Hyperhydricity-induced ultrastructural and physiological changes in blueberry (vaccinium spp.)

Hyperhydricity can cause significant economic loss for the micro-propagation industry that produces blueberry. In order to predict and control the occurrence of hyperhydricity, better understanding of the anatomical and physiological features of hyperhydric plantlets is required. In this study, we investigated the ultrastructural and physiological changes associated with hyperhydric blueberry plantlets. Compared to normal plantlets, hyperhydric plantlets exhibited reduced cell wall thickness, damaged membrane and guard cell structure, decreased number of mitochondria and starch granule, higher cell vacuolation, more intercellular spaces, and collapse of vascular tissues. In addition, excessive accumulation of reactive oxygen species (ROS) and ethylene, decreased stomatal aperture and water loss, as well as abnormity of stomatal movement were also evident in the hyperhydric plantlets. The results suggested that excessive ethylene and ROS produced in response to the stress arising from in vitro culture could lead to abnormal stomatal closure, causing the accumulation of water in the tissues. This would lead to subsequent induction of oxidative stress (due to hypoxia) and cell damage, especially guard cell structure, eventually giving rise to the symptoms of hyperhydricity. Reducing the content of ethylene and ROS, and protecting the structure and function of the stomata could be considered as potential strategies for inhibiting hyperhydricity or restoring the hyperhydric plants to their normal state.