Comparison of Net Photosynthetic Rate in Leaves of Soybean with Different Yield Levels

Li Da-yong,Zhang Zhi-an*,Zheng Dian-jun, ,Jiang Li-yan,and Wang Yuan-li

1 College of Agronomy,Jilin Agricultural University,Changchun 130118,China

2 Liaoyuan Academy of Agricultural Sciences,Liaoyuan 136200,Liaoning,China

Introduction

90%-95% of dry matter in over ground part of plant comes from photosynthesis.Theoretically,the maximum solar radiation utilization rate of crops is 5%-6%.However,the actually average solar energy utilization rate at present is only 1%,only some individual species reach 3.5%,indicating crops have a broad space potential to increase yield (Mann,1999).The key for the implementation of yield increasing is enhancing the solar energy utilization efficiency of the crops,while increasing photosynthetic capacity is the fundamental way to enhance the solar energy utilization efficiency.

Photosynthesis mainly takes place in plant leaves.The yields of crops are related to the leaf morphological structures and physiological traits (Xu et al.,1995;Li et al.,1999;Yang et al.,2008;Gao et al.,2009).Previous studies indicated that there were close relationships between yield and specific leaf weight,chlorophyll content as well as net photosynthetic rate (Wang et al.,2004;Pearce et al.,1969).The studies on the leaves of soybean,sunflower and ginseng demonstrated that crop yield was closely related to the leaf specific leaf weight,content of soluble sugar and net photosynthetic rate (Xu et al.,2003;Xu et al.,1994).However,systematic researches on the changes of specific leaf weight,chlorophyll content and net photosynthetic rate at different growth stages of soybean with different yield levels are not many.In the present study,the changes of specific leaf weight,chlorophyll content and net photosynthetic rate in the leaves of soybean with different yield levels at different growth stages and their relationships with yield were studied,aiming at providing some theoretical foundations for the high-yield breeding of soybean.

Materials and Methods

Materials

The studied nine soybean (Glycine max (L.) Merr.) cultivars were medium maturity varieties with the same maturity period,which were provided by the Soybean Germplasm Resources Room of Jilin Academy of Agricultural Sciences and the Innovation Center of Jilin Province Soybean Region Technology,Jilin Agricultural University.Low yield varieties: Jinyuan No.1,Jiti No.5 and Jilin No.5;middle yield varieties: Jilin No.9,Jilin No.15 and Jilin No.26;high yield varieties: Jinong No.7,Jinong No.16 and Jiyu No.66.

Methods

Experimental design

Experiments were carried out in the experimental station of Jilin Agricultural University during 2008-2009 employing randomized block design with 5 row plots.The row spacing was 0.65 m;the row length was 5 m;the area of each plot was 16.25 m2,with three replicates.The soybean seeds were sowed on 28 April,the density was 200 000 plants per hectare.Final singling was completed during seedling stage for all varieties and normal field management was adopted. The soybean was harvested on 27 September,threshing,air drying and cleaning were conducted uniformly.The soybean seed yield of the middle three rows subtracting 50 cm of both ends of each row was taken as plot yield.

Test methods

According to the Fehr grading standards (Sun,2005),the fourth leaf counting from the top of the plant at V4(the third compound leaf in stem was completely developed),R2(flowering stage),R4(pod bearing stage),R6(seed filling stage) and R7(maturity stage)was used to measure the specific leaf weight,chlorophyll content and net photosynthetic rate.Specific leaf weight was measured using punching,drying and weighing method (Zhang and Zhang,2006).Chlorophyll content was determined by spectrophotometry (Zhang and Zhang,2006).Net photosynthetic rate (Pn) of the middle leaves in the ternately compound leaf was measured by LI-6400 portable photosynthesis system.

Results

Yield comparison of three soybean varieties with different yield levels

As can be seen in Table 1,there was distinct difference in the yields among different soybean varieties with three yield levels.The mean yield of high yield varieties was respectively 25.9% and 65.1% higher than those of middle yield and low yield varieties,and the difference was extremely significant (P＜0.01).The mean yield of middle yield varieties was 31.2% higher than that of low yield varieties,and the difference was also extremely significant (P＜0.01).

