The role of nitrogen in frost tolerance root growth dynamics and hydraulic conductance of ecologically distinct pine species

Resumen

Background and Aims: environmental stress, such as frost and drought, determines the evolution and distribution of plants. In Mediterranean biome in which a cold wet season alternates with a dry and warm season, perennial plants undergo a complex process of cold acclimation to overcome the cold season and rapidly expand root systems during the wet season to survive the dry season. Environmental factors, such as temperature and water availability, affect cold acclimation, root development, and hydraulics. However, how nitrogen (N) affects these physiological processes and interacts with species ecology is controversial and poorly understood. The objective of this PhD Thesis is to analyze the effect of N on the frost tolerance, root growth dynamics and architecture, and root hydraulic properties in seedlings of four ecologically different pine species. The general hypothesis of the Thesis is that the adaptation of plant species to abiotic stress drives how N is used for growth and stress resistance. Key Results: shoot frost tolerance increased over winter while root frost tolerance did not change through time in any species. Pre-hardening N fertilization affected the frost tolerance of both roots and shoots, although the effect was species-specific: high N reduced the overall root and shoot frost tolerance in P. pinea and P. halepensis (the thermophilic species inhabiting mild winter areas and with weakest dormancy control), increased the frost tolerance in P. nigra (the psychrophilic species living in the coldest areas and with tightest dormancy control), and had no effect in P. pinaster (the mesophilic species that lives in intermediate cold sites). N supply in the fall consistently increased frost tolerance of shoots and roots in all species, indicating that plants used N surplus during cold acclimation to enhance frost tolerance. Differences in frost tolerance among species and N treatments were not explained by variations in organ N or soluble carbohydrate concentration, nor by timing of cessation of shoot elongation; although the most frost tolerant species ceased elongation earlier than the least frost tolerant species. Increased seedling N content enhanced root development, but species showed distinct root growth patterns and architecture in response to differences in N content. Pinus pinaster, which inhabits moderate climatic stress environments, showed the greatest root growth plasticity with variation in seedling N content. In contrast, root development was less plastic in the pines adapted to more stressful environments (P. pinea, which occurs in sites with strong summer drought and P. nigra, which inhabits cold winter areas). Nitrogen use efficiency for root growth increased strongly with increasing N reserves in P. pinaster, yet the opposite effect occurred in the other two species. This suggests that P. pinaster proportionally relies more on N reserves for root growth than the other pine species. New roots of high N content seedlings had lower specific root length than low N content seedlings in all species, suggesting that internal seedling N status plays a significant role in root architecture. Increasing N availability reduced Kl and Ks, which conflicts with a higher growth and photosynthesis rate of N fertilized plants. In contrast, high N availability increased Kr and especially Kfr, which is consistent with published results at interspecific level, where increase in plant hydraulic efficiency is linked to higher growth and photosynthesis. These contradictory results depending on the standardization method emphasizes the importance and difficulty of developing specific hydraulic comparison criteria that match the hydraulic functionality with water demand and other key ecophysiological processes of plants. In addition, according to the per species results, compared pines showed differences in their hydraulic efficiency, but these differences did not correspond with their ecology. Conclusions: despite their close phylogenetic relatedness, species ecology determined the effect of N availability on frost tolerance and root development. Specific differences might reflect a trade-off in the use of N: allocation of N to growth versus allocation to stress resistance. The contradictory results of the effect of N supply on root hydraulic efficiency shows the shortfalls of standardization methods to compare plant hydraulics.