Xylem Cavitation in Nodes and Internodes of Whole Chorisia insignis H. B. et K. Plants Subjected to Water Stress: Relations Between Xylem Conduit Size and Cavitation

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Abstract
Measurements of cavitation occurring in xylem conduits of different stem parts in whole Chorisia insignis H.B. et. K. plants subjected to water stress are reported. Pre-stressed plants were shown to undergo cavitation over 10 times greater than watered ones. The most vulnerable parts of plants were one-year-old twigs where cavitation reached a peak of over 50 acoustic emissions (AE) min−1 while in two-year-old twigs AE min−1 were about one half this value. Stem zones were found where cavitation was typically very low even during water stress: these were one-year-old nodes and junctions where branches meet. Measurements of the inside diameters of xylem conduits and distribution of conduit ends in stem parts where AE were detected, showed that nodes have a significantly larger percentage of narrow xylem conduits than internodes. Similar ‘constricted zones’ were found in junctions with respect to two-year-old twigs. Here about 50 per cent of the xylem conduits were as narrow as 20 to 50 µm in diameter. The distribution of xylem conduit ends show about 3 per cent of them ending in the nodes and 1 per cent in the internodes of one-year-old twigs. About 11.6 per cent of xylem conduits end in the junctions and about a half in two-year-old internodes. Our data would give further experimental evidence to the functional concept of ‘plant segmentation’ into zones (internodes) more efficient in water conduction, i.e. with wider xylem conduits but more vulnerable to cavitation and others (nodes and junctions) with opposite characteristics.