Phenotyping for Waterlogging Tolerance as a Proxy for Phytophthora medicaginis Resistance in Chickpea

Phytophthora root rot (PRR) caused by the soilborne oomycete Phytophthora medicaginis is a significant constraint to chickpea (Cicer arietinum) production across the northern grains region of Australia. In flooded soil, which is conducive to PRR disease development, up to 70% yield loss can occur in the most resistant Australian cultivars. Incorporating waterlogging tolerance in soybean (Glycine max) has been shown to improve quantitative resistance to Phytophthora sojae. Root growth of three chickpea genotypes was assessed at the seedling stage under waterlogging, PRR, and the combination of these abiotic and biotic constraints. Levels of waterlogging tolerance in chickpea are inherently low, yet selected genotypes displayed variability in root traits linked to improved waterlogging tolerance. The PRR moderately susceptible chickpea cultivar Yorker and PRR very susceptible Rupali demonstrated an eightfold increase in early adventitious root growth from the epicotyl region under waterlogging stress, compared with the PRR resistant interspecific backcross genotype 04067-81-2-1-1 (C. echinospermum × C. arietinum*2). Selection for primary root depth, which was significantly greater in 04067-81-2-1-1 under waterlogging, appears to improve PRR resistance compared with root replacement traits. Soilborne Phytophthora spp. are reportedly attracted to branch sites and leached exudates. We propose that compromised root barriers at emergence sites of adventitious roots under waterlogging conditions hasten hyphal entry, potentially increasing susceptibility to PRR. Hence, screening for root depth and absence of adventitious root development under waterlogged conditions may offer a novel proxy phenotyping method for PRR resistance traits at early stages of chickpea breeding.