Resumen de Nitrogen nutrition in pine seedlings: the role oforganic n in promoting tolerance and acclimation to abiotic stress
Nitrogen (N) nutrition plays an essential role in plant physiological mechanisms to cope with abiotic stress in pine species. However, how the availability of different N forms, and especially the organic N (amino acids, peptides, and proteins), affect plant tolerance and acclimation to abiotic stress is poorly understood. The general objective of this Thesis was to study the effect of the chemical N forms on seedling morpho-physiology of ecologically distinct pine species. Particularly, we examined how organic N affects different physiological mechanisms associated with tolerance and acclimation to frost and drought stress. The general hypothesis is that the potential energy- and C-savings from the organic N utilization would be expressed as a stimulated seedlings growth and (or) in boosting their tolerance and acclimation mechanisms to abiotic stress, but these effects would depend on the physiology and growth habit of the species. In a first experiment, we studied morpho-physiological responses to organic N (amino acids) supply at two N doses in Pinus halepensis and P. sylvestris seedings. Then, we assessed the ability of the organic-N fed seedlings to develop frost tolerance during the cold season compared with those under conventional inorganic-N fertilization (NH4NO3). In two subsequent experiments, the effects of the N form in plant. In the one hand, one-year-old P. ponderosa seedlings were fertilized for 10 weeks at 9 mM N with different N forms (either NH4+, NO3-, or organic N). On the other hand, seedlings of P. engelmannii and P. greggii were grown with either NH4+, NO3-, organic N, or a mixture of the three N forms, during their first growing season. In both experiments, we measured traits associated with intrinsic drought tolerance after fertilization and plant acclimation mechanisms to a prolonged drought period at varying intensities. It was demonstrated that pine seedlings could efficiently use amino acids as a primary N source, showing similar performance to those grown with inorganic N forms. At low N doses, both P. sylvestris and P. halepensis had acute P deficiency when grown with inorganic N, but not with organic N. When grown at high N rates, organic-N improved chlorophylls concentration. Organic-N supply boosted the accumulation of compatible solutes, such as soluble sugars and proline, during the cold hardening period. Consequently, we suggested that the extent the species depend on such compounds for cold acclimating will determine the benefits of organic N supply. Further, while organic N supply delayed the shoot growth cessation in P. sylvestris or stimulated secondary growth in P. halepensis during winter, it did not have detrimental effects on the frost tolerance of both species. Organic-N fertilization induced similar needle N concentration in P. ponderosa as NH4+, but higher than NO3-. Likewise, in P. greggii, organic N triggered higher shoot growth than NO3- and greater biomass allocation to roots than NH4+. Organic-N fed seedlings tended to have an earlier loss of turgor and photosynthesis than those grown with the inorganic N forms. Nevertheless, we found that damage to photosynthetic machinery due to high intensity drought is somehow diminished in seedlings grown with organic N, particularly in P. engelmannii. Seedlings of P. ponderosa and P. greggii grown with either organic N or NH4+ tended to performance under prolonged drought than NO3--fed seedlings. Notably, some drought acclimation responses, including biomass partitioning, leaf water use efficiency, or proline accumulation, were boosted by organic N fertilization. The general conclusion is that the potential energy- and C-savings of the organic N utilization are used in multiple functions, such as biomass construction, reserves, and support plant physiological acclimation to abiotic stress. Notably, fast-growing species are more plastic to N forms, and the benefits of organic N seems to be expressed in multiple functions, instead of maximizing a single process.
