Growth and morphology of spring wheat (Triticum aestivum L.) culms and their association with lodging: effects of genotypes, N levels and ethephon

Lodging behavior and morphological characters of 12 spring wheat (Triticum aestivum L.) genotypes (four Indian cultivars and eight Mexican cultivars/CIMMYT advanced lines) were studied under disease free condition with different N rates (180, 240 and 300 kg ha(-1)), and at 300 N, with ethephon (480 g ha(-1)) application at CIMMYT (Centro Internacional de Mejoramiento de Maiz y Trigo), near Ciudad Obregon, Sonora, Mexico, during 1997-1998 and 1998-1999. In both years, the crop lodged, mainly by buckling or bending of the culms, 25-30 days after anthesis. Ethephon application at DC 38 prevented lodging and was associated with reduction in plant height (10.2%), peduncle length (14.2%), length of third internode (8.6%). It also simultaneously increased tillers m(-2)(9.1%), stem wall thickness for the first (4.3%), second (6.3%) and third (8.1%) internodes and peduncle (3.6%) when compared at the same N level. Varietal differences in lodging behavior were significant during both years. The lodging tolerant genotype, Baviacora 92, had 31.9, 34.0, 40.7 and 34.1 % larger diameter for the first, second, third intemodes and peduncle, respectively, when compared to HD 2329, one of the most lodging susceptible genotypes. Furthermore, Baviacora 92 had first, second, third intemode and peduncle stem walls that were also 31.7, 33.5, 35.4 and 37.1% greater, respectively, than Pavon 76, which had the thinnest stem walls. Genotypic correlations were better than phenotypic correlations, which suggested that environmental effects did not reinforce the genotypic effects rather they weakened them. Tillers m(-2) correlated (r = 0.89, P < 0.01) significantly positively while stem diameter and wall thickness correlated significantly negatively with lodging score. Diameter of different intemodes including the peduncle, weight per stem and spike also correlated significantly negatively with tillers m(-2). Simple linear regression explained that 79% of the variation in lodging score could be due to tillers M and 49-65% due to diameter of different intemodes. The best sub-set of the regression showed that number of tillers m-2 and/or stem diameter of internodes were the key variables when deciding two or three important characters for selecting for varieties with lodging resistance. Three variables such as stem diameter of first, second internode and length of first intemode or tillers M height and length of stem or tillers M stem diameter of first, second intemode together explained about 89-91% of the variation in lodging score. Therefore, it was suggested that selection for lodging resistant cultivars should emphasize larger stem diameter and wall thickness of basal internodes and fewer tillers per unit area with heavy spikes. (C) 2003 Elsevier B.V. All rights reserved.