Field Crops Research ELSEVIER Field Crops Research 40 (1995) 95-99 Influence of photoperiod on culm length in wheat Gustav0 A. Slafer *, Gerald M. Halloran, David J. Connor Centre for Crop Improvement, Department of Agriculture, The University of Melbourne, Parkville, Vie. 3052, Australia Received 26 July 1994; accepted 24 October 1994 Abstract The effects of photoperiod on culm length (CL) were examined in a semi-dwarf (Condor) and a standard-height (Thatcher) wheat cultivar under five photoperiod regimes (natural photopericd, two constant photoperiods, and two under which photoperiod was artificially increased over natural photoperiod by 9.8 and 13.1 min/d). The five photoperiod treatments were randomly assigned at sowing and re-randomised at terminal spikelet initiation (TS) to give five different photoperiod treatment combi- nations before and after TS. The cultiv&s differed significantly in final CL, Thatcher being taller than Condor. Culm length was related to the average photoperiod during the period between seedling emergence (SE) and double ridge (DR). There appeared to be an indirect effect of photoperiod during the vegetative phase on subsequent culm elongation, which was much stronger in Thatcher than in Condor, but the differences in CL between the two cultivars were smaller under long compared with short photoperiods. The effect of photoperiod on CL appeared to be closely related to the final number of leaf primordia on the main culm and to the number of elongated internodes, both increasing with short photoperiods. In contrast, differences in CL between cultivars were mainly due to the lengths of the internodes. Keywords: Culm length; Photoperiod; Triticum aestiuum; Wheat 1. Introduction It has been shown by comparing cultivars that the final yield of a crop is frequently associated with har- vest index (see reviews by Loss and Siddique, 1994; Slafer et al., 1994~). Harvest index, in turn, has been found to be negatively associated with final culm length (CL) (e.g. Austin et al., 1989; Slafer and Andrade, 1993). The incorporation of the semi-dwarf habit in commercial wheat appears to have increased yield potential believed to be mainly due to increased harvest index (e.g. Brooking and Kirby, 1981; Borrell et al., 1991; Youssefian et al., 1992). * Corresponding author. 0378-4290/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10378-4290( 94)00098-O In addition, the culm-elongation (CE) phase has been shown to be a critical period in the determination of yield potential in wheat (e.g. Fischer, 1985; Savin and Slafer, 1991). During this period the number of grains per m* is determined (mainly because of its association with spike dry weight at anthesis), which largely determines yield potential (Slafer and Savin, 1994). Although CE is a growth parameter, it is strongly related, under field conditions, to developmental pro- gress. The culm begins to elongate soon after floral initiation, but is frequently evident only between ter- minal spikelet initiation (TS) and anthesis (An) (e.g. Cooper, 1956). In fact, results from several field exper- iments show that the stage of ‘spike at 1 cm’ above the crown, coincides with TS (Kirby et al., 1985; Masle,