| In maize and soybean strip intercropping,soybeans as a low niche crop from early flowering to mature stage is affected by the shade of higher maize,which cause significant changes in the growth,development and grain yield of soybean.Meanwhile,the photosynthetic capacity,dry matter accumulation and allocation,and the flower and pod formation and abscission are likely to have differences among different varieties in the shade,and then leading to the different changes in grain yield.Basing on the preliminary screening experiment,four different soybean cultivars ZH39,QH34,HD19 and HD20 were selected which the grain yield was similar in monoculture and significantly different in intercropping,while all of them had similar growth and development process under two planting patterns.Field experiments were in 2015-2017 and treatments included soybean monoculture（M0）and two kinds of different bandwidth of maize and soybean strip intercropping（INT）.The relationships between different decreases in grain yield and yield traits,plant morphology,photosynthetic capacity and reproductive characteristics of soybean cultivars under strip intercropping was studied.Moreover,the effects of dry matter accumulation and distribution,non-structured carbohydrate metabolism and flower and pod formation and abscission on yield decreasing were analyzed under strip intercropping.The main results are as follows:1.The grain yield of soybean cultivars was significantly different under maize and soybean strip intercropping（p<0.05）,and this difference was mainly caused by the pod number which changed response to intercropping shade.Under the strip intercropping conditions,the grain yield of soybean cultivar ZH39 and QH34 was significantly higher than that of cultivar HD19 and HD20.The average grain yield of ZH39 and QH34 was 1366 kg ha-1 and 1318 kg ha-1,respectively,which were 59.6%and 60.2%lower than that of monoculture,while the average grain yield of HD19 and HD20 was 990 kg ha-1 and 1027 kg ha-1,respectively,which were 71.5%and 69.5%lower than that of monoculture.Except for grain size,intercropping significantly reduced pod number per unit area,seed number per unit area,seeds per pod and harvest index.High-yielding cultivars had less decrease in pod number,seed number and harvest index,while the decrease of seed per pod were similar among cultivars.In addition,intercropping significantly reduced leaf area index（LAI）and net photosynthetic rate during shade period,but there was no significant difference among soybean cultivars.During the critical period of soybean seed set（R1 to R5）,the above-ground biomass and average crop growth rate of intercropped soybeans decreased by more than 50%,but the difference between cultivars was not significant.In R5 stage,the main reasons for the significantly higher seed number of high-yielding soybean cultivars were the lower decrease in reproductive organ biomass and seed set efficiency(Set Eff,seed g-1)and the higher increase in partitioning of total biomass to reproductive organs(Part Coef,g g-1).2.Under strip intercropping,the flower and pod abscission rate of high-yielding cultivars were significantly lower than that of low-yielding cultivars,which was the main reason that high-yielding cultivars had significantly higher pods number per plant in the maturity stage.Intercropping significantly reduced the number of reproductive nodes in main stems and the number of pods per node from 5th to 15th nodes,while the pods per node of high-yielding cultivars from 8th to 11th was significantly higher than that of low-yielding cultivars.The total flowers number of high-yielding cultivars decreased by 36.1%,while that of low-yielding cultivars decreased by 40.9%.In addition,the abscission rate of flower and pod of high-yielding cultivars increased by 2.8%and 0.2%,respectively,while that of low-yielding cultivars increased by 8.0%and 6.2%,respectively.3.The changes of endogenous hormones were the main factor of the difference of flower and pod abscission rate among soybean cultivars under strip intercropping.During the flowering and podding stage,the carbohydrate content of intercropped soybean decreased significantly,which was the fundamental reason for the decrease of total flower number.However,the carbohydrate content of high-yielding soybean cultivar was significantly lower than that of low yielding soybean cultivars under strip intercropping,indicating that the carbohydrate supply level was not the key regulator for the difference of total flowering number and flower and pod abscission rate among cultivars.In addition,the increase of IAA and DZ content and the decrease of ABA content in inflorescence at 10 days after flowering were higher than those in low-yielding cultivars.The results showed that the change of hormone content played a major role in regulating the difference of flower and pod shedding among soybean cultivars.4.The variation of plant morphology and yield reduction trend among soybean cultivars were different under strip intercropping.Although intercropping significantly increased the plant height,height of lowest pod,fifth internode length,SLn1/SDn1,SLn5/SDn5,SL/NN and SL/SDA,and significantly reduced the stem diameter of first node,stem diameter of fifth node,branch number and node number,but variations in the morphological characters of high-yielding cultivars was not less than the low-yielding cultivars.These results indicated that it was not reliable to select high yield cultivars based on plant morphology under strip intercropping.5.Under strip intercropping,the dry matter distribution among soybean cultivars was different from podding stage（R4）to seed filling stage（R6）,and the dry matter accumulation and pod dry matter distribution ratio of high-yielding cultivars at R6stage were significantly higher than those of low-yielding cultivars.During the dry matter partitioning,the high-yielding cultivars had more dry matter allocated to the leaves and pods in the R4 and R6 stage.During R2 to R6,the sucrose content in leaves and stems of the high-yielding cultivars was lower,but the decrease of sucrose content in R4 stage was larger,while the increase in R6 stage was smaller.The difference of sucrose content in leaves among soybean cultivars was mainly regulated by SPS,Su Sy and AI activities,while the difference of sucrose content in stems was regulated by SPS,AI and NI activities.Intercropping had no significant effect on starch content,AGPase and SSS activity in soybean leaves,while the intercropped high-yielding cultivars had lower starch content at R4 and R6 stages.According to the analysis of carbohydrate ratio,the sucrose content ratio of leaves/stems of high-yielding cultivars decreased lager than low-yielding cultivars in the R4 and R6stages,while the starch/sucrose content ratio of leaves and stems increased less in the R4 stage,and the starch/sucrose content ratio of stems decreased lager in the R6phase.The results showed that the high-yielding soybean cultivars outputed more sucrose from the leaves to the stem at R4 and R6 stages,but did not synthesize too much starch in the stem,and then more efficiently supplied the growth of young pod under strip intercropping. |
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