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The Effect of the Interaction of Citric Acid and Drought on the Growth of Spotted Gum (Corymbia maculata) Seedlings

Context Abiotic stress, and particularly drought, is a major threat to plant growth generally and world food security specifically and it is important for humanity to come up with ways to reduce the impact of drought and abiotic stress on plant growth. This is particularly important in the context of global climate change. Earlier research by a range of researchers has hinted that the use of cheap citric acid in treating plants may induce enhanced stress response pathways which may assist in enhancing drought tolerance. However, how altered stress response pathways affect plant growth patterns, and how these may affect drought tolerance, has not been well researched. Methods Spotted Gum seedlings were grown with and without initial treatment with citric acid, and with and without simulated drought. Key results Treatment with citric acid resulted in plants growing larger and more fibrous root systems compared to control plants. The effect was stronger under moderate drought. Implications Exogenous treatment of cotyledon roots with citric acid has tremendous potential for enhancing plant root systems under moderate drought. Resulting enhanced root systems could be expected to enhance a plant’s access to soil water and thus improve drought tolerance. Reduced shoot to root ratios could also be expected to improve drought tolerance of young plants in the early growth phase. Summary In addition to potentially having a negative effect on mine revegetation drought remains one of the major causes of agricultural loss globally, threatening food security. A range of research has hinted at the role of citric acid in plant stress response and particularly in drought tolerance (Godbold et al. 1984; Shlizerman et al. 2007; Sun and Hong 2011). It was reported that Arctic tundra soils contain high levels of citric acid (Jones 1998). Jones posed the question as to the relevance of citric acid in plant stress response and particularly to drought tolerance in environments where liquid water is limited. In order to discover whether citric acid might be used to enhance plant growth patterns leading to enhanced drought tolerance in woody species used in large scale mine rehabilitation, a series of trials were established. In these, the roots of cotyledons of commonly used species including Spotted Gum (Corymbia maculata) were soaked in various concentrations of citric acid in order to examine the effect on early plant growth. This paper discusses the results of one of these experiments conducted at the University of Newcastle as part of the author’s PhD program. A range of responses were noted in treated seedlings including the development of larger and more fibrous root systems. This response was stronger in plants subject to moderate drought and suggested that treatment enhanced an existing stress response pathway that affected root growth. This significantly enhanced root effect had not been previously noted in response to treatment with citric acid. Other beneficial effects were noted including the enhancement of shoot to root ratio and subsequent enhanced shoot growth as a result of larger and more fibrous root systems. Results from the study raised the question as to how widespread these effects are in the broader plant kingdom and what might be the relevance to food crop production? In this context, further research was undertaken on seeds and tissue culture of key crop species and the results, including significant effects on leaf gas exchange, and this will be reported in later papers. As such, it should be noted that this paper is part of a much larger research program in which the effect of citric acid treatment on cotyledons, seed and tissue culture of a range of woody C3 species is examined. Summary text for journal table of contents Drought, induced through global climate change and other factors, is likely to cause major conflict through its effect on plant establishment and food security in particular. Using Australian Spotted Gum trees as a subject, this experiment shows that the use of cheap citric acid on seedlings can produce growth effects such as enhanced fibrous root growth that, among other benefits, may make them significantly more drought-tolerant. The results may be beneficial to commercial forestry but may also have major implications for food security worldwide if observed effects are relevant to food crop species. Table of contents graphic