Relationships between allometric patterns of the submerged macrophyte Vallisneria natans, its stoichiometric characteristics, and the water exchange rate

Abstract

Hydrological variation is well-known to affect the morphological plasticity of aquatic macrophytes; however, whether these responses are size-dependent (“apparent plasticity”) or not (“real plasticity”) remains unclear, particularly with respect to the water exchange rate (WER). Here, we used allometric analysis to investigate the morphological plasticity and the carbon, nitrogen (N), and phosphorus stoichiometry of the submerged macrophyte Vallisneria natans cultured for 15, 30, 45, and 65 days under three levels of WER (0, 20, and 40% exchange of total volume day−1). We found that the negative effects of a higher WER on plant growth were primarily attributable to a deficiency in dissolved carbon dioxide, which resulted in shorter and thicker roots, smaller leaves (decreased width and length), higher specific leaf area, and lower below-: above-ground biomass ratios. Allometric analyses showed that the changes in root length reflected real plasticity, whereas other root and leaf adjustments indicated apparent plasticity. Furthermore, stoichiometric calculations indicated that N limited the growth of leaf tissues more than root tissues, thereby restricting the potential for real plasticity in leaf morphology. Therefore, V. natans adapts to WER variation largely through apparent plastic responses that can reduce the demand for plant nutrients.