Europ. J. Agronomy 24 (2006) 212–217 Does light quality initiate tiller death in wheat? D.L. Sparkes ∗ , S.J. Holme, O. Gaju Division of Agricultural and Environmental Sciences, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD, UK Received 2 September 2004; received in revised form 4 July 2005; accepted 2 August 2005 Abstract Field experiments at Sutton Bonington, Nottinghamshire, UK, in 2001–2003 involving wheat grown at contrasting plant populations and nitrogen availabilities created crops with widely varying tiller production and survival patterns. These crops were used to investigate the ‘trigger mechanism’ responsible for the start of tiller death. The date of maximum shoot number ranged from 5 February to 16 May at which time the developmental stage was anywhere between GS27 and GS39. There was no correlation between radiation available per shoot and the beginning of tiller death. The start of tiller death was closely related to the red to far red ratio (R:FR) measured at the base of the canopy and we propose a critical R:FR for the initiation of tiller death in winter wheat. We also suggest that this critical R:FR interacts with leaf nitrogen content—where leaf nitrogen content is higher, the critical R:FR is lower: in other words, when more nitrogen is available, the canopy is allowed to grow larger before tiller death starts and vice versa. © 2005 Elsevier B.V. All rights reserved. Keywords: Winter wheat; Tillering; Light quality; Red/far red ratio 1. Introduction Conventional descriptions of wheat development suggest that the rapid decline in tiller number starts at GS31 (ear at 1 cm) (Hay and Walker, 1989). However, as plant popu- lation is reduced, the period of tiller production is extended, thereby delaying the start of tiller death (Whaley et al., 2000). It therefore appears that competition between plants, rather than development, initiates the start of tiller death. Zhong et al. (2002) found that in rice, there was a critical leaf area index at which tillering stopped and that this critical value was influenced by the nitrogen status of the plant. They sug- gested that LAI influences tillering by changing light inten- sity and/or quality at the base of the canopy. This is sup- ported by work by Casal et al. (1985) who showed that low red to far red ratio (R:FR) can reduce tillering in grasses and Kasperbauer and Karlen (1986) who found that close-spaced wheat plants developed fewer tillers than wide-spaced plants and suggested that tillering is controlled by the R:FR. This study explores the relationship between tillering in wheat and light quality, as perceived at the base of the canopy, to determine ∗ Corresponding author. Tel.: +44 115 951 6074; fax: +44 115 951 6060. E-mail address: debbie.sparkes@nottingham.ac.uk (D.L. Sparkes). whether there is a critical R:FR (R:FR c ) which initiates tiller death. 2. Materials and methods Field experiments were established at University of Notting- ham Farm, Nottinghamshire, UK, in 2000/2001, 2001/2002 and 2002/2003 (hereafter referred to as 2001, 2002, 2003, respec- tively). The soil type was a stagnogleic argillic brown earth in 2001 and an argillic pelosol in the other years. Soil mineral nitro- gen, as measured at planting, was 24 kg ha -1 in 2001, 77 kg ha -1 in 2002 and 56 kg ha -1 in 2003. Field sites were planted with winter oats in the season preceding the first experiment and win- ter oilseed rape in the other 2 years. Crop protection chemicals were used prophylactically to minimise weeds, pests and dis- eases. Chlormequat was applied during early stem extension in all experiments to mitigate lodging risk. In 2002, ethephon was also applied during late stem extension. The work described here was carried out on a subset of treatments within a large factorial experiment, arranged in a split–split plot design with three replicates. The work pre- sented here was confined to three seed rates (40, 80 and 320 seeds m -2 ) and two timings of early nitrogen (40 kg N ha -1 applied as ammonium nitrate in mid-February or early-April). Plots (2 m × 24 m in size) were drilled using an Oyjord tractor 1161-0301/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.eja.2005.08.003