Physiological and biochemical responses of barley (hordeum vulgare l.) to nitrogen availability
Creators: name: Alves Negrini, Ana Clarissa
Publisher: name: The Australian National University
Year: 2016

description: Nitrogen (N) is one of the most important nutrients for plant growth and crop performance. Increases in the use of N have contributed to increases in agricultural production over the past 50 years. Large amounts of this fertilizer are applied to cereals crops to maximize yields; however, excessive fertilizer application is a source of several environmental problems, including pollution and increase in CO2 emissions. One way of minimizing N application to crops is by designing genotypes with increased nitrogen use efficiency (NUE). Previous studies showed that overexpression of alanine aminotransferase (AlaAT) resulted in significant increases in the NUE of canola and rice plants. This technology has been applied to barley, with preliminary studies showing promising results in terms of biomass increase and seed production. With the aim of having a more detailed understanding of the physiological mechanisms underpinning the performance of the AlaAT barley genotypes, an ecologically-focused approach was adopted to evaluate the efficiency of N use and growth performance in early vegetative stages of barley. To establish guidelines to study AlaAT overexpressing barley genotypes, a detailed growth analysis was first performed with wild-type plants grown on six N concentrations, to understand how N supply affects the carbon (C) and N economy of this crop at different stages of vegetative development. The results of these studies revealed interesting insights on the effect of N on the underlying growth parameters, and determined that the major factors affecting grown as a result of N availability are C and N allocation among above and below-ground organs. Furthermore, these studies revealed the importance of controlling for ontogenetic drift by comparing plants of similar masses when analyzing the effect of N availability on growth traits. Subsequently, analysis of the C and N economy of the AlaAT overexpressing barley genotypes revealed no increases in growth and N productivity compared to null controls. Biochemical measurements suggested that post-translational issues may have prevented increase in AlaAT activity and protein abundance during the vegetative period over which plants were grown. The role of N on agricultural production goes beyond the increase in plant growth and seed yield. N fertilization, in the form of NO3- also increases crop performance and survival during waterlogging. However, the mechanism behind this improvement in hypoxia tolerance has not yet been elucidated. With the aim of having a better understanding about N supply and hypoxia interaction, I analyzed the effect of NO3-availability on the global gene expression in response to hypoxia in barley. My results suggest that NO3- availability may be linked to the increase in ATP production, and that N supply affected numerous events within barley plants. This includes changes in N and C allocation that resulted changes in growth rate and biomass accumulation, protein expression, and transcriptomic responses to low oxygen stress. My thesis research suggests that increases in the efficiency of N use could be achieved by improving the capacity for N uptake and assimilation, associated with a reduction in the proportion of respiratory losses to maintain a large root system.