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

Variation in the water exchange rate (WER) of freshwater lakes has
recently been demonstrated to affect the phenotypic traits of submerged
macrophytes. However, whether these responses are size-dependent
(“apparent plasticity”） or not (“real plasticity”) remains
unclear. Here, we used allometric analysis to investigate phenotypic
plasticity, and the carbon, nitrogen, 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). Result showed that biomass accumulation, below-: above-ground
biomass ratio, root length, root diameter, leaf length, and leaf width
significantly decreased with increasing WER; however, specific root length
was unaffected, and specific leaf area (SLA) gradually increased with
increasing WER through the whole experiment. Allometric analyses showed
that shifts in leaf width and SLA were determined solely by changes in
plant size; all other morphological adjustments were largely
size-dependent. Stoichiometric calculations indicated that ramets were
mainly N-limited, and this limitation was significantly associated with
leaf growth, thereby restricting the potential for real plasticity in leaf
morphologies. Thus, V. natans adapts to WER variation largely through
apparent plastic responses that reduce the demand for plant
nutrients.