Nanoencapsulation improves the protective effects of a nitric oxide donor on drought-stressed Heliocarpus popayanensis seedlings.

Despite the important role played by nitric oxide (NO) in plants subjected to abiotic stress, NO donors application to induce drought tolerance in neotropical tree seedlings has not yet been tested. It is also worth investigating whether NO bioactivity in drought-stressed seedlings could be potentiated by NO donors nanoencapsulation. The aim of the current study is to evaluate the effects of chitosan nanoparticles (NPs) containing S-nitroso-mercaptosuccinic acid (S-nitroso-MSA) on drought-stressed seedlings of neotropical tree species Heliocarpus popayanensis Kunth in comparison to free NO donor and NPs loaded with non-nitrosated MSA. Nanoencapsulation slowed down NO release from S-nitroso-MSA, and nanoencapsulated S-nitroso-MSA yielded 2- and 1.6-fold higher S-nitrosothiol levels in H. popayanensis roots and leaves, respectively, than the free NO donor. S-nitroso-MSA has prevented drought-induced CO2 assimilation inhibition, regardless of nanoencapsulation, but the nanoencapsulated NO donor has induced earlier ameliorative effect. Both NO and MSA have decreased oxidative stress in H. popayanensis roots, but this effect was not associated with antioxidant enzyme induction, with higher seedling biomass, or with proline and glycine betaine accumulation. Nanoencapsulated S-nitroso-MSA was the only formulation capable of increasing leaf relative water content in drought-stressed plants (from 32.3% to 60.5%). In addition, it induced root hair formation (increase by 36.6% in comparison to well-hydrated plants). Overall, results have evidenced that nanoencapsulation was capable of improving the protective effect of S-nitroso-MSA on H. popayanensis seedlings subjected to drought stress, a fact that highlighted the potential application of NO-releasing NPs to obtain drought-tolerant tree seedlings for reforestation programs.

Nitrogen use plasticity in response to light intensity in neotropical tree species of distinct functional groups.

Environmental conditions influence the use of different nitrogen (N) sources by plants. We hypothesized that an increase in light intensity favors the use of nitrate (NO3 - ) relative to ammonium (NH4 + ) by seedlings of neotropical tree species from different functional groups, that is, Cecropia pachystachya (a shade-intolerant species), Cariniana estrellensis (a shade-tolerant canopy species), and Guarea kunthiana (a shade-tolerant understory species). We analyzed the growth and N metabolism in seedlings simultaneously provided with NH4 + and NO3 - , under lower (LL) and higher (HL) light intensity. 15 N incorporation into amino acids was monitored after incubation with 15 N-labeled NH4 + or NO3 - . Under HL, all species showed decreased leaf area ratio, and increased growth, nitrate reductase activity and assimilated N content. Cecropia pachystachya increased the use of both N sources under HL, with substantial increases in 15 N-amino acids derived from 15 NO3 - (12.5- and 4.0-fold in roots and leaves, respectively) and 15 NH4 + (4.5- and 3.0-fold in roots and leaves, respectively). Guarea kunthiana showed the greatest plasticity in N use, as the assimilation of 15 NO3 - in roots and leaves increased substantially under HL (11.2- and 17.0-fold, respectively). Cariniana estrellensis increased the assimilation of 15 NH4 + in roots and 15 NO3 - in leaves under HL. Therefore, the responses of N use strategies to light intensity varied with the species according to their ecological characteristics.

Nitrogen use strategies of seedlings from neotropical tree species of distinct successional groups.

Few studies have analyzed the strategies of neotropical tree seedlings for absorbing, translocating and assimilating the nitrogen. Here, we compared the nitrogen use strategies of seedlings from six tree species that are native to the Brazilian Atlantic Forest and that belong to different successional groups: Trema micrantha, Heliocarpus popayanensis and Cecropia pachystachya (pioneers), Cariniana estrellensis, Eugenia brasiliensis and Guarea kunthiana (non-pioneers). The effects of cultivating seedlings with nitrate or ammonium on the growth, physiology and nitrogen metabolism were analyzed. Nitrate-grown pioneer species had much higher leaf nitrate reductase activity than non-pioneer ones, but non-pioneer seedlings were also able to use nitrate as a nitrogen source. In addition to this remarkable difference between the groups in the capacity for leaf nitrate assimilation, substantial variations in the nitrogen use strategies were observed within the successional classes. Differently from the other non-pioneers, the canopy species C. estrellensis seemed to assimilate nitrate mainly in the leaves. Morphophysiological analyses showed a gradient of ammonium toxicity response, with E. brasiliensis as the most tolerant species, and T. micrantha and H. popayanensis as the most sensitive ones. Guarea kunthiana showed a relatively low tolerance to ammonium and an unusual high translocation of this cation in the xylem sap. In contrast to the other pioneers, C. pachystachya had a high plasticity in the use of nitrogen sources. Overall, these results suggest that nitrogen use strategies of neotropical tree seedlings were not determined solely by their successional position.