Fruit development, structure and histology in Nelumbo (Nelumbonaceae: Proteales)

Abstract

Abstract Nelumbonaceae, along with Proteaceae and Platanaceae, comprise a sister clade to the family Sabiaceae in the order Proteales, although these families were previously not considered to be related. In this work, we investigated fruitlet development in Nelumbo with a special focus on the principal events of fruitlet ontogenesis and its pericarp histogenesis, aiming to determine whether fruit structure and histology can provide characters supporting current systematic positions and affinities of these families and reveal potential synapomorphies for the order. We demonstrated for the first time that each carpel comprises a biseriate outer and inner epidermis and thick mesophyll with numerous air canals, which differentiate after pollination into a pericarp with a biseriate palisade exocarp and undifferentiated endocarp. The mesocarp differentiated into a protective hard peripheral zone and a soft inner zone. We propose the following additional autapomorphies for Nelumbonaceae: (1) the development of a specific zone of palisade cells of the subdermal layer of the exocarp with U-shaped cell walls thickening to form a light line around the locule, and potentially preventing apoplastic water transport; (2) the presence of a soft inner parenchymatous zone of the pericarp comprising numerous air cavities, channels and layers of spongy parenchyma, which can function as a seed cushion; (3) simultaneous growth of pollinated and unpollinated fruiting carpels up until the middle developmental stages; and (4) intensive development of the embryo and the seed at the late developmental stage in both Nelumbo spp., which has not been reported in any other member of Proteales. The presence of numerous vascular bundles in the fruitlets of Nelumbonaceae, Platanaceae and some Proteaceae is a synapomorphy for these three families forming a well-supported clade in Proteales.