Definition of phenology

Modern phenology is the study of the timing of recurring biological events in the animal and plant world, the causes of their timing with regard to biotic and abiotic forces, and the interrelation among phases of the same or different species. (Guidelines for plant phonological observations, Foundation for Sustainable Development, Netherlands)

Canola

Canola, Brassica napus, has proven itself to be a highly adapted crop across the world. It grows in diverse environments – from a crop that matures in 80 days in Canada to European crops that are almost 12 months old when harvested. However it is extremely unlikely that the European variety would flower at all when sown in the Canadian spring. Canola breeders have incorporated the features that allow canola to interact with the environment to produce an adapted variety. The key environmental influences are temperature and photoperiod and these determine the lifecycle of the crop so as to provide the best fit to the environment.

Overview of Crop Development

The canola plant goes through a series of developmental phases from sowing to harvest. While the crop is usually described in terms describing the vegetative appearance of the crop, ie “cabbaging” or “bolting”, there are several systems for describing the developmental stages the crop progresses through. The principal system of describing the growth stages used by Canadian agronomists is called the BBCH decimal system, which was developed BASF, Bayer, Ciba-Geigy and Hoechst. The BBCH decimal system is illustrated in Appendix 1. It is very similar to that of the Decimal Code for the Growth Stages of Cereals developed by the Dutch phytopathologist, Jan C Zadoks, in that it describes the growth in 10n  stages although stages 2 and 4 are not relevant. Other developmental systems have been developed, eg Harper and Berkenkamp, but are not widely used in Australia.

 

Sowing the crop – Growth Stage 0

After sowing, the seed adsorbs moisture and the various biochemical processes begin, resulting in the production of the first root and shoot. The root grows downward and develops root hairs that anchor the developing seedling. The new stem, or hypocotyl, begins growing up through the soil pushing the cotyledons or seed leaves. Emergence takes between 6 and 15 days depending on soil temperature, moisture and sowing depth.

As well as energy source to fuel the biochemical processes, the developing plant needs oxygen for respiration. Waterlogging results in oxygen being driven from the soil, as well as cooling the soil, resulting in slower growth rates. At this stage, all the energy required for the cotyledons to emerge is provided by the seed reserves. Deep sowing, small seed or any other factor that requires the plant to expend more energy getting the first leaf through to the surface such as crusting, apart from delaying emergence, results in weaker and smaller seedlings that may be more prone to weed and pest competition.

 

Vegetative phase – Growth Stage 1 – Rosette to Cabbaging

The growing point of canola is above the soil, between the two cotyledons. The exposed growing tip makes canola seedlings more susceptible than cereals to insect damage. Four to eight days after emergence the seedling develops its first true leaf. Subsequent leaves are produced at a rate determined by temperature. There is no definite number of leaves produced by a canola plant. A canola plant under good growing conditions normally produces nine to 30 leaves on the main stem depending on variety and growing conditions.

While the shoots continue to develop, a similar process is happening with the root system. Canola plants have a tap root system. The root system continues to develop with secondary roots growing outward and downward from the taproot. Root growth is due to cell division and enlargement at the tip of the root. Root development is relatively constant averaging nearly 2 cm per day as long as good soil moisture exists.

Roots do not grow in search of water or nutrients, they only intercept water and nutrients present in the soil pore space that they happen to contact. Factors limiting root penetration through the soil include a dry soil, soil compaction, weed competition for moisture and nutrients, salinity or cold soil temperatures.

 

Vegetative phase – Growth Stage 2

This growth stage refers to the development of vegetative side shoots (tillering) but is not applicable to describe the growth of many of the spring types grown in Australia but is applicable to the winter types, particularly if they are heavily grazed.

 

Reproductive phase – Growth Stages 3 – 6 – Budding, Bolting and Flowering

While leaves are still emerging from the plant crown, the growing point of the plant has already changed from the vegetative phase (producing leaves) to the reproductive phase (producing flowers). Depending on the variety, the change in phase is due to cues from the environment such as day length or a period of cold temperature. Often the canola varieties are categorised as “spring types” or the traditional varieties that are sown in the north of the dividing range where the change to the reproductive phase has a minimal influence from cool temperatures but the plants respond to day length. The “winter” types, also referred to as “dual purpose” for grazing and grain, generally have a strong requirement for a period of cold temperature before the change to the reproductive phase.

At or just prior to stem elongation, flower and branch initiation begins. Once stem elongation begins, the crop demand for nitrogen increases dramatically. Any nitrogen inputs to achieve target yield should be applied by this growth stage.

Stem diameter and height are influenced by seeding date, moisture, variety, soil fertility and plant population. Plants in low-density crops have thicker stems and are more resistant to lodging. Plants in high-density crops are thinner and more prone to lodging. Lodging aggravates the problem of uneven pod maturity and creates an ideal environment for the spread of diseases such as sclerotinia.

Flowers develop (Growth Stage 5) inside the plant rosette, which overlaps with GS 3. As the stem lengthens or “bolts”, the buds become free of leaves and the lowest flower stalks extend so that the buds assume a flattened shape. The remaining leaves attached to the main stem unfold as the stem lengthens and the small stalks holding the first unopened flower buds become more widely spaced.

