Grapes 101

Seasonal Concord Fruit Development in the Lake Erie Region

T. Bates

11/6/2017

Understanding fruit development is important in Concord grape production because fruit is the final agricultural product delivered to be processed into juice and other grape products. Payment to producers is based on fruit yield and juice soluble solids. Controlling crop size in relation to vine size is critical in delivering the highest possible yield of quality fruit. Fruit set and development is influenced by biological and environmental factors and it is important to understand and track how the crop is developing to make appropriate crop control management decisions. This article discusses Concord fruit development from bloom through the three stages of berry growth to harvest during the 2017 season.

Each growing season, Concord fresh berry weight is collected from a standard set of “phenology” vines maintained at the Cornell Lake Erie Research and Extension Laboratory in Portland, NY. The curve represents the 18-year mean in berry weight (error bars = standard deviation). Tracking the current season berry weight in relation to the long-term mean assists with a more accurate crop estimate.

This season, the staff at CLEREL recorded trace bloom in the standard phenology vines on 6/11/2017 and official bloom (50% cap fall) on 6/12/2017. The Concord flower cluster in this image is at over 90% full bloom on June 14th.

Most of the wild grapes you see on the roadside or in the woods have either all male or all female flowers. However, most of the cultivated grape varieties we grow have “perfect” or “hermaphrodite” flowers. Interestingly, cultivated grapes are also highly self-pollinated because the pollen will go from the anther to the stigma before the cap pops off. To catch and rehydrate the pollen, the stigma produces a sap (seen at the tip of the stigma arrow in the picture). Rehydration of pollen takes about 30 minutes and then the pollen uses stored starch in the pollen grain to grow down the style. The speed of pollen tube growth and the time it takes to reach the ovule is related to temperature (roughly 48 hours at 60 degrees F, 24 hours at 70 degrees, and 12 hours at 80 degrees). The colder it is, the slower the pollen tubes grow. Since the ovules are only receptive for a short time, cool weather during bloom can cause the pollen to miss the window and lead to poor fruit set.

Just after flowering, the pollinated pistils on the grape clusters will start to develop but not all of the ovaries will successfully develop into fruit. This Concord cluster has about 100 developing ovaries but will only retain and develop 25-30 fruit, on average. Expanding pollen must first fertilize at least one of four ovules while they are receptive. Successful fertilization induces the production of certain plant growth hormones for cell division (auxin) and cell expansion (gibberellin) in different tissue layers. The balance of these hormones is important for the successful retention of the developing ovary. Percent total fruit set is influenced by cultivar and certain management practices, such as pruning level. A variety of environmental stresses (light, temperature, carbohydrate, nutrient, and water stress) can also reduce fruit set. Many of our management recommendations, such as for weed control and mineral nutrition, aim to eliminate any vine stress during the fruit set and berry cell division phase in the 3-4 weeks after bloom. Unfertilized or stressed ovaries will eventually abscise or “shatter.”

Just one week after the start of bloom, Concord clusters are setting fertilized berries and dropping others. The cluster above has dropped about 40% of the pistils which were originally pollinated but not successfully fertilized. A corky abscission scar can be seen where the pedicel of aborted flowers have separated from the rachis (cluster stem).

Why is it important to track fruit set?

Current research objectives aim to improve mid-season crop estimation. Grape yield is a function of shoots/vine, clusters/shoot, berries/cluster, and final berry weight. These “yield components” can be influenced by biological factors such as vine size and vine water status, management factors such as pruning level, or environmental factors such as temperature during fruit set. Spatial data from the Efficient Vineyard project illustrate how yield components can vary from vineyard to vineyard as well as within a vineyard. We are testing the use of the Carnegie Mellon Image sensor to directly detect and count certain yield components – such as shoot number and berry number across a vineyard. We are also combining this information with other spatial data to direct vineyard sampling during the middle of the growing season to predict final crop size across whole vineyard blocks.

