Rhizosphere Acquisition Traits for Phenotyping Zn Efficient Rices (Oryza sativa L.)

Abstract

Zinc (Zn) deficiency is the most widespread micronutritional disorder in wetland rice crop [16, 27], contributing as high as 3 t/ha of yield loss [23]. Although total content of Zn in the soil may be sufficiently high, Zn availability is always reduced when the soil is calcareous, saline and/or high in organic matter [11, 35]; and most importantly when the soil is flooded regardless of its original pH; precipitation of Zn(OH)₂ occurs in acid soils as the result of pH increase after flooding [32], ZnS and bicarbonate precipitate in sodic or calcareous soil [27, 19], and there can be formation of ZnFe₂O₄ in the normal soil [28]. Using Zn efficient rice varieties is considered more appropriate because, for soil application, at least 10 kgZn/ha is needed in the field while plants take up less than 1 kgZn/ha, leaving the remaining Zn accumulated in the soil which might lead to toxicity if the soil conditions change [10]. Since 1971, IRRI tested more than 23,000 lines in both experimental and farmers’ fields and found 9% efficient. The disadvantage of field selection is poor repeatability due to environmental variation, while greenhouse screening technique using buffer chelate solution does not differentiate traits for rhizosphere acquisition and therefore marker-assisted selection techniques seem to be a most effective measure [9, 15]. However, as the segregation pattern suggests at least 3 independent incompletely dominant genes involved in this character [1, 18], quantitative trait loci (QTL) analysis is needed [33]. At present, lack of phynotypic characterization due to poor understanding of the physiology of tolerance and/or efficiency mechanism(s) is the main limitation [9, 24, 15].