Yingying NIU,Kang LIAO,Yang JIA,Hongxiang PANG,Guixiang XU,Zhenbin JIANG

Featured Fruit Tree Research Center,Xinjiang Agricultural University,Urumqi 830052,China

Responsible editor:Tingting XU Responsible proofreader:Xiaoyan WU

Over 90% of dry matters in fruit tree originate from leaf photosynthesis[1].Photosynthesis is the links and foundation for physiolog ical processes of fruit tree,such as growth,development and fruit set ting[2].Therefore,the high yield and high quality of fruit are closely related to photosynthesis.Xinjiang has rich resources of light and heat,hence,the research on photosynthetic characteristics of Korla fragrant pear can provide a theoretical basis for the improvement of yield and quality of Korla fragrant pear from the perspective of photosynthesis.At present,many scholars have studied the photosynthesis of apple,date,plum and other tree species[3],and many papers have been published about the diurnal variation of photosynthesis and the influence of light intensity on photosynthesis of a few pear species[4-8].The studies on the photosynthetic characteristics of Korla fragrant pear mostly focus on the differences among different tree shapes[9-12],but there are rare reports on the differences among different planting densities.Planting density has great influence on photosynthesis ability of fruit tree,resulting in differences in fruit yield and quality among different planting densities.Therefore,the optimum planting density for Korla fragrant pear was explored in this study so as to improve its fruit yield and quality.The Korla fragrant pear was used as experimental material.The leaf photosynthetic parameters of Korla fragrant pear under six different planting densities were determined,and then the differences in photosynthetic characteristics of Korla fragrant pear among different planting densities were analyzed,providing a practical guidance for the improvement of pear yield and quality and a theoretical basis for the further researches on Korla fragrant pear.

Materials and Methods

Materials

The experiment wascarried out in Sayd?ng Gardening of Xinjiang Bayingolin Mongolian Autonomous Prefecture(41°43′55″N,86°02′10″E).The 20-30-year-old orchards of Korla fragrant pear with evacuation layered tree shape,moderate tree vigor,good fruiting and uniform cultivation and management conditions,at six different planting densities (2.5 m×7.0 m,3.0 m×5.0 m,4.0 m×6.0 m,5.0 m×6.0 m,5.0 m×8.0 m,6.0 m×8.0 m),were selected as experimental materials.

Methods

Determination of photosynthetic parametersIn middle August of 2014,the photosynthetic parameters of Korla fragrant pear,including net photosynthetic rate(Pn),stomatal conductance (Gs),intercellular CO2concentration (Ci),transpiration rate(Tr)and water use efficiency (WUE =Pn/Tr)[7],were determined using the Li-6400XT portable photosynthesis measuring system during 10:00 to 11:00 am in sunny days.

Measurement of photoresponse curveDuring the determination of Pn,the gas path of the Li-6400XT portable photosynthesis measuring system was opened with CO2concentration in leaf chamber of 400 μmol/mol,air flow rate of 500 μmol/s and light intensity (red and blue light sources) gradients of 0,20,50,80,100,200,300,500,800,1 000,1 200,1 500 and 1 800 μmol/(m2·s).The photoresponse curve was drawn,and through the fitting process using the Farquhar model[13],the highest Pn,light compensation point (LCP),light saturation point(LSP)and apparent quantum yield(AQY)were all obtained.

Measurement of CO2 response curveIn a sunny day,the variation of net photosynthetic rate(Pn)with the variation of CO2concentration was monitored.The light intensity was set as the saturated light intensity obtained from the drawn photoresponse curve.The CO2concentration gradients were set as 50,80,100,150,200,400,600,800,1 000,1 200 and 1 500 μmol/mol.Then the CO2response curves were drawn.Based on the obtained linear regression equation,the highest Pn (Pnmax),CO2compensation point (CCP) and CO2saturation point(CSP) were obtained (under condition of saturated CO2concentration).

Analysis of test dataThe experimental data was processed and analyzed using Excel 2007 and SPSS19.0.

Results and Analysis

Effects of different planting density on photosynthetic parameters of Korla fragrant pear

As shown in Table1,Pn was highest [12.702 μmol/(m2·s)]for the planting density of 4.0 m×6.0 m and was lowest [10.839 μmol/(m2·s)]for the planting density of 2.5 m ×7.0 m.No significant difference was found in Gs between the planting densities of 2.5 m×7.0 m and 4.0 m×6.0 m (P＞0.05),but there were significant differences among the other planting densities(P＜0.05).Ci showed no significant differences among the planting densities of 2.5 m×7.0 m,4.0 m×6.0 m and 5.0 m×8.0 m(P＞0.05),which could be classified into one class;and it showed no significant differences among the planting densities of 3.0 m×5.0 m,5.0 m×6.0 m and 6.0 m×7.0 m (P＞0.05),which could be classified into another class.But there were significant differences between the two classes.Tr showed no significant difference between the planting densities of 3.0 m×5.0 m and 6.0 m×7.0 m,2.5 m×7.0 m and 4.0 m×6.0 m or 5.0 m×6.0 m and 5.0 m×8.0 m (P ＞0.05).In terms of WUE,the Korla fragrant pear under the planting density of 5.0 m×8.0 m showed the highest WUE; there were significant differences between the planting densities of 2.5 m×7.0 m and 3.0 m×5.0 m and 6.0 m×7.0 m (P＜0.05).

