Iron is an essential element for plant growth but its uptake by plants can be limited by biotic and abiotic factors (Kim and Guerinot, 2007). Dicots and nongraminaceous monocots respond to iron limitation through strategy I iron uptake. This strategy comprises the coordinated action of three complementary processes functioning at the plasma membrane (PM) of root epidermal cells: a) rhizosphere acidification, b) iron reduction, and c) transmembrane iron transport (Jeong and Conolly, 2009).
Rhizosphere acidification results from the action of PM-bound H+-ATPases that extrude protons from the symplastic space into the rhizosphere (Kim and Guerinot, 2007). This facilitates iron uptake by increasing the solubility of iron-containing compounds in the soil (Lemanceau et al., 2009), providing an adequate microenvironment for iron reduction and generating the proton motive force for ion uptake (Dell’Orto et al., 2000). Yet despite these benefits, there is wide diversity in the extent and plasticity of rhizosphere acidification among plant species.
Herbaceous and woody species like cucumber (Cucumis sativus), cork oak (Quercus suber), and plum (Prunus cerasifera) are capable of acidifying their rhizosphere by developing or enhancing proton extrusion in response to iron deficiency (Dell’Orto et al., 2000; Gogorcena et al., 2001; Gonzalo et al., 2011). On the other hand, wild apple (Malus baccata) and peach-almond hybrids (Prunus amygdalus × Prunus persica) are not capable of this response (Gonzalo et al., 2011; Wu et al., 2012). Moreover, some species like grapevine (Vitis vinifera) exhibit intraspecific diversity of responses; ‘Cabernet Sauvignon’ is capable of acidifying its rhizosphere whereas ‘Balta’ is not (Jimenez et al., 2007; Ksouri et al., 2006).
Although iron reduction and uptake by Vaccinum sp. have been investigated previously (Darnell and Cruz-Huerta, 2011; Poonnachit and Darnell, 2004), to our knowledge rhizosphere acidification in Vaccinium has not been quantified. Southern highbush blueberry, like all cultivated blueberry, is adapted to acidic soils (Coville, 1910; Finn et al., 1993) and experiences iron deficiency when grown in higher pH soils (Gough, 1997). On the other hand, VA is a wild species that exhibits greater tolerance to high pH soils (Lyrene, 1997) and greater efficiency at iron assimilation than SHB (Darnell and Cruz-Huerta, 2011).
Additionally, VA and SHB also differ in their ability to take up nitrate from the soil. Several studies indicate that VA exhibits greater nitrate assimilation than SHB (Darnell and Cruz-Huerta, 2011; Darnell and Hiss, 2006; Poonnachit and Darnell, 2004). Since nitrate anions are transported across the PM in symport with H+ (Pii et al., 2014; Santi et al., 2003), nitrate uptake in Vaccinium sp. could lead to pH increases in the rhizosphere, as it does in other woody plants (Jimenez et al., 2007; Sas et al., 2003). Hence, iron and nitrate uptake are closely related due to the antagonistic effect that these processes have on rhizosphere pH.
This study investigated the rhizosphere acidification capacity of two taxa in the genus Vaccinium—V. corymbosum interspecific hybrid (SHB) and VA. We hypothesized that a) SHB and VA would respond to iron deficiency by developing or enhancing rhizosphere acidification beyond levels necessary to offset the effect of nitrate uptake on pH, and b) VA has greater ability to acidify the rhizosphere than SHB and therefore is adapted to a wider range of soil conditions.
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