Macadamia is partially self-incompatible (Sedgley, 1983), although self-pollinated nut set is known to occur in at least some cultivars (Meyers, 1997; Vithanage et al., 2002; Wallace et al., 1996). Cross-pollination has been considered important to increase final nut set (defined here as 3 months following fruit initiation) in trees across many cultivars (Meyers, 1997; Wallace, 1999). However, when comparing separate trials, the consistency of the increase in final nut set was found to be variable even with the use of the same pollen donor and receiver cultivars (Wallace et al., 1996). Much of the current knowledge of Macadamia spp. floral biology and breeding systems has been outlined in a review by Trueman (2013); however, the breadth and scale of published research conducted across the many cultivars remains poor.
It is unclear whether self-pollination can occur within florets; pollen needs to be moved between florets by pollinators; pollination deficits are widespread within blocks of different cultivars; the amount of cross-pollination across an increasing number of racemes (hand cross-pollination intensity) influences potential nut yields. Such information can be useful for evaluating whether current pollinator management strategies are adequate, and for developing strategies to boost pollination if required.
Although self-pollination is known to occur in macadamia and may even be responsible for producing economic yields within isolated single cultivar blocks, published data showing the extent of self-pollination across multiple cultivars are largely lacking. A study by Meyers (1997) found that self-pollination of racemes resulted in limited nut set across some, but not all cultivars. In a block containing 27 rows of cultivar A16, self-pollination never exceeded 20% in a row (across 11 rows assessed), including the center row (13 rows from the nearest alternative cultivar) (Vithanage et al., 2002). Previous studies also demonstrate lower nut weight through self-pollination compared with hand cross-pollination (Meyers, 1997; Trueman and Turnbull, 1994; Wallace et al., 1996).
Despite the capability of at least some cultivars to produce fully developed nuts through self-pollination, there is a lack of studies exploring whether this pollination would likely benefit from an animal pollen vector, or (arguably less likely) wind or gravity (see Wallace, 1999). Macadamia presents viable pollen on or near the stigma (Trueman, 2013) but the floret is protandrous, with the stigma becoming receptive 1 or 2 days after anthesis (Sedgley et al., 1985) and reaches peak receptivity between 3 and 7 days after anthesis (Meyers, 1997). Therefore, autogamous pollination (within floret) could potentially occur if the pollen stays viable until the stigma becomes receptive. Alternatively, stigmas may receive self-pollen of variable age from other florets within the raceme, other racemes within the same tree, or from other trees of the same cultivar. This knowledge is important for evaluating whether pollinators are needed to transfer pollen to a floret, as wind pollination has been considered of minimal importance (Wallace, 1999). Wallace et al. (1996) explored how much final nut set in bagged racemes of cultivars A4 and 246 was affected by the transfer of self-pollen from other trees of the same cultivar compared with that from bagged racemes with no pollination. Few to no final nuts developed in these treatments. Evidence for the potential benefit of self-pollen movement between florets, to our knowledge, has not been obtained.
Cross-pollination has been demonstrated to increase final nut set across several cultivars compared with self-pollination (mostly measured at the raceme level) (Meyers, 1997; Wallace et al., 1996). A broad assessment of whether pollination is optimal across cultivars, even within a region, does not appear to have been examined. Such information is useful in assessing whether strategies to boost cross-pollination are useful and the scale at which this might be required. Moreover, we are not aware of studies that have assessed whether increased nut set within cross-pollinated racemes can be sustained within trees with increased hand cross-pollination intensity. Limits to resource allocation within a tree may limit final nut set when cross-pollinated florets or racemes are preferentially retained at the expense of self-pollinated flowers. If carbohydrates and/or certain minerals are limiting, trees will not set a full crop even with adequate pollination. Pollen tube growth, fertilization, and fruit set require adequate amounts of carbohydrate and boron in particular (Alcaraz et al., 2013; Boldingh et al., 2016; Tanaka et al., 2013). Therefore, simple measures of pollination deficit may overestimate the importance of hand cross-pollination because of limits to resources allocated to develop nuts. There is evidence of physiological limitation of nut yields, in that just 0.3% of original floret numbers result in a harvestable nut (Ito, 1980). Only 4% of immature nuts present at 3 weeks after pollination developed into harvestable nuts (Wallace et al., 1996).
