Citrus trees on many commercial rootstocks do not perform well in high-carbonate soils (Campbell, 1991; Castle, 1987; Castle et al., 2004; Cooper and Peynado, 1953, 1954; Ferguson et al., 1990; Hamze et al., 1986; Hodgson, 1967; Sagee et al., 1992; Sudahono and Rouse, 1994; Wutscher, 1979). Such rootstocks are limited by their inability to sufficiently extract micronutrients, including iron (Fe), that are rendered largely unavailable in these kinds of soils (Korcak, 1987; Manthey et al., 1994a). This limitation particularly applies to Poncirus trifoliata and its hybrids, which include some of the world's most popular rootstocks like Troyer and Carrizo citranges and Swingle citrumelo (Castle, 1987; Wutscher, 1979). The international significance of this problem is evident in the continued reporting of trials and germplasm releases concerning rootstocks specifically investigated for tolerance to high-carbonate soils (Tagliavini and Rombolá, 2001; Wei et al., 1994).
In the United States, citrange and citrumelo rootstocks are the mainstays of the Florida and California citrus industries, but both states also have areas of high-carbonate soils (Castle et al., 1993; Ferguson et al., 1990). Trees can be grown in those questionable sites if the grower is willing to choose other rootstocks with less desirable traits (Castle et al., 1992). Two examples are sour orange and rough lemon rootstocks, which are both well adapted to calcareous soils. Trees on sour orange produce excellent quality fruit, but are susceptible to citrus tristeza virus; those on rough lemon are high-yielding, but produce poor-quality fruit and are susceptible to citrus blight. Other management options for high-carbonate soils such as application of chelates are often expensive, so there is a strong incentive to develop new rootstocks (Castle et al., 2004; Grosser et al., 2004).
We began a screening project to search for superior citrus rootstocks suitable for calcareous sites based on the measurement of root Fe+3 reduction rates (Castle and Manthey, 1998). This method was selected because of its potential to minimize various interfering conditions that can occur in other screening methods mostly related to Fe chemistry (Cooper and Peynado, 1954; Hamze et al., 1986; Korcak, 1987; Manthey et al., 1993, 1994a; Pestana et al., 2005; Sagee et al., 1992; Sudahono and Rouse, 1994). For example, when using soil, pH affects the chemical state of Fe, and both pH and air-drying influence bicarbonate content, and the buffering system in solution studies affect Fe availability and uptake (Korcak, 1987).
Plants can respond to low-Fe stress through several inducible mechanisms, including, among others, electron release at the root surface (Briat and Lobréaux, 1997; Manthey et al., 1994b). By measuring a fundamental plant response, Fe reduction rate, in the simple test environment of a complete nutrient solution minus Fe, other possibly complicating factors could be minimized and the results would be more broadly applicable.
Our first screening project involved primarily common rootstocks and other citrus selections (Castle and Manthey, 1998). The selections ranked in terms of Fe3+ reduction rates in the same general order as their rankings developed from field-based and other screening trials, i.e., the lemon-type rootstocks such as Rangpur and Volkamer and rough lemon had the highest Fe reduction rates, the mandarins had intermediate rates, and the citranges and citrumelos along with trifoliate orange had the lowest rates. However, some selections ranked well below their expected ranking from field observation; also, measurements of Fe+3 reduction rates along with growth and leaf chlorosis suggested that the selections fell into several categories based on a composite of the three variables that improved the ranking procedure.
Two questions remaining after our first efforts were: 1) could the procedure be miniaturized so that individual plants might be rapidly and efficiently evaluated, thus enabling high-throughput screening for citrus breeders, a clear advantage in plant breeding; and 2) how do the solution culture and soil screening methods compare given that soil is a more natural environment and may produce results more closely linked to field experience? Thus, our objectives were to expand the range of material screened for tolerance to low-Fe stress to include additional rootstocks, citrus relatives, and other selections to confirm our previous results, develop a miniaturized procedure, and compare solution culture with soil screening.
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