Natural Genetic Variability in Nitrogen Metabolism

Abstract

Plant development is the result of various genetically controlled interacting metabolic processes, which are to a large extent modulated by environmental factors. In this context, the key role of absorption, distribution and accumulation of nutrient elements deserves the attention of the geneticist and the plant breeder, because these complex traits display a large variability between and within species, which may be important in terms of adaptation of the genotypes, or species, to various environments (Kjaer and Jensen 1995). Works dedicated to major nutritional variations were carried out to unravel reasons of both inefficiency in nutrient utilisation and efficiency of utilisation under nutrient stress. As early as 1943, Weiss studied the genetic variability of iron absorption efficiency in soybean. A susceptible genotype that developed the typical chlorosis of iron deficiency was crossed with a tolerant genotype. Inheritance studies showed that a single pair of alleles, Fe/fe, controlled the efficiency of Fe utilisation. The efficient allele, Fe, is dominant over fe, the fefe plants becoming chlorotic when submitted to Fe-limited conditions. Bell et al. (1958, 1962) demonstrated that the Zea mays mutant (ys1) discovered by Beadle (1929), exhibited yellow stripes between the main veins due to its inability to use iron (Fe³⁺). Potassium utilisation under severe K deficiency was investigated by Shea et al. (1967, 1968) in various bean genotypes. Very efficient strains were discovered, the most efficient producing almost 50% more dry weight than the less efficient, when supplied with a 0.13 mM KC1 nutrient solution. Efficient and inefficient strains were crossed and their progeny was evaluated for the capacity to grow under low K availability. The results suggested that this trait would be under the control of a single pair of alleles and that “efficiency” was recessive. Similar studies have been expanded to other species and other elements such as P, Ca and Mg. Differences in the use of Ca and Mg were explored in Zea mays, showing variations in yield among corn inbreds growing on soils known to induce Ca and Mg deficiencies (Clark and Brown 1974; Clark 1978). Wide genotypic differences in yield were obtained between bean strains growing on low phosphate (2 mg P), and between tomato strains grown on low nitrogen (35 mg plant^-1) (O’Sullivan et al. 1974). Genetic studies coupled to these various investigations showed that the utilisation of major cations and anions could be under polygenic control, except for K in bean, that appeared to be controlled by a single pair of alleles.