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Benzyladenine (BA), carbaryl (CB), daminozide (DM), and naphthaleneacetic acid (NAA) were applied postbloom as fruitlet thinning agents to mature ‘Empire’ apple (Malus domestica Borkh.) trees. BA, NAA, and CB reduced fruit set and yield per tree, and increased fruit size, percent dry weight, soluble solidscontent and return bloom. Fruit size was reduced, return bloom, length : diameter ratio and flesh firmness were increased, and fruit set and yield unaltered by DM. Although fruit set and yield were similar for BA, NAA, and CB, BA treated fruit were larger, indicating that BA increased fruit size beyond the effect attributable to chemical thinning alone. BA increased the rate of cell layer formation in the fruit cortex, indicating that BA stimulated cortical cell division. NAA, CB and DM had no effect on cell division rate. Mean cortical cell diameter at harvest was increased by NAA and CB and reduced by DM. Cell diameter at harvest in BA-treated fruit was similar to the control. These data support the hypothesis that BA-induced fruit size increase in ‘Empire’ apple results from greater numbers of cells in the fruit cortex, whereas the fruit size increase due to NAA or CB is a consequence of larger cell size. Chemical names used: N(phenylmethyl)-1H-purine-6-amine [benzyladenine (BA)]; 1-napthaleneacetic acid (NM); 1naphthalenyl methylcarbamate [carbaryl (CB)]; butanedioic acid mono (2,2dimethyl hydrazide) [daminozide (DM)]. Fruitlet thinning is an essential part of the commercial production of quality apples. Blossom or fruitlet thinning early in the season improves fruit size at harvest and increases return bloom, thereby reducing the biennial bearing habit of apple trees (Westwood, 1993). The cytokinin benzyladenine (BA) is now registered for use as a chemical thinner for apple as a formulation containing both BA and gibberellic acid (Abbott Laboratories, 1993). BA effectively thins many cultivars, including ‘Empire’ (Elfving and Cline, 1993a, 1993b; Greene et al., 1990) ‘McIntosh’, (Greeneand Autio, 1989), ‘Golden Delicious’ (Greene et al, 1990), and ‘Idared’ (Elfving, 1989). BA thins most cultivars at concentrations from 25 to 100 mg·liter (Bound et al., 1991; Byers and Carbaugh, 1991; Elfving, 1991; McLaughlin and Greene, 1984). BA also increases return bloom in many apple cultivars (Bound et al., 1991; Elfving and Cline, 1993a, 1993b; Greene et al, 1990) and increases fruit size more than would be expected from thinning alone (Greene et al., 1992; McLaughlin and Greene, 1984). Cytokinins promote cell division in apple tissue (Letham, 1958) and cell division is still underway in fruits when BA is applied for thinning (Denne, 1963a, 1993b; Schechter et al, 1993a, 1993b). These circumstances have led to speculation that BA used as a thinner may affect the rate and/or duration of cell division in the apple fruit. The objective of this investigation was to examine the effect of BA used as a chemical thinner on cell division and cell size in ‘Empire’ apple in comparison to the chemical thinners or publication 23 Dec. 1994. Accepted for publication 14 Apr. 1995. ch was supported by operating grant no. A6697 of the Natural Sciences ering Research Council (NSERC) of Canada held by J.T.A.Proctor and graduate scholarship held by Paul T. Wismer. We gratefully acknowlsistance of Larry Peterson, Dean Louttit, Jean Gerrath, Mary Born, and val. Mention of a product or trade name does not constitute a guarantee of the product by the Univ. of Guelph or the Horticultural Research Ontario nor an endorsement over similar products not mentioned. The lishing this paper was defrayed in part by the payment of page charges. al regulations, this paper therefore must be hereby marked advertiseto indicate this fact. dress: Tree Fruit Research and Extension Center, Wash. State Univ., estern Avenue, Wenatchee, WA 98801. naphthaleneacetic acid (NAA) and carbaryl (CB) and the growth regulator daminozide (DM), which inhibits cell division in apple under some circumstances (Martin et al., 1968). Materials and Methods All trees used for this investigation were mature, annually bearing ‘Empire’ growing at the Horticultural Research Institute of Ontario Horticultural Experiment Station in Simcoe, Ontario. The trees were growing in north-south rows and were trained to a conventional central leader form (Tehrani et al., 1988). Standard commercial practices for fertilization and pest control were followed (Ontario Ministry of Agriculture and Food, 1990). BA effects, Expt. 1. In 1990 a block of 68 lo-year-old ‘Empire’/ M.26 trees spaced at 3.7 × 5.5 m was used. The trees were of similar size and bloom density, and the single-tree plots were distributed in a completely randomized design. Full bloom occurred on 9 May. On 31 May (mean fruit diameter 10.5 ± 1.4 mm), 20 or 30 trees were each sprayed to the drip point with 50 or 100 mg·liter BA (ABG-3062, Abbott Laboratories, North Chicago, Ill.), respectively. This formulation contains no gibberellic acid. The