Preliminary Study On Flowering Phenology And Reproductive Characteristics Of Acacia Melanoxylon

Acacia melanoxylon is a leguminous evergreen tree of the genus Acacia,native to eastern Australia.The species is widely distributed,vertically distributed from the sea level to the elevation of 1500 meters,from north Queensland(16°S)south to Tasmania(43°S),and west to south Australia.Due to its rapid growth,good adaptability,strong tolerance,good soil performance,excellent materials and other characteristics,China began to introduce the cultivation of this kind of tree in the 1980 s.However,there are no reports on the phenology and reproductive characteristics of Acacia melanoxylon.Therefore,this paper takes Acacia melanoxylon phenology forested on the farm in Quanzhou,Fujian province as well as its reproductive characteristics as the research object,and systematically studied flowering phenology,breeding system and the development of male and female gametophytes through field observation,scanning electron microscope,paraffin section and other technologies.The main research results are as the followings:(1)The initial bud time of Acacia melanoxylon is in the middle of late May,and the group flowering stage is non-continuous,with strong synchronization and a concentrated flowering pattern.Because of the difference of inflorescence differentiation time,3 to 4 times of anthesiscycle occurs.The initial flowering period of the group is at the end of early June,but there is also a small topographic phenology.The flowering at the foot of the mountain is 1 to 2 days earlier than at the mountainside and the peak.The flowering stage lasts for 29 days until the end of early July.The initial fruiting stage of the colony is in the middle of late June,and a large number of fruits appeared after 3d,namely the fruiting stage.The first fruit ripening stage is in the early middle of August,and it enters into the full fruit ripening stage about 13 d later.After ripening,the fruit is severely twisted and cracked,and the whole fruit stage lasts about 63 days.Individual horizontal flowering also has strong synchronization,and shows the pattern of concentrated flowering.Individuals' first anthesis is on June 9,and then enter the full flowering period after 1 to 2 days.The last flowering period is in the middle of late August,and the whole flowering period lasts about 78 days.The observation showed that some plants bloomed again after the maturation of the fruit,and the flowering period could be extended to late September,but there was no fruiting phenomenon.(2)The inflorescences of Acacia melanoxylon are head shaped and spherical,and the buds on the inflorescence have strong synchronization.But there are also great differences in the degree of development.Inflorescences are open generally at night when,and it takes one night forthe buds to turn yellow and swell until the inflorescences are fully open,while a single flower can be fully open for only 4 to 5 hours.When inflorescences are in bloom,the corolla darkens in color and cracks appear,and then gradually splits into 5 petals,which unfold in a zigzag pattern.After corolla dehiscence,the pistil extends beyond the corolla before the stamen,and after the whole inflorescence is in full bloom,the pistil is higher than the stamen or slightly flush.Inflorescences in full bloom usually retain branches for 10 to 15 days.In the case of extreme weather such as rainstorm and typhoon,the inflorescences stick together after rain and are in mass shedding.Even if it does not fall off at that time,it will be difficult to restore the original status after the undermine of bad weathers,and it will wither 7 to 8 days later.According to the observation,after flowering for 5 to 6 days,the filaments began to wilt,and the stigma wilting was 1 to 2 days later than the filaments.After flowering for 11 days,the ovary began to grow and turn green,and the young fruit appeared about 14 days later,which was tender green and covered with brown villi in the beginning,and then the surface villi gradually decreased.(3)The buds Acacia melanoxylon are mainly flower buds,with only a small number of mixed buds.There are 1 to 11 head inflorescences in each axils,in the diameter of 5 to 11 mm,milky white,which contain 33 to 72 florets,bisexual and rarely unisexual.The buds surface are denselycovered with brown fluff and the corolla is light yellow and green;each flower usually has one pistil rolled by one carpel in the shape of the word"zhi”,colorless.Stigma morphological features are not obvious and indistinguishable from style.Ovary is fusiform and covered by puberulent;stamens are about 36 with slightly yellow anthers and colorless filaments;perianth flake is in thin winglike shape and slightly yellowish;the calyx is concrescent,bell-shaped and in the length of about half of the corolla.(4)The anthers of Acacia melanoxylon are pterygium and have longitudinal fissure;after cracking,they are scattered in the form of compound pollens,which are composed of 16 pollen grains(2 layers above and below,4 pollen grains in each layer and surrounded by 8pollen grains).The morphological characteristics of compound pollens are round or oval,and the pollen grains are block-shaped,usually regular in shape and orderly in arrangement and combination.The pollen germination pores are in the form of pores,and 3 to 4 pores of pollen germination sprout in the position of various corners.Stigma is in the size of 38.74 um and in concave cup shape with no papillae only one grain compound pollen can be accommodated;style closed,with guide tissue;the ovary is fusiform with lower mastoid spheroid,and hollow inside,and its surface is densely tomentose.(5)The hybridization index(OCI)of Acacia melanoxylon is 4.The pollination methods are mainly insect-pollinated,which areinsect-pollinated plants.The main effective pollinators are Apis cerana cerana,Apis mellifera ligustica,Eristalis arvorum and the so on.(6)The anthers of Acacia melanoxylon have 4 Chambers,and 2compound pollen grains are produced in each chamber;the anther wall is composed of 4 layers of cells,which are epidermis,the medicine chamber wall,the middle layer and the tapetal layer.The tetrad formed after meiosis is arranged in bilateral symmetry.There are one loculus of mature ovary and marginal placenta which is containing 4 to 16 anatropous ovules with double integument and hick nucellus.