Background and aims: Leaves of succulent Peperomia obtusifolia (Piperaceae), and its related species, contain a large multilayered hypodermis (epidermis) subtended by a very small single-layered photosynthetic palisade parenchyma, the latter containing spherical aggregates of crystals called druses. Each druse is in a central vacuole surrounded by chloroplasts. All hypodermal cell walls are thin, except for thick lowermost periclinal walls associated with the upper periclinal walls of the subtending palisade cells. These thick walls display 'quilted' impressions (mounds) formed by many subtending palisade cells. Conspicuous depressions occur in most mounds, and each depression contains what appear to be many plasmodesmata. These depressions are opposite similar regions in adjacent thin palisade periclinal walls, and they can be considered special pit fields that represent thin translucent regions ('windows' or 'skylights'). Druses in the vacuoles of palisade cells occur below these pit field regions and are surrounded by conspicuous cytoplasmic chloroplasts with massive grana oriented perpendicular to the crystals, probably providing for an efficient photosynthetic system under low-intensity light.
Methods: Leaf clearings and fractures, light microscopy and crossed polarizers, general and histochemical staining, and transmission and scanning electron microscopy were used to examine these structures.
Key results: Druses in the vacuoles of palisade cells occur below the thin pit field regions in the wall interface, suggesting an interesting physical relationship that could provide a pathway for light waves, filtered through the multiple hypodermis. The light waves pass into the palisade cells and are collected and dispersed by the druses to surrounding chloroplasts with large grana.
Conclusions: These results imply an intriguing possible efficient photosynthetic adaptation for species growing in low-light environments, and provide an opportunity for future research on how evolution through environmental adaptation aids plants containing crystals associated with photosynthetic tissues to exist under low-light intensity and with other stresses.