The increase in cell size and the activities of cell wall lytic enzymes during fruit growth supported by the mesocarp cell enlargement have been studied in peach of the freestone cultivar Redhaven. Four growth stages (S 1-S4) in peach fruit were observed. Cell enlargement and formation of intercellular spaces in the mesocarp were directly correlated with the increase in fruit diameter. Cellulase and exopolygalactu- ronase showed the highest activity during the first stage of fast fruit growth (SI), while the endopolyga- lacturonase activity increased during the last growth stage (S4), when the fruit reached its final size and ripening started

`Redhaven' peach [Prunus persica (L.) Batsch.] fruit growth, expressed as cheek diameter, displayed a double-sigmoid pattern in which four stages were defined (SI, SII, SIII, SIV). Free IAA concentration, as determined by polyclonal antibodies (PcAb) enzyme-linked immunosorbent assay (ELISA), paralleled fruit growth rate, peaking at 30 and 85 days after full bloom (AFB), concurrently with the exponential phases of growth. The highest peroxidase (EC 1.11.1.7) (POD) and IAA oxidase (IAAox) activities occurred during endocarp lignification. The main structural events described were mesocarp cell division within the first 2 weeks AFB and, later, cell enlargement, modifications of the epicarp cells, lignification of the endocarp, differentiation of the chloroplasts, and changes in their starch content. Chemical name used: indole-3-acetic acid (IAA).

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SUMMARYCell wall hydrolases, their mRNAs, and ultrastructural details of eell wall digestion have been studied in peach abscission zones (AZ) located at the base of flower bud (AZl) and the base of flower receptacle (AZ2), respectively.Induction of abscission was obtained by treatment of explants -with exogenous ethylene. Cell separation patterns of the two examined abscission zones have been compared with those of other already ktiown AZs of peach, i,e, the AZ3 located between fruit and peduncle and tbe leaf AZ, Analyses have shown similarities in response to etbylene treatment between AZl and leaf AZ and between AZ2 and AZ3, respectively. Results have been discussed eonsidering the precise position of AZl and AZ2 on the flower bud. The timing of functional differentiation, evaluated as tbe cells' ability to respond to induction by ethylene treatments, showed that AZl and AZ2 became functional after bud breaking and bud scale shedding. Later on, they lost their functionality at about 6-7 wk from antbesis, AZ3 became functional very precociously and could be activated 1 wk after anthesis in the fertilized flowers. In the latter zone the cells could also utidergo a morphological predifferentiation, even though it occurred a long time after the acquisition of the ethylene responsiveness. This flnditig shows that morphological differentiation is not necessarily a prerequisite for tbose cells to become competent to respond to tbe abscission inducing stimuli.