Out of Sight but Not out of Mind: Alternative Means of Communication in Plants

Current knowledge suggests that the mechanisms by which plants communicate information take numerous forms. Previous studies have focussed their attention on communication via chemicals, contact and light; other methods of interaction between plants have remained speculative. In this study we tested the ability of young chilli plants to sense their neighbours and identify their relatives using alternative mechanism(s) to recognised plant communication pathways. We found that the presence of a neighbouring plant had a significant influence on seed germination even when all known sources of communication signals were blocked. Furthermore, despite the signalling restriction, seedlings allocated energy to their stem and root systems differently depending on the identity of the neighbour. These results provide clear experimental evidence for the existence of communication channels between plants beyond those that have been recognized and studied thus far

Early growth of chilli seedlings depends on the presence and identity of their neighbour.

<p>(<b>a</b>) Seedlings growing next to a fennel (grey solid line and triangles) are marginally significantly taller than those growing next to an adult chilli plant (black solid line and squares; Pair-wise contrasts, P = 0.07) and significantly taller than seedlings in the empty control (black dotted line and white diamonds; Pair-wise contrasts, P = 0.01). The observed differences in above-ground growth among treatments (adult fennel plant, grey solid line and triangles; adult chilli plant, black solid line and squares; empty control, black dotted line and white diamonds) are amplified over time. Only plants that emerged by day 14 are included in these analyses (n = 32 per treatment). Error bars indicate standard errors. (<b>b</b>) Growth differences disappear when seedlings are allowed to grow in the absence of any adult plant after emergence (n = 80 per treatment). Error bars indicate standard errors.</p

Germination of chilli seeds is affected by the mere presence of an adult fennel plant.

<p>Because very little germination had occurred at day 4 and almost all seeds had germinated at day 11, germination data at days 6 and 8 are presented and used for statistical analysis. The median, inter-quartile range and range are represented by the middle bar, the top and bottom of box and the whiskers respectively. Outliers laying more than 3 times the inter-quartile range from the median are represented by the small circles. * n is total number of seeds as appropriate for binomial analysis.</p

Differences due to treatment overall and to 4 specific effects.

<p><i>Notes:</i> Shown are P-values for significance of effect obtained from a chi-squared test of deviance, AIC values for models with and without the effect, and, if significant, whether specific effects increased or decreased germination at days 6 and 8. Significance at P<0.05 indicated by * and model with lower AIC, which is the preferred model, is indicated by **.</p

Schematic representation of the custom-designed experimental unit (not in scale).

<p>(<b>a</b>) The seal at the base of the central cylindrical box ensured that chilli seeds arranged in a circle around the adult plant were chemically isolated from it. (<b>b</b>) All seeds and adult plants within a replicate unit were housed within 2 different sized square boxes, one inside the other, with the air in between the two boxes removed using a vacuum pump. The whole experimental unit was custom-made in colourless cast acrylic material (ModenGlas), which transmitted 92% of visible light, but was opaque to ultraviolet and infrared wavelengths.</p

Mean final root size of chilli seedlings is affected by the presence and identity of their adult neighbours.

<p>(<b>a</b>) Overall, maximum root length differed significantly depending on the neighbouring plant present in the sealed central box (n = 32 per treatment). Seedlings growing next to adult chilli plants had significantly shorter roots than those in the empty control or growing with the fennel (P = 0.015). (<b>b</b>) The presence of a neighbouring fennel during germination and emergence caused an increase in early root development of chilli seedlings when the communication channels are blocked, but not when unblocked (light grey bars) (F masked > F open and Control masked; P = 0.027; n = 80 per treatment). Differences disappeared when seedlings were allowed to grow away from a fennel plant (dark grey bars) (P = 0.94; n = 80 per treatment). Error bars indicate standard errors.</p