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.
Concord berry growth is currently in stage I of rapid cell division and expansion. In general, the 2017 season can be characterized by having favorable weather conditions during bloom and set, possibly leading to average to above average seed number (unless vines were severely water stressed in 2016). In addition, 2017 has had above average precipitation in area vineyards. Taken together, we are measuring above average fresh berry weight in Concord at 30 days post-bloom. The 2017 berry curve is currently tracking like 2015 and we should expect above average berry weight at harvest. In contrast, 2005 and 2016 were seasons that were warm and dry early which led to lower than average final berry weight. This information can be used for vineyard crop estimation procedures.
A cross-section of a Concord berry at 30 days post-bloom. In addition to seed development, the cell division and expansion of the mesocarp tissue has a considerable influence on fresh berry weight. 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).