Effects of CPPU (N (2 chloro 4 pyridyl) N' phenylurea) on fruit growth, maturity, and storage quality of kiwifruit

New Zealand Journal of Crop and Horticultural Science ISSN: 0114-0671 (Print) 1175-8783 (Online) Journal homepage: https://www.tandfonline.com/loi/tnzc20 Effects of CPPU (N (2 chloro 4 pyridyl) N' phenylurea) on fruit growth, maturity, and storage quality of kiwifruit K. J. Patterson, K. A. Mason & K. S. Gould To cite this article: K. J. Patterson, K. A. Mason & K. S. Gould (1993) Effects of CPPU (N (2 chloro 4 pyridyl) N' phenylurea) on fruit growth, maturity, and storage quality of kiwifruit, New Zealand Journal of Crop and Horticultural Science, 21:3, 253-261, DOI: 10.1080/01140671.1993.9513777 To link to this article: https://doi.org/10.1080/01140671.1993.9513777 Published online: 22 Mar 2010. Submit your article to this journal Article views: 1087 Citing articles: 24 View citing articles Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalinformation?journalcode=tnzc20

New Zealand Journal of Crop and Horticultural Science, 1993, Vol. 21: 253-261 0114-0671/93/2103-0253 $2.50/0 The Royal Society of New Zealand 1993 253 Effects of CPPU (N-(2-chloro-4-pyridyl)-N'-phenylurea) on fruit growth, maturity, and storage quality of kiwifruit K. J. PATTERSON K. A. MASON The Horticulture and Food Research Institute of New Zealand Private Bag 92 169 Auckland, New Zealand K. S. GOULD Plant Science Group School of Biological Sciences University of Auckland Private Bag 92 019 Auckland, New Zealand Abstract Effects of the synthetic cytokinin CPPU (N-(2-chloro-4-pyridyl)-N'-phenylurea; common name forchlorfenuron) on kiwifruit (Actinidia deliciosa) cv. Hayward fruit growth, maturity, and storage quality were examined. CPPU was applied to fruitlets 21 days after flowering, either as a 5 mg/ litre dip or as a 5 mg/litre spray. Dipping increased mean fresh weight of fruit at harvest by 43% and spraying by 33%. Most of the increase in fruit size was because of an increase in the volume of "small" isodiametric parenchyma cells in the outer pericarp. The volume of "large" ovoid parenchyma cells in the outer pericarp was not affected by CPPU treatment. CPPU application increased the rate of accumulation of soluble solids in fruit prior to harvest but did not increase the proportion of total dry matter. CPPU also caused increased growth of the outer pericarp at the stylar end of the fruit, resulting in small protuberances. Morphological attributes of fruit including the ratio of length to diameter, and flatness, were unaffected by CPPU treatment. Dipped fruit contained less tannin in the skin at harvest and appeared greener than untreated fruit at harvest and H93012 Received 15 March 1993: accepted 20 July 1993 after storage. Fruit firmness was similar in treated and untreated fruit at harvest, and their rates of softening in storage at 0 C were similar. Keywords kiwifruit; 'Hayward'; Actinidia deliciosa; CPPU; forchlorfenuron; growth regulators; cytokinin; fruit growth; fruit quality; fruit colour INTRODUCTION In New Zealand, management of kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson)) vines i s aimed at producing maxi mum-si zed fruit for the export market. Several key factors including adequate pollination, a balanced leaf-fruit ratio, and optimal nutrition all contribute to a large fruit size. Growth regulators al so offer the potential to increase fruit size by increasing the sink strength of the fruit. Hopping (1976b) demonstrated that applications of cytokinin and auxin were effective in promoting normal development of poorly pollinated fruit. More recently, Iwahori et al. (1988) showed that the synthetic cytokinin CPPU (N-(2-chloro-4-pyridyl)- N'-phenylurea; common name