Study On The Mechanism Of Seedlessness Of Loquat Induced By GA₃

Although loquat fruit has high nutritional,economic and medicinal value,it contains many seeds and the proportion of seed weight is about 30%,reducing the edible rate and commercial value. Therefore,it is the most urgent needs to reduce the number of seeds and achieve seedless fruit in loquat production and breeding.In theory,there are two main ways to achieve seedless fruit of loquat:one way is to breed a seedless variety.Usually,it is producing triploid by nature selection,hybridation through diploid and tetraploid,and artificial induction of triploid.At present,great progress has made in these areas and the triploid varieties will be tested and populated in the future.However,those cultivated triploid loquat whether it is from hybridation or nature selection,usually must be integrated with the use of plant growth regulator in order to gain the fruits of commercial value.Another way is to induce seedless fruit by plant growth regulators from seedy varieties.At present,GA₃ treatment is the main successful procedure inducing seedless fruit.Although,there are many studies about GA₃ inducing loquat parthenocarpy,but these studies emphasized on the inducing dose,on the inducing date,and on the effects on the fruit development,the fruit economy traits and the fruit quality.The mechanism of the formation of seedless fruit is still unknown.In this study,to reveal the reproductive biology and physiological mechanisms of GA₃ inducing seedless fruit in loquat,the effects of GA₃ on the loquat female and male organ growth and development,on the pollen viability and germination,on the embryo development,on the growth and development of fruit and fruit quality,and on the endogenous hormone content in flowers and fruits were studied with loquat cv.'DAWUXING'(middle maturity variety) and 'BIHUANGZAO'(early maturity variety) in Chongqing.The main findings were as follows: 1.GA₃ treatment did not affect the growth and development of female and male organ of loquat. Compared with the control,the floral organ shapes treated by GA₃ had not been seen significantly different;and there was no significant difference in anther and anther cytology development either, anther could develop to mature pollen.The development of loquat anther could be divided into sporogenous cell stage,microspore mother cell stage,tetrad stage,mononuclear microspore stage and pollen mature stage.2.Compared with the control,the pollen viability and germination did not be reduced when the Ioquat buds were treated by GA₃ before flowering.The rate of pollen germination and tube growth length can be improved when the buds were treated by 500-750mg/L of GA3.The pollen germination and pollen tube growth could be promoted when 5-200 mg/l GA₃ was added to medium.However, the pollen germination and the growth of pollen tube were reduced when the medium with GA₃ concentration over 200 mg/l.3.Loquat had anatropous ovule,which was developed from ovule anlage to nucellus,funicle, integument and other parts of ovule in the early stage of pistil development.Megaspore mother cell split to form the functional megaspore,the bi-nuclear embryo sac,the quad-nuclear embryo sac,and the eight-nuclear embryo sac.After fertilization,central cells formed the primary endosperm nucleus which split before the zygote.Loquat endosperm was nuclear endosperm.Fertilized egg cells formed the zygote,and then further developed into pre-embryo.The normal loquat embryo development went through globular embryo stage,heart-shaped embryo stage and cotyledon embryos stage,etc.4.GA₃ could affect the ioquat embryo development and inducing seedless fruit.After the flower was treated with 500mg/L GA₃,the normal egg apparatus,sclerenchyma and the central polar nuclei cell in embryo sac were still observed;fertilized egg cell was seen to form a zygote,and then further develop into pre-embryo;zygotic cell division remained at the pre-embryo stage and then embryo remained loose irregular shape;since the endosperm and nucellus development were abnormal and then degraded and disappeared,embryos were seen to degrade and disappear;although both inside and outside integument were able to develop normally in the early stages of embryo development,,but the cells between the inside and outside integument were degraded in the later of the development.Thus loquat fruit treated by GA₃ contained the inside integument and outside integument but the embryo and endosperm degraded completely.The reproductive biology mechanism of GA₃ inducing seedless fruit in loquat lies in abnormally development and degradation of the