To gain further understanding of the contribution of cross-pollination, self-pollination, the occurrence of pollination deficits within blocks, and whether cross-pollination intensity affects final nut set, we conducted field trials in Queensland, Australia. We assessed whether the transfer of self-pollen by hand between florets, racemes, and trees (typical of geitonogamous pollination) affected nut set compared with no movement (bagged), hand cross-pollinated (between different cultivars), and open (exposed to pollinators) racemes. We also assessed whether nut set was affected by pollination deficits across blocks by comparing open-pollinated (non-bagged) racemes with open racemes that also received supplemental hand cross-pollination. To test for resource limitation, we varied cross-pollination intensity by varying the number of hand cross-pollinated racemes on single trees.
Alcaraz, M.L., Hormaza, J.I. & Rodrigo, J. 2013 Pistil starch reserves at anthesis correlate with final flower fate in Avocado (Persea americana) PLoS One 8 e78467
Boldingh, H.L., Alcaraz, M.L., Thorp, T.G., Minchin, P.E.H., Gould, N. & Hormaza, J.I. 2016 Carbohydrate and boron content of styles of ‘Hass’ avocado (Persea americana Mill.) flowers at anthesis can affect final fruit set Scientia Hort. 198 125 131
Howlett, B.G., Nelson, W.R., Pattemore, D.E. & Gee, M. 2015 Pollination of macadamia: Review and opportunities for improving yields Scientia Hort. 197 411 419
Howlett, B.G., Evans, L.J., Kendall, L.K., Rader, R., McBrydie, H.M., Read, S.F.J., Cutting, B.T., Robson, A., Pattemore, D.E. & Willcox, B.K. 2018 Surveying insect flower visitors to crops in New Zealand and Australia. bioRxiv doi: http://dx.doi.org/10.1101/373126
Groeneveld, J.H., Tscharntke, T., Moser, G. & Clough, Y. 2010 Experimental evidence for stronger cacao yield limitation by pollination than by plant resources Perspect. Plant Ecol. Syst. 12 183 191
McFadyen, L.M., Robertson, D., Sedgley, M., Kristiansen, P. & Olesen, T. 2011 Post-pruning shoot growth increases fruit abscission and reduces stem carbohydrates and yield in macadamia Ann. Bot. 107 993 1001
Meyers, N.M., McConchie, C.A., Turnbull, C.G.N. & Vithanage, V. 1995 Cross pollination and intervarietal compatibility in macadamia Austral. Macadamia Soc. News Bull. 22 5 8
Meyers, N.M. 1997 Pollen parent effects on Macadamia yield. Univ. of Queensland, Brisbane, Australia, PhD Diss
Penter, M., Schoeman, S. & Nkwana, E. 2007 Cross-pollination for improved nut set in ‘Beaumont’ macadamias South African Macadamia Growers Assn. Yrbk. 15 13 16
Sedgley, M., Blesing, M.A. & Vithanage, H.I.M.V. 1985 A developmental study of the structure and pollen receptivity of the macadamia pistil in relation to protandry and self-incompatibility Bot. Gaz. 146 6 14
Tanaka, N., Uraguchi, S., Saito, A., Kajikawa, M., Kasai, K., Sato, Y., Nagamura, Y. & Fujiwara, T. 2013 Roles of pollen-specific boron efflux transporter, OsBOR4, in the rice fertilization process Plant Cell Physiol. 54 2011 2019
Vithanage, V., Meyers, N. & McConchie, C. 2002 Maximising the benefits from cross pollination in Macadamia orchards. Horticulture Australia Ltd., Sydney, Australia
Wallace, H.M. 1999 Bees and pollination of Macadamia, p. 71–73, Technical paper. The Australian Macadamia Society, Lismore, NSW, Australia
Wallace, H.M., Vithanage, V. & Exley, E.M. 1996 The effect of supplementary pollination on nut set of Macadamia (Proteaceae) Ann. Bot. 78 765 773