Surface characteristics of sweet cherry fruit: stomata-number, distribution, functionality and surface wetting

Abstract

The number, distribution, size, and function of stomata and wettability of the sweet cherry ( Prunus avium L.) fruit surface were investigated. The number of stomata per fruit differed significantly among sweet cherry cultivars, ranging from 143±26 per fruit in ‘Adriana’ to 2124±142 per fruit in ‘Hedelfinger’. The number of stomata per fruit was not affected by fruit mass (‘Burlat’). For a given cultivar, the stylar scar region had the highest stomatal density, followed by ventral suture or cheek. The stem cavity region was essentially astomatous. Stomatal density decreased as distance from the scar increased. Cross-sectional areas of stomatal pores had a log-normal distribution and differed among cultivars, with medians ranging from 39.0 to 105.2 μm 2 for ‘Van’ and ‘Sam’, respectively. The length/width ratio of stomatal pores increased in the course of a day in early stage II, but not in mature stage III fruit. Treating exocarp segments with ABA (0.1 mM) or sucrose (1 M) decreased length/width ratios of stomatal pores in early stage II fruit, but not in the mature stage III, suggesting that stomata were non-functional at maturity. Contact angles of 1 μl water droplets (71 mN m −1) with the sweet cherry fruit surface averaged 92.4±0.6° ( n=164) across years and cultivars and did not differ between regions (cheek, suture vs. stylar end). The critical surface tension of the sweet cherry fruit was not affected by developmental stage (stage II vs. mature stage III ‘Burlat’ fruit) or cultivar, and averaged 24.9 mN m −1 making Poiseuille-flow of water through open stomata unlikely.