cause serious oxidative damage and lead to cell death (Peerset al.2009).In particular,positive correlation in Chla/bwith DPI were observed in our study.There is a lower Chla/bunder low DPI in different portions of leaves,that is,a higher content of Chlbhelps to absorb the blueviolet component of diffuse light (Boardman 1977),indicating acclimatization to low light environment.In addition,the positive linear correlation between the content of carotenoid and DPI indicates that the excess light energy absorbed by Chl is dissipatedviathermal energy under high irradiance,thus protecting the chloroplasts from the damage of high light (Demmig-Adams and Adams 2006).

4.3.Systemic regulation of photosynthetic characteristics within the leaf

In many previous reports,the systemic regulation of photosynthesis between different leaves on a plant has been widely studied (Jianget al.2011;Liet al.2015;Devireddyet al.2018).For example,there have been studies on the impact of shading an individual leaf on the photosynthetic performance of other leaves on the same plant (Jianget al.2011).However,to the best of our knowledge,little attention has been paid to the effect of shading half of the leaf along the main vein on the photosynthetic characteristics of the remaining half.The experiment revealed that,for the shaded side along the main vein,LT,PT,ST,and LMA in the HL treatment were shown to be systemically regulated by the sun-exposed sides (Fig.4).This induces thicker leaf structures and higher LMA with a higher fraction of cell wall and/or defense tissue in shaded sides of the HL treatment (Poorteret al.2009;Devireddyet al.2018),which consumes energy that should be used to maximize photosynthesis.This study additionally observed that photosynthetic characteristics and anatomy traits of the sun-exposed sides of leaves changed significantly after shading the adjacent sides within individual leaves,sunexposed leaf side of the HL treatment seemed to be shaded,even if they were grown to a sunlight environment condition(Fig.4).The photosynthetic capacity of the sun-exposed sides in the HL treatment was decreased significantly when compared with that in the other treatments.This may be due to the transmission of ROS produced by shaded neighbors to the sun-exposed sides when measured with high light (Mittler and Blumwald 2015;Devireddyet al.2018),which reduces their photosynthetic capacity.In addition,significant changes in Chla,Chlb,Chl T,LD,and LMA were observed in the sun-exposed sides in the HL treatment.Therefore,this study suggests that there is a systemic regulation between the two sides of leaves.Similar systemic regulation that coordinates the response of different leaves within the same plant to light stress has been reported inArabidopsis(Jianget al.2011;Liet al.2015;Devireddyet al.2018).These systemic regulation requires the sun-exposed neighbors to reduce the light capture and transmissionin vivoto protect reactive centers from photooxidative damage by reducing Chl content and increasing mesophyll density and LMA (Scartazzaet al.2016) (Fig.4).Also,an increase in LD and LMA of finite assimilation production rather than LA in the sun-exposed sides were mediated by systemic regulation of shaded neighbors.It may be because that the expanision of the leaf sides with low photosynthetic capacity were inhibited,enabling the lower canopy leaves to receive more light energy.This is consistent with the lower LA found in shaded individual leaf,reflecting a tradeoff associated with partial and whole response in plants to maximize growth.

5.Conclusion

The heterogeneity of the photosynthetic characteristics between both sides of the main vein of the eastward and westward leaves is due to the self-shading or mutual shading on both sides in cupped leaves.Also,the development of the anatomy,morphology and function in one side of the main vein within a leaf is systematically regulated by the adjacent side.The photosynthetic capacity of both sides of the main vein in the southward leaf is relative high and uniform,and the direction of seeds entering the soil can be controlled artificially to promote the plant’s light energy efficiency in production.The present study provides theoretical guidance on accessing the feasibility of sampling.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (31 860355;U1903302)and the Regional Innovation Guidance Plan of Xinjiang Production and Construction Corps,China (2021BB001).We thank Wang Jingxuan and Wang Zixing from Shihezi University,China for their assistance with data collection in the phytotron and field,respectively.

Declaration of competing interest

The authors declare that they have no conflict of interest.

Appendicesassociated with this paper are available on http://www.ChinaAgriSci.com/V2/En/appendix.htm