Stem lodging in sunflower: Variations in stem failure moment of force and structure across crop population densities and post-anthesis developmental stages in two genotypes of contrasting susceptibility to lodging

Stem lodging is a risk to sunflower production in Argentina, and may contribute to fixing the upper limit to commercially viable crop population density, since yield is known to increase up to densities higher than those currently used. Reputedly, crops are particularly susceptible to stem lodging during grain filling and at harvest maturity, but the temporal and spatial unpredictability of lodging events under field conditions has hampered systematic research on this issue. In this study we used mechanical lodging to examine the relationships between stem failure moment of force and stem structure in plants of two sunflower hybrids of contrasting susceptibility to stem lodging grown at each of three crop population densities (5.6 plants m−2, 10 plants m−2 and 16 plants m−2). Measurements were performed at mid- and near-end of grain filling and at harvest maturity in crops grown in three separate seasons at two locations. Stem failure moment of force at all three developmental stages was significantly (p < 0.05) greater in the lodging-resistant hybrid than in the lodging-susceptible hybrid used in these experiments at 5.6 plants m−2, and fell with increasing crop population density and between 90% grain filling and harvest maturity in both hybrids. At harvest maturity differences in stem failure moment of force between hybrids were significant (p < 0.05) at all three crop population densities. Stem flexural rigidity (ability of the stem to resist bending) and stem deformation (horizontal displacement from the vertical of the stem immediately prior to breakage) exhibited responses to hybrid, crop population density and developmental stage that were broadly consistent with those of stem failure moment of force. Stem diameter at the breakage point fell with crop population density, but there was little difference between hybrids except at the harvest maturity, when the susceptible hybrid at the two highest crop population densities had smaller diameters than the resistant hybrid. Measured at the breakage point, the thickness of the epidermis plus cortex tissues (i.e., the effective thickness of the stem wall) which surround the friable stem pith fell with crop population density and between end of grain filling and harvest maturity in both hybrids. Differences in effective thickness of the stem wall between hybrids were small and often not significant, but there was a tendency for the susceptible hybrid to exhibit greater thickness of the effective stem wall, particularly at the two higher population densities. Stem failure moment of force was linearly related to thickness of the effective stem wall in both hybrids, the slope of the relationship being significantly greater for the resistant hybrid. We conclude that, within limits, the thickness of the effective stem wall may prove to be useful as a guide to genotype susceptibility to stem lodging in breeding programs and may offer a simple approach to modelling susceptibility. This work has also served to highlight the need to investigate the origins (presumably related to stem anatomy and/or cell wall properties) of the genotypic effects on the stem failure moment of force/the effective stem wall relationship. Finally, the demonstration of the existence of genotype differences in tolerance to stem lodging should encourage the execution of a broader survey aimed at identifying sources of tolerance to stem lodging at high crop population densities in sunflower.