Kamani Ratnayake (2012). MECHANISM OF ACTION OF POSTHARVEST COPPER TREATMENTS IN CUT ACACIA HOLOSERICEA FOLIAGE STEMS PhD Thesis, School of Agriculture and Food Sciences, The University of Queensland.

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Author Kamani Ratnayake
School, Centre or Institute School of Agriculture and Food Sciences
Institution The University of Queensland
Publication date 2012-06
Thesis type PhD Thesis
Supervisor Joyce, Daryl
Webb, Richard
Wearing, Alan
Total pages 228
Total colour pages 9
Total black and white pages 219
Language eng
Subjects 070605 Post Harvest Horticultural Technologies (incl. Transportation and Storage)
060705 Plant Physiology
060501 Bacteriology
Abstract/Summary Abstract Copper ions (Cu2+) have been used in postharvest pulse and vase solution treatments to extend the longevity of cut flowers without a clear understanding of the underlying beneficial mechanisms. Water deficit stress is probably the single most important cause of decreased longevity in cut flowers and foliage. Wilting and dehydration associated with diminishing postharvest water uptake is the major constraint on longevity for cut Acacia spp. The overall objective of this study was to investigate the potential mechanisms of action of postharvest Cu2+ treatments using cut foliage stems of Acacia holosericea, this species being representative of the poor postharvest water relations inherent in the genus Acacia. The efficacy of Cu2+ supplied as either pulse or vase solution additive to extend the vase life of cut A. holosericea stems was assessed using CuSO4. The treatments were optimised and either provision of 2.2 mM CuSO4 as a 5 h pulse after harvest followed by standing stems into deionised water (DIW) or provision of 0.5 mM CuSO4 in the vase water maximally extended vase life by 2.6-fold and 1.4- to 2.8-fold, respectively, as compared to non-treated stems in DIW. The extension of vase life was associated with improved water uptake rates and better maintenance of stem fresh weights. Experiments with a range of Cu2+ and Cu+ salts revealed that both Cu2+ and Cu+ cations were beneficial, regardless of the counter anion. The majority of Cu taken up by the cut stems accumulated in the basal stem region, suggesting possible Cu2+ involvement in suppressing bacteria and/or physiological blockage in the stem end region. A lysine based scanning electron microscopy (SEM) fixation protocol was adopted to preserve bacterial exopolysaccharides on and in the stem end of A. holosericea. The potential antibacterial action of Cu2+ against stem end and vase water colonising bacteria was then investigated by time series assessments of bacterial populations using SEM and Agar based colony counts on Petri plates. Initial suppression of bacterial growth by Cu2+ was only transient. Moreover, regular recutting of the stem ends to remove potential bacterial occlusion did not substantially improve either cut stem water relations or longevity. Also, the positive effects of Cu2+ treatments were not altered by the stem end recutting. Therefore, the primary mechanism of Cu2+ was unlikely to be inhibition of bacteria in either the vase water or on/in the stem end. Moreover, natural vase solution and stem end microbial populations had relatively insignificant effects on cut A. holosericea water relations and vase life. The effect of Cu2+ on wound induced xylem vessel occlusion was alternatively investigated for A. holosericea. The development of xylem blockage in the stem end region 10 mm proximal to the wounded or cut stem surface was examined over 21 days by light (LM) and transmission electron (TEM) microscopy. Xylem vessels of stems stood into the DIW control solution became occluded with gels which were secreted into vessel lumens from surrounding axial parenchyma cells. The gel secretion process was initiated within 1 – 2 days post-wounding and the gels were evident in the xylem from day 3. In contrast, Cu2+ treatments disrupted the cytoplasm of the surrounding parenchyma cells, thereby inhibiting the gel secretion and maintaining the vessel lumens free of occlusions. As compared to DIW controls, Cu2+ treatments can improve the water relations of cut A. holosericea stems by preventing an increase in the hydraulic resistance of vessel lumens as caused by gels. The physiological wounding response of gel secretion is evidently a major determinant in the postharvest water relations of cut Acacia foliage stems.  
Keyword Acacia holosericea
Electron microscopy
Vase life
Water relations
Wound reaction
Xylem occlusion
Additional Notes Colour pages: 42, 85, 107, 173, 180, 187, 198, 203, 228. Grayscale pages: 123-126, 128, 144, 146-150, 152, 175-178, 181-182. Landscape pages: 52-55, 103, 106, 154, 204, 205, 225.

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Created: Wed, 27 Jun 2012, 18:13:35 EST by Ms Ratnayake Ratnayake on behalf of Library - Information Access Service