Stomatal conductance and its relation to calcium accumulation in fruit of northern highbush blueberry

Authors: YANG, FAN-HSUAN - Oregon State University; DEVETTER, LISA - Washington State University; STRIK, BERNADINE - Oregon State University; Bryla, David

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2019
Publication Date: 1/2/2020
Citation: Yang, F., Devetter, L.W., Strik, B.C., Bryla, D.R. 2020. Stomatal conductance and its relation to calcium accumulation in fruit of northern highbush blueberry. HortScience. 55(1):96-102. https://doi.org/10.21273/HORTSCI14482-19.
DOI: https://doi.org/10.21273/HORTSCI14482-19

Interpretive Summary: Calcium is a key component of plant cell walls and is directly related to fruit quality in many crops, including blueberry. Foliar calcium sprays are commonly used in an attempt to improve fruit quality and storability. However, recent studies revealed these sprays have no effect on firmness or fruit calcium content when they are applied (at the labeled rate) to blueberries. The goal of the present study was to understand why and determine exactly when Ca uptake occurs in in the fruit. We found that Ca accumulated early on when the berries were small and green and then stopped completely once they began to change color. Movement of Ca was driven by transpiration, a process by which the plants absorb water through the roots and then give off water vapor through pores (stomata) in their leaves, or in this case, the berries. As the berries ripened, the stomata became clogged by the natural development of wax on the berries, and as a result, transpiration was severely limited. Thus, Ca sprays could be effective when they are applied early in the season, but their efficiency is probably limited by the small number of stomata on the fruit. Other methods to increase calcium in the berries may be more effective and include attention to irrigation (prevents water stress and loss of transpiration), proper pruning (under cropping reduces calcium in the fruit, while over cropping reduces berry size), and practices to promote healthy root development (improves Ca uptake) such as raised planting beds and regular applications of bark mulch and other organic soil amendments.

Technical Abstract: Accumulation of calcium (Ca) in fruit is largely driven by transpiration and varies depending on the concentration of Ca in the xylem fluid. The objective of the 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.). 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. 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/m2/s at petal fall and rapidly declined as the fruit developed. By the fruit coloring stage, conductance was low and remained < 15 mmol/m2/s throughout the ripening period. 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 contrast, other nutrients, including K and Mg, continued to increase throughout each stage of development, including fruit coloring and ripening. Although stomatal conductance is low in developing blueberries, it appears to be an important mechanism by which Ca is delivered to the fruit.