Unraveling species-specific and stressor-associated halophyte responses to plant growth-promoting bacteria (PGPB) under extreme salinity and temperature conditions.

Research on interactions between halophytes and plant growth-promoting bacteria (PGPB) has expanded in recent years; however, species-specific and stress-dependent responses remain poorly understood, particularly under extreme salinity and temperature. To address this gap, we conducted two complementary greenhouse experiments using two perennial halophytes with contrasting ecological traits, Arthrocaulon macrostachyum and Atriplex portulacoides. Plants were exposed to three conditions: control (171 mM NaCl; 16/25 °C), extreme salinity stress (1030 mM NaCl; 16/25 °C for 7 days), and extreme heat stress (171 mM NaCl; 28/40 °C for 3 days). Plant recovery was also assessed 72 h after stress. We evaluated the effects of a PGPB inoculum on plant growth, photosynthetic performance and antioxidant activity. Inoculation significantly enhanced growth, increasing biomass retention by 19% and 38% in A. macrostachyum under salinity and heat stress, respectively, and by 90% in A. portulacoides under heat stress, compared with non-inoculated controls. Both extreme salinity and heat stress reduced photosynthetic parameters, such as AN, Ce, gs, ETR, ΦPSII and ΦCO2, altered Vc,max, and increased photoinhibition, with species-specific shifts in photosynthetic limitations. In contrast, inoculation improved these photosynthetic parameters and reduced photosynthetic limitations, although responses varied by species and stress type. In A. macrostachyum, limitations were 9.71% in inoculated vs. 28.72% in non-inoculated plants under salinity, and 17.93% vs. 24.69% under heat stress. Similarly, in A. portulacoides, values were 33.08% vs. 64.69% under salinity and 8.82% vs. 27.32% under heat stress. These physiological improvements contributed to reduced photoinhibition. Inoculation also increased antioxidant enzyme activity, with CAT rising by 40-169% and SOD by 15-61% relative to non-inoculated stressed plants. No significant effects were observed after 72 h of recovery, suggesting that this period may be insufficient to detect early physiological restoration. Overall, PGPB effects were species- and stress-dependent, with A. macrostachyum responding positively to both stressors and A. portulacoides showing greater improvement under salinity, highlighting that PGPB effectiveness is context-dependent and shaped by the dominant stress factor and host ecological traits.