Seed production of Lotus uliginosus cv. Sharnae in response to plant population density

Abstract

Summary. An experiment was carried out under irrigation in the field to determine the influence of plant population density (7.5, 15, 30 and 60 plants/m2) on growth, development and yield of the pasture legume Lotus uliginosuscv. Sharnae. Reproductive development (flowering, pod set and stages of pod development) was recorded from 79 days after establishment and harvests were taken on 3 occasions during pod development: 118, 138 and 158 days after establishment. At each harvest, plants were divided into leaf, stem and pods. The pods were further separated into maturity classes on the basis of colour, and seed number and weight measured for each class. Harvesting late (i.e. 158 days after establishment) maximised seed yield (113 g/m2) and seed quality (27% immature, green pods). Seed yield per unit area up to 158 days after establishment was relatively insensitive to plant population up to 60 plants/m2 because the response of individual plants of lotus was highly plastic. At 158 days after establishment, dry matter per plant declined from 985 g at 7.5 plants/m2 to 713 g at 60 plants/m2. At the same time, seed number per plant and seed weight per plant were reduced from 27 400 to 3230 seeds and from 15.5 to 1.65 g, respectively, across the range of population densities. Despite the lack of a clear optimum plant population for maximum seed yield, some aspects of seed quality declined at high population density. For example, single seed weight fell by about 10% from 0.563 mg and the proportion of the harvest contributed by seeds from immature pods rose from 18 to 33%, as population density increased from 7.5 and 60 plants/m2. Seed quality and ease of management, rather than yield, may be the most important considerations when seed growers choose a sowing rate for a L. pedunculatus cv. Sharnae seed crop.