Does inoculation with associative bacteria improve tolerance to nitrogen deficiency in seedlings of Neotropical tree species?

Abstract

Nitrogen (N) deficiency generally affects plant physiology, influencing N metabolism and inhibiting photosynthesis and plant growth. The association with plant growth-promoting rhizobacteria (PGPR) can mitigate environmental stresses, such as N deficiency. No studies have examined whether inoculation with PGPR may counterbalance the effects of N restriction on the metabolism and growth of neotropical tree seedlings with different resource-use strategies. The current study aimed to determine the effects of inoculation with PGPR on the tolerance of seedlings of tree species submitted to N deficiency. Two species with acquisitive (Cecropia pachystachya and Heliocarpus popayanensis) and conservative (Cariniana estrellensis and Cabralea canjerana) resource-use strategies were chosen. The strains Ab-V5, of Azospirillum brasilense, and ZK, of Bacillus velezensis, were selected as PGPR for inoculation. Seedlings of the four plant species were grown in a mixture of soil and sand with (N+) or without (N-) N supplementation. The seedlings were also inoculated with Ab-V5 or ZK strains and, as a control, for each N treatment, some of the seedlings were not inoculated with PGPR (Nin). Thus, we assessed the effects of N deficiency, inoculation, and the interaction between these two factors on N metabolism, photosynthetic, and growth variables. Nitrogen deficiency limited photosynthesis in all plant species, in addition to decreasing the concentration of amino acids and proteins and the activity of enzymes of N metabolism. Only the growth of C. canjerana was not inhibited by N restriction. Inoculation with Ab-V5 led to increased growth of C. pachystachya seedlings, which was more pronounced under N supplementation (+70 % of total dry mass). The association with PGPR decreased nitrate reductase activity (-52 %) and increased glutamine synthetase activity (+370 %) in the leaves, among other changes in N metabolism. The growth of H. popayanensis and C. estrellensis seedlings was not affected by inoculation, but metabolic variables in these species were influenced by inoculation with ZK. Photosynthesis increased with inoculation with ZK in H. popayanensis and with Ab-V5 in C. estrellensis. In C. canjerana, both PGPR promoted seedling growth (+11 % of total dry mass). Nitrogen deficiency and inoculation with PGPR influence photosynthesis, growth, and N metabolism of seedlings of tree species that occur in the semideciduous seasonal forest. Moreover, the effects of inoculation are influenced by N supplementation during cultivation. The positive effects of PGPR on growth were higher in N-supplemented seedlings. Regardless of the functional group, inoculation with PGPR of seedlings of tree species of the Atlantic biome stimulates growth and development in optimal N conditions and increases tolerance to N restriction.