forchlorfenuron) could markedly increase fruit size of kiwifruit. A 70% increase in fruit size was reported by these workers but the mode of action of CPPU was not investigated. CPPU is now used by kiwifruit growers in Japan to improve fruit size. Individual fruitlets are dipped by hand in CPPU c. 20 days after flowering. This is a labour-intensive process and would not be feasible in New Zealand kiwifruit orchards. A spray application technique, however, could be more suitable. In light of the promising responses of kiwifruit to CPPU reported in Japan we examined the effects of CPPU on fruit growth, fruit maturity, and fruit quality. We were especially interested in the mode of action of CPPU on fruit growth and the relative merits of the dip technique compared with a spray method of application. Fruit morphology and anatomy were examined to establish whether CPPU

254 New Zealand Journal of Crop and Horticultural Science, 1993, Vol. 21 increases fruit size by enhancing cell division, cell expansion or both processes. MATERIALS AND METHODS Plant material A field trial was carried out on a commercial orchard near Tauranga in the Bay of Plenty, New Zealand. Fifteen 7-year-old 'Hayward' vines trained onto a T- bar trellis system were selected from the centre row of a typical block. Vines were planted at 5.8 m within-row spacing, with 5.2 m between rows. Six fruiting canes were randomly selected on each of the 15 vines. On 11 December 1989, 21 days after the mid-point of flowering, all of the fruitlets on 30 of the selected canes were sprayed to run-off with a solution containing 5 mg/litre CPPU (5 ppm) and 0.25 ml/litre "Agral" wetter. A 1.5 litre hand-held "Cambrian" sprayer was used to apply the solution. All the fruitlets on another 30 canes were dipped for c. 5 sec in the same solution and the fruitlets on the remaining 30 canes were left as untreated controls. Treatments were randomly assigned to canes on each vine. Fruit assessments One average-sized fruitlet per cane was tagged at the time of CPPU application and the growth of each fruit population was monitored with hand-held electronic calipers (Sylvac Instruments) at 2-3- weekly intervals until harvest on 17 May 1990. Fruit fresh weight (FW) was estimated from the relationship: FW = 0.454 x ( L x D max x D min /1000) 105 where L = fruit length (mm), D max = maximum diameter, D m j n = minimum diameter (Snelgar et al. 1991). Samples of 15 fruit were collected from each treatment 9 and 5 weeks before harvest and at harvest on 17 May. Measurements were made of soluble solids concentrations (SSC), ratio of length to mean diameter, and ratio of maximum diameter to minimum diameter. Fruit dry matter (DM) was assessed by measuring the moisture loss from a transverse slice (4-5 mm thick) taken from each fruit. On 17 May all remaining fruit were harvested and individually weighed. Fruit quality during storage To assess postharvest fruit quality, subsamples of 125 fruit per treatment were packed into single layer kiwifruit trays (25/tray) and stored for 20 weeks at 0 C. Fruit firmness (Bryce penetrometer, 7 mm diam. head), SSC (Atago refractometer, 0-20% range), and fruit pericarp colour (Minolta Chromameter, L* a* b* system) were measured at 2-5-week intervals during the storage period. The pericarp was prepared for the Chromameter by removing a 1 mm thick slice from one of the "flat" and one of the "round" sides of each fruit and then immediately measuring the exposed surfaces on the fruit. Colour was expressed as a "brightness" value (L*) and as a "hue" value, calculated from the equation: Hue = tan^bva*) (Francis 1975; Little 1975). Fruit structure In order to determine which fruit tissue was most affected by CPPU, 25 fruit from each of the three