Pistachio (Pistacia vera L.) is a deciduous dioecious tree grown commercially in the high desert and Mediterranean climates of Iran, California’s Central Valley, Turkey, Sicily, and Australia. Pistachio is an anemophilous species; air currents carry pollen from the male tree to the female tree. Both the male staminate and female pistillate inflorescences are panicles composed of hundreds of individual small flowers, the rachis. The florets of both female and male flowers are apetalous with fully exposed stigmas and anthers vulnerable to desiccating winds, extreme temperatures, and rain (Ferguson et al., 2005).
The receptivity of the stigma of an individual pistachio female floret is 2–3 d; thereafter, the stigma surface senesces and cannot be pollinated (González et al., 1995; Hedhly et al., 2003). The amount of viable pollen loaded on the stigma is the first step in pollination (Wilcock and Neiland, 2002). Acar and Eti (2008) reported 14 or 15 pollen grains per stigma is adequate for pollination in pistachio. On germination, the pollen tube grows the length of the style to the micropyle, the opening in the ovule’s outer layer. There, it enters the embryo sac (female gametophyte) and completes fertilization. Successful fruit set is collectively a function of stigma condition, pollen germination and tube growth, ovule longevity and the effective period between pollination (pollen landing on the stigma), growth of the germinated pollen tube through the style, and fertilization of the ovule (Herrero and Arbeloa, 1989; Kalinganier et al., 2000; Ortega et al., 2004; Zhang et al., 2018).
The combination of dry conditions, wind, and flail mowing during pollination has been observed to generate considerable dust. The diameter of orchard dust particles is between 1 and 100 μm, whereas the surface length of the female stigma and the diameter of pollen grains are ≈500 and 20 μm, respectively. Field dust has been demonstrated to injure the leaf stomata and spongy parenchyma beneath, preventing photosynthesis and inhibiting pollen loading on the floral stigmas, resulting in poor pollination (Sett, 2017; Waser et al., 2017). The female flower’s acidic stigmatic secretions that facilitate pollen hydration and germination can also be damaged by alkaline dust (Sett, 2017). McArtney et al. (2006) reported sulfur liming at bloom inhibited pollen tube growth and subsequently reduced fruit set in apple (Malus domestica).
There is little information on how dust affects the pollen and pistils, or their interaction. Most reports focus on the pollen–stigma interactions of the self-incompatible or foreign pollen (Goring, 2018; Hiscock et al., 2002). The pollen coat, composed of lipids and proteins, contains some signaling molecules such as pollen coat B-class (PCP-Bs) and S-locus proteins that are required for stigmatic receptors to trigger pollen recognition and start papillar responses (Doughty et al., 2000). The exotic PCP-Bs carried by the incompatible pollen have been reported to prevent pollen hydration in the Brassicaceae (Wang et al., 2017). On the attachment of the pollen and stigmas, vesicle-like structures in the papillae cells were observed moving toward and fusing with the papillar plasma membrane at the pollen–stigma interface. These vesicles transport the aquaporins and lipases (e.g., pectinase) that facilitate water distribution that precipitates grain hydration, and cell wall expansion that facilitates the pollen tube penetrating the papillar layers (Elleman et al., 1992). However, these stigma reactions have not been observed in the flowers pollinated by self-incompatible pollen grains (Safavian and Goring, 2013). We hypothesized that dust, as an abiotic substance, will not provoke signaling-related biochemical reactions and produce the same effect as an incompatible pollen.
We addressed the following questions. First, does dust on the stigmatic surface reduce pollination and fruit set in pistachio? Second, do herbicide residues in the dust harm pollination and yield as a direct effect of the herbicide harming the stigma and pollen? Third, does dust influence pistachio nut growth and shell splitting? The final value of harvested pistachios is a function of the size and percentage of successfully harvested filled and split nuts. Blank, partially filled, and unsplit nuts can be successfully harvested or remain on the tree; in either case, yield and quality, and, therefore, net return suffer. The objective of this study was to investigate if orchard dust harms pistachio pollination and how.
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