Photoresponse curve of Korla fragrant pear under different planting density

The photoresponse curves of Korla fragrant pear under different planting densities were shown in Fig.1.The net photosynthetic rate of Korla fragrant pear was increased with the increase of planting density.Within the light intensity range of 0-200 μmol/(m2·s),the net photosynthetic rate of Korla fragrant pear was increased linearly with the increased light intensity;when the light intensity was increased to 800 μmol/(m2·s),the increase of net photosynthetic rate was slowed.It was indicated that after reaching the LSP,the leaf Pn of Korla fragrant pear was trended to be stable.

The Pnmax,AQY and Rd of KorlaDifferent lowercase letters in the same column indicate significant differences at the 0.05 level;different capital letters in the same column indicate significant differences at the 0.01 level.fragrant pear differed among different planting densities(Table2).Pnmax referred to the value of Pn under the condition of light saturation.At the planting density of 4.0 m×6.0 m,the Pnmax[19.326 μmol/(m2·s)]and AQY(0.044 mol/mol) of Korla fragrant pear reached the peaks.It was indicated that the Korla fragrant pear under the planting density of 4.0 m×6.0 m had the strongest utilization ability for weak light.Pnmaxwas lowest [16.846 μmol/(m2·s)]in the Korla fragrant pear under the planting density of 5.0 m×6.0 m,and AQY was lowest (0.031 mol/mol) in the Korla fragrant pear under the planting density of 6.0 m×7.0 m.It was suggested that the Korla fragrant pear at the planting density of 6.0 m×7.0 m had the weakest utilization ability for weak light.Rd was increased with the increase of planting density.

Table1 Photosynthetic parameters of Korla fragrant pear under different planting density

The fitting was carried out using the Farquhar model,and the LCPs and LSPs of Korla fragrant pear at different planting densities were obtained(Table2).LCP was decreased with the decrease of planting density,The Korla fragrant pear at the planting density of 6.0 m×7.0 m had the lowest LCP,illustrating its strongest utilization capacity for weak light; at that planting density,Korla fragrant pear had the highest LSP,illustrating its strongest adaptability to strong light at the sametime.

Table2 Photoresponse curve of Korla fragrant pear under different planting density

Table3 CO2 response curve of Korla fragrant pear under different planting density

CO2 response curve of Korla fragrant pear under different planting density

As shown in Fig.2,the leaf net photosynthetic rate of Korla fragrant pear at different planting density was increased with the increase of CO2concentration.In the CO2concentration range of 0 -400 μmol/mol,Pn showed a linear increase with the increase of CO2concentration,but the increment was small; when the CO2concentration was higher than 400 μmol/mol,the leaf Pn of Korla fragrant pear was increased continuously;when the CO2concentration reached 800 μmol/mol,the increase speed of leaf Pn was slowed down.After reaching the CO2saturation,the leaf Pn was basically stable.

Different planting density showed different Pnmax,CE and Rd in Korla fragrant pear (Table3).Among all the planting density treatments,Pnmaxreached the peak[65.368 μmol/(m2·s)]at the planting density of 3.0 m×5.0 m,and reached the lowest point [51.250 μmol/(m2·s)]at the planting density of 5.0 m×6.0 m.At the planting density of 4.0 m×6.0 m,the CE and Rd of Korla fragrant pear were all highest.CE was lowest in Korla fragrant pear at the planting density of 3.0 m×5.0 m,and Rd was lowest at the planting density of 5.0 m×6.0 m.

There were differences in CCP of Korla fragrant pear among different planting densities.The Korla fragrant pear at the planting density of 6.0 m×7.0 m showed the lowest CCP,illustrating its strongest utilization capacity for low-concentration CO2.At the planting density of 3.0 m×5.0 m,Korla fragrant pear showed the highest CSP,illustrating its stronger adaptability to high CO2concentration.

