Effects of Plant Growth Regulators on Plant Phenotypes and Yield Components of Grass Seed Crops
Creators: Pandey, Bishnu
Publisher: name: University of Alberta Library
Year: 2023

<p>Creeping red fescue (Festuca rubra L.), meadow brome (Bromus riparius Rehmann) and timothy grass (Phleum pratense L.) are the most important grass species grown for seed production in the Peace River region of Alberta and British Columbia. Creeping red fescue is a popular turf grass species, whereas meadow bromegrass and timothy are commonly used for pasture, hay, and silage production for livestock feed. The seed productivity of these grass crops is limited by lodging under abundant rainfall conditions. Lodging can negatively impact seed yield and quality in forage crops, and make harvesting more difficult, time-consuming, and prone to seed loss through shattering. Plant growth regulators (PGRs) are an effective means of controlling lodging and seed loss, especially when grasses are grown under high nitrogen (N) fertility and non-limiting soil moisture conditions. A study was conducted over two years in 2021 and 2022 to quantify the morphological development of these grasses in response to PGRs at Beaverlodge Research Farm, Agriculture and Agri-Food Canada (AAFC) in northwestern Alberta, Canada. This study aimed to identify beneficial PGRs for regulating plant photosynthesis, growth, lodging tolerance, seed yield, and yield components of these grasses. A field experiment with split-plot design included PGR treatments as main plots and top-dressed spring nitrogen (urea) at 40 kg ha-1 as subplots on three and four-year-old stands of these grasses for two consecutive years. Three PGRs, including trinexapac-ethyl (TE), chlormequat chloride (CC) and ethephon (ETH), applied at 0.20 kg, 1.12 kg, and 0.60 kg active ingredient (ai.) ha-1 were used separately on each grass at the two-node stage of crop development (BBCH 31-32) and compared to untreated grasses. The results showed that TE was efficacious in shortening internode and tiller height and reducing lodging in meadow bromegrass and timothy. However, PGRs did not alter grass seed attributes such as panicle length, seed weight, and seed number, as well as the total seed yield in all three grasses. TE demonstrated similar levels of marginal economic returns to the check, indicating comparable profitability for timothy and creeping red fescue. Spring nitrogen was beneficial in increasing biomass and seed yield in timothy and meadow bromegrass without significant lodging. The effects of PGRs were more pronounced in improving grass morphology under normal rainfall conditions. However, under moisture deficit climatic conditions, PGRs had inconsistent and minimal effects on grass plant morphology. <br>
A greenhouse study was also conducted to investigate the effects of PGRs on several variables related to vegetative growth, root-shoot allometry, chlorophyll pigmentation, and photosynthetic efficiency at the juvenile stage of these grass crops. The study found that TE reduced root, shoot, and total biomass up to 40 days in timothy, and up to 55 days in creeping red fescue after application, but no PGRs affected meadow brome biomass at any point of time. The PGRs had no effect on growth variables such as root area, root length, root to shoot ratio, leaf area, tiller number, final biomass, and other growth parameters like relative growth rate, net assimilation rate, leaf weight ratio  and specific leaf area. Both TE and CC reduced the plant height and lodging severity of the three grasses. PGRs did not enhance or hinder the quantum yield of photosystem II (Fv/Fm) from 4 h through 12 days after PGR application. However, TE and CC increased chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid contents (µg g-1 FW) in timothy up to 70 days after application. Despite these limited responses, a principal component analysis  revealed that the ETH and check treatments exhibited higher total biomass, shoot biomass, root-to-shoot ratio, and plant height, which were associated with higher lodging severity, while TE and CC exhibited higher contents of chlorophylls and carotenoids along with greater NAR in all tested grasses. Overall, TE was more efficient and efficacious in reducing plant height and lodging severity of timothy and meadow bromegrass under field conditions and did the same for all grasses including creeping red fescue, in the greenhouse. Multiple studies of PGRs under a range of environmental scenarios can improve our understanding of how these compounds influence grass morpho-physiology. Likewise, long-term field studies under different lodging conditions could lead to the development of more efficient PGRs utilization strategies that maximize forage yield and seed harvestability. </p>