A Study On Fruit Drop Pattern And The Mechanisms Of Early Fruit Drop In Macadamia

Excessive fruit drop is a common problem in macadamia growing countries,and is particularly serious in China.Technical measures to reduce fruit drop and increase yield are badly needed in macadamia industry.To explore the mechanisms of fruit drop and its regulation has become one of the most important issues in macadamia research.In this study,we selected a high-yield variety?Nanya-2?and a low-yield variety?B3/74?as comparative materials.Pollen vigor,photosynthesis,fruit growth,shoot growth and fruit dropping were investigated,attempts to master the rules of fruit drop.Effect of temperature on early fruit drop was studied.We compared carbohydrate content,endogenous hormones and sugar metabolism enzymes between the two varieties during early stage of fruit development.At the same time,effect of CPPU on fruit retention was assessed in relation to its effects on carbohydrate accumulation and metabolism and endogenous hormone contents.In addition,in order to reveal the association of carbohydrates with abscission changes in carbohydrates and endogenous hormones in fruit tissues were analyzed during fruit abscission induced by shading.Differentially expressed genes related to carbohydrate metabolism and utilization,endogenous hormone metabolism and stress responses were selected by transcriptome sequencing and bioinformatics analysis means.To reveal transcriptional information and metabolic pathways of fruit perception starvation stress.The results obtained are as follows.1.Fruit drop of macadamia throughout development could be divided into three stages in Zhanjiang.The first stage occurred within two weeks after anthesis,when massive flowers fell off from the pedicel.The second stage was from 3 weeks to 10 weeks after anthesis,about 70%to 80%of initial set fell off,and the third stage happened from 11weeks after anthesis to maturation.2.High temperature above 35?accelerated fruit drop in macadamia,and drought increased the fruit drop caused by high temperature.Effects of high temperature and drought at different fruit age were different.When high temperature and drought occurred earlier,fruit drop became more serious,suggesting young fruit are more susceptible to adverse conditions.3.Compared with low-yield variety?B3/74?,high-yield variety?Nanya-2?had higher pollen viability and its stigma had a higher ability of capturing pollens with a higher initial fruit set.Higher initial fruit set and lower accumulative fruit drop rate led to higher productivity in?Nanya-2?.4.Compared with low-yield variety'B3/74','Nanya-2'had a higher photosynthetic rate,but the carbohydrate content in leaf retention was low,and the branches and fruit is high in carbohydrates,it suggested that'Nanya-2'had a stronger ability to mobilize and transport of carbohydrates from the leaves with higher concentration of sugars in the fruit.'Nanya-2'has higher activities of enzymes related to sucrose metabolism in fruit,improved the content of available sugars such as sucrose and glucose,which was favorable for fruit set.On the other hand,fruit of high-yielding variety had higher contents of IAA,GA₃ and ZR and a lower content of ABA,and thus a higher ratio of IAA+GA₃+ZR/ABA,which favors fruit set.5.Shading weakened the ability of carbohydrate supply from the leaves,resulting in significant decrease in fruit carbohydrate content thereby causing massive fruitlet drop.Shading enhanced the activity of acid invertase?AI?,neutral invertase?NI?,sucrose synthase?SUS?and sucrose phosphate synthase?SPS?in fruit.Shading decreased the contents of IAA,GA₃ and ZR,but increased the content of ABA,resulting in a lower ratio of?IAA+GA₃+ZR?/ABA than in the control,which may be the reason of massive fruit drop.6.More than 22.6 M high quality sequence?the amount of data about 5.0 Gb?were obtained by Illumina HiSeq?2500 sequenced,and 111,919 unigenes were obtained by de novo assembly.46.3%?51,798?unigenes were functionally annotated by Blast alignment with six public protein database.7.909 significant differentially expressed genes?DEGs?were selected by comparing the expression profiling before and after shading,and 646 genes were up regulated and 263genes were down regulated by shading.181 DEGs were annotated 50 metabolic pathways of KEGG.10 genes were selected from 909 DEGs for fluorescence quantitative real-time PCR?QRT PCR?expression analysis.The results QRT PCR confirmed RNA-seq results.They included genes related to ethylene?ACS and ACO?and ABA?NCED1.2?biosynthesis,sucrose metabolism?SUS4,NI and SPS?,?-d-glucosidase?BG?and stress responsive transcription factors?HDZIP,bZIP124,ABI5?and all of them were up-regulated by shading treatment.8.CPPU not only significantly reduced early fruit drop but also delayed the occurrence of early drop peak.During early fruit growth,CPPU accelerated the transportation of carbohydrates from leaves to bearing shoots,and enhanced the activities of AI,NI,SUS and SPS in the pericarp,and increased the content of total soluble sugars in the pericarp and seed,especially the contents of glucose and sucrose in pericarp,suggesting CPPU increased sink strength of the fruit.The results agreed with the up-regulation of sucrose metabolism related genes?SPS,SUS4 and NI?.The difference of carbohydrate content between the treatments and the control mainly existed in the pericarp,suggesting that the pericarp might be an important site to sense sugar content and to produce signals that regulate fruit abscission.CPPU increased the ratio of?IAA+GA₃+ZR?/ABA in fruit,as the treatment increased the contents of IAA,GA₃ and ZR and decreased the content of ABA in pericarp.The decrease of ABA agreed with the down-regulation of NCED1.2,the key gene in ABA biosynthesis.The treatment also induced increased expression of ethylene biosynthesis genes?ACS and ACO?and inhibited?-d-glucosidase gene?BG?.The molecular physiological changes induced by CPPU might be involvled in promoted fruit set and development.Based on the results of our study,a mode of the mechanism of early physiological fruit drop of macadamia was constructed integrating the effects of various internal and external factors influencing fruit abscission.