Effect Of Light Distribution Within The Canopy On Maize(Zea Mays L.)Stalk Lodging Resistance Charactistic

High plant density is an important way to increase maize yield.However,high plant density often decreses the stalk strength and results in lodging.This reduces grain quality and increases both harvest losses and costs.High plant density increases the leaf area index(LAI)and mutual shading among individual leaves.This shading significantly decreases photosynthetically active radiation(PAR)in the lower canopy from the mid-vegetative to early grain-filling stages.The photosynthetic capacity decreases when leaves are either shaded or under weak light conditions.This caused carbohydrate transport to maize stalk to decline.Therefore,we hypothesized that light intensity may be the main factor affecting maize stalk strength formation and lodging.To test this hypothesis,three field experiments involving plant density,shading and defoliation were designed to explore how the light environment in a maize canopy affects stalk strength formation and lodging rate.The morphological,mechanical,anatomical,and biochemical characteristics of the third basal internode were compared in three experiments.This study will provide new insights about maize lodging under close planting.This information could be helpful for developing new maize cultivation practices and for helping crop breeders to develop lodging resistant maize cultivars.The main results are as below.1 Effect of plant density on maize basal intenrdoe developmentThe development of the basal internode begins with a period of rapid thickening and lengthening,followed by an increase in total and structural carbohydrates,and then an increase in stalk strength.The critical period of rind penetration strength(RPS)formation was from about 14 to 25 DPV6.High plant density caused rapid elongation and thickening to begin and end earlier.High plant density increased the internode length by increasing the rate of rapid elongation.High plant density reduced the diameter and dry weight per unit length(DWUL)of the third internode by shortening the duration of rapid internode growth.This change also decreased the stalk strength.2 Effect of light intensity whithin maize canopy on lodging resistance under high plant densityHigh plant density reduced the light intensity within maize canopy,decreased the DWUL and RPS of the third basal internode,and increased the lodging rate.Compared with natural light(CK),the 30% light exclusion(30%-S)treatment decreased DWUL by about 40%,decreased RPS by about 40%,and increased lodging rate by more than 10% in both cultivars.The 60% light exclusion(60%-S)treatment decreased DWUL by about 50%,decreased RPS by about 50%,and increased lodging rate by about 20% in both cultivars.This indicated that low light intensity in the maize canopy is the main reason for the decrease in DWUL and RPS and for the increase in lodging rate.3 Effect of shading and leaf removal treatments on maize stalk strength and lodgingShading increased the plant height and ear height of maize.However shading decreased the gravity height.Shading increased the basal internode length,decreased the internode diameter and RPS.These caused the lodging rate to increase.Further analysis showed that shading significantly decreased the DWUL,water-soluble carbohydrate,cellulose,lignin and hemicellulose content.And there is a great effect on water-soluble carbohydrate content.This indicated that shading decreased the water-soluble carbohydrate content,and cause structural carbohydrates to decline.The low structural carbohydrate decreased the RPS and increased the lodging rate.Removing whole leaves or half leaves above the three-ear-leaves(RAE and RAE/2)at flowering significantly increased the internode structural carbohydrate content,and increased RPS of the third basal internode by 5.4-11.6% and 7.7-8.2%,and reduced lodging by 4.9-7.8% and 4.2-4.6%.Removal of the first three leaves below the three-ear-leaves(RBE)before flowering significantly reduced the internode structural carbohydrate content,and reduced the RPS of the third basal internode by 9.1-17.4% and increased lodging by 7.0-11.2%.Removal of the three lowest green leaves(RB)in the canopy before flowering had no effect on the structural carbohydrate content and RPS of the basal internode,and the lodging rate.The results indicated that the key factors affecting the basal internode carbohydrate accumulation and strength formation were the leaves from the ear to halfway below the ear.4 The relationship among carbohydrate,anatomical characteristics and mechanical strength in maiz basal internodeThe RPS did not increaserapidly until after the termination of rapid morphological growth.The mid-to-late stage of dry matter accumulation was critical for RPS formation.The rapid accumulation of cellulose and lignin began and ended earlier under high plant density.High plant density reduced the rate of cellulose and lignin accumulation,thus stunting the development of the cortex.This caused declines in the rate of RPS formation and in RPS.The measures should be implemented as early as possible in the growing season to increase lodging resistance at high plant density of maize.These measures need to reduce the rate of rapid internode elongation and increase the rate of rapid cellulose and lignin accumulation.5 Effect of high PAR in maize canopy on lodging resistanceThe leaves from the ear to half way below the ear played a very important role in determining the mechanical strength of the basal internode and lodging resistance.Increasing PAR at the ear and IPAR from the ear to halfway below the ear could enhance lodging resistance by increasing the structural carbohydrate content and mechanical strength of the basal internode.The above results suggest that stalk strength and lodging resistance could be improved by breeding maize plants with relatively smaller and upright leaves in the upper canopy.This type of maize plant can increase PAR at the ear and IPAR below the ear under close planting.This is a main technological approach to achieve high yield in maize production under field condition.