Kelp extracts as biostimulants: An investigation of when and why they work

Abstract

This thesis describes a series of experiments designed to explore aspects of the benefits of kelp (brown algae; class Phaeophyceae) extracts as biostimulants for plant growth and development. The mechanisms for claimed benefits of kelp in horticulture, viticulture and agriculture are not well defined. The topic is introduced with a literature review (Chapter 1) discussing research into the use of kelp extracts in agriculture until 2019. The review reflects the state of knowledge of the use of kelp in agriculture at the time of the commencement of my candidature. It provides an outline of the evolution and qualities of kelp and a comparison between the structure and composition of kelp and green plants. Defence mechanisms of plants and algae are discussed. The species of kelps used in agriculture and the various methods of extraction of the commercially available kelp products are reviewed. The various modes of action of kelp extracts in agriculture that have been proposed are presented and the molecules such as phytohormones and carbohydrates that have been proposed by researchers as being responsible for kelp bioactivity in green plants are discussed. The overall purpose of the research carried out in this project was to improve understanding of the mechanisms responsible for the response of terrestrial plants to soil and foliar application of kelp extracts, identify the circumstances under which kelp extract application produces a growth and/or yield benefit and explore any observed differences between different kelp products. The research component of this thesis is organised in a “thesis by publication” style, with Chapter 1 (Literature Review) acting as a general introduction, and Chapters 2-4 each containing specific introductions relevant to those sections. Chapter 2 describes four field experiments conducted to investigate the potential benefit of kelp extract treatment to broccoli growing on slaking soil on the grounds of the Waite Campus of the University of Adelaide. Broccoli was chosen for these experiments because it is suited to the Mediterranean climate of Adelaide and prior research has shown that broccoli responds to kelp under Australian conditions. These four field experiments are similar in design and each explores a different aspect of kelp treatment. In order to limit repetition, aspects of Materials and Methods common to each of the four experiments are presented at the outset. The first field broccoli experiment (Experiment 1; Section 2.1) was a comparison between five commercially available kelp products, applied as either soil drench or foliar application and differing in either the kelp species extracted or the method of extraction or both. This experiment was to have been the centrepiece of the broccoli field experiments, but no statistically significant response to any of the kelp extract treatments was found. While this is an important result in itself, it should be interpreted as providing a lack of evidence of an effect of kelp rather than evidence of a lack of an effect of kelp. Several “side-experiments” were established in the same season on the same site, and these provided further important information, and further refined conditions under which kelp had no significant response effect and others in which a response to kelp was found. The second and third experiments (Experiment 2 (Section 2.2) and Experiment 3 (Section 2.3)) were designed to explore interactions between applied kelp extracts and macro-nutrients. In Experiment 2, an extract from the kelp species Ascophyllum nodosum was applied to broccoli in conjunction with varying levels of phosphorus (P). While the response to P was significant, no interaction between the kelp and P was detected. In Experiment 3, two commercially available kelp products, extracted from A. nodosum by different means, and two forms of nitrogen (N) fertiliser were studied to observe potential interactions between kelp and N on the production of broccoli. No response to either kelp extract was detected. In Experiment 4 (Section 2.4), kelp extract was compared with commercially produced alginate as a soil treatment prior to planting broccoli seedlings. Alginates are extracted from the cell wall of brown algae. They have been shown to have properties consistent with being beneficial to soil structure and this field study was undertaken to determine the importance of kelp extract and/or alginate on the development of broccoli growing in soil susceptible to slaking. Although established in the same season as Experiments 1-3, this experiment was established approximately one month later, when ambient air temperatures were much cooler. Furthermore, a higher rate of P than for Experiments 1 and 3 was adopted because of observations from Experiment 2 where there was early response of broccoli plants to higher rates of P. In contrast to Experiments 1-3, significant treatment effects were found for Experiment 4, with both the kelp extract and the alginate positively stimulating broccoli production. These results, and the results of the other experiments are summarised at the conclusion of Chapter 2 (Section 2.5). Chapter 3 describes a glasshouse pot experiment with broccoli, designed to follow on from Experiment 2 in the field. In this experiment, kelp was applied with a range of P fertilizer at a range of rates. Variation within treatments in the field was high and appeared to be associated with localised waterlogging experienced across the field experiments. It was felt that more uniformity and hence less within-treatment variation would be experienced in a glasshouse pot experiment. In the glasshouse, variation in growth parameters of broccoli within treatments was found to be very low when compared with variation in the field, but plant responses were very different to those observed in the field. There was no perceived response to P. Consideration is given as to what might be done differently in order for a repeat of the glasshouse experiment to resemble the field more closely. Pot size and temperatures were considered important. Chapter 4 describes a glasshouse experiment investigating the efficacy of the five commercially available kelp extractions on tomato production. There is much evidence that tomato plants respond to foliar and soil kelp applications but I have found no comparisons between the activity of these products on tomatoes in the literature. While some significant treatment effects were found, this experiment was compromised by the limitations imposed by the Covid-19 epidemic. It had been intended to look for interaction between kelp and mycorrhizal fungi, but this aspect of the experiment was omitted owing to inaccessibility to facilities when required. The kelp comparison experiment was terminated prematurely, due to the uncertainty that the pandemic was causing. Tomatoes were harvested at early fruiting and yields recorded. Results for all five of the kelp extracts gave a higher tomato yield at harvest than the untreated plants, but only the results for Kelp D (fermented Ascophyllum nodosum extract) were statistically significant. The mechanism for the plant response to kelp extract treatment was considered to be very different to that for the broccoli experiment. Chapter 5 provides overall project conclusions based on the results from the field experiments with broccoli and the results from the glasshouse experiment with tomatoes. I have speculated as to the mechanisms for the plant responses to kelp where they have occurred. The response of broccoli to treatment in Experiment 4 is consistent with the activity of the alginate component of kelp in the soil, while the observed response of tomatoes in the greenhouse is consistent with stimulation of plant hormone production by the kelp extract. These two experiments with statistically significant responses to kelp application thus appear to have been the result of very different response mechanisms. The results of these experiments highlight the need for further research to identify what triggers such responses. After the conclusion, there are four brief appendices. The first was a simple experiment with wheat treated with a kelp extract at different rates. No benefits of the kelp applications were found in this experiment. Appendices 2 and 3 describe two experiments designed to study the effects of each of the five commercial kelps on seed germination and seedling emergence. The kelp applications were found to be of no benefit in these experiments. The fourth appendix describes the initial stages of a metabolomic study of broccoli curds retained from Experiment 4. There are (anecdotal) claims of improved food quality from kelp treatments and this second experiment was an ambitious exploratory study hoping to find evidence of differences in molecular composition between curds of treated and untreated broccoli. The work was suspended due to the constraints of Covid-19. The outcome and implications of this work, including suggested future research, are discussed in the conclusion.