Hypernodulating soybean mutant line nod4 lacking ‘Autoregulation of Nodulation’ has limited root-to-shoot water transport capacity

Although hypernodulating phenotype mutants of legumes, such as soybean, possess a high leaf N content, the large number of root nodules decreases carbohydrate availability for plant growth and seed yield. In addition, under conditions of high air vapour pressure deficit (VPD), hypernodulating plants show a limited capacity to replace water losses through transpiration, resulting in stomatal closure, and therefore decreased net photosynthetic rates. Silva Lopes et al. used hypernodulating (nod4) (282.33 ± 28.56 nodules per plant) and non-nodulating (nod139) (0 nodules per plant) soybean mutant lines to determine explicitly whether a large number of nodules reduces root hydraulic capacity, resulting in decreased stomatal conductance and net photosynthetic rates under high air VPD conditions.

Hypernodulating (nod4, A) and non-nodulating (nod139, B) soybean mutant lines either inoculated (nod4 inoc and nod139 inoc) with Bradyrhizobium diazoefficiens (strain BR 85, SEMIA 5080) or non-inoculated (control – nod4 ctr and nod139 ctr). Below (left side) is an overview of the root system of nod4 inoc mutant, highlighting the high number of nodules on the proximal root zone. Source: Silva Lopes et al. 2019.

Plants were either inoculated or not inoculated with Bradyrhizobium diazoefficiens (strain BR 85, SEMIA 5080) to induce nitrogen-fixing root nodules (where possible). Absolute root conductance and root conductivity, plant growth, leaf water potential, gas exchange, chlorophyll a fluorescence, leaf ‘greenness’ [Soil Plant Analysis Development (SPAD) reading] and nitrogen content were measured 37 days after sowing.

Hypernodulated plants were more vulnerable to VPD increases due to their limited root-to-shoot water transport capacity. However, greater CO2 uptake caused by the high N content can be partly compensated by the stomatal limitation imposed by increased VPD conditions.

Further reading

Caroline Silva Lopes, E., Pereira Rodrigues, W., Ruas Fraga, K., Machado Filho, J. A., Rangel da Silva, J., Menezes de Assis-Gomes, M., … Campostrini, E. (2019). Hypernodulating soybean mutant line nod4 lacking “Autoregulation of Nodulation” (AON) has limited root-to-shoot water transport capacity. Annals of Botany. https://doi.org/10.1093/aob/mcz040