Quantifying the nitrogen allocation and resorption for an orchid pseudobulb in relation to nitrogen supply

Abstract As the main organ for storing water and nutrients, pseudobulbs are of central importance to the growth and survival of orchids. Quantifying nitrogen allocation and resorption of pseudobulbs is important for understanding the survival and adaptation strategies of orchids, but studies in this area are still lacking due to the complicated integration effect between pseudobulbs of different ages. In this study, nitrogen allocation to each organ and nitrogen resorption of Pleione aurita, an epiphytic orchid characterized by annually renewed pseudobulbs, were examined quantitatively under three nitrogen levels using 15N labeling. We found the old pseudobulb can act as a sink for exogenous nitrogen, although it had a much lower sink activity than the developing new pseudobulb. The single leaf accounted for the largest proportion of both exogenous and pseudobulb-stored nitrogen among all organs and in three nitrogen treatments, and the fruit tended to use pseudobulb-stored nitrogen preferentially regardless of exogenous nitrogen supply. The resorption of nitrogen in the leaf and pseudobulb was highly efficient, but decreased with increasing nitrogen supply. Overall, positive biomass accumulation was only observed in plants with exogenous nitrogen supply, while all fruit-bearing plants lost more than 30% of their biomass after a growing season regardless of nitrogen supply. The P. aurita plants exhibited a strong low nitrogen tolerance with only a slight loss of biomass for those nitrogen deficient and fruitless plants. Surprisingly, an increase in the nitrogen pool of the whole-plant was always observed regardless of biomass loss in fruit-bearing plants after nitrogen addition. Our results suggest an important role of the pseudobulb in the nitrogen economy of P. aurita, and the high dependence of the reproduction process on the pseudobulb-stored nitrogen. The findings will help to understand the adaptive strategies of orchids, and contribute to the conservation of endangered species.