Secondary branches arise from buds that develop in axils of upper leaves and occasionally from axils of some lower leaves on the main stem. These secondary branches develop one to four leaves and a flower bud cluster. The canola plant initiates many more branches with flower clusters than it can support, then aborts back according to the plant's set carrying capacity and environmental conditions. The ability to produce secondary branches is useful as it allows the crop to compensate for poor stand establishment and damage due to hail, pests and diseases. Development of branches is not fixed until the end of flowering. Removal of branches by hail can initiate replacement. Environmental stress can reduce the degree of branching and if the second to fourth primary branches (from the top) are affected, total flower production and therefore total seed yield can be seriously reduced.

Flowering is described by Growth Stage 6. All canola varieties grown in Australia are of the Brassica napus type and, as such, are self-pollinated and generally do not need pollinating agents such as bees. While the crop is very attractive to bees, their presence is unlikely to have any direct effect on yield. Wind and feral bees usually make the use of honey bees unnecessary.

Flowers begin opening early in the morning and pollen is shed and dispersed by both wind and insects. Flowers remain receptive to pollen for up to three days after opening. If favourable warm, dry weather occurs, nearly all the pollen is shed the first day the flower opens. In the evening, the flower partially closes and opens again the following morning. Fertilization occurs within 24 hours of pollination. After pollination and fertilization, the flower remains partially closed and the petals wilt and drop (two to three days after the flower opened). The young pod becomes visible in the centre of the flower a day after petals drop.

During flowering, the branches continue to grow longer as buds open into flowers and as flowers develop into pods. In this way, the first buds to open become the pods lowest on the main stem or secondary branches. Above them are the open flowers, and above them, the buds which are yet to open. All of the buds that will develop into open flowers on the main stem will likely be visible within three days after the start of flowering.

Canola plants initiate more buds than they can develop into productive pods. The flowers open, but the young pods fail to enlarge and elongate, and eventually fall from the plant.

Flowers that are open during heat stress may fail to pollinate. Normally, fertility of flowers that open later will be unaffected if stress has been alleviated. Areas on the main stem or branches with no pod development are symptoms of stress. Under severe stress, loss of unopened buds increases, signalling the end of flowering. If the severe stress occurred at early flowering the plant may resume flowering through increased branching if very favourable conditions return.

 

Grain development and ripening Growth Stages 7 – 9 - Podding

Growth Stage 7 describes pod development. By mid-flowering, when lower pods have started to elongate, the stem becomes the major source of photosynthate for plant growth. Leaf area is beginning to decline and is also shaded by the flowers. The early developed pods have a competitive advantage over later formed pods. Older pods at the base of the flowering branches are well along in development while new flowers are still being initiated at the tips. At this stage, the stem and pod walls are both major sources of food for seed growth since the pod photosynthetic surface area has greatly increased.

Any stress leading to a change in the supply of photosynthate can abort pods or reduce the number of seeds in each pod. The stress can be either the plant’s inability to photosynthesise enough to meet demand or where soil moisture is limited or too higher temperatures occur.

The number of seeds that develop in each pod will be influenced by the availability of photosynthate supplies at the time when seed expansion occurs. Lack of supplies at this growth stage will result in smaller pods with fewer, lighter seeds, especially in the later secondary branches and tops of branches. Substantial stress at seed expansion leads to shorter pods and/or lack of expansion around missing seeds.

Growth Stage 8 describes the development of the grain. At the stage where seeds in the lower pods have turned green, most of the leaves on the plant have fallen. The pod walls have become the major source of photosynthate although the stem is still important.

About 35 to 45 days after the flower opens, seed filling is complete.

Usually the earliest formed pods are the largest and develop more and larger seeds. At 40 to 60 days after first flower or 25 to 45 days after the end of flowering, the seeds in the lower pods will have ripened and fully changed colour. When 40 to 60% of the seeds on the plant have begun to change colour, the majority of seeds have reached physiological maturity. This is the optimum stage for windrowing. Windrowing before physiological maturity can result in reduced yields due to incomplete seed development.

Although the potential number of pods per plant and seeds per pod are set at flowering, the final number is not established until a later stage. Seed filling requires adequate soil moisture and nutrients. Seed abortion, or reduction in seed weight, can be caused by anything that interferes with plant functions during this time.

In canola, the seed accounts for about 23 to 31% of the total plant dry matter produced, depending upon growing conditions.

Growth Stage 9 describes the drying down of the seed and harvest.

 

The drivers of Plant Phenology

 

Temperature

Temperature influences the development principally through two areas, rate of growth and the plant’s response to temperature for influencing developmental phases.

 

Temperature and Vegetative Growth

As a general statement, plant vegetative growth increases as temperature increases. There is an upper and lower temperature limit where growth ceases. The lower limit, or base temperature, for growth in Australia is generally accepted as 3° C, although studies have shown a broader range of lower limits between 0° and 5° C.

Optimal temperature for growth is in the range of 20° -25° C.