treatments (at harvest) were bisected transversely. The freshly-exposed face of one of the halves of each fruit was photocopied. Cross-sectional areas of the pericarp (exocarp plus mesocarp) and core region were determined from each photocopy by tracing their outlines over a digitising tablet (Summagraphics, Fairfield, Connecticut, United States). Differences in cross-sectional areas among treatments were evaluated by a one-way analysis of variance. We assumed that the cross-sectional areas of the core and pericarp bear a relationship to the volumes of these tissues. Tissue sections Small blocks of tissue containing exocarp and outer layers of the mesocarp were removed from the equatorial region of three similar sized fruit from each treatment. They were fixed overnight under vacuum in a 2% paraformaldehyde and 2.5% glutaraldehyde mixture, and buffered with 0.025M phosphate at ph 6.9 (Karnovsky 1965). Specimens were dehydrated through a methyl cellusolve series (Feder & O'Brien 1968), infiltrated, and embedded in a glycol methacrylate resin (Technovit 7100, Kulzer and Co., Wehrheim, Germany), sectioned transversely with glass knives at 3 4 u r n, and stained with 1 % toluidine blue in 1 % sodium borate solution (Feder & O'Brien 1968). Sections (two per fruit, selected at random) were photographed on Kodak Technical Pan black and white film (Eastman Kodak, Rochester, New York, United States). Prints were enlarged to give a 70- fold final magnification of each section. Crosssectional areas of cells in the 20 outermost layers of mesocarp were determined from the prints using a digitising tablet. L'ata from the two sections through

Patterson et al. Effects of CPPU on kiwifruit 255 160 120 60 '5 80 40 X^ F o A D 0 IDec Uan lfeblmar lapr IMay Uun I ^ l untreated dipped sprayed Fig 1 Effects of CPPU on the growth of 'Hayward' kiwifruit. Bars represent LSD values (P < 0.05). Table 1 Effects of CPPU on the relative proportions of pericarp and core tissue, and on the cross-sectional areas of small and large parenchyma cells in the outer pericarp of 'Hayward' kiwifruit (LSD value at P < 0.05. = not significant). Tissue areas (%) Treatment Pericarp Core Untreated Dipped Sprayed LSD 93.5 93.8 93.9 6.5 6.2 6.1 Small Large parenchyma parenchyma cell area cell area (um 2 ) (um 2 ) 15 030 19 540 18 390 1000 148 800 146 100 146 400 each fruit were pooled. At least 60 cells were measured for each fruit. CPPU residues To determine whether any CPPU remained on the skin of the treated fruit at harvest, a subsample of three fruit from each treatment was assayed for CPPU. Fruit in each sample were peeled thickly and the peel was homogenised with acetone. Homogenates were filtered, concentrated under vacuum, and the chlorophylls and lipids were removed by partitioning into hexane. The extracts were then analysed for CPPU by HPLC using a Partisil ODS-3 column with U V detection at 216 nm. Breakdown products of CPPU were not assayed. Sensory evaluation Members of a panel trained in the descriptive analysis of kiwifruit were asked to rate the overall acceptability of appearance of untreated and CPPU-sprayed fruit. Data was collected on a 150 mm line scale anchored at both ends and in the centre. The left anchor was labelled "dislike very much", the centre point "neither like or dislike", and the right anchor was "like very much". Nine panellists were used to score fruit from the two treatments. RESULTS Fruit growth CPPU significantly increased fruit growth (Fig. 1). By mid January (52 days after fruit set), control, dipped, and sprayed fruit had mean weights of 47, 63, and 62 g respectively. At harvest in mid May (178 days after fruit set) mean fruit weights were 110,158, and 146 g respectively. The dip treatment, therefore, increased the mean fruit size by 44% and Table 2 Effects of CPPU on the relative increases in fruit volume and small parenchyma cell volume in the outer pericarp. Treatment Untreated Dipped Sprayed Mean fruit volume a at harvest (ml) 114 164 152 a Fruit density was 1.04 g/ml. Increase in fruit volume (%) 44 33 Estimated parenchyma cell volume (u.m 3 ) 1385 x10 3 2054 x 10 3 1875 x10 3 Increase in small parenchyma cell volume (%) - 48 75

