| Summary: | One way to help plants deal with unfavorable temperatures is to graft elite cultivars onto adaptable rootstocks. Citrus macrophylla is a newly introduced rootstock in Egypt. Still, it needs to be evaluated in terms of its response to chilling stress compared to the most widely used rootstock (Citrus volkameriana). Thus, the present work assessed the physiological, biochemical, and anatomical responses of C. volkameriana (Volkamer) and C. macrophylla (Alemow) rootstocks and the cultivars ‘Olinda Valencia orange’ and ‘Murcott Tangerine’ grafted onto them to chilling stress. Plants were subjected to low-night temperatures for 48 h at 20/4 °C ± 1 (day/night), followed by three days of recovery at 30/25 °C ± 1 (day/night). Under low-night temperatures, all plants recorded a significant increment in malondialdehyde (MDA), peroxidase (POX), superoxide dismutase (SOD), proline, and leaf diffusion resistance (LDR). At the same time, a significant reduction was found in the photosynthetic apparatus (stomatal conductance (GS), transpiration (TR), and chlorophyll), polyphenol oxidase (PPO), and catalase (CAT). Moreover, anatomical damage was observed in total leaf thickness, palisade tissue thickness, spongy tissue thickness, midvein vascular bundle thickness, upper epidermis thickness, lower epidermis thickness, and spongy ratio (SR). Our results also indicated that the Volkamer, Olinda Valencia orange/Volkamer, and Murcott Tangerine/Volkamer were slightly damaged and showed a relatively high level of antioxidant activity (SOD and POX), LDR, and a more significant decrease in TR. However, they were less affected anatomically than Alemow rootstock and the scions grafted on it. These results demonstrated that grafting on Volkamer could improve the physiological, biochemical, and anatomical structural responses of the scions grafted on it to low-night temperature stress compared with grafting on Alemow.
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