Conclusions and Discussion

Among six different planting densities,there was significant difference in Pn of Korla fragrant pear between the planting densities of 4.0 m×6.0 m and 2.5 m×7.0 m (P＜0.05),but there were no significant differences among the other four planting densities (P ＞0.05).Gs is closely related to Pn[5].The Korla fragrant pear with greater Pn usually has greater Gs.Wang et al.[3]considered that the photosynthesis not only depends on the planting density but also depends on the other climatic factors,such as air temperature and humidity.In this study,the highest Ci was shown at the planting density of 2.5 m×7.0 m,which was consistent with the study result about the relationship between planting density and Ci[3].There were significant differences in Gs of Korla fragrant pear between the planting densities of 2.5 m×7.0 m and the other treatments,which was different,to some extent,from the study results of Zhao et al[14].Significant differences were also found in Tr and WUE of Korla fragrant pear among different planting densities,indicating that the orchards of Korla fragrant pear at different planting densities had different water use efficiencies.At the planting density of 6.0 m×7.0 m,the Korla fragrant pear showed the highest Tr but the lowest WUE; on the contrary,at the planting density of 5.0 m×8.0 m,the Korla fragrant pear showed the lowest Tr but the highest WUE.The photosynthetic characteristics of Korla fragrant pear differed among different planting densities,indicating that the planting density has great effect on photosynthesis.

Under the condition of low light or low CO2concentration,leaf Pn of Korla fragrant pear was increased dramatically with the increase of light intensity or CO2concentration[15].Sun et al.[10]and Mei et al.[11]found that the utilization rate of light or CO2in Korla fragrant pear leaves was increased with the increase of light intensity or CO2concentration,and the Pn will be basically stable at a certain level when the light intensity or CO2concentration reaches a certain value.In this study,the variation trends of the response curves of Korla fragrant pear at the six different planting densities were basically the same.

The Pn value of leaf at light saturation is called as Pnmax,which can reflect the leaf photosynthetic potential.The fragrant pear at the planting density of 4.0 m×6.0 m has higher photosynthetic potential,and when the light intensity reaches the saturation level,the Pn is no longer increased with the increase of light intensity,showing better light fastness.Zhang et al.[5]proposed that this kind of light fastness of fragrant pear is native to Southern Xinjiang (Korla,Aksu,Kashgar,Hotan,Atush) which has windy,drought and high-transpiration desert climate.AQY is an important index that reflects the utilization rate of light,utilization efficiency of weak light and photosynthetic potential of leaves.The AQY of Korla fragrant pear at the planting density of 4.0 m×6.0 m was highest,indicting higher light use efficiency in leaves.LCP and LSP of photosynthesis are also important indexes reflecting the utilization ability of leaves for light energy.Low LCP endows plants with high shadow resistance[8],while the high LSP endows plants with high light resistance.In general,the LSP is often high in plant that has high LCP[16].In this study,the LCP of Korla fragrant pear at the planting density of 6.0 m×7.0 m was low,but its LSP was high,indicating that at this density,the adaptability of Korla fragrant pear to weak light was strong.However,the AQY of Korla fragrant pear at the planting density of 6.0 m×7.0 m was low,indicating at this planting density,the utilization efficiency of light energy in Korla fragrant pear was low,i.e.,the utilization ability of Korla fragrant for weak light was weak.

The concentration of CO2directly affects plant photosynthesis[1],and CSP and CCP reflect the photosynthesis efficiency in plant.But in the saturation phase,CO2concentration is no longer a limiting factor for photosynthesis.The Pn of Korla fragrant pear at the planting density of 3.0 m×5.0 m at the saturated CO2concentration was highest,so its photosynthetic capacity was higher.The CE was highest in the Korla fragrant pear at the planting density of 4.0 m×6.0 m.High CE indicates higher photosynthetic rate under low CO2concentration[16].It was indicated that the utilization rate of CO2in Korla fragrant pear at the planting density of 4.0 m×6.0 m was higher.The plant that has low leaf CCP can utilize the low concentration of CO2.In this study,the CCP was lowest in the Korla fragrant pear at the planting density of 6.0 m×7.0 m,indicating its use efficiency of low-concentration CO2was higher.The CSP was higher in the Korla fragrant pear at the planting density of 3.0 m ×5.0 m,so its adaptability to high-concentration CO2was stronger.

The study results showed that the leaf photosynthetic characteristics of Korla fragrant pear were different among different planting densities.At the highest planting density(2.5 m×7.0 m),the canopy of Korla fragrant pear orchards was closed and the light was insufficient,so the Pn of Korla fragrant pear at the planting density of 2.5 m×7.0 m was lowest.The Pn and Pnmaxwere all highest at the planting density of 4.0 m×6.0 m,indicating the highest photosynthetic potential of Korla fragrant pear at the planting density of 4.0 m×6.0 m.In addition,the AQY was also highest in the Korla fragrant pear at the planting density of 4.0 m×6.0 m,indicating the highest use efficiency of low light; at this density,the Korla fragrant pear had medium-level LCP and LSP.According to the study results above,it could be concluded that the Korla fragrant pear at the planting density of 4.0 m ×6.0 m had good adaptability to weak light and strong light.The LCP was increased with the increase of planting density.The Korla fragrant pear at the planting density of 6.0 m×7.0 m,the lowest planting density,showed lower LCP but higher LSP,so it also had strong adaptability to weak light and strong light.In the Korla fragrant peat at the planting density of 4.0 m×6.0 m,the use efficiency of low-concentration CO2was relatively high,the Rd was the highest,and the photosynthetic capacity was also relatively strong.

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