256 New Zealand Journal of Crop and Horticultural Science, 1993, Vol. 21 Fig. 2 Light micrographs of transverse sections through the outer pericarp of: A, untreated; and B, CPPU-dipped kiwifruit. Small parenchyma (SP) cells were larger after CPPU treatment, whereas large parenchyma (LP) cells were unaffected (Bars represent 0.2 mm). N 30 25 ^ * «3 xi 20 <u.s 15-4» I in <«-< 10 o 5 o c i V h \ untreated - dipped sprayed u / / ;\ >. \ -. \ \\ A\ \ V V \\ 50 100 150 200 Fruit weight (g) 250 Fig. 3 Effects of CPPU application on the distribution of fruit fresh weights at harvest. Vertical lines indicate the limits of fruit weights normally packed according to the New Zealand Kiwifruit Marketing Board grade standards. Fruit above 160 g are packed into the special classes "Large Jumbo" and "Extra Large Jumbo". the spray treatment by 33%. During the period of fastest growth, from mid December until mid January, growth rates of control, dipped, and sprayed fruit were 1.5, 2.2, and 2.1 g per day respectively. Fruit structure and cell volumes Core and pericarp areas were measured in samples of similar-sized fruit from each treatment. The pericarp accounted for the majority of the fruit cross-sectional area in each sample (Table 1 ). CPPU did not affect the relative proportions of pericarp/ core tissues. Thus there was no evidence of a marked differential effect of CPPU on either core or pericarp tissue. As the pericarp accounted for the majority of the fruit volume (c. 94%), we focused only on the dimensions of the cells making up this tissue. The outer pericarp was composed of small, isodiametric parenchyma cells containing abundant starch, and large ovoid parenchyma cells containing very few starch grains (Fig. 2). The mean cross-sectional area of the small parenchyma cells was markedly increased by CPPU application, by 30 and 22% in dipped and sprayed fruit respectively (Table 1 ). The mean cross-sectional area of the large parenchyma

Patterson et al. Effects of CPPU on kiwifruit 257 Fig. 4 Changes in soluble solids of CPPU-treated kiwifruit preharvest and during storage for 20 weeks at 0 C. Bars represent LSD values (P < 0.05). g a o S a <u Ö o 14 12 10 8 Fruit on vine w SJSÖ i S / / Fruit in store CO 0 4. ta O untreated A dipped O sprayed 10 5 Harvest 5 10 15 20 Weeks before and after harvest cells, however, was not affected by CPPU. Parenchyma cell volume was estimated from the formula: vol = 1.337tr 3 (assuming that the shape was approximately spherical). The proportionate increases in fruit volume caused by CPPU treatment were in close agreement with the estimated increases in volume of the small parenchyma cells (Table 2). Fruit number and size distribution CPPU, applied either as a spray or dip, had no effect on the number of fruit remaining on the canes at harvest (i.e., there was no premature fruit drop). Fruit size (Fig. 3) was shifted towards the larger size classes and 64% of the fruit from the dip treatment fell into the 25 count (143-160 g), 22 count "large jumbo" (160-185 g), and 18 count "extra large jumbo" (>185 g) size grades (NZKMB 1991). Fruit weight was more variable after the dip and the spray treatments than in untreated fruit. (Standard deviations were 28.1, 24.3, and 16.2 for dipped, sprayed, and untreated fruit respectively.) Fruit shape