CSIRO PUBLISHING www.publish.csiro.au/journals/ajar Australian Journal of Agricultural Research, 2008, 59, 80–85 Additive genetic variance for stem strength in field pea (Pisum sativum) C. P. Beeck A,C , J. Wroth A,B , and W. A. Cowling A,B A School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. B Canola Breeders Western Australia Pty Ltd, 15/219 Canning Highway, South Perth, WA 6151, Australia. C Corresponding author. Email: cbeeck@cyllene.uwa.edu.au Abstract. Weak stem strength in field pea (Pisum sativum) is a major restriction to yield, seed quality and ease of harvest. Three aspects of stem strength: load at breaking point, flexion and compressed stem thickness, showed substantial genetic variation among a diverse range of six parents including modern cultivars, landrace accessions, and interspecific progeny. Diallel analysis of parents and F 1 progeny was conducted using a simple additive-dominance model, which was adequate for load and compressed stem thickness. There were significant additive genetic effects for load and compressed stem thickness with no evidence of dominance or maternal effects, and also significant additive genetic effects for flexion which was subject to more complex genetic control. Valuable alleles for these stem strength traits were present in commercial cultivars and landrace types of field pea. Efficient and practical breeding for improved stem strength will involve several recurrent selection cycles with moderate selection pressure for compressed stem thickness in early generations, followed by verification of improvements in lodging resistance in subsequent field trials. Compressed stem thickness is relatively easy to measure on individual plants in the field and is closely associated with load. Additional keywords: compressed stem thickness, diallel, load. Introduction Field peas (Pisum sativum L.) have weak stems and normally lodge before harvest, causing canopy collapse. The consequences of canopy collapse are loss of yield, decreased seed quality and increased difficulties with mechanical harvesting. To some authors, lodging is the largest problem in the field pea crop (Hedley et al. 1983; Heath and Hebblethwaite 1985; Holland et al. 1991). Lodging also increases the risk of disease such as blackspot (Mycosphaerella pinodes) due to increased humidity in the collapsed sub-canopy (Davies 1977; Dantuma 1983). Lodging is largely due to the weak strength of the basal nodes of the stem in relation to the weight of the upper parts of the plant (Davies 1977; Heath and Hebblethwaite 1984). Breeding for lodging resistance in field pea has focused on leafless or semi-leafless varieties. This plant morphology results in individual plants supporting each other to stand in a display of ‘structural mutualism’ (Givnish 1995), with varying levels of success in increasing lodging resistance (Stelling 1989; Davies 1993). The semi-leafless morphology is present in Australian field pea cultivars Kaspa and Excell (Leonforte 2003) and is the predominant morphology in European and Canadian pea cultivars. Crops with this morphology stand upright for longer but often lodge to varying degrees before harvest, as a result of weak basal nodes (Davies 1993). Lodging resistance may be improved further if the strength of stems at the basal nodes was enhanced genetically, thereby increasing the standing ability of the crop. This may lead to lower disease levels, easier mechanical harvest and improvements in seed yield per hectare. In the past, stem strength of field pea has been assessed through visual estimates of lodging in field plots (Stelling 1989), with few reports on quantitative physical measures such as load and flexion which may be assessed on individual stems (McPhee and Muehlbauer 1999; Kemsley et al. 2004). Quantitative physical measures of stem strength in cereals involved measuring the bending forces on stem samples (Dolinski et al. 1993; Crook and Ennos 1994; Ames et al. 1995; McPhee and Muehlbauer 1999; Kemsley et al. 2004). We have applied this technology to field pea stems and evaluated the genetic control of stem strength in field pea. Compressed stem thickness at the base of the stem was closely associated with load at breaking point, but less closely associated with flexion (Beeck et al. 2006). In the current work, load, flexion and compressed stem thickness were measured on the F 1 and parents of a diallel cross in order to evaluate their genetic control. Diallel analysis is useful to evaluate the genetic control of quantitative traits. The analysis allows an interpretation of the role of major and minor genes and additive, dominance or maternal effects controlling the traits. Minor genes controlling stem strength were implicated in the work of Tar’an et al. (2003) who reported two QTLs associated with lodging resistance in field pea. The QTLs were linked to plant height and disease resistance, but these authors did not measure physical attributes of stem strength. Knowledge of the genetic inheritance of physical stem strength will guide the development of optimal © CSIRO 2008 10.1071/AR07069 0004-9409/08/010080