Acacia holosericea is a potential cut foliage species, however, this species has an inherent short vase life. The short vase life of A. holosericea is associated with reduced water uptake due to microbial and physiological xylem vessel occlusions following mechanical injury incurred at harvest. It was found that mechanical wounding at harvest initially induced elevated endogenous ethylene production within 1 to 3 h for phyllodes, and 2 to 6 h for stems. Treatments with the Ag+ based ethylene inhibitors 4 mM silver thiosulfate (STS), 40 mg L-1 nano silver particles (NS), or 200 mg L-1 silver nitrate (AgNO3) promoted ethylene production by 1.1 to 5.2-fold over control levels. Vase life was significantly prolonged by the AgNO3 and NS treatments, while a significant reduction occurred by the STS treatment. The negative effect on vase life of STS could be to phytotoxicity, which was evident as browning of the veins in phyllodes. Treatments with 100 µL L-1 ethylene gas, 1, 10, 100, and 1000 µL L-1 ethephon, or 1 mM jasmonic acid (JA) also shortened the vase life.
Based on light microscope observations, the short vase life of A. holosericea was associated with gel secretions into the lumens of water conducting xylem cells in the stems. Treatments with 4 mM STS and 100 and / or 200 µL L-1 ethylene did not differentially influence gel formation. Xylem vessel occlusion is a recognised wound healing response and defence process in plants. The gels were found to develop soon (ca. 3 h) after wounding. The biochemical makeup of the gels changed from basophilic to acidophilic as they matured, and the proportion of xylem vessels occluded increased over time.
The gels obstructing xylem flow reduced the hydraulic conductance (Kh) of the stem of this species. Compared with deionised water (DIW), the use of either de-aerated DIW or 10 mM KCl in measurements did not significantly improve xylem hydraulic conductance (Kh xylem) or phyllode hydraulic conductance (Kh leaf). Anatomical characterisation suggested that the total cross sectional vessel lumen area per stem TS area (mm2 / mm2) and the relative degree of lignification (staining intensity) of xylem tissue are also crucial determinants of reduced Kh with the increased tissue developmental maturity stage. The unstable values and lack of significant reduction in Kh leaf for the A. holosericea foliage stems during vase life were largely dependent on the changing physiological status of the wilting phyllodes across time. Relatively low Kh in stem segments and adjacent phyllodes from 15 to 25 cm distal to the cut end was consistent with maximum gel formation in those stem regions. Overall, the prevalence of occlusions by gels in the xylem of cut A. holosericea foliage stems could account for the reduced water uptake that elucidates the inherent short vase life of this species.