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Kin selection is often used to explain the evolution altruism towards relatives through favouring the evolution of kin recognition. However, it remains unclear whether kin recognition is affected by plant pair density and different degrees of relatedness. A two-factor experimental design of kinship (three kinship degrees including siblings, closely related strangers and distantly related strangers) and pair density (including relative small, medium and large pair densities) was conducted in this study. Plant competitive traits including rosette size, specific leaf area (SLA), stem elongation, root and leaf allocation, seed biomass and vegetative biomass were measured to reflect interactions among plants living with different relatives of Arabidopsis thaliana accessions [Columbia (Col-0), Landsberg erecta (L er) and Wassileskijia (Ws)] in three different pair densities. The SLA only showed kinship effect, and siblings showed higher SLA than non-siblings in each pair density. The plant total biomass was only affected by pair density, which increased with decreases of pair density. The rosette size, stem elongation, root allocation and leaf allocation showed interactive effects of kinship and pair density. In the large pair density, the rosette size of siblings was lower than distantly related strangers, compared to closely related strangers; the stem elongation and root allocation were lower, while the seed biomass of siblings was higher than the closely or distantly related strangers. In the medium pair density, plants living with siblings or with closely related neighbours showed higher root allocation than with distantly related neighbours. In the small pair density, the plant rosette acted similarly to of which in the large pair density, and siblings showed higher root allocation than the two strangers. The other traits in each pair density showed no significant differences among kinship treatments. A relatively large pair density achieves kin recognition by deducing root competition ability and mutual shading, with increased efficient light capture and fitness. Similar such root and efficient light capture strategies were selected in medium pair density. Except the efficient light capture strategies, small pair density also displays allocation trade-offs between roots (decreased) and leaves (increased) for siblings. Moreover, kin responses on those attributes are also adjusted by kinship degree. Thus, kinships and pair density are important variables to mediate kin interactions. [ABSTRACT FROM AUTHOR]