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Australian Journal of Experimental Agriculture Volume 40, 2000 © CSIRO Australia 2000
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Australian Journal of Experimental Agriculture, 2000, 40, 859–866
Relationships between kernel oil content, fruit removal force and abscission in macadamia S. J. TruemanA, S. RichardsAB, C. A. McConchieB and C. G. N. TurnbullA ADepartment
of Botany, The University of Queensland, Qld 4072, Australia; e-mail:
[email protected] BCSIRO Plant Industry, Horticulture Unit, Long Pocket Laboratories, Private Bag 3, Indooroopilly, Qld 4068, Australia.
Abstract. Delayed or extended fruit abscission in many cultivars of macadamia, Macadamia integrifolia (Proteaceae), necessitates multiple harvests from the orchard floor. However, it is not known whether there is a link between fruit maturation and timing of abscission. In this study, relationships between kernel oil content, nut size, removal force and abscission were investigated in 2 commercial orchards, one in Queensland (24°S) and one in New South Wales (29°S). Abscission lasted 6 months (March–September) at both sites due to differences in timing of abscission between cultivars and extended abscission within cultivars. Abscission was consistently associated with declines in fruit removal force, from more than 2 kgf in early March towards 1 kgf at the peak of abscission. Later abscission in cvv. A16 and 246, compared with cvv. 344 and 741, was correlated with more gradual declines in removal force. Fruits containing small nuts and kernels tended to possess lower removal forces and these fruits were often heavily represented in the population of fruits that abscised earliest. There was little or no difference in kernel oil content between fruits of different removal force or between sample dates for any cultivar, either for samples taken from the tree or from the orchard floor. Attainment of maximal oil content and onset of fruit abscission are therefore independent processes in macadamia. If abscission can be accelerated, it may be possible to advance the harvest of late-abscising cultivars such as A16 and 246 which retain mature nuts on the tree up to several months after maximal oil accumulation.
Introduction Fruits of macadamia, Macadamia integrifolia Maiden & Betche (Proteaceae), are usually harvested from the orchard floor by mechanical sweepers following natural fruit abscission. Despite the relatively short and synchronised flowering period of many cultivars in Australian orchards, abscission can occur over several months within a single cultivar and varies greatly in timing between cultivars. Harvest intervals of 4 weeks or shorter are recommended, particularly during wet weather, to avoid kernel deterioration due to germination, fungal growth or exposure to sunlight (Mason and Wells 1984; Nagao and Hirae 1992; Liang et al. 1996). Most macadamia plantings comprise more than 1 cultivar, and harvesting in many orchards therefore extends over several months and usually involves multiple passes over the same rows. Promotion of abscission using ethephon sprays (Kadman and Ben-Tal 1983; Gallagher and Stephenson © CSIRO 2000
1985; Trochoulias 1986; Nagao and Sakai 1988; Richardson and Dawson 1993) or mechanical tree shaking (Gillespie et al. 1975; Nagao and Hirae 1992) has been investigated as a means of reducing the duration and number of harvests in macadamia orchards. Advantages of minimising the number of harvests include lower management costs and reduced soil disturbance. The potential commercial benefit from accelerating abscission will depend on the kernel quality of the harvested fruits. However, the relationship between maturity and fruit abscission is poorly understood in macadamia. Harvested macadamia fruits are dehusked and the resultant nuts-in-shell (NIS) are dried and then cracked to release the kernels. Growers are paid a premium for the percentage of Grade 1 kernels (specific gravity c.72%), as well as for kernel recovery as a percentage of NIS. Oil accumulation occurs during the second half of the fruit development period, but several reports indicate that full oil accumulation occurs well 10.1071/EA00004
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before fruit abscission both in Hawaii (Jones 1937, 1939; Liang and Myers 1975) and in Australia (Baigent 1983; McConchie et al. 1996). The aim of this study was to determine the relationships between attainment of maximal kernel oil content, decline in fruit removal force, and timing of fruit abscission in macadamia cultivars. Materials and methods Experiments were conducted on mature, bearing trees in commercial macadamia orchards at Winfield, Queensland (24°32′S, 152°01′E) and Victoria Park, New South Wales (28°74′S, 153°30′E). Two cultivars with mid-season abscission, ‘HAES 344’ (344) and ‘HAES 741’ (741), were used at both sites. Two cultivars with lateseason abscission were also used: ‘Hidden Valley A16’ (A16) at Winfield, and ‘HAES 246’ (246) at Victoria Park. Ten