Plant growth-promoting bacteria alter early growth under nitrogen deprivation and subsequent nitrogen-related physiological responses in sugarcane cultivars

Plant growth-promoting bacteria (PGPB) can enhance plant growth and nutrient acquisition; however, it remains unclear whether early plant responses to bacterial inoculation under nitrogen (N) deprivation influence later N-related physiological responses following N supply. This study evaluated whether differences in early growth responses to PGPB inoculation under N deprivation are associated with contrasting physiological responses to nitrate supply in sugarcane cultivars. Two experiments were conducted using RB867515 and RB966928 inoculated with a mixture of five PGPB strains. In the first experiment, plants were grown in a sterile N-free substrate to assess early growth under N deprivation. In the second, inoculated plants were supplied with nitrate and evaluated over the short-term at 0, 4, 24, and 72 h after N application. Nitrate and ammonium concentrations, total N concentration, and the activities of nitrate reductase (NR) and glutamine synthetase (GS) were determined. Inoculation significantly increased shoot and root biomass only in RB966928 under N deprivation. Following nitrate supply, this cultivar exhibited higher shoot N concentration and increased NR and GS activities. In contrast, RB867515 showed more transient physiological responses following nitrate supply, despite the absence of an early growth response to inoculation under N deprivation. The greater biomass accumulation observed in RB966928 under N deprivation was associated with stronger physiological responses after nitrate supply, whereas RB867515 exhibited limited growth responsiveness and less consistent changes in N-related traits. These findings highlight cultivar-specific associations between early growth responses to bacterial inoculation under N deprivation and subsequent short-term physiological responses to N availability, providing new insights into the temporal dynamics of sugarcane-bacteria interactions.