Unravelling how a mysterious substance may boost plant growth

Humic substances are organic compounds derived from soil, many of which have a positive effect on plant growth. One group of humic substances is fulvic acid, which actually appears to be a mix of different compounds. This and other humic substances are of significant interest for their potential to support growth of important food crops. However, what these substances actually do to promote plant growth remains unknown – do they have some direct nutritional benefit to plants, or can they act in a similar way to known plant hormones?

It is important that we understand this, both so that we know how to get the most out of humic substances to support growth of important plants, and so that we can be aware of any possible deleterious effects of their use. In a recent paper available Open Access in the Journal of Experimental Botany, Nicola Capstaff and colleagues based at Norwich Research Park investigate how fulvic acid may boost the growth of the important leguminous plant Medicago sativa (alfalfa) in both experimental and field environments.

Capstaff and colleagues obtained two different commercial sources of fulvic acid and analysed their chemical composition. Interestingly, they found that the two commercial samples were very different in chemical composition despite being made from similar starting materials by the same procedure. This highlights the difficulty in interpreting previous work on humic substances, as seemingly equivalent substances can actually have very different compositions.

The authors tested the effect of the application of both of these commercial fulvic acid sources on growth of Medicago sativa in greenhouse experiments, and found that application of fulvic acid significantly increased the vegetative growth of two of the three cultivars that they tested. Moreover, carefully selected nutritional controls used by Capstaff and colleagues indicated that the effect of fulvic acid on Medicago sativa growth was not produced by a nutritional benefit of the substance. Increased vegetative growth of Medicago sativa by application of fulvic acid was also reproducible in field trial settings.

Medicago sativa is a leguminous plant, meaning that it forms symbiotic associations with nitrogen-fixing bacteria in root nodules as an additional method of obtaining fixed nitrogen. Capstaff and colleagues investigated whether the growth boost provided by fulvic acid to Medicago sativa may due to a positive effect of fulvic acid on this process. Fulvic acid treatment did not significantly increase the number of root nodules on the Medicago sativa plants, but did increase the proportion of root nodules at a mature, actively nitrogen-fixing state. Furthermore, the authors also found that both fulvic acid sources boosted the growth of Sinorhizobium meliloti – a species of nitrogen-fixing bacteria.

After ruling out a direct nutritional effect of fulvic acid on the Medicago sativa plants, Capstaff and colleagues analysed the impact of fulvic acid application on gene expression in Medicago sativa. Interestingly, they found that application of fulvic acid particularly caused increased expression in Medicago sativa roots of genes associated with nitrogen metabolism, root nodulation processes and signalling.

Capstaff and colleagues therefore take us closer to understanding how one type of humic substance, fulvic acid, actually promotes plant growth in the case of Medicago sativa, in which it seems to promote nitrogen-fixing processes through a non-nutritional effect. Whilst this focusses on the possibility that this and other humic substances may act in a hormone-like way, the molecular analyses of the fulvic acid samples failed to find hormone-like molecules in the samples that the authors used. Capstaff and colleagues highlight the in-field potential of humic substances and indicate how these specifically may effect Medicago sativa. Hopefully this will continue to apply to other plants in a world with ever-increasing pressures on food security.