Defining features of age-specific fertility and seed quality in senescing indeterminate annuals

A trade-off between fertility and offspring viability underpins plant reproductive response to sub-optimal environmental conditions. Senescence involves internal resource limitation, and it is a sub-optimal body condition. We tested if senescence affects age-specific fertility and seed viability (quality) in indeterminate annuals.

Fertility in individual pods on the monopodial indeterminate inflorescence of Arabidopsis thaliana and its big-seeded relative Brassica nigra was quantified. The reproductive phase was divided into three phases: (1) early-senescence (initial flowers) (2) mid-senescence and (3) late-senescence (wilting leaves). Seed-viability probability as a function of pod position on the inflorescence (a proxy for parent's age) and seed position within pod was verified by germination tests in Brassica and then analysed using a binomial logistic regression model.

Age-specific fertility increased gradually, peaked, and then declined significantly during senescence in Arabidopsis and Brassica. Acropetal size distribution of rosette leaves was similar to that of pods (age-specific fertility) in Arabidopsis. Seeds positioned closest to stigma tended to be heavier and more viable than others in highly fertile pods, characteristic of mid-senescence phase in Brassica. Pod position (parent's age) was a significant predictor of seed-viability probability or seed quality, which improved in old and senescing Brassica.

High viability probability of seeds produced in low-fertility pods during late-senescence phase suggests weakening of maternal control over seed-size optimization (bigger, fewer, and better seeds) in internally resource-depleted older parent plants. Proximity to stigma can increase seed quality. The unexpected increase in fertility and seed viability during early-senescence phase is likely due to highly conserved developmental constraints on leaf and pod phenotype. Indeterminate annuals can shed light on fertility, offspring quality and senescence relationships in all plants that reproduce sexually and indeterminately.