Crop/ Forage/ Soil Management/ Grassland Utilisation Department of Agronomy, Faculty of Agriculture, Ataturk University, Erzurum, Turkey Response of pea (Pisum sativum L.) to mepiquat chloride under varying application doses and stages E. Elkoca and F. Kantar AuthorsÕ address: Dr E. Elkoca (corresponding author; e-mail: eelkoca@atauni.edu.tr) and Prof. Dr. F. Kantar, Department of Agronomy, Faculty of Agriculture, Atatu¨rk University, 25240 Erzurum, Turkey With 2 figures and 4 tables Received May 2, 2005; accepted August 6, 2005 Abstract Grown as a monoculture, peas (Pisum sativum L.) exhibit severe lodging after flowering and lodging causes yield reductions considerable. This study was conducted to investigate the effects of dose (untreated, 25, 50, 75 and 100 g a.i. ha )1 ) and stage (late vegetative, early blooming and early pod filling) of mepiquat chloride (MC) applica- tion on the growth, lodging control, seed yield and yield parameters of pea (Pisum sativum L.) under field conditions in Erzurum, Turkey in 2002 and 2003. Application doses of 25, 50, 75 and 100 g a.i. ha )1 significantly reduced stem height by 5.3 %, 7.2 %, 7.5 % and 6.4 % and increased stem width by 7.5 %, 12.7 %, 12.3 % and 15.7 % respect- ively, when compared with the untreated control, and thereby reduced the tendency of the crop to lodging. Increases of the seed yield under different application doses of MC ranged between 13.7 % and 20.1 % over the untreated control. However, in all parameters investigated, except for stem width, higher application doses of MC gave no clear advantages compared with the application dose of 25 g a.i. ha )1 . Seed yield was also significantly influenced by application stage of MC and application at early blooming stage of crop, MC significantly increased seed yield by 11.4 % and 10.2 % when compared with the late vegetative and the early pod filling stages respectively. Furthermore, the interaction of application dose and stage was significant, and spraying of pea plants with 25 g a.i. - ha )1 MC at early blooming stage has the most beneficial effects on the characters evaluated. Key words: lodging — mepiquat chloride — pea — yield Introduction When peas (Pisum sativum L.) are grown as a monoculture, they exhibit severe lodging after flowering (Heath and Hebblethwaite 1985, Stelling 1997). Lodging causes light interception in the crop canopy (Hebblethwaite 1977) and thus reduces the efficiency of photosynthesis, suppresses or inhibits anthesis (Burbidge 1977) and provides an ideal microclimate for the spread of fungal disease (Daniels et al. 1982). Moreover, the harvesting of lodged crops can be difficult, and failure to harvest all the grain may lead to further yield losses. Therefore, reductions in seed yield of pea have been attributed to lodging (Stelling 1994). However, manipulation of crop morphology using chemical plant growth regulators (PGRs) such as mepiquat chloride (MC) to reduce height and lodging increases the utilization of solar radiation and alters assimilate distribution in favour of seed development, and hence would appear to have considerable potential in many crops (Daniels et al. 1982). Application of PGRs, particularly growth retard- ants, may maintain internal hormonal balances and efficient sink–source relationships and thus enhance crop productivity (Singh et al. 1987). Plant growth retardants are often applied to agronomic and horticultural crops to achieve agronomic targets without lowering crop productivity. Because of their specific properties, plant growth retardants have found widespread use in agricultural and horticultural practices. The economically most important applications (Grossmann 1990) are improving the lodging resistance and canopy structure of crops (e.g. cereals and oilseed rape), reducing vegetative growth in favour of generative growth (e.g. peanuts, cotton, orchard trees and J. Agronomy & Crop Science 192, 102—110 (2006) Ó 2006 Blackwell Verlag, Berlin ISSN 0931-2250 U.S. Copyright Clearance Centre Code Statement: 0931–2250/2006/9202–0102 $15.00/0 www.blackwell-synergy.com