Ecophysiological and anatomical responses ofVallisneria natansto nitrogen and phosphorus enrichment

Abstract

Here, we describe an experiment using four nitrogen (N) and phosphorus (P) concentrations to investigate the effects of nutrient enrichment on the submersed macrophyte Vallisneria natans (tape grass) grown in a sand culture medium. The objective of this study was to examine the influence of nutrient enrichment in the water column on V. natans, especially with regard to anatomical structures. The results showed both the absolute growth rate (AGR) and intrinsic efficiency of light energy conversion of PSII (Fv/Fm) decreased with increasing nutrient levels. Root morphological characteristics, including the total root length (L), root surface area (SA), projected root area (PA), total root volume (V), average root diameter (AD), total root length per volume (LPV), total tips (T) and total forks (F), also showed a generally negative relationship with increasing nutrient concentrations. The anatomical structures of stolons and leaves also changed with nutrient enrichment. The shrinkage of aerenchyma and disappearance of starches and chloroplasts were the main structural changes leading to poor growth. These phenomena, especially the anatomical changes, might be the mechanism underlying the effect of nutrient enrichment on V. natans growth. Nous décrivons ici une expérience utilisant quatre concentrations d'azote (N) et phosphore (P) afin d'étudier les effets de l'enrichissement en éléments nutritifs sur le macrophyte immergé Vallisneria natans cultivé dans un milieu de culture sableux. L'objectif de cette étude était d'examiner l'influence de l'enrichissement en nutriments dans la colonne d'eau sur V. natans, en particulier en ce qui concerne les structures anatomiques. Les résultats ont montré à la fois que le taux de croissance absolue (AGR) et l'efficacité intrinsèque de la conversion de l'énergie lumineuse du PSII (Fv/Fm) ont diminué avec l'augmentation des niveaux d'éléments nutritifs. Les caractéristiques morphologiques racinaires, y compris la longueur totale des racines (L), la surface des racines (SA), la zone des racines prospectée (PA), le volume total des racines (V), le diamètre moyen des racines (AD), la longueur totale des racines par volume (LPV), le totalité des extrémités (T) et des ramifications (F), ont également montré une relation globalement négative avec RÉSUMÉ Réponses écophysiologiques et anatomiques de Vallisneria natans à un enrichissement en azote et en phosphore Mots

Untitled

“…observation). Similarly, several authors suggest observed declines in macrophytes in other systems was a direct effect of increased NO x -N (Burkholder et al 1992;Burkholder et al 1994;Wang et al 2012), suggesting closer investigation of this phenomenon is warranted. Burkholder et al (1992) report that Zostera marina (eelgrass) exhibited highly negative physiological effects (even death) when dosed with 0.05, 0.1 and 0.5 mg L -1 NO x -N. Although a marine species, this plant shows extreme sensitivity to increased nitrate evidenced through loss of carbon storage in roots unrelated to shading by algae.…”

“…Been (Ni 2001). The most definitive observations of inhibition were in the form of shrinkage of arenchyma tissues and disappearance of starches and chloroplasts observed in increased NO x -N and NH 4 -N concentration treatments of Vallisneria natans (Wang et al 2012.). It is important to note that ammonia, whether sourced from high concentrations in water or derived (via intracellular nitrate reduction) from elevated nitrate availability to SAV is the source of toxicity.…”

“…However, the potential of NO x -N toxicity or inhibition of SAV is not well understood, nor is it intuitive given our understanding of mechanisms of toxicity for higher organisms. Observations coinciding with elevated inorganic N (NO x -N and or NH 4 -N) in plants include stunted growth, iron deficiency, amino acid accumulation, oxidative stress and structural tissue damage (Burkholder et al 1992;Smolders et al 1997;Smolders et al 2000;van der Heide et al 2008;Wang et al 2012). To better determine potential inhibitory mechanisms of NO x -N, a closer look at the process of nitrogen assimilation is necessary.…”

“…The energetic demand of reducing the resulting ammonia toxicity, in concert with unregulated ANR, could represent a very significant stress on SAV (Smolders et al 2000;Wang et al 2012). …”

“…Buildup of free amino acids in tissues is considered an indication of "nitrogen overload" or impending toxicity due to excessive nitrogen availability (Smolders et al 1996;Smolders et al 2000;Wang et al 2012). Specific types of amino acids that accumulate in tissues (for instance arginine, glutamine, asparagines) are dependant on the stresses involved (toxicity, mineral deficiency, grazer pressure) and the species and constant exposure due to high flow conditions), the potential for increased biomass of SAV to ameliorate potential toxicity is not anticipated.…”