In our first Hindsight, Dr. Ken Thompson takes us back to his PhD research and points out one –perhaps forgotten– follow up study from Van Assche & Van Nerum that may have you asking if there is such thing as plant intelligence.

by Ken Thompson

During my PhD, back in the 1970s, I built a machine to expose seeds to various amplitudes of diurnal temperature alternation, and then used that machine to screen the seeds of a large number of species. The ecological implications of my results were, I thought, pretty clear, but I’ll spare you the details.

What I didn’t worry about at all was exactly what my seeds were responding to. But others were interested in that question, as I discovered much later when I read Van Assche & Van Nerum (1997).

In a series of neat experiments, Van Assche & Van Nerum showed two things, both of which appeared to me to show that plants are smarter than we usually give them credit for. In the first place, on the principle that what goes up must come down, and in accord with Darwin’s observation that ‘nature always economizes her means’, Rumex seeds respond only to a rise in temperature. A reduction in temperature, whether fast or slow, large or small, has no effect.

Second, they showed that the rate of temperature change is crucial. A rate of warming of 3·3 °C h^–1 may not seem all that different from 2 °C h^–1, but the former produced much more germination than the latter. As I had concluded during my PhD, compared to those at the surface of bare soil, diurnal temperature alternations are rapidly damped by both increasing soil depth and by a plant canopy. Thus sensitivity to temperature alternations (especially in darkness) acts as a detector of both depth of burial and of vegetation gaps. Van Assche & Van Nerum measured soil temperatures at different seasons and depths and concluded that a response to 3·3 °C h^–1, but not to 2 °C h^–1, ensured ‘that only seeds in open places near the soil surface will germinate in the spring’.

Which is interesting enough in itself, but in the light of recent debates about ‘plant intelligence’, Van Assche & Van Nerum’s results seemed to me to demonstrate that plants have a remarkably accurate perception of rates of change, and thus of the passage of time. Ask yourself: could you tell the difference (even if your life depended on it) between a rate of change of 3·3 °C h^–1 and 2 °C h^–1? No, I thought not.

Van Assche & Van Nerum’s paper, somewhat to my surprise, has never aroused much interest. According to Web of Science^TM, it has been cited just 14 times, and those 14 papers have generally caused even less excitement; another Van Assche paper, on the comparative germination ecology of nine Rumex species, with 24 citations, is the most cited by a wide margin. A slightly sad fate for a paper that, if nothing else, is a neat demonstration of how to do interesting ecology on a small budget.

Van Assche, J.A. & Van Nerum, D.M. (1997) The influence of the rate of temperature change on the activation of dormant seeds of Rumex obtusifolius L. Functional Ecology, 11, 729-734

Ken Thompson is a professor at the University of Sheffield and has been a senior editor on Functional Ecology for over a decade. He’s also written several popular science books and writes a regular column on plants for The Telegraph.