Kamani Ratnayake

Treatment with Cu2+ ions extends the longevity of cut Acacia holosericea foliage stems

Summary Wilting associated with diminishing post-harvest water uptake is a major constraint on longevity for cut stems of many Acacia spp. Cu2+ ions have known biocidal activity and may also act as inhibitors of plant wound reaction enzymes. A series of experiments were conducted using Cu2+ ions as a pulse, or as a vase-solution additive to evaluate their potential to extend the vase-life of cut A. holosericea foliage stems. Provision of Cu2+ ions in the vase-water at 0.5 mM, or as a 5 h pulse at 1.0 mM, resulted in 1.4- to 2.8-fold, and 1.7- to 2.1-fold extensions of vase-life, respectively, compared to de-ionised water controls. The pulse treatment was optimised, and a 5 h pulse with 2.2 mM Cu2+ ions resulted in a 2.6-fold extension of vase-life. Thus, Cu2+-pulsing can effectively delay wilting in cut A. holosericea foliage stems.

Characterisation of xylem conduits and their possible role in limiting the vase life of cut Acacia holosericea (Mimosaceae) foliage stems

Early desiccation limits the vase life of Acacia cut flowers and foliage and may be attributable to poor hydraulic conductivity (Kh) of the cut stems. Acacia holosericea A.Cunn. ex G.Don has been adopted as the test species to investigate the postharvest water relations of the genus Acacia. To understand potential constraints on Kh, xylem conduits in cut A. holosericea stems were anatomically characterised by light and scanning and transmission electron microscopy. Vessels with simple perforation plates and tracheids were the principal water conducting cells. Bordered vestured intervessel pits were present in xylem vessel elements. The majority of conduits (89%) were short at 1-5cm long. Only 2% were 15-16cm in length. Mean xylem conduit diameter was 77±0.9µm and the diameter profile showed a normal distribution, with 29% of diameters in the range of 70-80µm. Simple perforation plates can offer relatively low resistance to water flow. On the other hand, bordered vestured pits and short xylem conduits can confer comparatively high resistance to water flow. Overall, the presence of bordered vestured pits, together with a high proportion of short xylem conduits and high stomatal densities (232±2mm-2) on unifacial phyllodes, could contribute to early dehydration of A. holosericea cut foliage stems standing in vase water. Further research will relate these anatomical features with changes in Kh and transpiration of cut foliage stems.

Cu2+ inhibition of gel secretion in the xylem and its potential implications for water uptake of cut Acacia holosericea stems

Maintaining a high rate of water uptake is crucial for maximum longevity of cut stems. Physiological gel/tylosis formation decreases water transport efficiency in the xylem. The primary mechanism of action for post-harvest Cu(2+) treatments in improving cut flower and foliage longevity has been elusive. The effect of Cu(2+) on wound-induced xylem vessel occlusion was investigated for Acacia holosericea A. Cunn. ex G. Don. Experiments were conducted using a Cu(2+) pulse (5 h, 2.2 mM) and a Cu(2+) vase solution (0.5 mM) vs a deionized water (DIW) control. Development of xylem blockage in the stem-end region 10 mm proximal to the wounded stem surface was examined over 21 days by light and transmission electron microscopy. Xylem vessels of stems stood into DIW were occluded with gels secreted into vessel lumens via pits from surrounding axial parenchyma cells. Gel secretion was initiated within 1-2 days post-wounding and gels were detected in the xylem from day 3. In contrast, Cu(2+) treatments disrupted the surrounding parenchyma cells, thereby inhibiting gel secretion and maintaining the vessel lumens devoid of occlusions. The Cu(2+) treatments significantly improved water uptake by the cut stems as compared to the control.