# Scion–Rootstock Interactions Enhance Freezing Stress Resilience in Citrus reticulata Through Integrated Antioxidant Defense and Carbon–Nitrogen Metabolic Adjustments

## TL;DR

Grafting citrus scions onto specific rootstocks improves their ability to withstand freezing temperatures by boosting antioxidant defenses and metabolic adjustments.

## Contribution

The study identifies UFR5 as a rootstock that significantly enhances freezing tolerance in citrus through genotype-dependent physiological and metabolic mechanisms.

## Key findings

- UF-950 grafted onto UFR5 showed the highest freezing tolerance with reduced oxidative damage and sustained metabolic fluxes.
- Antioxidant enzyme activity and osmolyte accumulation varied significantly among rootstocks, influencing scion resilience.
- Rootstock genotype strongly modulates scion physiology under freezing stress, highlighting the importance of genotype selection.

## Abstract

Frequent and increasingly severe freezing events threaten citrus production in northern Florida, underscoring the need for strategies that enhance freezing resilience in citrus cultivars. Grafting scions onto tolerant rootstocks provides a physiologically integrative approach to improve stress tolerance. This study aims to elucidate how these interactions modulate physiological and metabolic responses under freezing stress, thereby identifying mechanisms that contribute to enhanced freeze resilience in citrus. Here, we grafted Citrus reticulata (cv. UF-950) onto eight rootstocks (Bitters, Blue-1, C-146, Sour Orange, UFR07TC, UFR09TC, UFR5, and US942) to evaluate scion–rootstock interactions under normal (20 °C) and freezing (−6 °C) conditions. Freezing stress caused a sharp increase in oxidative stress markers, lipid peroxidation, and membrane damage while reducing photosynthetic performance across most combinations. Antioxidant capacity, osmolyte accumulation, and carbon–nitrogen metabolic responses varied significantly among rootstocks, revealing strong genotype-dependent modulation of scion physiology. Among the tested combinations, UF-950 grafted onto UFR5 displayed the highest freezing tolerance, characterized by robust activation of antioxidant enzymes, elevated proline and glycine betaine accumulation, reduced oxidative damage, and sustained carbon–nitrogen metabolic fluxes under freezing stress. These results demonstrate that rootstock genotype governs the extent of scion defense activation and metabolic homeostasis under freezing conditions. Our findings identify UFR5 as a promising rootstock for enhancing freezing resilience in citrus and provide mechanistic insight into how scion–rootstock interaction orchestrates integrative stress tolerance pathways. Future work should focus on multi-omics dissection of rootstock-mediated signaling networks and long-term field validation to optimize rootstock selection for enhanced cold resilience under variable climatic conditions.