Cannabis seed germination, dormancy and seed storage
As the primary starting material of nearly all plant species is seeds, it is important to understand the processes involved in germination, seed development, storage and dormancy. In general, germination is an extremely short-lived process, and is bookmarked by dormancy and radicle emergence. It begins with water uptake and ends at the point at which the radicle has broken through the seed coat layers and can be seen (see diagram below). In some species – and according to some experts, germination ends when photosynthesis begins – the difference in these definitions is unimportant and will not be discussed further in this article (Bewley 1997, Koornneef et al 2002, Kucera et al 2005). The germination mechanism is the changes in the seed which results in the activation of the embryo. For germination initiation, seeds generally absorb water – a process called imbibition – triggering the enlarging of the embryo, and mobilization of the seed’s food preserves, which then prompts protein synthesis and finally radicle emergence (Hasanuzzaman et al 2013, Muller et al 2006).
Cannabis seeds have an indeterminate flowering patterning, which means if left to grow naturally flower initiation continues over time, resulting in seeds of different developmental stages on different parts of the same plant (Canadian Hemp Trade Alliance 2019). Therefore, what prevents some healthy seeds from germinating before others? And how do seeds know when to germinate? For answers to these we must first look upstream at the process of seed dormancy.
As different species undergo different transitions to get to germination, defining dormancy is not easy. However, generally, it is defined by a very slow metabolic rate, lack of responsiveness to signalling mechanisms and inhibition of germination. Dormant seeds are seeds which are fully viable and able to germinate, but do not germinate under optimised conditions (Bewley and Black 1994, Vleeshouwers et al 1995, Lange 1996, Bewley 1997, Graeber et al 2012). Dormancy is an important evolutionary trait which can be both a measure of plant fitness and viability. Mutations which cause disruption to dormancy can be devasting for a lineage, as it can cause germination to be triggered when conditions are not favourable and, for example, can result in early germination causing reduction in vigour or plant death (Leon-Kloosterziel et al 1996, Abe et al 2019). However, for crops and many other important harvestable plants, lack of a dormancy mechanism can be a desirable trait as it means germination can be triggered at the point of sowing, resulting in synchronised growth (Kilian et al 2009, Gubler et al 2005).
When a seed is produced, the initial dormancy is highly induced and freshly harvested seeds will be more dormant than those which have been stored, for example. In cool, dry storage, seeds will generally mature/ripen and this coincides with a reduction in the dormancy (Holdsworth, Bentsink and Soppe 2008). This could be regarded as the last step in seed development and this mechanism can be used to a cultivator’s advantage.
A recent study on hemp seeds has shown that breaking dormancy is highly enhanced by pre-chilled storage. Otherwise known as cold stratification, this process involves incubating wet/moist seeds in cold conditions, for example, at 10oC for 5 days, and then transferring to 20-30oC. In the study published in the Journal of Agricultural Hemp Research by Elias et al 2020, the authors found that dormancy was reduced by a pre-chill treatment and that this is extremely important when the age of the seeds is unknown, 5 days cold treatment was enough to break the dormancy. Furthermore, this pre-chill treatment also acts as a hydropriming treatment, which triggers the enzymatic breakdown of the food reserves resulting in faster germination and growth and enhanced uniformity of growth (Elias et al 2020).
Cold stratification can be viewed as a method of accelerating the breaking of dormancy, but what happens when seeds are not pre-chilled before sowing? In dry, ambient storage, dormancy does still break down, but at a much slower rate. Release of dormancy is much less understood than dormancy initiation, and generally it is a very specific process for each species. Each species has a particular temperature which drives the breaking of dormancy (Probert 2000, Penfield et al 2005). However, the temperature sensing mechanisms within seeds have remained elusive for many years, and only recently have some Heat Shock Proteins been identified as being part of that process (Ma et al 2019).
With cannabis seeds, storage methods are important as the influence of intensive indoor breeding has resulted in a lack of selection pressure on the seeds natural cycle, which usually involves seed shedding, followed by seasonal conditioning through the autumn and winter months. It is important to store seeds in the correct conditions, and at PharmaSeeds, seed handling and storage is a key principle for our operations. In most cases, seeds are very resilient and it is unusual for germination rates to be zero, however inappropriate storage can result in a reduction of the germination rate. Generally cold storage is best, however considering that this increases the release of dormancy, consistency is more important than anything else. If temperature and humidity is in flux, there is greater chance of the seed becoming inviable. Also, if the seeds are stored at cold temperatures, then there is no way of further pre-chilling before germinating and other germination elicitors may be needed to enhance the chances.
The subsequent parts of this series will look at other types of seed treatments, priming and coating technologies, as well as a look at the type of endogenous influencers of germination which may be applied exogenously to enhance germination.
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