Studies On The Physiological Indices And Phloem Unloading Pathway In Fig Fruit

To uncover the phloem unloading pathway of fig fruit, and in order to study the relationship between ABA and ethylene content and fruit ripening during the development of fig fruit, five-stage fruit of Masui Dauphine fig(Ficus carica L., Moraceae) were used to research, including small green, big green, initial red, partial red, and full red, at about 15, 30, 65, 70 and 85 d after fruit setting. A combination of electron microscopy and phloem-mobile symplasmic tracer carboxyfluorescein(CF), the phloem unloading pathway was studied in the developmental fruit, meanwhile, the changes of fruit size and color, soluble sugar and starch contents, respiratory rate, ABA level and ethylene production level were also examined.The growth of fig fruit follows a typical sigmoid growth curve with distinct three developmental stages: stage I, the first rapid growth phase; stage II, the lag phase; stage III, the second rapid growth phase, namely, turning and ripening stages; the transition from stage II to stage III represents the onset of ripening. During stage I and stage II, fruit soluble sugar contents is lower, while the onset of the fruit ripening, a rapid increase in both fruit volume and soluble sugar contents with main glucose and fructose was observed. The contents of starch were decreasing rapidly before the onset of ripening, then after, decreased slowly. Electron microscopy observations showed that the sieve element-companion cell(SE-CC) complex of the vascular bundles is apparently symplasmically connected through plasmodesmata with surrounding parenchyma cells(PCs) during the stage I and stage II, but a transition occurred from symplastic to apoplastic after the onset of the fruit ripening. Interestingly, an abundance of plasmodesmata was always observed between parenchyma cells during the fruit development. Imaging of carboxyfluorescein movement showed that the dye is released from the functional phloem strands before the onset of ripening, whereas the CF was confined to the phloem strands after the onset of ripening.There is an decrease in ABA content during ripening in fig fruits. But there is an increase in ethylene production during fig fruits ripening, and during the onset of ripening a respiratory climacteric rise was concomitant with a sudden burst of endogenous ethylene production.The results obtained from the present study indicate that the phloem unloading pathway switches from a symplasmic pathway before onset of ripening to an apoplasmic pathway in the late stage; subsequently, post-phloem transport of sugar follows a symplasmic pathway in fig fruit. Fig fruit is a typical climacteric fruit. Ethylene initiates a chain of metabolic and physiological events leading to fig fruit ripening. In addition, ABA might directly take part in the regulation of onset of the fruit ripening.