endosperm and the nucellus, ultimately resulting to the embryo abortion.5.There was no effect on trend of zeatin content change in loquat fruit treated by GA₃,but it could significantly increase zeatin content in loquat fruit treated by GA₃ during the period from early January to early February.The zeatin content in both the control and the sample treated by GA₃ of loquat cv.'DAWUXING' and 'BIHUANGZAO' had two peaks during the entire fruit growth and development:in mid-November before flowering and early January of the next year.6.The trends of the IAA content change in the fruit of both the control and the GA₃ treatment were similar before late January and after mid-February.However,the trend in control increased significantly while that in the GA₃ treatment decreased continually from late January to mid-February. In addition,the IAA content of the fruit treated by GA₃ was decreased significantly before mid-January(globular embryo period).7.GA₃ affected the trend of ABA content change in the loquat fruit and significantly decreased the ABA content.The ABA content of the control had three peaks during the period of fruit growth and development:in late November after flowering and late January in next year of the second phase of physiological fruit dropping and middle and late April.8.The trends of the ZT/ABA ratio change in both the control and the GA₃ treatment were similar but the change range was little before early January.However,the ZT/ABA ratio in the sample treated by GA₃ sharply increased while that in the control decreased continually from early January to late March.9.The trends of IAA/ABA ratio change in both the control and the GA₃ treatment were similar but the change range was little before mid-February.However,the IAA/ABA ratio in the GA₃ treatment sharply increased after mid-February and reached the peak in late March and then declined before harvest while that in the control decreased continually and change range was little in this period.10.Embryo degeneration induced by GA₃ was closely related to the fruit endogenous hormones: high level of IAA and ZT and low level of ABA were very important to the pollination and fertilization,the early cell division and differentiation;dramatic increase of IAA and ABA content in the ovary were closely related to the splitting of primary endosperm nucleus and fruit zygote,and normal embryo and endosperm development;a favorable balance of ZT,IAA and ABA content would benefit to the loquat fruit setting and growth,and to the normal development of embryo.The IAA and ABA content in the fruits of GA₃ treatment were significantly lower than those in the control before mid-January,resulting disruption of endosperm and embryo development,and the disappearing of endosperm and embryo.ABA is not only a growth preventing hormone,but also an important substance to the loquat embryo development because a favorable level of ABA content could promote cmbryo and endosperm development.It was one of key factors which resulted the disappearing of endosperm and embryo when ABA content in fruits was reduced significantly. 11.Whether it was the middle maturity loquat cv.'DAWUXING' or the early maturity loquat cv. of 'BIHUANGZAO',in both the GA₃ treatment and the control in Chongqing,the process of the loquat fruits growth and development could be divided into three stages:(1) Flowering and fruit setting stage(from mid-November to early February).In this stage,the fruit weight,the fruit weight daily gain,and the longitudinal diameter of fruits changed slightly.(2) Young fruit growth and development stage(from mid-February to mid-March).In this stage,the fruit weight,the fruit daily gain,and the longitudinal diameter of fruit changed greatly.(3) The rapid expansion of cells and fruit maturation stage(from late March to the harvest).In this stage,the fruit weight,the fruit weight daily gain,the longitudinal diameter,and the transverse diameter of fruits sharply increased.12.In the first and the second fruit growth and development stage,compared with the control,the GA₃ treatment increased significantly the fruit weight,the fruit diameter of longitudinal and transverse, the length of spikes and the first shaft axis and the pedicel,and the fruit setting rate.13.GA₃ treatment could induce loquat parthenocarpy,and the rate of seedlessness could be 100%. The fruit shape index,edible rate,Vitamin C,and titratable acid content in the GA₃ treatment were higher than those in the control.However,the soluble solids,protein,soluble sugar,and carotenoid content in the GA₃ treatment were lower than those in the control.The maturation was delayed by the GA₃.