trees per cultivar were selected at each site in early March 1998, before fruit abscission had commenced. The ground under each tree was raked to remove any fruits remaining from previous seasons or currentseason fruits that had fallen prematurely due to pests or disease. Five of the 10 trees per cultivar were used to monitor fruit abscission and changes in fruit removal force and nut quality. Ten racemes were selected and tagged on each of these trees, and the number of fruits remaining was monitored through to October 1998. Ten freshly abscised fruits, when available, were sampled from underneath the same trees on the counting dates. The fruits sampled underneath trees in early March were the first fruits to abscise. The removal forces of 10 fruits per tree, from untagged racemes, were also determined on the same days using a linear force meter (Shimpo Instruments, Lincolnwood, Illinois) with attached aluminium collar, and the fruits were retained. The other 5 trees per cultivar were used to test for nut size and quality differences between fruits of different removal force. Removal forces were determined for fruits on these trees on several of the counting dates and, for each tree, the fruits were pooled into the following removal force classes: 2 kgf. From each tree, 10 nuts from each removal force class were later selected randomly for size and quality determinations. Fruits from all experiments were dehusked, and the NIS were dried in fan-forced laboratory ovens at 45°C for 6 days. Nuts were then cracked, individual NIS and kernel weights were determined, and kernel recovery (the ratio of kernel to NIS) was calculated. The oil content of each kernel was determined from its specific gravity measured using a pan immersed in a 95% (v/v) ethanol solution (McConchie et al. 1996), with the formulae: Oil content (%) = 284.7 – 212.57 × specific gravity, where: Specific gravity = (0.7995 × weight in air)/ (weight in air – weight in 95% ethanol). Analyses of variance were conducted on the data sets for NIS and kernel weight, kernel recovery and oil content, with trees regarded as blocks in a randomised block design. Means are reported with standard errors.
Results Fruit abscission commenced in March for cvv. 344 and 741 at Winfield (Fig. 1), and most of the crop had fallen by the 13 May harvest (5.8 ± 3.5% and 2.6 ± 2.2% of fruits remaining on the trees, respectively). Little abscission had
occurred by this date on cv. A16, with 94.6 ± 1.9% of the fruits remaining. Abscission of cv. A16 fruits continued through to September, with 3.4 ± 1.0% of fruits remaining on the trees at the 11 September harvest. For cvv. 344 and 741 at Victoria Park, abscission occurred about 1 month later than at Winfield, with 9.0 ± 7.5% and 4.1 ± 2.7%, respectively, of the fruits remaining on the tree at the 24 June harvest. Fruit abscission for cv. 246 was more protracted, such that 10.8 ± 8.6% of the fruits remained at the 18 August harvest (cf. 3.0 ± 3.0% and 0 fruits for cvv. 344 and 741, respectively). Fruit removal forces at Winfield were high (>2.5 kgf) for all 3 cultivars on 5 March but declined to about 1 kgf by 15 April for the 2 cultivars with the earlier fruit abscission (344 and 741) (Fig. 2). A similar decline was not observed in cv. A16 until the 2 June harvest. At Victoria Park, large declines in removal forces were evident between the 9 March and 28 April harvests for cvv. 344 and 741, but were not observed until the 29 May harvest for the cultivar with the later abscission (246). Sampling from under trees of cvv. 344 and A16 indicated that fruits with low NIS or kernel weight or low kernel recovery may represent a large component of the first fruit abscission (Figs 3a, b, c and 4a, b, c). Size or kernel recovery differences were not detected between sample dates for freshly abscised fruits of cvv. 246 or 741. Sampling of fruits remaining on the trees also suggested that nuts of low NIS or kernel weight or low kernel recovery may be the first to abscise (Figs 3e, f, g and 4e, f, g). This was evident on cv. 344 where increased NIS weight was observed on the second sample date at Winfield (25 March) and higher kernel recovery was evident in the last sample at Victoria Park (29 May). Likewise, for cv. 741 at Victoria Park, fruits of higher NIS and kernel weight and higher kernel recovery remained on the trees beyond the first sample date (i.e. beyond 9 March). There were also some differences between individual sample dates for cvv. A16 and 246, but overall there was no distinction between pre-abscission samples and the later samples. Maximal oil accumulation had occurred by the first sampling dates (5 March at Winfield and 9 March at Victoria Park), with no increase in oil content detected in subsequent samples either from the ground (Figs 3d and 4d) or from the tree (Figs 3h and 4h). There was a small decline in oil content in the 15 April cv. 741 sample from the ground at Winfield, and also for some samples of fruits remaining on the tree (cvv. 741 and A16 at Winfield and cv. 246 at Victoria Park). Sample means from all cultivars at both sites, both from the ground and from the tree, greatly exceeded the oil content required for Grade 1
