Seed growth and development of three perennial ryegrass cultivars after treatment with 'Moddus' straw shortener.

Abstract

First year crops of three diploid perennial ryegrass (Lolium perenne L.) cultivars, ‘Meridian’, ‘Bronsyn’ and ‘Grasslands Impact’, that contain the AR1 endophyte, were sown on 1 April and 14 May 2008. A subsequent application of Moddus (a.i. 250 g/l Trinexapac ethyl) plant growth regulator at three rates was used to examine the relationship between seed and stem dry weight in relation to thermal time. Seed filling of ‘Meridian’, ‘Bronsyn’ and ‘Grasslands Impact’ followed a sigmoidal growth pattern. The lag phase was 150 growing degree days (°C days) and the duration of the linear period constant at 294 °C days. The application of Moddus increased seed yield by approximately 26% for each 800 ml/ha applied from 1715 (0 mlha) to 2195 (800 ml/ha) and 2722 kg/ha (1600 ml/ha). The time to 95% of final seed weight was constant between treatments at 443 °C days. Seed yield increase from Moddus was achieved by increased rate of seed filling per seed head, 0.24 mg/°C days/head, which increased the number of seeds/m². For all cultivars, 1600 ml/ha of Moddus produced the highest seed yield and the shortest total stem length. There was a 0.15 m reduction in length between 0 and 1600 ml/ha of Moddus with all internodes shortened, including the seed head length. Stem dry weight increased to a maximum at between 310 and 400 °C days following anthesis. Thus, stems competed with growing seeds from anthesis, throughout the lag phase until approximately 75% of final seed weight. When seed demand for assimilate was low, lag phase and early seed growth, the stem was a competing sink. As seeds developed their sink capacity increased, thus drawing assimilate from the stem. At harvest, stems were 25% heavier than at athesis which suggests they were a net sink for assimilate post anthesis and that there was further assimilates available for seed production. Moddus also decreased absolute lodging at harvest where stems were horizontal when no Moddus was applied compared with stems leaning on a 45 ° angle for 800 ml/ha and upright for 1600 ml/ha. Moddus increased the harvest index (HI) of plots, 13.5 – 19.8%, and individual stems, 20 – 40%, primarly through an increase in seed mass/ha while straw mass remained constant for ‘Meridian’ and ‘Bronsyn’. In ‘Grasslands Impact’ the increase in HI was limited through an associated increase in straw DM when Moddus was applied. The change in harvest index per day (dHI/dt) on main stems was linear, which suggests this could be a useful method for incorporation into crop simulation models. The dHI/dt was influenced by treatments which influenced lodging. Competition for assimilate between stems and growing seeds was a major factor limiting the seed yield. This has implications for plant breeders and seed producers where both should aim to reduce stem length which is likely to increase the rate of seed filling. There was a clear advantage to applying Moddus at 1600 ml/ha for all cultivars which highlighted the advantage of breeding for a shorter stem.