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

Abstract

Background and Aims 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. Here, we 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. Methods 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. Key Results Besides the reduced growth of hypernodulating soybean mutant nod4, such plants showed decreased root capacity to supply leaf water demand as a consequence of their reduced root dry mass and root volume, which resulted in limited absolute root conductance and root conductivity normalized by leaf area. Thereby, reduced leaf water potential at 1300 h was observed, which contributed to depression of photosynthesis at midday associated with both stomatal and non-stomatal limitations. Conclusions 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.

Severe water deficit strongly impacts physiological and morphoanatomical traits of soybean cultivar during its vegetative stage

“…This establishes a hydraulic pathway known as the soil-plant-atmosphere continuum. In this context, high root hydraulic conductivity plays a pivotal role in supplying water to the aerial parts of plants (Figueiredo et al 2014;Lopes et al 2019). Thus, investigating how plants respond to regulated de cit irrigation can help optimize water productivity (Fernández et al, 2020) and shed light on their tolerance and sensitivity to water de cit at different phenological stages (Coelho et al 2020).…”

Critical compromise: Trade-off between symbiosis and water uptake

“…A soybean AON-deficient mutant showed compromised root hydraulic conductivity and leaf water potential, indicating that AON contributes to water uptake and maintains leaf water status ( Caroline Silva Lopes et al 2019 ). Numerous investigations have been conducted on deducing the mechanism of AON; however, a few questions remain unanswered.…”

“…Nonstructural carbohydrates are essential for meeting immediate carbon requirements in environmental conditions such as water-deficit stress. Caroline Silva Lopes et al (2019) showed that soybean AON-deficient mutants have poor ability to transport water from root to shoot, leading to less shoot water content and uncontrolled nodule number ( Caroline Silva Lopes et al 2019 ). These findings led to the hypothesis that the shoot of AON-deficient mutants is deprived of water.…”