Lemon maturation causes anatomical and biochemical changes at the flavedo tissue level

Abstract

Plants mobilize the photosynthates by three transport pathways: apoplastic, symplastic through plasmodesmata (PD), and transcellular. In flavedo of postharvest mature lemons, a high activity of cell wall-bound invertase (WI), an enzyme associated with transcellular transport of monosaccharides, has been detected. In order to elucidate whether this high enzymatic activity is related to restricted transport in the symplastic pathway with fruit maturation, the aim of the present work was to compare anatomical and biochemical parameters in peel tissues of immature and mature lemons. Anatomical structure focusing on cell walls, callose deposition, WI activity, and sucrose content were analyzed in peel tissues of immature and mature lemons. The parenchyma of flavedo tissue of immature lemons presented an elevated number of primary pit fields (PPF). These PPF, associated to PD or cell wall interruptions, had the appearance of a string of beads. However, in mature lemons, the number of PPF was scarce due to callose deposition. WI activity and apoplastic sucrose content increased significantly in flavedo of mature lemons in comparison to immature lemons. Present findings lay structural and functional bases relevant to understand differences between immature and mature lemons, which would help to design agricultural practices in pre- and post-harvest management.