Agriculture Reference
In-Depth Information
nitrogen uptake. A clear understanding of the genetic mechanisms and
inheritance of NUE is lacking for low-input systems, since most mecha-
nisms governing NUE have been studied in high-input production systems
[62,66]. Several adaptations have been suggested for advanced NUE, such
as improved root development and architecture, along with delayed leaf
senescence, increased arbuscular mycorrhizal colonization or nitrogen fi x-
ing symbioses, and increased activity of specifi c enzymes [7,58,67]. Un-
der relatively simple genetic control compared to other benefi cial molecu-
lar and physiological traits, improvements in root structure, such as length
and thickness, as well as density by increasing the production of root hairs
and adventitious roots are effi cient ways to improve the ability for crops to
acquire and absorb soil nutrients [68].
Breeding for delayed leaf senescence, increased enzyme production,
and symbiotic relationships are more diffi cult than for qualitative (Men-
delian) traits, largely because they are more complex and under polygenic
control. Nevertheless they can be subject to genetic improvement through
either traditional or advanced breeding [7]. Delayed leaf senescence is es-
pecially related to the physiology of the hormone cytokinin, and its as-
sociated genes seems to be key to molecular breeding of this trait under
low-input conditions [69,70]. Bertin and Gallais [63] showed the NUE
was negatively correlated to leaf senescence at low-input. By delaying leaf
senescence, a genetic gain in yield can be seen, due to the greater capacity
to uptake nitrogen during fruit maturity [62]. Also by breeding for high
chlorophyll content, delayed leaf senescence can be achieved, leading
to increased nitrogen uptake. Grain yield has been positively correlated
to chlorophyll content in low-N input systems, this correlation not be-
ing signifi cant under heavy nitrogen application [71,72], has been ignored
by breeding programs and serves as an example of the need for specifi c
breeding at low-input systems to improve sustainability on a global-scale.
Spano et al. [73] determined that “stay green” genotypes of wheat had
increased maintenance of leaf chlorophyll, which lead to 10-12% increase
in grain weight. It was also determined that as a result of the increased N-
uptake, due to increased leaf area duration and chlorophyll content avail-
ability, “stay green” varieties had early development of adventitious roots
and increased root density than senescent varieties also contributing to
yield gains [73].
