I am looking for a chemical reaction which can generate a lot of Gas in a short period of time, on the order of seconds, to build up pressure and allow a plant to propel a seed like projectile.
The main limitation is that the reaction should be plausible from a natural evolution perspective. For instance, Potassium + Water is not plausible because how would it evolve ? This is not a rhetorical question but rather a "Vibes" based one. Of course you could construct a plant which naturally evolved the ability to make C4, or any other compound. But even if in World it naturally evolved no reader would say "Yeah, this checks out". So i am looking for a reaction that isnt obviously a product of my, or i guess your, idea but could be seen as something that naturally evolved.
In addition, it would be good if the reaction was a repurposed trait. So say two chemicals a plant could feasibly already have, which were simply repurposed for this goal. Since this is something evolution likes to do.
Whilst the plant(s) in this context have undergone evolution on an exoplanet, since this is something that has not evolved on Earth it is ok to just consider this question in the context of Earth biology. I can take it from there and we all get spared a lot of extra unneeded context.
I have done a bit of googling into this, to check the few reactions i knew from memory. Most dont appear plausible. For example, Acetic Acid + Sodium bicarbonate. Both of these are fairly simple compounds and Acetic acid could presumably be part of another chemical process inside a plant. But Sodium Bicarbonate ? Idk... Other stuff, like Ammonium Nitrate + Water seems questionable from the NH4NO3 side.
So yeah, i hope i explained by issue adequately. Thanks for reading !
EDIT; Additional Context upon request
A core concept of the Exoplanets evolution are BPNs, Biochemical Pulse Networks. A BPN, loosely speaking, is a large collection of networked Plants, here called Lignicolae, which can communicate with each other and are organized in a hierarchical system.
Some Plants, Intrinsic Members, are responsible for processing information and controlling the collective, redistributing resources etc. Generative Members are responsible for making Energy, Perceptive Members collect information and so on. Note, these niches are generated by AED (Artificial Ecological Drift). The idea is that the BPN will apply new selective pressures which steer evolution. Say we have two plants, both of which can detect Sunlight and transmit this information to the BPN. But one, through random mutations, does it a little bit better. The BPN can understand this difference and will allocate more resources to the slightly mutated and for its purpose better, plant. Which intern applies a new selective pressure, as for this species it suddenly becomes valuable to develop better Vision. Because better Vision means more energy from the BPN. This is AED in a nutshell and it has resulted in stuff like this;
On the left you have a Teptelit, and on the right a Talantal. Both are Perceptive members which through AED has vastly over exaggerated sensory structures. The Teptelit has a enormous Compound eye, whilst the Talantal can hear and generate sounds.
With this basic setup, this question is about Protective members, which fall into a few classes. The one i am struggling with are those which launch projectiles.
What they have to overcome is a Frontline. Opposing BPNs will intoxicate the bordering soil and build Fortification structures. So "No-Mans land" can be 100s of meters wide. Additionally, the Frontline between two BPNs is almost always deprived of resources maybe one or two kilometers inwards and entirely supported by resource streams coming from deeper within the Forest. As such, the plants in question would have to launch their projectiles at least a few 100 meters. They also need to be fast enough to dig themselves into the ground upon impact and deliver a heavy ish payload.
What the payload does depends on the species. The example species i am working on would launch a package that aims to disrupt operations on the other side. So imagine a big seed that gets fired to the other side, quickly connects itself to the supply lines and causes havoc by sending in biochemical stimuli to confuse the BPN. But this is just one possible specialization. Another will be launching seeds to the other side to establish a bridgehead. So we are talking about maybe a few kg on the high end over a few 100 meters.
As for why this is more practical than having birds do it. There are two reasons. BPNs have very tight control over anything directly connected to them. They can, through for example Talantals, communicate with Animals, but that is less precise. There are many cases where BPNs will use Animals for their advantage. For instance there are Protective members which grow across the Frontline and are protected by small insect like creatures. So this is a thing, its just that there are multiple ways of doing the same thing. The 2nd reason is resource related. Opposing BPNs do have ways of dealing with intruding Animals, and are not really Afraid of going scorched earth. Any intrusion / Push is a multi decade long process and it is, over the long term, more sustainable to have static attack vectors. But again, this stuff is still used.
I hope this cleared stuff up xD