Alternanthera reineckii

Alternanthera reineckii, commonly known as Scarlet Temple, is a classic choice for hobbyists looking to introduce and maintain intense red coloration in a planted aquarium. Its appeal lies in the deep magenta to scarlet hues, which are most prominent on the abaxial (underside) of the leaves. Achieving and maintaining this coloration is directly linked to the tank's conditions. High-intensity lighting is the most crucial factor; under low light, the plant tends to lose its red pigment, reverting to olive green or brownish tones, and may become 'leggy' as it sheds its lower leaves while stretching towards the light source. Supplementing with CO2 injection significantly enhances both its growth rate and color vibrancy, allowing it to flourish rather than just survive.

This plant is a heavy root feeder and benefits immensely from a nutrient-rich substrate or the regular use of root tabs. It also absorbs a significant amount of nutrients from the water column, so a comprehensive liquid fertilizer regimen is highly recommended. Pay special attention to iron (Fe) supplementation, which is vital for red plants to produce their signature pigments without compromising chlorophyll production.

In aquascaping, its vertical growth habit makes it a versatile midground plant. It can be planted in dense groups to form a 'street' or 'hedge' of color, guiding the viewer's eye through the layout. Propagation is straightforward for this stem plant: simply trim the top half of a healthy stem and replant it into the substrate. The trimmed top will quickly develop roots, and the bottom, rooted portion will sprout new side shoots, resulting in bushier growth. It is often sold in its emersed form, which has rounder, less colorful leaves. When introduced to an aquarium, it will undergo a transition period, shedding its emersed leaves and growing new, more delicate submersed foliage. Do not be alarmed by this initial 'melting,' as it is a natural part of its adaptation process.

Alternanthera reineckii is an amphibious herbaceous perennial from the family Amaranthaceae. Its native distribution in South American floodplains and marshes explains its physiological adaptability to both emersed and submersed conditions. Morphologically, it exhibits a classic stem plant structure with a decussate phyllotaxy, where opposite pairs of leaves are arranged at 90-degree angles to the pairs above and below. The leaves are simple, with a shape ranging from lanceolate to ovate. Submersed leaves are typically more elongated and possess a thinner cuticle compared to their emersed counterparts, facilitating direct nutrient uptake from the water column.

The characteristic red to magenta pigmentation is due to the accumulation of anthocyanin pigments within the leaf vacuoles. The synthesis of these flavonoids is a complex process influenced by environmental stimuli. High light intensity, particularly in the blue and UV spectra, is a primary trigger for upregulating anthocyanin production, which serves a photoprotective function. The process is also linked to the plant's nutrient status; while a sufficient supply of iron (Fe) is a necessary precursor for pigment synthesis, a controlled limitation of nitrates (N) and phosphates (P) can further enhance red coloration as a mild stress response. However, severe nutrient deficiency will lead to stunted growth and necrosis.

As a member of the Caryophyllales order, A. reineckii is a true eudicot. In an aquarium environment, reproduction is almost exclusively vegetative. Fragmentation via pruning is the most common propagation method, with cuttings readily developing adventitious roots from stem nodes, especially when planted in a nutrient-rich substrate. In its emersed state, the plant can produce small, sessile, whitish flowers in the leaf axils, which are characteristic of the genus. Its high nutrient uptake rate classifies it as an effective tool for nutrient export in a closed aquatic system, sequestering nitrogenous and phosphorous compounds into its biomass, thereby competing with algae and contributing to the stability of the ecosystem. This biological filtration capacity, combined with its oxygen production through photosynthesis, makes it a functionally significant as well as aesthetically valuable component of a planted aquarium.