We know much more about photosynthesis than about the flowering of the plant. This sometimes leads to surprises, especially with new crops. The grower has to take into account the juvenile phase, effect of temperature, light, size of the plant, day length, and the interaction between hormones, sugars and other compounds in the plant.
Before a plant can flower it first has to become an adult. Many plants have a juvenile phase. Even under optimal conditions they are unable to flower during this stage.
This is logical because a plant flowers in order to reproduce. Therefore the flowers must be of sufficient quality to actually achieve this. They have to be developed to the extent that they can be pollinated, for example by insects. And after pollination all kinds of processes need to start for the fertilisation and development of seeds and fruits. All of this costs a lot of energy. So from the plant’s point of view it’s considered ‘wise’ to postpone these processes until sufficient assimilates are available in the plant.
From juvenile to adult phase
The duration of the juvenile period varies enormously, from a few days to a few decades in trees. Of course, for the grower this can be very unprofitable if you have to wait a very long time for it to become productive. That’s why it’s good that a cutting or graft taken from a plant that is already in the adult phase also remains an adult.
The switch from juvenile to adult phase happens quite abruptly. The moment at which this occurs can depend on the size of the plant, age, number of leaves and growth factors.
Hormonal factor
From a wide range of research it’s clear that a hormonal factor plays a part in the transition from vegetative to generative. Suddenly the apical bud changes in shape as a forerunner to flowering. For a long time researchers looked for a hormone that stimulated flowering. The unknown flowering hormone was even given a name, namely florigen. But, it is now clear that florigen does not exist.
Although gibberellins play a role in many plants – this group of hormones was for a long time the leading candidate for the role of florigen – the situation is still ambiguous. In some plants gibberellins actually slow down flowering. Bearing this in mind, it’s also remarkable that growth inhibitors, such as daminozide, that slow the activity of gibberellin, do prevent the long and thin development of flowering plants, but not the flowering itself.
Another hormone group, the cytokinins, plays an important role in the induction of flowering. But again no general rules apply.
It seems that an interaction between hormones, such as gibberellins, cytokinins and ethylene, as well as sugars and other substances, such as polyamines, causes the induction of flowering. It’s different for every crop. The limited knowledge about the mechanism of flowering makes it difficult to effectively influence flowering. This is especially the case for new ornamental crops. Usually, the practical research focuses on achieving the most appropriate cultivation measures, without knowing exactly what happens inside the plant.
Leaves under first truss
Fortunately, a lot of research has already been done on the major horticultural crops. One of the many crops examined is tomato. A grower would like the plant to start producing quickly, and in terms of the tomato this means: the number of leaves under the first truss has to be limited.
In theory, a certain amount of assimilates must first be present in the tomato plant before it can start to flower. Indeed, research shows that any procedure taken to increase the amount of assimilates speeds up flowering. More light means fewer leaves under the first truss. A higher temperature at a low light intensity also leads to more leaves under the truss because the plant consumes more energy at a higher temperature.
As well as having a minimum quantity of assimilates, distribution is also important. At a lower temperature the top of the plant – the apex – has an advantage as it competes with the leaves.
This knowledge is difficult to translate into other crops. In fact the influence on flowering should be examined separately for each crop.
Short day = long night
A special phenomenon is the sensitivity of a flower to day length. In this respect, the origin of the plant makes a big difference. At the equator the length of day and night are the same and tropical plants are not day length sensitive. Plants from higher altitudes, that flower in the spring or even in the autumn, do tend to be sensitive to day length.
Sensitivity to day length exists as a result of natural selection. Therefore it’s also possible to remove this sensitivity by selection. By consistently selecting and further propagating the most insensitive plants it’s possible to solve this inconvenience. This doesn’t work sufficiently well with all crops, so we do encounter short day plants such as poinsettia, chrysanthemum and kalanchoe and long day plants such as gypsophilia, trachelium and carnation.
The naming is actually wrong. A short day plant is actually a long night plant because it’s all about the length of the dark period. And if this is broken – even for just a very short period – the whole effect of the dark period is lost.
Length of dark period
The plant registers the length of the dark period in its leaf but flowering takes place elsewhere. Therefore there has to be some communication between the leaf and the point where flowering occurs. This is carried out by a hormone that is produced in the leaf and then travels to the point of flowering.
How does the plant measure the length of the dark period? Previously researchers thought that the pigment phytochrome slowly broke down during the night into another form and that this was a signal to the plant to start flowering. But it’s more complicated than that. There is an interaction between the endogenous rhythms in the plant (‘the biological clock’). As a result the same length of darkness can sometimes produce different effects, whereby temperature can also play a role.
Some short day plants need just one long night. One of the most well known short day plants in horticulture is the chrysanthemum and it actually needs several weeks of long nights. If a grower stops the dark period prematurely, abnormalities occur. Just after a few short days the growth point becomes generative and stops producing leaves. Yet a grower has to continue with the long night regime for several weeks. It’s likely that multiple genes are involved in the flowering of chrysanthemum and it’s not simply a transition from vegetative to generative based on one gene that can be turned ‘on’ or ‘off’.
Mutual competition
Once the plant has switched from vegetative to generative and then flower buds have actually formed, many things can still go wrong. The buds can dry out or fall off and the flower may not open properly. This is mostly a question of how well the flower bud and the flower have been supplied with water, minerals and assimilates.
The flower has to compete with other parts of the plant and sometimes loses the fight. Optimal climate conditions, providing enough light and water, reducing the competition with the young leaves (by picking leaves) are all ways to ensure that flowering is successfully achieved.
Summary
A plant can only flower when it is mature. In horticulture, we bypass the juvenile phase by using cuttings and grafting. The transition from vegetative to generative appears to be controlled by a hormones. Flowering is the result of an interaction between several substances, for example, gibberellins. We still know too little about flowering which is sometimes difficult when working with new crops. A lot of research has been carried out on the major horticultural crops such as tomato and chrysanthemum. The latter is the best-known short day plant, although we should call it a long night plant.
Text: Ep Heuvelink (Wageningen University) and Tijs Kierkels. Images: Theo Blom (University of Guelph, Ontario, Canada).