CPPU caused slight changes to fruit shape. The most obvious effect of the dip or spray treatment occurred at the stylar end of the fruit where there was a slight protrusion around the base of the styles. Instead of the characteristic "flat" stylar end, treated fruit had small raised ridges c. 1-3 mm higher than normal. When sprayed and untreated fruit of similar size were assessed for external visual appearance by trained sensory evaluation panellists, no significant difference in acceptability of appearance was found (scores were: untreated mean = 92.4, standard error (SE) = 5.1, n = 19; CPPU-sprayed mean = 95.4, SE = 4.5, n = 20). The ridges at the stylar ends of the fruit were not commented on by the panellists. The "ribbing" noted by Lawes & Woolley (1990) was not apparent on either dipped or sprayed fruit in this trial. Fruit morphometrics, including the ratio of length to mean diameter and the ratio of minimum to maximum diameter (or "flatness") were not influenced by CPPU treatment (Table 3). Neither was pedicel length affected by CPPU (Table 3). Table 3 Effects of CPPU on fruit shape, pedicel (stalk) length, and dry matter (DM) content of ' Hay ward ' kiwifruit at harvest (LSD value at P < 0.05. = not significant). Untreated Dipped Sprayed LSD Length/ diameter ratio 1.27 1.24 1.25 "Flatness" (D min /D max ) 0.88 0.86 0.87 Pedicel (stalk) length (mm) 62.7 57.4 57.3 DM (%) 15.44 13.70 14.05 0.38

258 New Zealand Journal of Crop and Horticultural Science, 1993, Vol. 21 Où M, c«4) Ö 12 1 Fruit on vine I Fruit in store O A D untreated dipped sprayed Fig. 5 Changes in fruit firmness of CPPU-treated kiwifruit preharvest and during storage for 20 weeks at 0 C. Bars represent LSD values (P < 0.05). i-h S B 10 5 Harvest 5 10 15 Weeks before and after harvest 20 lighter -a g 60 55.«50 darker O untreated A dipped ü sprayed I I Harvest 5 10 15 Time in storage (weeks) Fig. 6 Changes in outerpericarp "brightness" of kiwifruit treated with CPPU and stored for 20 weeks at 0 C. Bars represent LSD values (P < 0.05). Fruit maturation and quality during storage Fruit maturation was advanced by CPPU. At 9 and 5 weeks before harvest, SSCs in treated and untreated fruit were similar. However, at harvest there was a significant difference in SSCs between treated and untreated fruit. The time required for treated fruit to I 20 reach commercial maturity (6.2% soluble solids concentration) was c. 1 week shorter than for the untreated fruit. After 20 weeks in store, SSCs in fruit from all treatments were not significantly different (Fig. 4). Fruit DM contents were assessed before harvest on 10 April. Untreated fruit had a significantly higher proportion of DM than either dipped or sprayed fruit (Table 3). Before harvest, dipped and sprayed fruit were significantly softer than untreated fruit (Fig. 5). However, during coolstorage this difference decreased, and from 10 to 20 weeks fruit from all treatments were not significantly different in firmness. Fruit colour was affected by CPPU treatment in two ways. At harvest and during storage at 0 C, the outer pericarp tissues of dipped and sprayed fruit were consistently darker (i.e., had lower L* values) than those in untreated fruit (Fig. 6). Colour readings (i.e., calculated hue angles) for the outer pericarp of CPPU-treated and untreated fruit were similar at harvest and after 20 weeks storage at 0 C (data not shown). During the sectioning of experimental fruit, we also observed that there was much less tannin deposited in the sub-epidermal layers of dipped fruit than in the untreated fruit (Fig. 7). CPPU residues at harvest Residual CPPU in the peel of dipped or sprayed fruit was below the level of detection of 20 (Xg/kg and was therefore considered to be negligible.

