Physiological And Biochemical Analysis Of The Dormancy Of Melliodendron Xylocarpum Seeds

Melliodendron xylocarpum Handel-Mazzetti,a tree species endemic to China,has multiple economic values for which can be used as an ornamental,oil and wood plant.Under natural conditions,most seeds of this species are in a dormant state and germinate in the second year after the maturation.To reveal the dormancy mechanism of M.xylocarpum and provide technical reference for dormancy releasing in the practice,we used fruits in the first and the second year of stratification as experimental materials,and surveyed dynamic changes of morphological structure,water permeability and pH values of pericarp,nutrient contents,metabolism-related enzyme activities,and endogenous hormone contents with their ratios of endosperm.Our study shows that:1.Fruits of M.xylocarpum have an obovate shape.A five-layer structure including corky outer pericarp,woody and thick mesocarp,woody and thin endocarp,membranous seed coat,and fleshy embryo was observed in our study.The woody and hard pericarp forms mechanical obstacles to seed germination.Fruits are obscurely 5-lobed,and have a hollow air chamber,which contributes to the seed dispersal through water.2.Seeds of M.xylocarpum have physical dormancy.Woody and hard pericarp not only leads to mechanical obstacles to seed germination,but also impedes the exchange of water and air with the outside,affecting the imbibition and germination of seeds.After one year's stratification,the outer pericarp and mesocarp of M.xylocarpum rotted gradually,and the water permeability increased and pH values decreased significantly.Etching under natural conditions might improve the air and water permeability of the pericarp.3.Contents of soluble sugar,starch,dissoluble fat and soluble protein in the endosperm of M.xylocarpum decreased rapidly during the 456th to the 486th day of stratification,and corresponding consumption rates were higher than those during the first year of stratification.The activity levels of amylase,proteinase,acid phosphatase and malate dehydrogenase in the endosperm increased rapidly during the 456th to the 486th day of stratification,significantly higher than those the first year of stratification.Seeds in the first year after maturation are still in the dormant stage,metabolic activities were relatively weak,which may be closely related to the poor permeability of pericarp.4.During the 456th to the 486th day of stratification,the contents of ABA and IAA in the endosperm decreased,while GA₃ content increased,and ZR content was at a high level,suggesting dormancy of M.xylocarpum was releasing.During this stage,ABA and IAA inhibited germination,while GA₃ promoted germination.5.In the practice,effective measures such as cutting and acid etching can be used to increase air and water permeability of pericarp of M.xylocarpum,and then soaking seeds for more than10 days before stratification can contribute to the moisture absorption.At the same time,replenishing enough water before germination is useful to ensure seeds of M.xylocarpum are fully imbibed.