How trees mature – understanding development phases

It’s probably obvious that many plants, like humans, go through different developmental phases throughout their lifetimes. In plant biology, a developmental phase describes a period of time during which a stem produces a specific type or combination of organs, such as shoots & leaves (vegetative organs) or flowers & cones (reproductive organs). In fact for plants it is individual stems, not entire plants, which go through these phases and so a single plant can have stems which each are in a different phase.ref

All trees start in a vegetative phase – the initial growth phase when the tree is establishing. Usually this means that only foliage is produced, and no cones, flowers or fruit – in fact the growing tip is not capable of producing flowers during this phase. The vegetative phase can have stages within it, for example juvenile foliage may be produced before adult foliage, however in vegetative phases, stems are programmed genetically to produce only shoots and leaves. Although we often refer to defined ‘juvenile’ and ‘adult’ foliage in trees, it is apparently a bit more complicated, with variation within the phases as well. In fact many different attributes are affected by the stem’s phase, including the size and shape of leaves (as seen in conifer needle and scale foliage), phyllotaxy (the arrangement of the leaves), plastochron (the time between leaf primordia emerging), internode length, adventitious root production, trichome distribution and cell size.ref These are all – as you have probably worked out – genetically controlled, apparently by ‘microRNAs’ref, specifically miR156 and miR172 as shown below.

Examples of juvenile and adult foliage are shown below, these show stems which have undergone a vegetative phase change.

A phase change is when a stem starts producing a different type of organ from its growing tip (meristem). For example, it starts to produce buds which will become flowers. Phases changes are usually – but not always – stable – that is they don’t tend to move in reverse order. Once a tree has reached the reproductive phase on a particular stem, it should retain that capability since the meristem has changed to the new phase. As an illustration of this is that when propagating cuttings and air layers, once they are successfully rooted, the stem will maintain the properties it had on the tree (until that stem goes through the next phase change). If it had flowers before, it will continue to flower. In some species – particularly conifers – if the stem was horizontally oriented it will continue to grow horizontally.ref

One good example of phase differences on a tree is suckers. Suckers are shoots which emerge from the base of a tree, and as they are derived from buds which have not passed through the same growth process as the rest of the tree, they are usually juvenile vegetative shoots, even if the main branches of the tree have reached a flowering phase.

One study found a logical sequence of developmental phases based on biochemical factors which turned on certain genes.ref They found that substances which are important for embryo development in the seed promote the initial vegetative phase. It’s then sugars – the product of photosynthesis – which contribute to an ‘adult’ vegetative phase change. So continued photosynthesis and the production of more sugars over time, promote phase change. Plant growth regulators (aka phytohormones) also play a role, with Gibberellin A3 shown to revert ivy back to juvenile foliageref, although the exact interplay with auxins and other substances is still not clear as of 2020.ref A key finding from this study was that defoliation delays vegetative phase change – so don’t defoliate or prune if you’re trying to develop mature foliage!

When a stem moves into a reproductive phase, the structure of the growing tip changes so that floral organs (which become flowers) are produced instead of shoots & leaves. In woody perennials (ie. trees) which have reached the reproductive phase, stems can transition between vegetative and reproductive, allowing them to continue to grow, as well as reproduce.ref For example, they may produce vegetative buds at the start of the new stem, reproductive buds in the middle and more vegetative buds at the end.ref This is all regulated by genes. One study on poplar identified two genes which control this transition based on environmental conditions – illustrated in the diagram below. The gene FLOWERING LOCUS T1 drives reproductive onset – in experiments, FT1 caused vegetative meristems to transition to reproductive when it was expressed in response to winter temperatures. As a result, the organs developing inside the winter bud moved from vegetative (formed earlier) to reproductive (formed later when it was colder). This created a bud with both forms of stem waiting to emerge in spring. Its partner gene FLOWERING LOCUS T2 then took over during warmer weather and drove vegetative growth.

If you are working with material which has not yet flowered, you would probably like to know how long it will be before it does, and what you can do to encourage your tree to flower. This is where horticulturalists use the concept of ‘growing degree days’. Growing degree days is a measure of the amount of heat that a plant has received over its lifetime (this would also be associated with the amount of light, which as we read above drives sugar development which in turn encourages phase change). Growing degree days (“GDD”) are used when planning crops and flowering annuals & perennials. They are calculated as follows:

GDD = t (days) x ( (TMAX+TMIN)/2 −TBASE)) ; where TMAX and TMIN are daily maximum and minimum air temperature, and TBASE is a known baseline temperature.

For example, I have recently been trying to grow the Australian plant Sturt’s Desert Pea (Swainsona formosa). Studies have shown that this species requires 874 GDD for 100% of axillary branches to flower and 988 GDD for 100% of main stems to flower.ref This is an extremely high light and temperature requirement for what is effectively an annual, so I have a heat lamp (and a grow lamp) providing daytime temperatures of 28oC and evening temperatures of 18oC. The base temperature for the calculation is 5oC.

So the number of days theoretically required to achieve 100% flowering on axillary stems using my setup will be a minimum of 874 / ((28 + 18)/2 – 5)) = 874 / 18 = 49 days.

You can see that if this relationship is true then global warming will shorten the flowering time of plants since plants will achieve their GDD faster. And this is what has been observed; in the UK researchers found that plants are flowering a month earlier due to climate change.ref

For trees which have to first achieve reproductive maturity, then generate floral organs, it’s likely that both growing degree days and other environmental accumulations (such as a cold period known as vernalisation, light levels and total rainfall) are involved.ref The key point is that these are accumulations of the factor in question, which implies that time is needed, as well as the correct conditions.

What does this all mean for bonsai?

Firstly if you are obtaining material for bonsai, consider what type of phase you want for the tree. If you want a flowering tree straight away, you need to take a cutting or air layer from a stem which has reached the reproductive phase. A sucker, or seed, will start from scratch right at the beginning of the tree’s development – and depending on the species it may never flower or fruit the entire time that you own it! As has been noted elsewhere in this blog, if you have a tree with juvenile foliage and you keep pruning it back, it may never reach an adult foliage or reproductive phase, because it may not have accumulated the amount of sugar or growing degree days to move to that phase. So when sourcing a new tree, if you want fruit or flowers you should make sure that it has produced these already.

Also, the environmental conditions which your tree is naturally used to are important for its phase transitions. Using the above example, if you put a poplar indoors where it never gets the cold temperature signal to activate FT1, it won’t create flowers or seeds. When you have a non-native tree in your collection, it’s a good idea to research its usual climate and to try to replicate it as much as possible.