![]() ![]() As a result, the geometrical scaling of the phloem sieve elements has been revealed in the stems of forest trees 4, 6, 8, and in tapering branches of shrubs 5. The geometrical scaling of the conductive elements composing the phloem along the trunks of trees have remained unexplored until recently, when the development of microscopy techniques has allowed detailed visualisation of these micro conduits, most remarkably the quantification of the micro pores composing the sieve plates 7. The hydraulic function of the phloem is intimately linked with the geometry of the individual sieve tube elements, which are connected in series composing a continuous tube and, thus, follows the laws of an electric circuit with resistances to the flow (the connections between tubes or sieve plates). This continuous piping system hydraulically drives substances at long and short distances, and works according to the osmotically generated pressure flow hypothesis, proposed almost a century ago 3, but empirically tested in relatively few woody plants 4, 5, 6. The phloem is the vascular tissue that transports photoassimilates from the source photosynthetic organs towards the sink tissues, which are the growing organs of the plant, including the roots, the meristems, fruits, or the flowers and inflorescences 1, 2. The pipe model theory applied to the continuous tubing system of the phloem appears as a good approach to understand the hydraulic transport of photoassimilates in fruit trees. These evaluations in mango fit with previous measurements of the phloem architecture in the stems of forest trees, suggesting that, despite agronomic management, the phloem sieve cells scale with the tapering branches. A scaling geometry of the sieve tube elements-including the number of sieve areas and the pore size across tapering branches-resulted in an exponential conductivity towards the base of the tree. Callose was present in the sieve plates, but also in the walls of the phloem sieve cells, making them discernible from other phloem cells. We revealed that the anatomy of the phloem changes from current year branches, where it was protected by pericyclic fibres, to older ones, where the lack of fibres was concomitant with laticiferous canals embedded in the phloem tissue. We combined fluorescence and electron microscopy to evaluate the structure of the phloem tissue in the tapering branches of mango trees, and used this information to describe the hydraulic conductivity of its sieve tube elements following current models of fluid transport in trees. Previous studies examined the xylem structure in the stems of mango, but the anatomy of the phloem has remained elusive, leaving the long-distance transport of photoassimilates understudied. Mango ( Mangifera indica L., Anacardiaceae), the fifth most consumed fruit worldwide, is one of the most important fruit crops in tropical regions, but its vascular anatomy is quite unexplored. ![]()
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