Root response of soybean genotypes to low phosphorus availability from juvenile to adult vegetative stages

Due to the rapid exhaustion of global phosphorus (P) resources, P-efficient crops are required. In this study, we used various soybean genotypes collected from around the world and investigated the morphophysiological responses of their roots to low-P conditions at the cotyledon emergence (VE), unifoliate leaves emergence (VC), and fourth trifoliate leaflet emergence (V4) growth stages. First, we compared the growth of 81 soybean genotypes under different P conditions at the VC stage. The root morphology of most genotypes did not differ according to P conditions. However, GmWMC138 showed increased root weight under low-P conditions at the VC stage, and was therefore selected for further comparative analysis with genotypes with similar seed weights. Four selected genotypes were compared in terms of their seed storage P content and responses of plant growth and phytase and acid phosphatase activities to low-P conditions at the VE and VC stages. The inorganic-P (Pi) levels and shoot growth at the VE and VC stages of GmWMC138 were less affected by low-P conditions compared to the other genotypes. In this genotype, root fresh weight at the VC stage, phytase activity in roots at the VE stage, and acid phosphatase activity in roots at the VC stage increased under low-P conditions. The differences in these enzyme activities may have led to the maintenance of root Pi content and subsequent increase in the root fresh weight at the VC stage under low-P conditions. In addition, the low-P responses of growth and P content at the V4 stage were compared among the selected genotypes. In GmWMC138, fine-root length increased and total P content was maintained under low-P conditions compared to normal-P conditions. These results imply that in soybeans morphological changes in roots in response to low-P conditions at juvenile growth stages, such as the VE and VC stages, may contribute to P-deficiency tolerance in subsequent growth stages, such as the V4 stage.