Reproductive allometry in four annual weeds

Abstract

Differences in plant biomass/plant seed production relationship imply that plants suppressed by competition to the same extent, may not suffer the same reduction in seed number setting. This size-dependent process evidenced by the quantitative relationship between plant growth and allocation to reproductive structures can be considered from an allometric perspective. The aim of this study was to describe the reproductive allometry of four annual weed species of the Pampas cropping systems by modelling the plant biomass/plant seed production relationship in Digitaria sanguinalis, Amaranthus hybridus, Setaria verticillata and Chenopodium album. To achieve this goal, two experiments developed during two consecutive growing periods (Experiments 1 and 2) were set up under greenhouse conditions. Weed species were grown in monocultures and in binary mixtures, with each other and with soybean crop, to obtain plants of different size after the effect of intra and interspecific competition. Data per plant regarding biomass versus seed number production considering monocultures and mixtures from Experiments 1 and 2 were analysed for each species using linear regression. Parameters of the linear functions fitted for C. album, S. verticillata and D. sanguinalis did not differ between experiments and data were pooled. Thus, three single specific models were obtained with the same intercept of zero (y-axis intercept when x = 0 was zero) and different slopes, 1122, 69 and 130 seeds g plant biomass–1 respectively. In the case of A. hybridus, the slopes were different in Experiments 1 (123 seeds g plant biomass–1) and 2 (74 seeds g plant biomass–1). Moreover, reproductive allocation changed allometrically under highly competitive stress since small plants failed to set seeds, unless a plant biomass threshold was attained (x-axis intercept when y = 0 was positive). Besides the ecological interpretation in reproductive output, implications in weed management are considered.