Anatomical characteristics of the buds of different pomegranate cultivars

Buds are organs very important for plant development, but very few investigations have been carried out on pomegranate buds since works generally focused on horticultural aspects of the species. The present work aimed to study the buds of four pomegranate cultivars in order to understand the mechanism behind the bud differentiation towards the formation of the flowers. The cultivars used in this research are grown at the pomegranate repository located at the ‘P. Martucci’ experimental station in Valenzano (University of Bari ‘Aldo Moro’, Italy). In particular, the cultivars were: Comune San Giorgio (Italian ecotype), Wonderful, Haku Botan, Ki-Zakuro. Buds were collected from June 2021 until leaf fall, every 14 days from different twigs (spurs, short, mid and long twigs), carried to the lab for the successive analyses (cross- and transverse-sections by using a steel blade, stained with 0.05% toluidine blue and successively observed under an optical microscope and a stereomicroscope). In addition, 3D morphological studies were carried out on intact dormant buds (collected at the end of October) by high-resolution micro X-ray computed tomography (µCT) analysis at the Micro X-Ray Lab (www. microxraylab.com) of the University of Bari ‘Aldo Moro’. The buds showed some little differences mainly related to the stage of development. For all the cultivars, the lateral buds on the twigs resulted generally smaller and narrower, whereas more rounded and larger on the spurs. The buds had various protective scales and two small transition leaves, and no reproductive primordia were observed until the dormant phase, very differently from other fruit tree species (sweet cherry, peach, apricot, etc.). The X-ray images of dormant buds showed clear protective scales rich in starch grains for thermal insulation of the meristematic apex, two small transition leaves and the absence of differentiated reproductive organs, thus suggesting the flower formation only at bud break, after shoot development, when further analyses will be accomplished.