To what extent can we extrapolate what we know of terrestrial plants to our freshwater aquatic plants?
In terrestrial plants, blue light suppresses the hormones auxin, leading to shorter stem inter-nodal distance between leaf nodes. This is exemplified by experiments such as the one below:
In nature, most aquatic plants are marginal/marsh plants. Gaseous exchange underwater is 10000 times slower than in air - it's much easier for these marginal plants to grow when they have the aerial advantage as this gives them free access to CO2/O2 in air.
When it's access to CO2/O2 is cut off sharply by submergence. This leads to rapid accumulation of the hormone ethylene in the plant meristem, which induces rapid stem elongation in an attempt to break back up the water surface to access air. When CO2/O2 levels are poor, plants will put all energy available into stem elongation, even sacrificing the maintenance of older leaves, in order to access surface air to survive. This is why bare lower portions on stem plants is quite commonly seen in setups with no CO2 injection.
Inter-nodal distance in many aquatic plants is thus generally more impacted by gaseous exchange & flooding response. To prevent overly long internodes, ensure good O2 and CO2 levels in your tank, which alleviates ethylene accumulation.
Flowering cycles in aquatic plants are similarly more connected to access to surface air than just red light spectrum. These are just a demonstrate how the physiology of aquatic plants can different quite a bit from terrestrial plants due to adaptations to living in water.
That being said, if CO2 and O2 levels are already optimized, based on anecdotal evidence by hobbyists, stronger red/blue spectrum does seem to give slightly more compact growth forms.
These are a few factors that give more compact growth forms for aquatic plants. Some species will react more strongly to certain factors than others:
- Higher overall light (PAR) levels
- Lower temperatures
- Slower growth parameters / slower growth rates (including lower CO2 and nutrient levels)
- Being planted in open space