Table1 Significant analysis of yield difference

Changes of specific leaf weight in leaves of soybean with different yield levels at different growth stages

From Fig.1,we can see that the specific leaf weights of soybean varieties with different yield levels at different growth stages showed different changes,appearing a trend of first declining and then increasing from seedling stage to maturity stage.The minimum value and maximum value appeared at the flowering stage and seed filling stage,respectively.

Fig.1 Changes of specific leaf weight in leaves of soybean at different growth stages

The specific leaf weights of varieties with higher yields were higher than those with lower yields at each growth stage.Among them,the biggest difference in specific leaf weight between high yield and middle yield varieties appeared at the seed filling stage,the value of high yield varieties was 5.7% higher than that of middle yield varieties.The difference at the maturity stage was the smallest,and the specific leaf weight of high yield varieties was 1.1% higher than that of middle yield varieties.The biggest difference in specific leaf weight between middle yield and low yield varieties appeared at the flowering stage,and the value of middle yield varieties was 3.9% higher than that of low yield varieties.The difference at the seed filling stage was the smallest;the specific leaf weight of middle yield varieties was only 0.1% higher than that of low yield varieties.Comparing the specific leaf weights between high yield and low yield varieties,the difference at the flowering stage was the biggest,the value of high yield varieties was 23.3% higher than that of low yield varieties.The difference at the seedling stage was smaller;the specific leaf weight of high yield varieties was 5.9% higher than that of low yield varieties.

Changes of chlorophyll content in leaves of soybean with different yield levels at different growth stages

Fig.2 showed that the chlorophyll content in the leaves of high yield varieties was higher than that in the middle yield and low yield varieties.At the seed filling stage,the chlorophyll content in the leaves of high yield varieties was 14.5% and 20.2% higher than that in the middle yield and low yield varieties,respectively.At the maturity stage,the chlorophyll content in the leaves of high yield varieties was 10.6% and 26.8% higher than that in the middle yield and low yield varieties,respectively.At the late growth stage,the difference in chlorophyll content between the leaves of high yield and low yield varieties gradually became bigger.The chlorophyll contents in high yield varieties stayed at higher levels,indicating the slow down of aging,which laid the foundation for the enhancement of photosynthesis.

Fig.2 Changes of chlorophyll content in leaves of soybean at different growth stages

Changes of net photosynthetic rate in leaves of soybean with different yield levels at different growth stages

The net photosynthetic rates in leaves of soybean varieties with three different yield levels were the highest in high yield varieties,followed by middle yield varieties,and the lowest in low yield varieties at each growth stage (Fig.3).During the whole growth stage,the net photosynthetic rates for each individual yield level variety slowly increased from the seedling stage,reached the peak values at the pod bearing stage,then declined,and finally reached the minimum values at the maturity stage.At the seedling,flowering,pod bearing,seed filling and maturity stages,the net photosynthetic rates of high yield varieties were respectively 18.6%,22.0%,32.0%,18.9% and 57.3% higher than those of low yield varieties,the difference was significant (P＜0.05).At the seed filling and maturity stages,the net photosynthetic rates of high yield varieties were respectively 12.4% and 32.8% higher than those of middle yield varieties,the difference was significant (P＜0.05).The enhancement of net photosynthetic rate in high yield varieties demonstrated their better substance synthesis capacity,which laid the foundation for yield increase.

Fig.3 Changes of net photosynthetic rate in soybean leaves at different growth stages

Relationships between soybean yield and specific leaf weight,chlorophyll content as well as net photosynthetic rate in soybean leaves

As can be seen from Table 2,the yield had significantly and extremely significantly positive correlations with the specific leaf weight in soybean leaves at the flowering and maturity stages (r=0.6528* and 0.8390**).At seedling,pod bearing and seed filling stages,the yield had positive correlations with the specific leaf weight,but did not reach significant level.

Table2 Relationships between soybean yield and specific leaf weight,chlorophyll content as well as net photosynthetic rate in soybean leaves at different growth stages

At the flowering and maturity stages,the yield had significantly positive correlations with the chlorophyll content.Especially in maturity stage,the positive correlation was extremely significant (r=0.8602**).At seedling,pod bearing and seed filling stages,the positive correlations between yield and chlorophyll content were not significant (r=0.2530,0.5733 and 0.5690). There were significantly or extremely significantly positive correlations between soybean yield and net photosynthetic rate at the flowering,pod bearing,seed filling and maturity stages (r=0.7725**,0.8683**,0.6694* and 0.8573**).At the seedling stage,the positive correlations between yield and net photosynthetic rate did not reach significant level (r=0.6187).

