In order to maximise the chances of survival, plants have developed strategies which allow them to suppress growth when faced with stress in their environment. This trade-off has evolved over millions of years, enabling plants to redirect their energy and genetic resources to optimise their fitness. As the main goal of all living organisms is securing their genetic makeup in the next generation – in other words, long-term survival of the species – this can often be in direct conflict with what humans desire from their plants, i.e. yield optimisation/enhance crop production.
For this article, it is helpful to think of only 3 major transcriptional rearrangements the plant can undertake which determine the focus of the plant’s resources:
2) Stress Response.
3) Seed development (last phase of life usually).
It’s often a trade-off – when a plant is in full growth mode, exposure to stress can tilt the balance, dramatically reducing growth to deal with that stress.
Yield Optimisation – The Tradeoff
An example of this is found in the balance between optimal growth and defence against pathogens. Halting growth can result in poor quality, low-yielding harvests, and this is especially true if the plant assigns too many resources into responding to the pathogen (Fig 1). Once the plant becomes locked into these stress response pathways, it reaches a point where its priority is to produce seed, and so will switch from full stress response to full seed development, leaving growth and yield well behind.
If a plant funnels its resources into growth and ‘ignores’ the need to respond to stress, the plant may be rendered defenceless, leaving disease-free to take over and resulting in symptoms (Fig 2). However, this is highly dependent upon the capabilities of the pathogen – if it does not easily spread throughout the plant, it can actually be advantageous for the plant to ignore these stress signals and keep prioritising growth. Much like in humans with cold symptoms, the immune response is more to blame for the symptoms than the pathogen’s actions. In such cases, ‘ignoring’ the stress-signalling cues can be beneficial.
When a plant is highly susceptible to a pathogen, it can often devastate the plant entirely by killing it, but in other examples it simply makes the plant small and sickly, resulting in retarded growth. The level of susceptibility can correlate to growth rate, and common middle ground is often preferred. In the agricultural world, some breeding companies have a four-part view on this:
Yield, Yield, Yield, Resistance.
This strange ratio is indicative of the need to ensure a good harvest over hardened resistance, which can often result in yield reduction, and gives a clear measurement of what breeders are targeting. Most likely, keeping a balance is good for the plant, since exposure to a little stress as a seedling can lead to activation of the stress response pathways. However, as the plant is so young, it cannot switch from that to seed development. The typical outcome is a normal level of growth, but a heightened readiness for stress. (Fig 3).
Of course, the genetic starting point is key to all of the above. Understanding the environment, how the plants respond to stress, and what they offer in terms of self-protection, are central to producing a successful crop. In instances where a quality product is a priority over yield (for example, in the medical cannabis flower market), it is vital to ensure plants are stress-free and without unacceptable levels of pathogens in the finished product. This is paramount to the way we work at PharmaSeeds, where understanding the growing environment, the genetic capabilities of the cultivars, and matching this with the physiology and desired finished product are key considerations before making suggestions on cultivars.