Often, in discussions about orchids – particularly phalaenopsis – the subject of “split leaves” comes up. That is, the plants’ leaves seem to “crack” down the mid-rib, separating at least part of the leaf into two pieces.
The cause of split leaves are a bit of an enigma. I have heard a variety of theories related to low humidity levels, too much water uptake, or a combination like the cuticle layer becoming less flexible due to a combination of low humidity and the drying effect of excessive light levels, while the softer tissues underneath try to grow or swell.
There must be some mechanical aspect of the issue, and the elasticity of the outer part of the leaf structure seems like a reasonable avenue to consider, but what would cause that to be an issue? I have two theories on that, both chemical, but one is the direct effect, while the other is environmental. The “direct” one being related to calcium, and the “environmental” one related to mineral ion concentration.
In the environments in which these plant originate, the food supply is very low – some measurements of “throughfall” and “stem flow” from the forest canopy in the tropics has it in the 15-25 ppm total dissolved solids (TDS) range. Orchids, therefore, have adapted to having very low concentrations of dissolved solids in their water & nutrient supplies, and accordingly, in their vascular fluids (sap), as well. One plant biologist and university professor I know estimated it to be roughly one-fourth of the concentration of that of terrestrial plants. The vast majority of the nutrient ions in that supply is nitrogen, but very low levels of the other essential mineral ions are also present.
I focus on calcium, as it is essential for a wide range of plant physiological processes, and is relatively immobile within the plant, so must be constantly supplied in order for newly-growing tissues to survive. It tends to be accumulated in cell walls, where among other things, it affects cell extension. It seems logical that an insufficient supply of calcium
Plants can take up nutrition passively by diffusion, based upon concentration gradients – if there is more available outside of the roots, more will be absorbed – and actively, through nutrient “pumps” that will take in more than diffusion would dictate. I think it’s probably safe to disregard the active uptake part, as it seems unlikely that a plant would “shoot itself in the foot”, resulting in leaf damage that might open pathways for infection. So if we focus on the passive uptake pathway, we can picture the following scenario:
In the environments in which these plant originate, the food supply is very low – some measurements of “throughfall” and “stem flow” from the forest canopy in the tropics has it in the 15-25 ppm total dissolved solids range. Orchids have adapted to having very low concentrations of dissolved solids in their water & nutrient supplies, and accordingly, in their vascular fluids (sap), as well. One plant biologist estimated it to be roughly one-fourth of the concentration of that of terrestrial plants. Now let’s expose them to higher concentrations of minerals in their water supplies, and what happens?
Their coping mechanisms might not be able to control the passive intake, leading to much higher concentrations in their internal fluids, and with the concentration of minerals being higher, the plant will compensate by absorbing a lot more water. If the rate of absorption of water is such that it outdoes the flexibility of the outer cell layers, then Boom!, split leaves can occur.
As a point of observation, I too, have experienced the occasional split leaves on phalaenopsis, but that appears to have been totally eliminated since I reduced my fertilizer concentration to about 35 ppm N – still about 250-300 ppm TDS, or ten times that seen by plants in nature – but maybe it’s enough.
I am not ruling out some chemical factors that might play a role in reducing the elasticity of the outer layers of the leaves, but I think the environmental factors may be more significant.