Discussion

Chlorophyll is the basic substance for leaves to absorb,transfer and convert light energy.Hence,chlorophyll content is closely related to the leaf photosynthetic rate (Zhang et al.,1986).The results in the present study demonstrated the chlorophyll content in the leaves of high yield soybean varieties appeared an increasing trend as compared with that of low yield varieties.The soybean yield had a significantly positive correlation with the chlorophyll content in the leaves at the flowering and maturity stages,indicating that the increasing of chlorophyll content in the leaves of high yield varieties laid the foundation for the enhancement of photosynthetic rate and yield increase.

Specific leaf weight is an important index of leaf morphology and physiological traits.Owing to the simple and exact determination method,it is commonly used for the comparative determination among varieties or strains (Gan et al.,1998).Specific leaf weight is also an index of source strength,which has close relationship to sink strength.The higher specific leaf weight and its longer duration are the performances of higher source strength.McClendon (1962) and Orgren (1977) demonstrated that soybean specific leaf weight was closely related to photosynthetic rate.

The present study showed that the specific leaf weight in leaves of high yield soybean varieties was the highest,followed by middle varieties,the low yield varieties were the lowest.The soybean yield had significantly positive correlations with the specific leaf weight at the flowering and maturity stages,indicating that the increasing of specific leaf weight in the leaves of high yield varieties also laid the foundation for the enhancement of photosynthetic rate and yield increase. Zhang et al.(1986) pointed out that the yield was positively correlated with the mean photosynthetic rate at the main growth stage for each soybean variety,the correlation coefficient r was 0.8592,this trend could be maintained in different years.Evans and Rawson (1970) considered there was no stable correlation between leaf photosynthetic rate of crops and productivity.Dong et al.(1979) studied eight soybean varieties and pointed out that mean net photosynthetic rate was obviously correlated with neither biological yield nor economic yield.

Leaf photosynthesis is a dynamic process,which changes along with the growth stages (Patterson and Hoss,1980).The study of Chu et al.(1988) indicated yield was positively correlated with net assimilation rate at the early and middle growth stages,while negatively correlated at the late growth stage,no correlation was significant.The research results of Singh et al.(1993) demonstrated that yield was positively correlated with the net photosynthetic rate at flowering and filling stages.Wells et al.(1982) considered soybean seed yield had a positive correlation with apparent canopy photosynthesis curve at pod bearing and physiological maturity stages.Du et al.(1999) pointed out that soybean yield was positively correlated with photosynthetic rate at R4stage.Zhou et al.(1990) also considered soybean seed yield had a significantly positive correlation with the leaf photosynthetic rate at pod bearing and seed filling stages (r=0.88**).Xu and Shen (1992) considered the negative correlation between yield and leaf photosynthetic was a false appearance.Xu (1999) also pointed out 95% of the plant dry matter came from assimilated CO2by photosynthesis.This basic fact determined the positive correlation between crop yield and leaf photosynthetic rate,neither no correlation nor negative correlation.The positive correlations were not a few exceptions but regular performance.This internal positive correlation might be covered by the complex changes of other factors,for example,the yield of species with high leaf photosynthetic rate might be lower than that of species with low leaf photosynthetic rate but large leaf area and long photosynthesis functional period because of its small leaf area and short photosynthesis functional period.

The results in the present study indicated that there were significantly or extremely significantly positive correlations between soybean yield and net photosynthetic rate at the flowering,pod bearing,seed filling and maturity stages.At the seedling stage,the positive correlation between yield and net photosynthetic rate did not reach significant level (r=0.6187).The results were basically in accordance with those of Zhang et al.(1986) and Xu et al (1999).

Conclusions

The specific leaf weight,chlorophyll content and net photosynthetic rate of high yield soybean varieties were higher than those of low yield varieties.The yield of soybean had significantly positive correlations with the specific leaf weight,chlorophyll content and net photosynthetic rate.Among them,the correlation between yield and net photosynthetic rate was closer than those between yield and specific leaf weight as well as chlorophyll content.With the improvement of modern technology,the net photosynthetic rate could be measured quickly and exactly.Hence,net photosynthetic rate could be used as an effective index in the selection of high yield soybean,which could reduce breeding blindness.

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