Can plasticity compensate for architectural constraints on reproduction? Patterns of seed production and carbohydrate translocation in Perilla frutescens

Abstract

1 The architecture of Perilla frutescens was manipulated by growing plants in a greenhouse under two photoperiodic regimes, representing conditions near the northern and southern ends of the species' range, to investigate its effect on patterns of seed production. Within each photoperiodic regime, plants were labelled with 14C at one of three developmental stages at one of three leaf positions to assess the effects of architecture on carbohydrate translocation. 2 Perilla flowers in response to short days, i.e. southern-treatment plants flowered earlier and were smaller and more sparsely branched than northern-treatment plants. Branch number limits inflorescence number in Perilla, and southern-treatment plants therefore made fewer inflorescences. Inflorescences are, however, indeterminate, and the southern-treatment plants made more flowers per inflorescence than the northern-treatment plants, such that architecture did not constrain flower number. 3 As a result, plants in both treatments made a similar number of seeds and did not differ in total seed mass. 4 Increased seed production per inflorescence was associated with reduced carbohydrate movement between branches and more 14C-labelled assimilate per seed at the labelled node. 5 In summary, plasticity at the level of the inflorescence largely compensated for architectural constraints on inflorescence number, and was associated with differences in carbon translocation patterns between the photoperiod treatments.