Nitric Oxide Signaling in Photosynthetic Resilience: Linking Photoprotection and Carbon Gain in Tropical Crops

Tropical crops are increasingly exposed to climate-driven, recurrent combinations of heat, excessive irradiance, high vapor pressure deficit (VPD), drought, flooding, salinity, and pathogen pressure that collectively impair photosynthesis and reduce crop productivity. Unlike individual stress events, these interacting stresses simultaneously disrupt photoprotection, electron transport, carbon assimilation, stomatal regulation, and chloroplast redox homeostasis, ultimately limiting whole-plant carbon gain under fluctuating environments. Nitric oxide (NO) has emerged as a central signaling molecule coordinating these adaptive responses, yet its roles in photosynthetic regulation remain dispersed across molecular, physiological, and crop-specific studies. This review presents an integrated framework linking chloroplast-level NO signaling with whole-plant photosynthetic performance in major tropical crops, including maize, sugarcane, banana, cassava, and oil palm. We propose that NO regulates photosynthetic resilience through three interconnected axes: (i) reversible protein S-nitrosylation that modulates photosynthetic complexes and Calvin–Benson cycle enzymes; (ii) maintenance of chloroplast redox homeostasis through coordinated reactive oxygen species (ROS) and reactive nitrogen species (RNS) signaling, antioxidant networks, and S-nitrosothiol metabolism; and (iii) ABA-mediated stomatal regulation that optimizes the trade-off between carbon assimilation, stomatal conductance, and water-use efficiency under environmental stress. We further discuss the emerging roles of S-nitrosothiols (SNOs) and S-nitrosoglutathione (GSNO) as central regulators of NO homeostasis and potential biochemical indicators linking chloroplast signaling with whole-plant carbon performance. By synthesizing current evidence across climate-relevant stress combinations, including heat + high light, drought + high VPD, flooding + hypoxia, salinity + pathogen interactions, this review provides a conceptual framework for understanding NO-mediated photosynthetic resilience and identifies key research priorities for developing NO-based strategies to enhance productivity and climate resilience in tropical cropping systems.