Management of N in macadamia orchards is of primary concern for growers. Plant N status is a factor that affects plant vigor, yield, and quality of the macadamia kernel. Accumulation of N occurs in macadamia leaves during the fall and winter periods (Stephenson and Cull, 1986). Leaf N declines in the spring during active growth and flowering and is lowest during late summer. Analysis of leaf tissue for nutrient concentration is the accepted method to assess the nutritional status of macadamia crops (Cooil et al., 1953; Guest, 1953; Hirae, 1976; Wallace, 1971). Optimal leaf tissue N recommendations range from 1.45% to 2% (Bittenbender and Hirae, 1990) in Hawai’i and from 1.4% to 1.5% in Australia (Stephenson and Cull, 1986). Nagao and Hirae (1992) reported a reduction in macadamia tree growth when N leaf concentrations were less than 1.22% and is implicated in yield variability (Stephenson and Cull, 1986). The current practice of collecting leaf samples is time-consuming and expensive for growers to adopt. The current recommended selection criteria are that: 1) the terminal bud of the branch is not flushing, 2) the branch tissue is woody directly up to the terminal bud to ensure uniform maturity, and 3) the apical meristem bud is not swelling (Hirae, 1976). Selected leaves are exposed to full sun, mature, undamaged, and on the second node from the terminal bud (Hirae, 1976). Leaf selection criteria for tissue analysis are subjective and timing the collection based on plant ontogeny could make collection even more complicated (Nagao and Hirae, 1992; Stephenson et al., 1986). Leaf N concentration fluctuates depending on the maturity of the leaf, where N is high in recently matured leaves and is readily mobilized from the oldest leaves during vegetative flushes (Nagao and Hirae, 1992). There are few diagnostic laboratories available to growers in Hawai’i, and these are often several hours away, require air freight, or have limited availability, which could lead to less-accurate results. As a result of the aforementioned reasons, only a small subset of trees is selected for analysis, which can lead to inaccurate recommendations for the rest of the orchard. Having an affordable and rapid method to assess N levels on-site would be a great benefit to macadamia growers.
Chlorophyll and other components related to chlorophyll account for ≈75% of leaf N (Loomis, 1997), making chlorophyll a fairly accurate indicator for N status in leaves. The SPAD-502 Chlorophyll Meter (Konica Minolta, Osaka, Japan) is a portable, handheld device developed to quickly and nondestructively measure foliar N. Chlorophyll meters measure relative chlorophyll concentration. The SPAD-502 emits two wavelength regions of light that peak at 650 and 940 nm through the leaf (Minolta Co., 1989). Chlorophyll absorbs light at 650 nm and reflects light at 940 nm. The amount of each wavelength that is transmitted through the leaf is collected by a photodiode in the meter and converted to volts and subsequently valued. These values are used in an equation to form an empirical relationship by the meter’s microprocessor. The reading output (SPAD) is in an arbitrary unit between 0 and 100 that is proportional to the amount of chlorophyll.
The chlorophyll meter originally was developed for measuring foliar N in rice [Oryza sativa (Chubachi et al., 1986)] and has been positively correlated with destructive chlorophyll measurements in rice, wheat (Triticum aestivum), soybean (Glycine max), and corn (Zea mays) (Ahmad et al., 2012; Monje and Bugbee, 1992). The meter’s use has since been expanded to include research in hardwood and fruit trees (Chang and Robinson, 2003; Hardin et al., 2012; Neilsen et al., 1995; Netto et al., 2005). A chlorophyll meter could be useful for assessing tissue N levels in macadamia orchards, but there has been no reported data correlating SPAD values and leaf tissue N for macadamia. The objective of this study was to evaluate the relationship between SPAD values obtained on site to corresponding N tissue analysis.
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