Winter Dormancy
Blackcurrants remain dormant from the time the leaves fall in October until the following March when the buds again swell and burst in response to rising temperatures and increasing day length. Exceptionally, after a very prolonged cold winter, bud break might be delayed until April. A very mild winter may lead to bud development during February, when the absence of the requisite minimum period of winter chill will be accompanied by uneven bud break on sensitive varieties.
There is increasing evidence that the amount of cold experienced by blackcurrant cultivars, in some regions, in some winters, is inadequate; leading to delayed and uneven bud break, with consequent adverse effects on yield and quality. The disruption of the normal pattern becomes apparent from the reluctance to leaf out by buds, apart from those at the tips of the shoots. Eventually a proportion of the delayed buds come into leaf, but they might not flower or crop as well as normal. Other buds remain dormant throughout the entire season; although when dissected they are likely to contain a normal complement of leaf and flower initials.
Flowering
The majority of buds on healthy shoots should develop flower initials with the exception of a small percentage of tip and basal buds that remain vegetative. The production of flower initials by the buds commences in June, provided the nitrogen status is sufficient to initiate the process. By the end of summer, flower formation should be nearing completion, and by October the embryo flower clusters can be identified within the interior of the buds when they are dissected.
Flower formation is liable to disruption when the bushes are under stress from drought, disease or insect attack. Leaf loss occasioned by disease infection causes premature bursting of buds and flowers in late summer/early autumn, and a generalised reduction in vigour and yields the following year.
The fruitlet develops after a certain number of ovules have been fertilised and viable seeds produced. Where an individual fruit has produced fewer than 35 - 40 fertile seeds, it might not be able to develop any further, and could drop prematurely some weeks later. This premature dropping is part of the process familiar to all growers called “run-off.” Where the requirements for satisfactory cropping have not been met in full, the crop prospects could be adversely affected but for the majority of plantations, frost remains the biggest single limiting factor.
An important factor over which growers can exercise control is the availability of soil moisture as any deficits can be corrected by means of irrigation, and by maintaining a high level of organic matter in the soil. This helps to ensure the mobility of the nutrients that are necessary to ensure satisfactory development of the leaf canopy & the fruitlets. Phosphates requires moisture for its uptake by the fine feeding roots, and is recognised as the most critical element affecting fruit setting.
During periods of cold dull weather accompanied by a drying wind, the rate of plant growth will be reduced, as also will be the availability of nutrients if the soil is dry. Growers frequently use foliar feeds to enable the leaf canopy to absorb nutrients in a soluble form when climatic conditions are unfavourable. The interval in days between the dates of first open flower & start of harvest is remarkably constant for each variety, and is only marginally affected by seasonal variations.
Blackcurrant flowers are attached to a stem or “strig” with the oldest flower at the base nearest the bud and the youngest at the tip. Each “strig” carries from 10 to 20 flowers and some buds also produce 1 or 2 shorter secondary strigs. Following bud burst and the emergence of the first 2 leaves, the flower clusters begin to be visible and from this stage they become susceptible to frost.
The flowers change colour from pink/green to red as the clusters emerge fully to hang from the bud cluster in the characteristic “grape” stage. The flowers open in succession, starting at the base of the strig. The basal flowers are less exposed and obtain better nutrition than the tip flowers which may be frosted or fail to set fruit. Basal fruits may be twice the weight of later pollinated tip fruits, set more seeds & have longer to grow.
The anthers are the male organs producing the pollen. They burst open when they are ripe, releasing the powdery pollen that is distributed by the wind when it is dry, as well as by bumble bees and other insects, mainly flies. The pollen grains may drift or be carried some distance before alighting on a stigma. These become sticky when receptive, helping to retain the pollen. The stigma is sited in the centre of the flower at the termination of the style, down which the pollen tubes must grow to effect the fertilisation of the ovules or embryo seeds.
When a pollen grain succeeds in landing on a style, it will germinate by sending out a pollen tube that will grow at a rate governed by the temperature. In warm weather it could reach the ovule within 48 hours of germinating; conversely low temperatures slow down the growth rate so that the tube might not reach its objective until 7 - 9 days later. This would be beyond the period when the ovule can be fertilised.
