Characteristic Of Internode Growth Pattern In Soybean Affected By Shading And GA₃ Application

As an important grain and oil crop,soybean(Glycine max L.)is an important source of edible vegetable oil and high quality plant protein.With the rapid development of China's economy,the consumption of soybean increases rapidly,and the contradiction between production and demand is prominent.It is urgent to improve the supply level of soybean.Low yield per unit area severely limits the development of efficient soybean production.Increasing planting density is an effective way to improve yield per unit area,but the risk of lodging increases with the increase of planting density.Soybean internode length and internode diameter are closely related to lodging and are regulated by shading and hormones.Therefore,it has important theoretical and production significance to study the effects of shading and GA₃regulation on the internode elongation of soybean plants.This experiment was carried out in the experimental base of Northeast Agricultural University from 2016 to 2019.Two soybean varieties,HN48(high stalk variety)and HN60(dwarf variety),were selected for testing.The law of internode length and diameter was determined by measuring internode length and diameter and observing the characteristics of cell changes.Natural light(NL),Shading of the apical meristem(SAM),Covering of all internodes with aluminum foil(CAF),the whole-plant shading with plastic shading nets(PSN)were used to verify the internode elongation and enlargement.The internal anatomical structure of stem and chloroplast ultrastructure were observed,and the stem fiber composition,regulating hormones and gibberellin-related enzyme gene were determined to explain the shading effect.In addition,GA₃was applied to stem,leaf and root to analyze its regulatory effects on internode elongation and GA₃content.The results showed that:(1)Obvious hierarchical relationship was observed in soybean internode elongation process.When the number of nodes(N)of the main stem of soybean was greater than 3,internode N began to elongate.Internode N-1 exhibited the most rapid elongation,and internode N-2 elongated slowly.Internode N-3 stopped elongating,but the increase in internode diameter did not cease as internode elongation stopped.Shading treatment only affected the length and diameter of the internodes but did not change the elongation and diameter mode of the internodes.After shading treatment,the order of the length of the internodes was as follows:PSN>CAF>SAM>NL.Order of internode diameter was NL>SAM>CAF>PSN,and the order of internode diameter/internode length size was NL>SAM>CAF>PSN.(2)At the beginning of internode elongation of soybean,pith cells stretched laterally and longitudinally,cell volume increased,and internode length and diameter increased rapidly.Subsequent increase in pith cells and primary xylem resulted in the internode diameter increasing.The increase of secondary xylem area after internode elongation was the main reason for the continuous diameter of soybean internode,at which time the pith cells area was compressed.With the increase of shading degree,the area of xylem decreased,the cortex gradually became compact and narrow,the longitudinal stretching degree of pith cells increased,and the arrangement was neat and tight,and the ratio of xylem to pith area decreased.At the same time,starch grains in the chloroplasts of stem cells decreased,but the number of osmiophilic grains increased.(3)Shading treatment had a significant effect on carbohydrate content in stalks.With the increase of shading degree,the content of lignin,cellulose,hemicellulose,starch,sucrose and soluble sugar decreased significantly.The content of endogenous gibberellin(GA₃)and salicylic acid(SA)increased while that of abscisic acid(ABA)decreased.Exogenous GA₃promoted internode elongation after removal of growth points,indicating that GA₃regulating internode elongation was mainly derived from growth point synthesis.Shading led to upregulation of key enzymes of gibberelins metabolism,Gm GA3ox6,Gm GA20ox1-D and Gm GA2ox4.(4)GA₃applied outside the stem had a significant effect on the internode length and GA₃content.The main manifestation was that the sensitivity of exogenous GA₃was different in different growth stages of the internode.The more vigorous internodes were affected by the gibberellin,and there was no effect on the internodes that had only a small amount of elongation and had stopped elongation.In addition,the content of GA₃in the stem increased after the stem was coated with GA₃.(5)Applied GA₃on the leaves had a significant effect on the internode length and gibberellin content.Cutting off the leaves would weaken the elongation ability of the adjacent upper internodes.The younger the internodes,the greater the impact,and it would not affect the internodes that had stopped elongating.Cutting out the leaves and applying GA₃and applying GA₃outside the leaves had a significant promotion effect on the internodes that were elongating,but it had no effect on the internodes that stopped elongation.After the leaves are coated with GA₃,the content of GA₃in the stem increased,and the content of GA₃in the leaves decreased.The effect of GA₃on the internode length and the content of GA₃outside the leaves were less than that of GA₃outside the stem.(6)The application of GA₃to the root had a significant effect on the internode length and GA₃content.External application of GA₃to root would increase the internode length of elongating internodes,but it had no effect on the internode elongation of plants lacking growth points and leaves,indicating that the leaves are essential for the roots to absorb the exogenous GA₃and transport upwards.The root and stem simultaneous applied GA₃had no effect on mature internodes.In addition,the application of GA₃to the roots would increase the content of GA₃in the stem.(7)The effect of external application of uniconazole on internode elongation was contrary to that of GA₃.The younger the internode was affected by uniconazole,the shorter the internode length was.The inhibition and promotion of internode elongation can be broken by the external application of uniconazole and GA₃.