Emersed to submersed: why new aquarium plants change and drop leaves

Most aquarium plants start life out of water

Tropica, an aquarium-plant nursery, states that nine out of ten tropical aquarium plants are amphibious in the wild: they survive dry seasons above water and flooded seasons below it ^[1]. Nurseries exploit this. Tropica grows most of its plants in greenhouses with only the roots in water, in mineral wool in pots ^[1], and Aquasabi notes that most potted plants on the market are grown this way ^[2]. Tissue-culture cups are no different in that respect: they hold a nutrient gel with air above it, so the plants inside grow in their emersed form too ^[2].

The exceptions are plants that only ever live under water. Tropica names Egeria, Cabomba and Vallisneria as pure water plants that it cultivates submerged ^[1]; its Vallisneria ‘Tiger’ article calls that plant 100 % aquatic, with no emersed form ^[10]. A few tissue-culture products are grown in liquid medium for the same reason, among them Potamogeton gayi and Fontinalis antipyretica ^[2], and Tropica’s Eleocharis ‘Mini’ cup, which Tropica says lets the plant grow under water almost immediately ^[9].

Two kinds of leaf

Plants that can grow in both worlds often make different leaves in each, a habit botanists call heterophylly. A 2019 review summarizes the difference: leaves formed under water are thin and narrow and lack a cuticle and stomata, while leaves formed in air are thicker, broader, coated with a cuticle and carry stomata ^[6]. The switch is controlled by plant hormones and the environment; for water wisteria (Hygrophila difformis) the review lists abscisic acid, ethylene, gibberellin, humidity and temperature as triggers ^[6].

What happens to the old leaves was measured directly in water wisteria. Leaves grown on land and then submerged raised their underwater photosynthesis within five days, but their structure did not change. The plant’s full acclimation came from new leaves of the submerged type, which are thinner, have fewer stomata and can take up bicarbonate as a carbon source ^[7]. Our reading (an inference): a land leaf can work under water for a while but cannot become a water leaf, so an emersed-grown plant replaces its foliage over the following weeks.

What to expect after planting

Plant form	Typical behavior	Source
Emersed-grown stem and carpeting plants	Old leaves may stall or die back; new submerged leaves follow. Staurogyne repens: emersed leaves die off and can be removed at planting	[8]
Cryptocoryne and Bucephalandra	May lose adult leaves; new leaves soon sprout. See crypt melt	[3]
Tissue-culture cups	A few days of slow or stalled growth; stem plants may already show submerged-type leaves under the lid	[3]
Pure water plants (Vallisneria, Egeria, Cabomba)	Already submerged; no leaf change needed	[1, 10]

Tissue-culture plants have their own quirk. Leaves in a sealed cup form little cuticle, so they are more delicate than emersed-grown plants, and some show glassy, swollen tissue (hyperhydricity) caused mainly by leaves pressing against the lid; Aquasabi says this is reversible and affected plants often regrow healthy shoots after planting ^[3].

Why this matters for algae

The weeks of conversion are weeks of slow plant growth, and that is when new tanks get algae. Aquasabi’s account of a new tank links its brown and green algae phases to plants that are still converting from their emersed form and growing only slowly ^[5]. The practical consequence is the advice in our diatoms and thread algae guides: plant densely, keep the photoperiod short at first, and change water often.

The dry start

A dry start delays the conversion on purpose. The tank is planted, misted and sealed under film so the plants grow emersed for a few weeks before it is flooded ^[4]. Aquasabi notes that the emersed-to-submersed conversion then happens only after flooding, so the length of the dry phase is less important than whether the plants are healthy and the substrate covered; mold and bacteria are the main risk while it is sealed ^[4].