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Chlorosis of Concord grapevines: Soil microbial Fe-chelation to enhance Fe bioavailability.

The Problem:random-chlorosis-3

Leaf yellowing, or chlorosis, occurs on more than 50% of the ‘Concord’ vineyards in central WA, with vines displaying symptoms every year resulting in significant decline in grape yield and quality. The initial symptoms of chlorosis develop around bloom, after which time chlorotic leaves turn brown or burned.  It significantly reduces vine size, vine uniformity and productivity, and even causes vine death (Davenport et al., 2003).  The yellowing of the leaves resembles classic Fe-deficiency induced chlorosis, and Fe deficiency in orchard and vineyard systems is well documented in calcareous soils around the world (see reviews by Tagliavini and Rombolá 2001, and Marschner et al. 2011).

Many studies have attempted to determine the precise cause of this type of chlorosis in the hopes of developing effective management strategies (Korcak, 1987) and the application of synthetic iron chelates has been employed in many vineyards (Tagliavini and Rombolá, 2001).   However, application of Fe-chelates is only a temporary solution, requiring re-application each year, and therefore does not represent a sustainable nor cost effective cure to iron chlorosis (Smith and Cheng, 2006).  While links have been drawn between high soil pH, calcium carbonate levels, as well as high soil moisture and low soil temperature (Ao et al., 1987; Davenport and Stevens, 2006) all work, to date, supports the concept that some factor beyond these is involved in grape chlorosis in areas of similar soils in Washington state.

Although Fe is relatively abundant in soils, Fe acquisition is limited by solubility because the dominant forms of Fe in soils are poorly soluble and consequently unavailable biologically (Marschner, 2003; McBride, 1994).   Plants and microorganisms have developed similar mechanisms to address Fe deficiency, mobilization, and uptake of Fe in the environment and there is intensive competition for uptake (Marschner et al., 2011).

Strategy I plants, to which group perennial trees and grapes belong, increase iron availability by releasing protons that lower the soil pH and complex Fe, holding it in a soluble form that can diffuse to the root surface.  Strategy II plants, however, along with microorganisms, release siderophores into soil to chelate the Fe.  Siderophores are low molecular-weight, amino acid derivatives which chelate Fe, allowing uptake and storage (Neilands, 1993, 1995).  Soil microbes produce a wide variety of different types of siderophores (Neilands, 1984b) each with a different affinity for Fe and capacity to enhance Fe bioavailability.  The localized production of microbial siderophores in the rhizosphere can result in high concentrations of siderophores (Marschner et al., 2011), but their contribution to plant iron nutrition will vary greatly depending on the exact composition of the microbial community (Sullivan et al., 2013) and their associated classes of secreted siderophores. Soil microbial production of siderophores has been found to increase Fe solubility and improve plant Fe acquisition as a result, for many different plant species (Chen et al., 1998; Crowley et al., 1992; de Santiago et al., 2009; Rroco et al., 2003; Shenker et al., 1999).

However, virtually nothing is known about the soil microbial community composition of the Concord grape vines of Washington or their capacity to produce siderophores.  We propose to embark upon this critical knowledge gap and hypothesize that differences in the soil microbial community composition and siderophore production may be tied to chlorosis of Concord grape vines.

Objectives of the Research:

  1. Determine how the composition of the soil microbial community associated with chlorotic Concord grape vines is different from the soil microbial community of healthy vines.
  2. Determine how the Fe-chelating capacity of the soil microbial community may differ between chlorotic Concord grape vines and healthy vines.
  3. Asses plant growth promotion characteristics and potential of soil microorganisms associated with healthy Concord grape vines for potential inocula.