Evaluation Of Overwintering Ability Of Forsythia Germplasms And Breeding For Cold Tolerant Cultivar With Large Amount Of Flowers

Forsythia species are traditional shrubs blooming in early spring in China.Because of the bright color and beautiful posture,it is widely used in the gardens.However,most cultivars of Forsythia could not survive the low temperature in severe winter areas in northern China（area in north of Beijing）,which affected the landscape application of Forsythia.Forsythia mandschurica has strong overwintering ability,but the amount of flowers is small.In order to accurately evaluate the overwintering ability of Forsythia germplasms,and provide excellent Forsythia plants for the application of gardening in severe cold areas of northern China,10 Forsythia species and cultivars commonly used in gardens were evaluated the cold resistance in Pingquan city of Hebei Province（40°24′0″N～40°40′17″N）.The F₁ population obtained by F.intermedia‘Lynwood’×F.mandschurica was used to screen excellent plants with strong cold resistance and large amount of flowers.The results are as follows:1.After overwintering in the field,the above-ground branches of F.suspensa,‘Lynwood’,‘Spectabilis’,‘Goldrausch’,‘Courtalyn’were all dried up.40%,75%,30%and 5%of the branches of F.koreana,‘Courtasol’,‘Kumsum’and‘Courtaneur’were dry respectively,and the proportion of dry shoot length to the whole shoot were 10%,15%,10%and 20%,respectively.Only the branches and leaves of F.mandschurica grew normally.The hydroponic experiment showed that the shoots of‘Courtalyn’and‘Lynwood’died on December 20,and the shoots of‘Spectabilis’and‘Goldrausch’died on January 20.Based on the results of field observation and hydroponic experiment,the order of overwintering ability of the 10 Forsythia species and cultivars were:F.mandschurica>‘Courtaneur’>‘Courtasol’>F.koreana>‘Kumsum’>F.suspensa>‘Spectabilis’、‘Goldrausch’>‘Lynwood’、‘Courtalyn’.2.During natural overwintering period,the relative conductivity,soluble sucrose content,soluble protein content,superoxide dismutase activity,and peroxidase activity of the shoots of 10 Forsythia species and cultivars increased with the overwintering time.The membership function method was used for comprehensive cold resistance evaluation,and the order of cold resistance of 10 Forsythia species and cultivars were:F.mandschurica>‘Courtaneur’>‘Courtasol’>F.koreana>‘Kumsum’>F.suspensa>‘Spectabilis’>‘Goldrausch’>‘Lynwood’>‘Courtalyn’.The result was basically consistent with the result obtained from field observation combined with hydroponic experiment.3.The genetic coefficients of variation of 12 traits in hybrid population of F.intermedia‘Lynwood’×F.mandschurica were between 11.33%and 36.89%.The frequency distribution of each trait was close to the normal distribution,which conformed to the characteristics of quantitative traits.The cold resistance of 48.89%of the hybrids were between their parents.Mixed genetic model analysis revealed that corolla lobe length,corolla lobe width,length-width ratio of corolla lobe,leaf length,leaf width,length-width ratio of leaf,shoot length,cold resistance were controlled by minor-polygene.Corolla diameter and crown width were controlled by a pair of additive-dominant major genes.Flower density was controlled by two pairs of additive-dominant major genes.Plant height was controlled by two pairs of equally additive-dominant major genes.The heritability of major-gene for corolla diameter,crown width,flower density and plant height were 76.05%,60.30%,72.22%and 64.75%,respectively.4.Among the 131 hybrids of‘Lynwood’×F.mandschurica,there were 95 plants with higher flower density than F.mandschurica,and 8 plants with strong cold resistance were selected,which were the pre-selected plants with dense flowers and strong cold resistance.The growth performance of cutted seedlings of the hybrid progenies were widely separated after the overwintering in open field in Pingquan.Among them,there were 10 plants with the proportion of length of dry shoot less than 30%,including 4 pre-selected strains,which can be used as excellent strains with dense flowers and strong cold resistance.The results of the field experiment were basically consistent with the result of the cold resistance evaluation by the relative electrical conductivity measurement and the calculation results of the semi-lethal temperature(LT₅₀).The results of our research provided new materials for gardening in spring in northern China,and laid the foundation for breeding of new cultivars of Forsythia with excellent comprehensive traits.