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

* 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 neighbourwere 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. Key words: Anticipatory maternal effect, bet-hedging, game theory, neighbour detection, Phaseolus vulgaris, kidney bean, root competition, seed-setting, seed size variation, size inequality, skewness. INTRODUCTION A considerable degree of variation in seed size within plants is commonly observed (Michaels et al., 1988; Silvertown, 1989; Ruiz de Clavijo, 2002; Vo¨ller et al., 2012). Such variation is often interpreted as an adaptive bet-hedging strategy (Harper et al., 1970; McGinley et al., 1987; McGinley and Charnov, 1988; Venable and Brown, 1988; Geritz, 1995). Many studies also reveal that plants modify the pattern of variation (i.e. distribution) to cope with their abiotic environmental conditions (e.g. temperature, Wulff, 1986; light, Galloway, 2001; nutrients, Galloway, 2001;water, Parciak, 2002). Herewe demonstrate that seed size distribution may also be modified in response to the presence of a below-ground neighbour. Within a species, seed size (following common practice, seed size refers to seedweight in this paper) often correlates positively with the competitiveness of the offspring (e.g. Houssard and Escarre´, 1991; Eriksson, 1999; Lehtila¨ and Ehrle´n, 2005; Dubois and Cheptou, 2012). Based on the trade-off, induced by resource limitation in plants, between competition (favours large seeds) and colonization (favours a large number of small seeds), Geritz (1995) extended an optimal offspring size model (Smith and Fretwell, 1974) by considering seedling competition and using