Floral Differentiation: Bud Swell to Bloom
As buds are reactivated in the spring, floral development resumes. Each fine-branch meristem produces clusters of 3-4 flower primordia before bud burst, with formation of a calyx (fused sepals at outer edge of flower). After bud burst, individual flower organs (corolla made up of fused petals and pollen-producing stamens) first appear, followed by the pistil, ovaries and individual ovules (female organs). This process may be largely complete by about 15 days after bud burst.
Anthesis (Flowering)
About two weeks after ovules are formed, the calyptra (fused petals surrounding flower parts) separates at the base, and is shed. In our climate, most flowers open within 5-7 days; but cool temperatures can delay and extend the bloom period. Bloom is well-synchronized in our climate with its severe winters, but can extend over 3-4 weeks in Mediterranean climates with mild winters (see: Climate, Duration of Bloom, and the Window of Risk for Grapevine Diseases). Anthers (pollen-laden structures at the tip of stamens) release pollen after cap fall.
Pollination
Commercial cultivars with perfect flowers (both male and female parts) are often self-pollinated, but most wild grapevines have either male or female flowers, and require cross-fertilization. Once the pollen lands on the stigma (receptive part of the pistil), the pollen germinates and produces a pollen tube, which fuses with the ovule.
Importantly, the rate of growth is critical, because the ovules are only receptive for a short time after bloom. At high temperatures (25-30° C) fertilization occurs within 12 h; at 20°C, this process takes 24 h, and at low temperatures (15°C), 48 hours. With cooler temperatures, growth of the pollen tube may be so slow that fertilization doesn’t occur.
Factors Affecting Flower Formation and Fertilization
Weather conditions, the light environment, and vine stressors such as water or nutrient deficits can have dramatic effects on the processes leading to inflorescence and flower formation, and ultimately fruit set – many of which are beyond a grower’s control. Here are a few of them:
- Light environment. Shoots that are shaded in a dense canopy tend to have less fruitful buds. In practice, the period of inflorescence induction for the first 6-10 nodes retained at pruning tends to occur during 3 or 4 weeks centered around bloom.
- Supply of photosynthates to buds. Assimilates (sucrose, N), whether produced by leaves or from stored reserves, are key factors influencing inflorescence induction in developing buds. Stored reserves that fuel early-season growth are largely exhausted by bloom (see Sources and Sinks: Allocation of Photosynthates during the Growing Season), so reductions in leaf area from cluster-zone leaf removal or poor early-season canopy development can reduce bud fruitfulness and return bloom the following year. One example comes from a 2017 study where we completely defoliated Riesling vines at fruit set (we left the shoot tips on, and they produced a new canopy by veraison). In 2018, these vines had a 50% reduction in cluster number, and 25% of shoots were clusterless, compared to 4% in the ‘Standard’ treatment.
- Stored Reserves. If vines enter the dormant season with low amounts of stored carbohydrates, floral development in the spring can be compromised. This can be the result of over-cropping, disease that reduces leaf area (think downy mildew), or early leaf fall in cool seasons. Goffinet (2004) measured carbohydrate reserves in minimally-pruned Concord vines, some of which were defoliated at veraison – and therefore over-cropped. The defoliated vines depleted starch reserves the following spring a week earlier than balance-pruned vines, and produced fewer mature leaves to export newly produced photosynthate to the flowers. High crop-to-leaf areas during the ripening season strongly influenced over wintered reserves and primary bud potential.
- Low temperatures, cloudy weather and extended bloom. Temperatures at or below 15° C can delay pollen tube growth, resulting in poor set. Cloudy weather that results in less carbon assimilation than on sunny days can limit the supply, also resulting in poor fruit set.
Understanding the timing and sequence of floral development – and the factors that affect it – can help growers diagnose some of the reasons for poor fruit development. Some practices (crop level adjustment, timing and extent of cluster-zone leaf removal) are manageable, while others (weather) are not.
