In mature macadamia orchards adjacent trees inter grow to create a continuous canopy. Orchards at this stage of development may display reduced yields. It is suspected that overcrowding which leads to increased competition for light may contribute to this decline. Trials have been conducted comparing nut yield in the upper and lower canopy of macadamia trees in orchards where every second tree had been removed with yields in an intact orchard in order to understand the effects of resource supply and allocation within trees. Leaf nutrient content was also measured at three levels in the canopy. Results are presented concerning the productivity of different positions within trees. The implications for the macadamia industry are discussed.
Mature macadamia orchards may begin to display declining yields as they approach 10-15 years in age. Yield reduction in macadamia has been attributed to a variety of causes including nutritional disorders, diseases and pests. More recently it has been suggested that it is caused by a partitioning imbalance in available resources resulting from overcrowding of orchards (Landsberg 1987). Pruning trees to allow more light into the canopy and thinning orchards by tree removal has been shown to increase yields, supporting this conclusion (McFadyen 1995). We present preliminary results from an experiment comparing yield within the upper and lower canopy in two cultivars in thinned and unthinned macadamia orchard blocks and describe nutrient profiles within the canopy.
Experiments were conducted in a mature orchard at Victoria Park in northern NSW using two macadamia varieties, cv. 660 and cv. 344 in September 1994. Six trees of cv. 660 and three trees of cv. 344 were selected within the unthinned orchard. Three trees of each variety were selected within a thinned section of the orchard. Planting distances within the unthinned orchard were 7 x 4 m and within the thinned part of the orchard were 7 x 8 m. The thinned orchard resulted from the removal of every second tree within a row with the removed trees offset in adjacent rows.
Four flowering branches were selected in the upper canopy (c. 5-7m above the ground) and in the lower canopy (c. 1.5 m above ground) within each tree. On each of these branches five to six racemes were selected and evenly thinned to 50 flowers. All other racemes were removed. At anthesis flowers were cross pollinated on at least two dates by sliding a tube coated with pollen of the compatible cv. 508 over each raceme. Leaf number on two of the four branches in each position was reduced to 400 and after pollination was complete these branches were girdled. Fruit set was monitored at monthly intervals from pollination until May 1995.
Approximately ten leaves were sampled from three positions on each of the study trees. Leaves were sampled from the upper canopy (7 m), the mid-canopy (3.5 m) and the canopy base (1.5 m). The mid-veins were removed and the area of the lamina measured. Leaves were then dried, weighed, ground and the amount of N, P, K and trace elements present measured. Only results for leaf nitrogen content will be presented here. Leaf nitrogen was measured by Kjeldahl digestion.
All statistical results presented in this paper were determined using analysis of variance.
| Significant factors affecting total yield | ||
| cultivar | P<0.01 | 660>344 |
| position | P<0.001 | top>bottom |
| girdling | P<0.05 | girdled>ungirdled |
| position x girdling | P<0.01 | top girdled>top ungirdled> bottom girdled = bottom ungirdled |
Cultivar 660 overall produced significantly more nuts on the study
branches compared with cv. 344. Within both cultivars more nuts
were produced in the upper canopy than in the lower canopy. Girdling
branches caused a significant increase in nut yield per branch
compared with ungirdled branches. There was a significant interaction
between the girdling treatment and position, with girdled branches
producing more nuts than ungirdled branches in the upper canopy
and both girdled and ungirdled branches in the upper canopy producing
more nuts than either treatment in the lower canopy. Nut retention
was greatest on girdled compared with ungirdled branches from
pollination until maturation and in the upper compared with the
lower canopy.
| Significant factors affecting nut dimensions | ||
| cultivar | P<0.001 | 344>660 |
| girdling | P<0.05 | ungirdled>girdled |
The cultivar had a significant effect upon nut dimensions with cv. 344 producing larger nuts than cv. 660. Although girdling increased nut number per branch it actually resulted in smaller nuts overall.
| Significant factors affecting nut dry weight | ||
| - nut in shell | ||
| cultivar | P<0.05 | 344>660 |
| position | P<0.05 | top>bottom |
| girdling | P<0.01 | ungirdled>girdled |
| - kernel | ||
| position | P<0.001 | top>bottom |
| girdling | P<0.001 | ungirdled>girdled |
As with nut dimensions, cultivar was a significant factor in nut in shell (NIS) dry weight with cv. 344 nuts weighing more than cv. 660. No significant effect of cultivar upon kernel dry weight occurred indicating that the difference in mass between cv. 344 and cv. 660 NIS dry weight may be accounted for by greater shell mass in cv. 344. Nuts harvested from the upper canopy possessed both greater NIS dry weight and greater kernel dry weight. Nuts produced on ungirdled branches weighed more than those produced on girdled branches both as NIS and kernel dry weight. This would be expected as nuts produced on ungirdled branches had greater dimensions.