remaining 18 trees were left unsprayed to serve as control. More trees were used in the two chemically thinned groups than in the control group to avoid excess fruit load reduction from repeated removal of fruit samples for microscopic tissue analyses. At final harvest all fruit from each tree were counted and weighed to determine total yield per tree. The number of normally developed seeds per fruit was counted in a 30-fruit sample from each treatment. In Spring 1991, one representative limb per tree was selected, the circumference measured and all bloom clusters counted (Forshey and Elfving, 1979a, 1979b; Lombard et al., 1988). Chemical thinner effects, Expt. 2. In 1991 a block of lo-yearold ‘Empire’/M.7 trees spaced at 4.9 × 6.7 m was used. Trees were grouped into ten randomized complete-blocks of five trees according to tree size, bloom density and location. Before bloom, one representative limb 12 to 15 cm in circumference was tagged on each tree for use in bloom and fruit set counts (Forshey and Elfving, 1979a, 1979b). Full bloom occurred 13 May. On 27 May (mean J. AMER. SOC. HORT. SCI. 120(5):802-807. 1995. fruit diameter 13.1 ± 1.4 mm), one tree in each block was sprayed with either BA 100 mg·liter (ABG-3062), NAA 15 mg·liter, CB 1000 mg·liter or DM 1500 mgliter’. The fifth tree in each block was not sprayed and served as the control. Blossom clusters were counted on each tagged limb before full bloom and the number of fruit counted after June drop. At final harvest the total number and weight of fruit on each tree were recorded. Twenty fruit per tree were sampled for length, diameter, fresh and dry weight measurements. Fruit length and diameter were measured by laying the fruit end-to-end and side-toside in a V-shaped trough and measuring total length and diameter. After removing the pedicel, each fruit was weighed fresh (FW) and then cut into slices and oven-dried to a constant dry weight (DW) at 80C. Ten of these same fruit were cut and the numbers of aborted and apparently viable seeds were determined. For the remaining 10 fruit, flesh firmness was determined with a penetrometer (Effegi, Alfonsine, Italy; tip diameter 11.1 mm) on two sides of each fruit. Juice was collected during the firmness test and soluble solids content (SSC) was determined on a composite sample using a hand refractometer (Fisher Scientific, Unionville, Ont.). Return bloom counts were taken on the tagged limbs in spring 1992. Cell division and diameter, Expt. 1. To examine the number of cell layers and cell diameter in the fruit cortex, five well exposed fruit from 1 to 2 m height in the canopy were harvested randomly from each treatment on each sample date and preserved and stored in formalin-acetic acid-alcohol (FAA) [per 100 ml: 50 ml ethanol (95%), 5 ml glacial acetic acid, 10 ml formalin (37% formaldehyde), 35 ml distilled water] (O’Brien and McCully, 1981). Small fruits were preserved whole, while larger fruits were cut into pieces to allow for rapid fixation of the inner tissue. The samples were preserved so fruit cortex tissue could be examined at the cellular level at a later date following embedding, sectioning and staining of the tissue. For paraffin embedding and microtome sectioning, whole apples or apple pieces were removed from the FAA and rinsed in running water for 5 min. Equatorial transverse sections 3 to 4 mm thick were cut from each apple. For small apples (<2.5 cm in diameter) the whole disc was used, while for larger fruit, 2 quarters of the disc from opposite sides of the fruit were used. Discs were held in 70% ethanol until the paraffin embedding procedure began. The tissue discs were dehydrated in a graded alcohol series and cleared with xylene, embedded in paraffin and sectioned at 5 to 10 μm on a rotary microtome (Leica 1512, Willowdale, Ont.). Sections were stained with the periodic acid/Schiff’s reaction (O’Brien and McCully, 1981) and permanent mounts made. Using a light microscope (Zeiss Photomicroscope III, Oberkschen, W. Germany, magnification 63 to 400×), the number of cell layers in the cortex was determined using methodology similar to Denne (1960). For each apple the number of cells in a radial line from a sepal vascular strand and from a neighboring petal vascular strand to the epidermis was counted. Four counts were made per fruit, two from each side of the fruit. Cortex width was determined by measuring the distance from the same vascular strands to the epidermis. The mean radial cell diameter was calculated from width of the cortex divided by mean number of cells across the cortex. In 1991, fruit sampling was similar to that of 1990. Fruit sampling began at full bloom and at each sample date six fruit per treatment were harvested randomly. For fruit sampled up to 35 days after full bloom (DAFB), the paraffin embedding procedure and cellular analysis were the same as used in 1990. For fruit sampled 35 DAFB and later, sections were prepared by hand. Hand sectioning. Frui

Benzyladenine Affects Cell Division and Cell Size during Apple Fruit Thinning