Variations in size structure, growth and reproduction in Japanese plantain (Plantago asiatica L.) between exposed and shaded populations

Abstract

Abstract Plantago asiatica is a perennial herb that is distributed over a wide range of east Eurasia. The population structure, growth and reproduction in exposed (E‐) and shaded (S‐) populations of P. asiatica were examined in the Kanto District of eastern Japan. In both populations, the plant size structure showed a bimodal distribution during spring to early summer, in which the two modes corresponded to smaller seedlings and larger overwintered plants, respectively. In autumn, this distribution became unimodal due to seedling growth. However, this change occurred later in the S‐population because of suppressed growth in the seedlings. In the S‐population, flowering also began later in the growing season and the threshold plant size for flowering was larger than that of the E‐population. Biomass allocation to the rhizomes was greater in autumn in the reproductive plants of the S‐population. Growth and biomass allocation in plants grown from seeds collected from each population were compared under phytotron conditions. Near a saturated photon flux density, E‐population plants had a higher relative growth rate than S‐population plants. Therefore, E‐population plants should allocate resources to reproductive organs sooner. Shaded population plants were not vigorous in their growth and reproduction. Seed size (dry weight per grain) was significantly greater in the S‐population than in the E‐population, both in the field and under phytotron conditions. These results suggest that ecotypic differentiation in life‐history strategies, which is mainly due to light availability, occurs among local P. asiatica populations. The effects of severe trampling on early reproduction in the E‐population are also discussed.