After bloom and fruit set, Concord berries enter a growth phase of both cell division and cell expansion. At 4-weeks post-bloom, Concord berries will reach approximately 50% of their final fresh weight but will still be in the middle of the rapid fruit growth (stage I). Berry size and weight in a cluster, vine, or vineyard varies at both 30 days after bloom and at harvest (as seen in the photo). Variation in berry growth is a function of both cell number (through cell division) and cell size (through cell expansion) – and these are controlled by both biological and environmental factors.

The developing seeds produce auxin, cytokinin, and gibberellin and it is the balance of these hormones which influences the amount of cell division and expansion in the fruit. A berry with more seeds will tend to be larger than one with fewer seeds because of the seeds’ influence on cell division and expansion. While all parts of the fruit are developing during stage I, it is the division and expansion of the mesocarp (flesh or pulp) that makes up most of the berry volume. Environmental factors, such as water availability, will also influence berry weight at 30 days after bloom by influencing cell expansion – again primarily in the mesocarp tissue. Cell number will double two times during stage I through cell division. In addition, cell volume will increase through cell wall loosening and the expansion of cell vacuole volume. Presumably, the cell division hormone, cytokinin, diminishes through stage I, which slows cell division as the berries enter the lag phase of berry growth (stage II).

At 45 days after bloom, Concord berries are between 60-65% final fresh weight and are entering stage II of berry development. This stage is also referred to as the lag phase and will last until veraison (approximately 69 days after bloom in Lake Erie Concord). Lag phase is dominated by seed development and maturation. By veraison, the seeds will reach their final size and lignify in preparation for dispersal. The growth of the mesocarp and exocarp slows during the lag phase. Again, it is the balance of plant hormones, such as auxin, cytokinin, and abscisic acid, which controls the cell division and expansion in berry tissues and prevents the seeds from germinating too early.

During stage III, the cell walls of the mesocarp change physically and chemically to soften and accumulate water and sugar. The seeds turn brown from tannin accumulation and harden through desiccation and become ready for dispersal. Veraison also triggers the accumulation of anthocyanins (purple pigments) in the grape skins (exocarp).

In Stage III of berry development from veraision to harvest, the seeds finish maturing and the fruit ripens to attract animal feeding for seed dispersal. Just before veraison, the berries are hard and green with relatively high organic acid (30 g/L) and low sugar concentrations (7.5 oBrix). Over a four to five week period from veraison to harvest, Concord fruit will become soft and dark purple with relatively low organic acid (10 g/L) and high sugar concentrations (16 oBrix). In Lake Erie Concord, veraison occurs 69 days after bloom, on average. Veraison may start a few days earlier in warm, dry years with moderate vine water stress and lower berry weight and it may be delayed in cool, wet seasons with high vine water status and larger berries. Veraison marks a physiological change in the fruit characterized by a rapid increase in water and sugar accumulation in the mesocarp (flesh) and anthocyanin accumulation in the exocarp (skin). There is also a degradation of organic acids and chlorophyll and the fruit will become soft as the cells walls of the mesocarp change and weaken.

Desired fruit chemistry for producing single strength juice is 16 oBrix (+/- 0.5) and 1.0-1.1% (10-11 g/L) titratable acidity at approximately 30-40 days post-veraison. Environmental conditions (precipitation, sunlight, temperature) as well as viticulture management (crop load) can influence berry weight and the rate of sugar accumulation.

At approximately 100 days after bloom, Concord seeds are fully mature and the fruit has reached the right sugar, acidity, color, and texture to be eaten by animals and promote seed dispersal. In the processing industry, we use these attributes to harvest and process the fruit into grape juice. Most of the water, natural sugar, and fruit acidity can be pressed out of the mesocarp. The deep purple pigments are primarily in the grape skins and need to be extracted during processing to give Concord products the characteristic purple color. After juice processing, Concord seeds can also be collected, dried, and pressed to extract grapeseed oil.