2018 ASHS Annual Conference

Accumulation of Ca in fruit is largely driven by transpiration and varies depending on the concentration of Ca in the xylem fluid. The objective of present study was to evaluate the relationship between fruit stomatal functioning and Ca accumulation during different stages of development in northern highbush blueberry (Vaccinium corymbosum L.). The information is needed to develop feasible practices for increasing Ca levels in the fruit. Stomata were scarce on the berries and were concentrated primarily on the distal end near the calyx. Density of the stomata was greatest at petal fall, averaging 5–108 stomata/mm² from the proximal (pedicel end) to the distal end of the berries. In comparison, mean density on the abaxial surface of the leaves averaged 496 stomata/mm². Stomata were wide open at the early green stage of berry development and only had a slight deposit of wax along the guard cells. As the berries expanded during the initial period of rapid growth (stage I), most of the stomata remained near the distal segment of the berries, and by the late green stage, almost none were found in the middle and proximal segments. The majority of these stomata were completely covered with wax once the berries began to change color and ripen. Stomatal conductance of the berries averaged 45 mmol·m^-2·s^-1 at petal fall and rapidly declined as the fruit developed. By the fruit coloring stage, conductance was low and remained < 15 mmol·m^-2·s^-1 throughout the ripening period. Leaf stomatal conductance measured at the green stage averaged ≈ 100 mmol·m^-2·s^-1. Dry matter accumulated in the berries in a typical double-sigmoid pattern, with an initial period of rapid growth (Stage I) from petal fall to fruit coloring, followed by a short lag period of growth (Stage II) during fruit coloring, and finally a second period of rapid growth (Stage III) during fruit ripening and prior to harvest. Calcium likewise accumulated rapidly during the initial stage of berry development, but in this case, accumulation slowed considerably between the late green and fruit coloring stages, and stopped completely during fruit ripening. In general, Ca accumulation appeared to end sooner in the early- and mid-season cultivars (Duke and Bluecrop) than in the late-season cultivars (Aurora and Elliott). Although stomatal conductance is low in developing blueberries, it appears to be an important mechanism by which Ca is delivered to the berries.