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Copyright Statement This copy of the thesis has been supplied on the condition that anyone who consults it is understood to recognise that the copyright rests with the author and that no quotation from the thesis and no information derived from it may be published without the author’s prior consent. Allelopathic Potential of the Invasive Alien Himalayan Balsam ( Impatiens glandulifera Royle) by OWEN PETER SMITH A thesis submitted to Plymouth University in partial fulfilment for the degree of DOCTOR OF PHILOSOPHY University of Plymouth School of Biomedical and Biological Sciences Faculty of Science and Technology June 2013 Allelopathic Potential of the Invasive Alien Himalayan Balsam ( Impatiens glandulifera Royle) By Owen Peter Smith Abstract Investigations were carried out into the allelopathic potential of the invasive alien annual Himalayan Balsam (Impatiens glandulifera Royle) using a series of bioassays, including ones developed or adapted for this study. They were evaluated for their suitability to detect three of the four main modes of allelochemical release, namely leaching, exudation and decomposition. Assays which involved the measurement of lettuce radicles and hypocotyls gave reliable results and allowed a range of different Impatiens material, both living and dead, to be assessed and ranked according to the allelopathic effects demonstrated. Attempts were made to isolate resource competition from allelopathy using separately grown but connected donor and receiver plants and a density dependent design where single I. glandulifera plants were grown in pots with variable numbers of receiver plants. Results proved inconclusive. Initial experiments showed that the allelopathic potential of I. glandulifera varied according to the organ from which the material was derived. Pods, leaves and stems produced the greatest inhibition of lettuce seedlings. Effects on germination were not significant at most of the concentrations tested. Live roots of I. glandulifera plants produced pronounced orange staining of the agar into which they were placed and showed clear evidence of distance dependent inhibition of lettuce radicles. Effects were limited to growth rather than germination of the test plants. Germinating I. glandulifera seeds caused a significant inhibition of lettuce radicle elongation when the two species were grown together in an agar medium. The inhibitory effects increased significantly with increasing exposure time. Increasing I. glandulifera seedling number also produced significant reductions in lettuce radicle length. Dormant seeds, by contrast, stimulated growth. Dead seeds did not produce significant changes to the growth of the test plants. When rhizosphere soil was gathered from pot grown I. glandulifera plants, the results were mixed. Initial samples inhibited growth, whereas those collected from dying plants over a period of weeks stimulated growth. Further experimentation is required before the indications of allelopathic interactions demonstrated here can be applied to the behaviour of wild populations of I. glandulifera. ii Contents Copyright Statement .........................................................................................i Abstract ............................................................................................................ ii Contents .......................................................................................................... iii Tables ............................................................................................................ xvii Figures ............................................................................................................xix Plates .............................................................................................................xxix Acknowledgements .....................................................................................xxxi Author’s Declaration ................................................................................. xxxii Presentations and Awards ....................................................................... xxxiii List of abbreviations ...................................................................................xxxv 1 General Introduction .................................................................................... 1 1.1 Introduction ............................................................................................. 2 1.2 Definitions of the terms alien and invasive species ............................. 2 1.2.1 Alien Species ...................................................................................... 3 1.2.2 Invasive species ................................................................................. 3 1.3 Harmful effects of invasive alien species .............................................. 5 1.4 The Role of Alien Plant Introductions in the United Kingdom .......... 8 1.5 The genus Impatiens in the United Kingdom ........................................ 9 iii 1.5.1 Relative invasiveness of Impatiens species in the United Kingdom ................................................................................................................... 10 1.6 Impatiens glandulifera in the United Kingdom..................................... 11 1.6.1 History ............................................................................................. 11 1.6.2 Native Habitat ................................................................................. 11 1.6.3 Distribution of I. glandulifera in Europe and the United Kingdom ................................................................................................................... 12 1.6.4 Plant description ............................................................................. 13 1.7 Ecological Impact of Impatiens glandulifera on native plant communities ................................................................................................ 18 1.8 Other deleterious effects of I. glandulifera ........................................... 19 2. Allelopathy as a potential contributor to the invasive success of Impatiens glandulifera ...................................................................................... 20 2.1 Introduction ........................................................................................... 20 2.2 Allelopathy – a missing component in the invasive success of Impatiens glandulifera? ................................................................................. 24 2.2.1 Allelopathy – definitions ................................................................ 25 2.2.2 Historical references to allelopathy ............................................... 27 2.2.3 Types of chemical compounds identified as allelochemicals ..... 28 2.2.4 Mode of release of allelochemicals ................................................ 30 iv 2.2.5 Factors influencing the release of allelochemicals ....................... 31 2.3 Phytoalexins and Phytoanticipins ....................................................... 34 2.4 Mode of operation of allelochemicals.................................................. 36 2.4.1 Indirect ............................................................................................. 36 2.4.2 Direct ................................................................................................ 36 2.5 The challenges in establishing proof of allelopathy ........................... 37 2.6 Types of experiments used to identify allelopathic effects ............... 42 2.6.1 Bioassays .......................................................................................... 43 2.7 Use of activated charcoal in allelopathy experiments ....................... 45 2.8 Measurement of allelochemicals in situ .............................................. 46 2.9 Effects of plant density on allelochemical uptake .............................. 47 2.10 The role of allelopathy in plant invasions ......................................... 48 2.11 Evidence for allelopathic effects in Impatiens .................................... 49 2.12 Naphthoquinones, a potential chemical basis for allelopathy in I. glandulifera .................................................................................................... 50 2.13 Taxonomic basis for I. glandulifera’ s invasiveness ............................ 56 2.14 Rationale for the present study .......................................................... 57 3. Materials and Methods .............................................................................. 60 3.1 Introduction ........................................................................................... 60 v 3.2 Location of study. .................................................................................. 60 3.3 Plant Production .................................................................................... 61 3.3.1 Seed Collection ................................................................................ 61 3.3.2 Seed storage and stratification ....................................................... 62 3.3.3 Seed germination ............................................................................ 63 3.3.4 Treatment of seedlings grown in sand .......................................... 67 3.3.5 Treatment of Seedlings grown in compost ................................... 69 3.4 Growing Conditions ............................................................................. 71 3.4.1 Temperature ...................................................................................