Upper limit is generally regarded as 35° C, but crops have been shown to be able to acclimatise if they have previously been exposed to high temperatures.

In an attempt to describe the influence of temperature on crop growth, the concept of Degree Days is used to be able to predict crop growth stage. Degree Days are calculated by taking the daily average temperature (maximum plus minimum divided by 2) and subtracting the base temperature.

For example, if the daily maximum was 20° C and the minimum was 10° C, then the average daily temperature was 15° C, then subtracting the base temperature of 3° C, therefore the crop experienced 12 degree days for growth.

 

Development of the Plant in Response to Temperature

Germination and Emergence

Germination is the process of the seed breaking dormancy and producing the first root and shoot to break through the seed coat and emergence for the coleoptile to break the soil surface and the first leaf appears. Temperature, moisture and oxygen are needed to instigate this process. Minimum temperatures for wheat germination are in the range of 3.5° to 5.5° C, the optimum being 20° – 25° C and the upper limit being 35° C.

An estimate of the time needed for the crop to germinate can be calculated using degree days, with the range being 80 - 100 DD or if the soil temperature is 16° C, then approximately 7 days.

 

Vegetative Growth

Following emergence, the plant then produces leaves at a rate determined by temperature. The appearance of one leaf to the next is a constant in thermal time and the accepted figure is 80 DD. This period is also known as the phyllochron. There are variations to this figure, influenced by sowing date, but for the majority of crops sown in the late April to mid-June period, the phyllochron is 80 DD.

If the daily mean temperature is 15° C as per the previous calculation, then a leaf will appear every 6.7 days, or if it cooler and the daily mean is 10° C, then it will take 11.5 days for the next leaf to emerge. This information can be used to predict leaf timing for herbicide operations.

The transformation from vegetative to reproductive growth occurs after approximately 500 – 800 degree days after emergence.

 

Flowering and Pod Fill

High temperatures (> 32° C) at flowering will hasten the plant’s development, reducing the time from flowering to maturity. High temperatures during flowering shorten the time the flower is receptive to pollen, as well as the duration of pollen release and its viability.

High temperature can decrease total plant dry matter, the number of pods that develop, number of seeds per pod, and seed weight resulting in lower yields. Very hot weather combined with drought and high winds may cause bud blasting wherein the flower clusters turn brown and die resulting in serious yield losses.

While temperature is the key driving plant development, the plant’s response is modified by vernalisation and day length.

 

Vernalisation

Vernalisation is the requirement to be exposed to cold temperatures in order for the reproductive phase to begin. Temperatures in the order of close to 0°C are needed to meet this requirement depending on varietal characteristics. Canola varieties that have little or no vernalisation requirement are often referred to as spring types, and temperatures in the range of 7 to 18° C for brief periods will be sufficient for vernalisation. Canola varieties that have a strong vernalisation requirements are called winter types, and lower temperatures of between 0° and 7° C for several weeks are needed for vernalisation.

Vernalisation is a useful tool in that it gives an environmental cue to the plant on when is the most suitable time to transform to the reproductive phase, and so offer a greater sowing window.

Spring types will simply go through the vegetive phase and transform to reproductive based on the temperatures and/or day length the plant is experiencing.

 

Day length

Canola develops and flowers more rapidly when grown under long day conditions.  Canola is not an obligate day length plant (ie a plant that will not commence the reproductive phase until a certain day length requirement is met), but the change to reproductive phase can be delayed if the day length is too short. While varieties have differing responses, a day length longer than approximately 11 hours is required by the canola plant to trigger reproductive development.

This results in an autumn sown canola crop remaining in the vegetative phase during the winter, accumulating biomass, and hence yield potential, as well as delaying flowering until the risk of frosts is reduced. However early sown canola may experience the 11 hour photoperiod shortly after emergence which may result in the plant flowering prematurely.

Day length requirement is a desirable characteristic as it allows flexibility in sowing dates.

 

What does all this mean?

The aim of growing a canola crop is to have the crop grow sufficient biomass to flower late enough to reduce the risk of a major frost event, but early enough to ensure grain fill is not adversely affected by either moisture or temperature stress. Therefore the optimum flowering window is generally known for each district. However getting a crop to flower in this period is not easy as there are many variables such as sowing date, seasonal temperatures and available moisture that influence the crop phenology.

Canola varieties already possess the characteristics that give a reasonable sowing window that will flower at the optimal time for maximising grain yields. One key advantage with canola over many crops is the much greater flowering period, which while a frost may kill the flowers for that day, there will be new flowers tomorrow to compensate.

A characteristic such as vernalisation allows canola to be sown at the first opportunity early in the season and be confident that it will remain vegetative until its temperature requirements are met and flower in late spring. In a high rainfall zone where summer rain is assured, theoretically spring sowings would remain vegetative until late winter. In lower rainfall areas, the delay to the reproductive phase can mean that flowering occurs too late for the optimum conditions for grain fill.

Similarly a variety with that is sensitive to day length has a wider sowing window as the vegetative period would be extended, developing biomass and yield potential, and the flowering period delayed until the optimum.