Patterson et al. Effects of CPPU on kiwifruit 259 Fig. 7 Light micrographs of transverse sections through the outer pericarp of: A, untreated; and B, CPPU-dipped kiwifruit showing the reduced tannin deposition (arrowed) in the fruit treated with CPPU. (Bars represent 50 um.) DISCUSSION Fruit growth Our results indicate that the synthetic cytokinin CPPU can significantly increase kiwifruit fruit size while maintaining optimal long-term storage qualities. The effects of CPPU on fruit growth are consistent with the results of Iwahori et al. (1988) who obtained a 71 % increase in fruit size when 40 mg/litre CPPU was applied 24 days after anthesis. The increased magnitude of the CPPU effect found by Iwahori et al. (1988) was most probably because of the higher concentration used. Although the dip treatment gave the greater increase in fruit size, the spray method of application was still very effective. This method could be more practical in the New Zealand orchard situation, particularly if CPPU could be applied using conventional spray equipment in combination with a standard post-flowering insecticidal spray. In our experiment we targeted the spray onto the fruit, not the leaves. It is not yet known if CPPU applied as an all-over vine spray will significantly affect leaf function, or, most importantly, if it will affect vegetative meristems. From our observations we conclude that the CPPU concentrations used had no detrimental effect on the appearance of the leaves when shoots near the fruit were also sprayed. The effects of CPPU on fruit size were evident soon after application. The time of application (21 days from the mid-point of flowering) was well within the 35-day cell division phase of 'Hayward' fruit growth reported by Hopping (1976a). Thus, although CPPU might have been expected to stimulate cell division, this did not appear to be the situation. Results indicate that CPPU stimulated cell expansion in the pericarp sufficiently to explain the measured increases in total fruit volumes. The increase in fruit weight that was stimulated by dipping fruit in CPPU, resulted from additional accumulation of both DM and water. The CPPUtreated fruit only had c. 2% more water than untreated fruit, whereas the difference in fruit weight was 44%. Thus fruit expansion in CPPU-treated fruit was not just the result of increased water uptake but also involved significant DM accumulation. The experiment was conducted on six individual canes per vine carrying moderate crop loads (c. 900 fruit/vine). If CPPU was applied to every fruit on more heavily loaded vines then a smaller increase in

260 New Zealand Journal of Crop and Horticultural Science, 1993, Vol. 21 fruit size might be expected. A 30% increase in fruit size on a vine carrying 1000 fruit (i.e., mean fruit weight increasing from 100 to 130 g) would require the vine to produce or reallocate an extra 4.5 kg of carbohydrate to the fruit sink (assuming a fruit DM content of 15%). Such an increase would be equivalent to an untreated vine with 1000 fruit supporting an additional 20 kg of fruit (c. 200 more fruit). Relationships between total fruit weight and fruit size in 'Hayward' are well understood and indicate that as total fruit weight per vine increases fruit size is reduced significantly. Thus CPPU influences on fruit size would be expected to depend on crop load. The source of the additional carbohydrate necessary for the growth of CPPU-treated fruit poses an interesting question. Where does it come from? In other crops such as apple, artificially raised sink demands can increase the tree's photosynthetic rate and carbohydrate production significantly (Avery et al. 1978). However, there is no published evidence that this is possible in 'Hayward' kiwifruit. An alternative source of carbohydrate could be from vine reserves and/or reallocation from competing sinks (e.g.,meristem growth). In girdled kiwifruit laterals, for example, reallocation of carbohydrate to the fruit sink can result in marked increases in fruit size (Snelgar et al. 1986). CPPU has also been shown to stimulate higher fruit yields in other fruit crops including apple (Nickell 1986; Greene 1989), grape (Nickell 1985, 1986), cranberry (Devlin & Koszanski 1988), and pear (Nickell 1986). The yield increases, however were not all the result of fruit size increases. In the instance of cranberry and grapes, CPPU also stimulated fruit set when applied at or just before flowering. Under normal conditions fruit set is very high and is not a problem with kiwifruit. However, final fruit size is related to seed number (Pyke & Alspach 1986) and fruit with low seed numbers do not achieve the minimum fruit size for export. Thus there may be scope to use CPPU as an aid to fruit growth where pollination is poor. Fruit maturity and storage quality CPPU advanced fruit maturity by c. 1 week as judged by the time take for fruit to reach 6.2% soluble solids. Although soluble solids concentrations in CPPUtreated fruit were significantly higher at harvest, this difference was not maintained during storage and by 20 weeks fruit from all treatments had similar concentrations of soluble solids. Iwahori et al. (1988) also noted that CPPU did not affect soluble solids concentrations in ripe kiwifruit. The differences in the firmness of fruit on the vine induced by CPPU were not maintained during storage. Although CPPU-treated fruit were softer than untreated fruit at harvest, CPPU had no major effect on the rate at which the fruit softened in storage, either in the early rapid phase of softening (0-10 weeks) or in the later slower phase (10-20 weeks). In contrast, Iwahori et al. (1988) found that when CPPU was applied to fruit 24 days after anthesis, the rate offruit softening after harvest was increased. In their study, CPPU-treated fruit were 0.9 kg softer after ripening than untreated fruit. The difference between the results of Iwahori et al. ( 1988) and those of this study might be explained in part by the different concentrations used. Other workers using CPPU concentrations in the 20-40 mg/litre range have also noted detrimental effects of CPPU on fruit firmness. (Biasi et al. 1991; Lotter 1991). It is critical that fruit firmness is assessed in any future work on the effects of CPPU on kiwifruit quality. In the present study, CPPU stimulated a darker green colour to develop in the outer pericarp of treated fruit just below the exocarp. This was probably due to increased chlorophyll production. This effect contrasts with the results of Kurosaki & Mochizuki (1990) who found that application of 10 mg/litre CPPU to developing 'Monty' kiwifruit significantly reduced the chlorophyll content, and by implication reduced the intensity of the green colour in the outer pericarp. These workers found that at harvest, CPPUtreated fruit had approximately half the chlorophyll concentration of untreated fruit. Increases in the intensity of green colour in the outer pericarp just below the exocarp were also found when 'Hayward' fruit were treated with CPPU (Lawes et al. 1991 ). An explanation for the contrasting effects CPPU on kiwifruit pericarp colour is that the cultivar 'Monty' behaves differently from 'Hayward'. Further studies are required to clarify this suggestion. CPPU effects on fruit colour have also been reported for other fruit. Devlin & Koszanski (1988) found a major reduction in anthocyanin and flavonol pigments in developing cranberry fruit treated with 10 mg/litre CPPU 5 days after full bloom. We also observed that there was a significant difference in tannin deposition in the sub-hypodermal layer of cells in CPPU-treated fruit. Tannins in the skin normal lymasktheunderlyinggreen chlorophyll colour and impart the characteristic "tan" colour to the fruit. A reduction in tannins would therefore impart a "greener" appearance to the skin of the fruit. This finding was consistent with observations made

