Structural changes of large lipid droplets in stomatal complex of Trochodendron aralioides and their possible functional significance

Abstract

Light, scanning and transmission electron microscopy were used to study lipid droplets (LDs) in the stomatal complex cells of Trochodendron aralioides. This species has peculiar features of the stomatal complex and primitive tracheal elements. Its guard cells are located on subsidiary cells. The stomatal pore lies on the bottom of the outer cavity, formed by hypertrophied stomatal ledges. In the stomatal complex cells we discovered lipid droplets (LDs), microbodies and numerous mitochondria. The accumulation of LDs differs between subsidiary and guard cells. The mature guard cells contain small LDs. Large LDs are typical of the subsidiary cells. They coalesce forming larger LDs. The possibility of these LDs breakdown was shown. This process can lead to filling a considerable volume of subsidiary cells with swelling LDs and products of their breakdown. Such contents of subsidiary cells located under the guard cells supposedly decrease the probability of hydropassive stomatal movements. Our results do not reject the possibility of deposition in subsidiary cells of large LDs which could be mobilized to maintain stable stomatal function in T. aralioides. The subsidiary cells are connected by plasmodesmata to spongy tissue cells which also accumulate numerous LDs. It is proposed that the features of the T. aralioides stomatal complex compensate imperfections of this species xylem that conducts water slowly.