Diversity of phenotypic variation of polyploid plants new germplasm in pear (Pyrus communis L.)

Astract:【Objective】Pear (Pyrus communis L.) is a nutrient-dense fruit with strong consumer demandand high commercial value. However,most cultivated pear varieties are often susceptible to diseases andfruit quality is decreasing with the increase of planting time. Therefore,genetic improvement of existingpear cultivars or breeding new cultivars is a continuous target for pear breeding. Polyploid breeding offruit crops has an important value in new cultivar selection and fertility improvement. In order to providepolyploid germplasm for selecting innovative cultivar in pear,new polyploid plants were obtained fromdiploid pear cultivar‘Fertility'(Pyrus communis L.) by in vitro colchicine treatment of leaf explants. Thephenotypic variations of polyploid plants and their diploid control were compared and evaluated.【Meth-ods】The in vitro plantlets and field plants of the diploid pear cultivar‘Fertility'and polyploid clones de-rived from it were selected as materials. For in vitro plantlets,the leaf traits,including leaf length,leafwidth,leaf thickness,petiole length and petiole width,were measured. Leaf index were calculated by theratio of leaf length to leaf width. The variations of leaf colour and leaf margin were observed and photo-graphed. For plants grown in the field,the plant height above the ground was measured,ten plants wererandom selected and measured for each ploidy clone. The length and the number of internodes of currentyear's new shoots were measured and counted,three new shoots were selected from each plant,and totalthirty new shoots were measured for each ploidy clone. The average internode length was calculated bynew shoot length/the number of internode. The base diameter of current year's new shoots were measured.All collected data were analysed using DPS v3.01 statistical software.【Results】The phenotypic character-istics of in vitro polyploid plantlets had a wide range of variation. Polyploid plantlets had evidently thickershoots and shorter stem internodes than diploid plantlets. The leaf shapes of polyploids showed a varietyof variations,including oval,ovate,and oblong. Leaf shapes of mixploid were irregular,including abnor-mal or deformed leaves. Leaf base of most polyploids became more broaded than that of the diploid control. The variation of leaf bases of polyploids included obtuse,oblique and cuneate. The variation of leaftips of polyploids included obtuse,acuminate and spike. The leaf thickness of all polyploid clones wassignificantly thicker than diploid control. Leaf colour of tetraploid plants was darker than diploid control.All polyploid clones had significantly shorter petiole length than diploid control. All polyploid clones hadsignificantly longer leaf length than diploid control. The ratio of leaf length and leaf width was significantdifferent between polyploid clone and diploid control. Two triploid clones had significantly higher leaf index than diploid control,and all other polyploids had significantly lower leaf index than diploid control.The traits of leaf margins between polyploid plants and diploid plant were markedly different. The leafmargins of different ploidy plants showed a wide range of variations,including margin entire and marginsharply pointed teeth. In the in vitro proliferation,many deformed leaves were observed in mixploids,andfew deformed leaves were observed in diploid,triploids and tetraploids. For the plants growing in thefield,compared to the diploid plants,polyploid plants grew slower and had significantly dwarfer plantheight,significantly shorter internode length,and significantly smaller trunk diameter at 30 cm above theground. Generally,in the current year,polyploid plants only had one time growth,namely growth in thespring,but diploid plants had two times growth,namely growth in the spring and in the autumn,respec-tively. The mixploids showed poor endurance to natural environment,mixploid plants were not obtainedin the field.【Conclusion】Pear polyploids derived from somatic cells by in vitro colchicine treatmentshowed a wide range of variation in morphological characteristics. Generally,compared to the diploid con-trol,polyploids had thicker leaf thickness and shorter petiole length. Leaf index was significantly different between polyploids and diploid. Leaf thickness,petiole length and leaf index can be used as effectiveindicator for early selecting polyploid variation from diploid cultivar. Under the same environment,poly-ploid plants had slower growth and shorter plant height than its diploid control. Mixploids could grow wellin tube,but they were difficult to survive in the field.