The presence of a below-ground neighbour alters within-plant seed size distribution in Phaseolus vulgaris

Abstract

Background and aims: Considerable variation in seed size commonly exists within plants, and is believed to be favoured under natural selection. This study aims to examine the extent to which seed size distribution depends on the presence of competing neighbour plants.

Methods: Phaseolus vulgaris plants rooting with or without a conspecific neighbour were grown in soil with high or low nutrient availability. Seeds were harvested at the end of the growth cycle, the total nitrogen and phosphorus invested in seed production were measured and within-plant seed size distribution was quantified using a set of statistical descriptors.

Key results: Exposure to neighbours' roots induced significant changes in seed size distribution. Plants produced proportionally more large seeds and fewer small ones, as reflected by significant increases in minimal seed size, mean seed size, skewness and Lorenz asymmetry coefficient. These effects were different from, and in several cases opposite to, the responses when the soil nutrient level was reduced, and were significant after correction for the amount of resources invested in seed production.

Conclusions: Below-ground neighbour presence affects within-plant seed size distribution in P. vulgaris. This effect appears to be non-resource-mediated, i.e. to be independent of neighbour-induced effects on resource availability. It implies that, based on current environmental cues, plants can make an anticipatory adjustment of their investment strategy in offspring as an adaptation to the local environment in the future.

Keywords: Anticipatory maternal effect; Phaseolus vulgaris; bet-hedging; game theory; kidney bean; neighbour detection; root competition; seed size variation; seed-setting; size inequality; skewness.

Fig. 1.

Relative numbers and masses of seeds in different size categories of Phaseolus vulgaris plants growing in the presence (open bars with red cross-hatching) and absence (grey bars) of a below-ground neighbour at (A, B) high and (C, D) low nutrient levels. Data were first standardized for each plant (as a percentage) and then averaged to represent the individual level. Data are means ± s.e.

Fig. 2.

Total reproductive output, i.e. (A) total pod mass, (B) total seed mass, (C) seed number, and resource investment in seed production, i.e. total (D) nitrogen and (E) phosphorus content, of Phaseolus vulgaris plants growing in the presence (open bars with red cross-hatching) and absence (grey bars) of a below-ground neighbour at high and low nutrient levels. Data are means ± s.e.

Fig. 3.

Descriptors of within-plant seed size distribution, i.e. (A) maximal seed size, (B) minimal seed size, (C) mean seed size, (D) coefficient of variation (CV), (E) skewness, (F) adjusted Gini coefficient and (G) Lorenz asymmetry coefficient, of Phaseolus vulgaris plants growing in the presence (open bars with red cross-hatching) and absence (grey bars) of a below-ground neighbour at high and low nutrient levels. Data are means ± s.e.