Genetic and environmental influences on tree nutrition in Pinus radiata (D. Don) : impacts of nitrogen source in tree growth and root-associated communities.
Thesis DisciplinePlant Biology
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The main objective of the investigation presented here was to increase our understanding of how genotype-by-environment interactions influence nitrogen nutrition of pines. Nitrogen in soil ranges from simple inorganic forms (NH+₄ and NO−₃) to organic forms (amino acids, peptides and proteins), and microbially-mediated processes limit the availability of these molecules. Plants can only take up a portion of the N present in the soil solution, mainly in form of NH+₄ and NO−₃ and free amino acids. While an increasing number of studies have investigated species preference for different N forms and the resulting changes in plant development, the significance of intraspecific variation in plant resource use of organic and inorganic N forms and whether such variation translates to variation in root-associated communities, have not been established.
Growth studies in young radiata pine genotypes (Pinus radiata D. Don) showed that the amino acid L-arginine, alone or in combination with NO−₃ can promote growth as ef- fectively as, and sometimes even more effectively than, inorganic fertiliser (NH₄NO₃) at the same N concentration. The study was performed in both short-term greenhouse con- ditions (6 months) and longer-term field conditions (2.5 years), and results were confirmed in both systems. The study under field conditions revealed that the observed genotype variation to N source was further influenced by edaphic factors and interspecific competi- tion with understory vegetation. In addition, root traits in three contrasting genotypes in response to organic and inorganic N form showed changes in the degree of root colonisa- tion with ECM fungi and variation in root morphological traits, with no differences in a plant’s capacity to take up any N source. In light of the significant impact of genotype and N form in growth and root traits, I then studied whether root-associated below-ground communities had a determining role in the observed variation.
In order to seek evidence of linkage between above and below-ground responses, I
investigated how the root microbiomes of two full-sib P. radiata genotypes with distinct physiological responses to organic versus inorganic N were altered in both greenhouse and field experiments. The results showed that intraspecific variation in tree response to N form, and the associated changes in soil physicochemical properties, have signifi- cant consequences for the root microbiome of P. radiata. The investigation showed that N supplied in either organic or inorganic form can differently influence plant and rhizo- sphere soil nutrient cycling, and these direct and indirect mechanisms can be associated with shifts in the diversity and composition of rhizophere microbial communities. Results from field conditions revealed that C:N stoichiometry of both rhizosphere soil and plant tissue decreased with the addition of L-arginine compared to the control, while adding an equivalent amount of N as NH₄NO₃ did not shift this ratio in either rhizosphere soil or tree needles. In addition, of the communities considered in the studies (rhizosphere bacteria, rhizosphere fungi, and root fungi) rhizosphere bacterial communities demon- strated genotype-specific responses to N treatment for both diversity and composition, measured in both the field and greenhouse. However, the effect of tree genotype and N treatments differently influenced rhizosphere and root-associated fungal communities, and this in turn differed between growth conditions, which suggests that edaphic and climatic factors evolve as strong filters of these communities.
Based on the results obtained in this study, it is clear that gene-by-environment interactions are important determinants of plant growth and physiological responses to organic and inorganic N source, and are strong determinants in shaping host-associated communities. This research suggests that, while subtle changes in genetic background can to some extent predispose trees to prefer one N source over the other, organisms comprising the root microbiota (including bacterial and fungal taxa) might be key determinants of soil N availability and plant capacity to access N sources. Therefore, the form of the N source differently influences above- and below-ground plant traits among genotypes of the same species, and these can affect the diversity and composition of the rhizosphere, either directly (by changing the nature of plant-microbe interactions) or indirectly (by altering soil properties in ways that favor particular microbial taxa).