Patterson et al. Effects of CPPU on kiwifruit 261 during grading and during sensory evaluation by trained pannellists that CPPU-treated fruit appeared to be greener than untreated fruit. Other workers (Biasi pers. comm.) have also observed that CPPU treatment imparts a greener appearance to the skin of freshly-harvested fruit. The result suggests that CPPU may have some influence on the visual sensory quality of kiwifruit. CONCLUSIO CPPU has great potential to increase the size of kiwifruit when applied as a low-concentration, postflowering spray. We have shown that CPPU applied to fruit as a 5 mg/litre spray does not impair fruit quality during long-term storage at 0 C, whereas other workers using higher concentrations have reported detrimental effects on fruit firmness. In the development of this chemical for use on kiwifruit, several practical and scientific questions should be addressed. In particular, we need to establish how increases in fruit size are affected by crop load, how the effects of CPPU are influenced by seed number, and how the vine provides the additional carbohydrate necessary for increased fruit growth. ACKNOWLEDGMENTS We thank Ms K. McMath and Ms J. Gilbert of the Sensory Evaluation Unit, HortResearch (formerly DSIR Fruit and Trees) for their assistance; and Mr G. Dyer for the sample of CPPU, the use of his orchard, and for his hospitality. REFERENCES Avery, D. J.; Priestley, C. A.; Treharne, K. J. 1978: Integration of assimilation and carbohydrate utilisation in apple. Pp. 221-231 in: Proceedings of a Conference on Photosynthesis and Plant Development, Belgium, 1978. Marcelle, R.; Clijisters, H.; Van Poucke, M. ed. Biasi, R.; Costa, G.; Giuliani, R.; Succi, F.; Sansavini, S. 1991 : Effects of CPPU on kiwifruit performance. Ada horticulturae 297: 367-374. Devlin, R. M.; Koszanski, Z. K. 1988: Effect of CPPU on yield and development of 'Early Black' cranberry. Proceedings of the Plant Growth Regulator Society of America 15: 136-140. Feder, N.; O'Brien, T. P. 1968: Plant microtechnique: some principles and new methods. American journal of botany 55: 123-142. Francis, F. J. 1975: The origin of tan -1 a/b. Journal of food science 40: 412. Greene, D. W. 1989: CPPU influences 'McIntosh' apple crop load and fruit characteristics. HortScience 24: 94-96. Hopping, M. E. 1976a: Structure and development of fruit and seeds in Chinese gooseberry (Actinidia chinensis Planch.). New Zealandjournal of botany 14: 63-68. Hopping, M. E. 1976b: Effect of exogenous auxins, gibberellins, and cytokinins on fruit development in Chinese gooseberry (Actinidia chinensis Planch.). New Zealand journal of botany 14: 69-75. Iwahori, S.; Tominaga, S.; Yamasaki, T. 1988: Stimulation of fruit growth of kiwifruit, Actinidia chinensis Planch., by N-(2-choro-4-pyridyl)-N'-phenylurea, a diphenylurea-derivative cytokinin. Scientia horticulturae 35: 109-115. Karnovsky, M. J. 1965: A formaldehyde-glutaraldehyde fixative of high osmolarity for use in electron microscopy. Journal of cell biology 27: 137A. Kurosaki, T.; Mochizuki, T. 1990: Effect of KT-30 treatment on fruit growth and some components of 'Monty' kiwifruit. Journal of the Japanese Society for Horticultural Science 59: 43-50. Lawes, S.; Woolley, D. J. 1990: Remarkable gains in fruit size achieved. New Zealand kiwifruit, February 1990: 26. Lawes, S.; Woolley, D. J.; Cruz-Castillo, J. G. 1991 : Field responses of kiwifruit to CPPU (cytokinin) application. Acta horticulturae 297: 35-356. Little, A. C. 1975: Off on a tangent. Journal of food science 40: 410-411. Lotter, J. de V. 1991 : A study of the preharvest ripening of 'Hayward' kiwifruit and how it is altered by N-(2- choro-4-pyridyl)-n'-phenylurea (CPPU). Acta horticulturae 297: 357-366. Nickell, L. G. 1985: New plant growth regulator increases grape size. Proceedings of the Plant Growth Regulator Society of America 12: 1-7. Nickell, L. G. 1986: Effects of N-(2-choro-4-pyridal)-N'- phenylurea on grapes and other crops. In: Proceedings of the Plant Growth Regulator Society of America 13: 236-241. NZKMB 1991: New Zealand Kiwifruit Marketing Board Grade Standards for 1991. Auckland, New Zealand Kiwifruit Marketing Board. Pyke, N. B. Alspach, P. 1986: Interelationships of fruit weight, seed number and seed weight in kiwifruit. New Zealand agricultural science 20: 153-156. Snelgar, W. P.; Manson, P. J.; Martin, P. J. 1992: Influence of time of shading on flowering and yield of kiwifruit. Journal of horticultural science 67(4): 481-487. Snelgar, W. P.; Thorp, T. G.; Patterson, K. J. 1986: Optimal leaf:fruit ratios for fruit growth in kiwifruit. Acta horticulturae 175: 115-120.