The Difference Of Fruit Morphology、anatomy And Physiology Of Atlantic Giant Pumpkin And Its Small Fruit Ancestor Hubbard

Pumpkin variety with extreme huge fruits,"Atlantic Giant"Cucurbita maxima),is a unique material for studying fruit size and has important commercial value in the tourism and other fields.“Atlantic Giant”was originated from its ancestor squash "Hubbard",but there is a significant difference in fruit weight and volume between two varieties.The physiological reasons for these differences are still unclear.To this end,the weight,volume,density,vascular bundle structure,fruit cell size and number,leaf photosynthesis,sugar contents and activities of key enzymes associated with assimilation traslocation of“Atlantic Giant,and "Hubbard" at different developmental stages were investigated to eluciadate the physiological difference between two varieties.Furthermore,the F2 population with“Atlantic Giant,as the female parent and "Hubbard" as the male parent was constructed,and the single fruit weight,fruit volume,cell size and cell number per fruit were determined of each F2 plants for further exploration of the key genes that determine the fruit size.The results are as follows:1.Under the cultivation condition of this experiment,the fruit of "Atlantic Giant,began the rapid growth period between 10-20 days,and the weight and volume of fruits reached their maximum values at 60 days after pollination;while "Hubbard" fruit entered the rapid growth period between 0-10 days,and the fruit size was basically unchanged after 40 days after pollination.The density of the two pumpkin fruits significantly differed only in the ovary phase,with no remarkable difference in subsequent development stages.The cell size and cell number of the "Atlantic Giant,fruit were significantly greater than those of "Hubbard".The size and number of fruit cells in the F2 population distributed discretely,indicating they belong to quantitative traits.In addition,the trait“number of cells per fruit" showed a slight distribution tendency to the low-value parent.2.The leaf area and the net photosynthetic rate,the content of stachyose in leaves of"Atlantic Giant" is significantly greater than those of "Hubbard",but there was no significant difference of actitities of leaf stachyose synthase between two varieties.3.The peduncle diameter,the total area of xylem and phloem per peduncle,the concentration of soluble sugars in the phloem sap of "Atlantic Giant,were significantly higher than those of "Hubbard".Interestingly,glucose and fructose were significantly exist in the"Atlantic Giant" phloem sap,but not in "Hubbard" phloem sap,suggesting that "Atlantic Giant"may have other phloem loading methods in addition to the "Polymer trap" mechanism,a well-known symplast loading pathway in the cucurbits family.Alkaline alpha-galactosidase activity in the peduncle of "Atlantic Giant" fruits was significantly lower than that of "Hubbard"at most growth stages.The content of stachyose in "Atlantic Giant",fruit in the fast growing period is significantly higher than "Hubbard",while the content of stachyose in "Hubbard" fruit is extremely low,indicating that at least part of stachyose translacated from the leaves was not decoposed,and translocated directly into the "Atlantic Giant" fruit.In summary,the significant difference of the fruit size between "Atlantic Giant" and"Hubbard" fruit is associated with multiple aspects,including cell size,cell number,photosynthesis,total area of xylem and phloem,the stachyose content,and the amount and type of soluble sugar in leaves and phloem sap.