Canopy position affected leaf nitrogen content and specific leaf weight. This was a consistent effect across all four treatments with the greatest amount of leaf nitrogen (mg/cm2) being found in the upper canopy and the amount decreasing through the middle to the base of the canopy (Figure 1). Specific leaf weight was greatest in the upper canopy and least at the canopy base (Figure 2).
Canopy position significantly affects nut yield in terms of both quantity and size with the upper canopy producing the greatest number and largest nuts. Leaf nitrogen and specific leaf weight are also greatest in the upper canopy compared with the lower canopy. Coupled together these results may have important implications for orchard design and pruning methods which may be employed to improve yields.
Nitrogen per unit leaf area has been found to be a sensitive indicator of the light microenvironment experienced by different parts of a tree's canopy (Weinbaum et al 1989). Leaf nitrogen, an essential component in effective photosynthesis, was found in highest concentration at the top of the canopy of the study trees. The greatest flush of new leaf growth was also observed to occur at the top of trees. This suggests that photoassimilates are of greatest availability in the upper canopy of macadamia as has been shown in other tree crops (Southwick et al 1990). Fruit have been found to be strong sinks for photoassimilates and leaves will preferentially supply adjacent developing fruits (Southwick et al 1990). It is possible that developing fruits in the upper canopy of macadamia are acting as immediate sinks for the photoassimilates produced in nearby leaves resulting in these fruits being both larger and more numerous. Decreased nitrogen in the lower canopy is probably the result of shading. Further, specific leaf weight declines from the upper to the lower canopy as a result of decreasing light availability. The combination of decreased nitrogen per unit leaf area, specific leaf weight and leaf number toward the base of the canopy all provide evidence that photosynthetic resources are least at the bottom of the canopy and that this may explain the overall greater performance in nut size and yield in the upper canopy.
In replicated trials Meyers et al (1994) have observed differences in nut size when different pollen sources are supplied to different cultivars even when initial set is apparently similar. Therefore the observation that cv. 660 produced more nuts than cv. 344 over all treatments may be explained by differential compatibility with the cv. 508 pollen parent. The effect of this pollen source on the fruitfulness of these two cultivars is unknown. However, pollen of cv. 508 has been observed to produce greater pollen tube growth and initial nut set than self pollen in both cv's 344 and 660 (Sedgley et al 1990). Our results confirmed these observations since high initial sets were observed on both cultivars. Further studies with a greater range of pollen sources may assist to resolve these differences. Such studies would clarify whether either of these cultivars is intrinsically capable of producing more nuts than the other.
Previous studies of leaf nitrogen content have shown an increase in nut number but reduction in nut size with increasing leaf nitrogen content ( Stephenson and Gallagher 1989). In these previous trials leaves were sampled from the second whorl of leaves on a mature spring flush at 1.2 m above the ground. Our results show that there are large differences in leaf nitrogen content from the base to the top of the canopy. These differences varied with cultivar and tree planting distance. The highest levels of leaf nitrogen also produced the greatest number and largest nuts on both the cultivars sampled. The effect of resource supply on nut size is unknown in macadamia. Under Australian conditions nuts are known to achieve their maximum dimensions in December after which kernel development occurs (Trueman and Turnbull 1994). Maximum demand for dry weight occurs during the period of rapid growth prior to cessation of nut expansion (Vivian-Smith 1995). This suggests that proximal limits to resource supply may restrict nut growth. Further work is needed to reconcile the results of Stephenson and Gallagher (1989) with our present results, but reduction in nut size does not appear directly related to leaf nitrogen content. This work also suggests that further research may be needed to determine the optimum means of sampling to determine tree nitrogen status.
Proximity to active leaves seems a prerequisite for optimal nut production in macadamia. This prerequisite needs to be taken into consideration in the development of pruning treatments and orchard design for maintaining productive orchards.
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