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kernels (i.e. 72%). The percentage of Grade 1 kernels was 100% for most samples, including all 6 on-tree samples in early March, and exceeded 94% in every case (data not shown). Fruits with low removal force generally contained nuts with lower NIS weight (10 cases out of 14) (Table 1) and kernel weight (9 cases out of 14) (Table 2). Significant fruit abscission had already occurred on 3 of the occasions when no differences in kernel weight were detected (cv. 741, 25 March at Winfield, and cv. 246, 29 May and 25 June at Victoria Park) (Fig. 1). Abscission had just
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commenced on the other 2 occasions (cv. 741, 2 April and cv. 246, 29 April, both at Victoria Park). Neither kernel recovery nor kernel oil content was related to fruit removal force, with generally less than 1% differences among the 3 removal force classes (data not shown). Overall the sample means were similar to those presented in Figures 3 and 4. Discussion Fruit abscission both at Winfield and Victoria Park lasted about 6 months. The prolonged harvest period
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Figure 1. Percentage (mean ± s.e.) of macadamia fruits [cvv. A16 (), 246 (), 344 () and 741 ()] remaining on the trees from March to October 1998 at Winfield, Queensland, and Victoria Park, New South Wales (n = 5 trees).
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resulted from strong cultivar differences in timing of abscission and from very prolonged abscission in some cultivars. For example, harvesting of cvv. 344 and 741 could be completed by the end of May at Winfield, at which time abscission of cv. A16 was just commencing. Abscission of cv. A16 fruits continued for several months, requiring continued harvesting until September. Abscission commenced a little later at Victoria Park, possibly the result of lower fruit growth rates in the cooler conditions of New South Wales (Stephenson and Trochoulias 1994). Harvesting was again required until September, in this case
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due to delayed and extended abscission in cv. 246, as well as to extended abscission in cv. 344. Flowering in Australian orchards is strongly seasonal, usually occurring during a brief period in August or September. Differences in flowering time between cultivars are typically no greater than several weeks, and differences between trees within a cultivar may be only a few days (Moncur et al. 1985; Stephenson and Trochoulias 1994; Meyers et al. 1995; Gallagher 1996). Such small differences in flowering time were evident at both study sites and cannot account for the very prolonged
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Figure 3. Nut-in-shell weights, kernel weights, kernel recoveries and kernel oil contents of macadamia fruits [cvv. A16 (), 344 () and 741 ()] sampled from the ground (a–d) or from the tree (e–h) from March to June 1998 at Winfield, Queensland. Significant differences between means (± s.e.) for a cultivar are indicated by different letters (A16: a, b; 344: c, d, e; 741: f, g) (P < 0.05, n = 5 trees).
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period of fruit abscission. However in other production areas such as Hawaii, flowering may occur over several months, further prolonging the fruit abscission and harvest period (Nagao and Sakai 1990; Nagao and Hirae 1992; Nagao et al. 1994). Abscission was associated with declines in fruit removal force for all cultivars, from more than 2 kgf initially towards 1 kgf at the peak of fruit abscission. The later start of fruit abscission in cvv. A16 and 246 correlated with more gradual declines in removal force in these cultivars. Declines in removal force have also been
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reported before fruit abscission for cvv. 333 and 344 in Hawaii (Sakai and Nagao 1985) although the precise timing of fruit abscission was not defined in that study. Nut weights, kernel weights and kernel recoveries differed amongst cultivars, but these patterns were essentially consistent with the typical commercial characteristics of each cultivar (Gallagher et al. 1999). One exception was cv. 741 having much smaller kernels, kernel recoveries and oil contents at Victoria Park (e.g. 1.5–1.8 g kernels) than at Winfield (e.g. 2.6–2.7 g kernels). Kernel weight, kernel recovery and oil content
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Figure 4. Nut-in-shell weights, kernel weights, kernel recoveries and kernel oil contents of macadamia fruits [cvv. 246 (), 344 () and 741 ()] sampled from the ground (a–d) or from the tree (e–h) from March to June 1998 at Victoria Park, New South Wales. Significant differences between means (± s.e.) for a cultivar are indicated by different letters (246: a, b; 344: c, d; 741: f, g) (P
