Alterations in Photosynthetic Apparatus, Antioxidant Systems, and Leaf Anatomical Structure of Some Citrus Genotypes Grafted on The Recently Introduced 'Macrophylla' Rootstock in Response to Chilling Stress

One-year-old transplants of 'Olinda Valencia orange', 'Murcott Tangerine', and 'Limoneira 8A Lisbon' genotypes were grafted on 'Macrophylla' rootstock and subjected, along with 'Macrophylla' rootstock, to 24 h of the night chilling (nocturnal) at 20/4 degrees C +/- 1 (day/night) for three days, (8 h per day), followed by three days of recovery at 30/25 degrees C +/- 1 (day/night). The results showed that leaf diffusion resistance (LDR), proline, malondialdehyde (MDA), peroxidase (POX), and superoxide dismutase (SOD) increased significantly in all genotypes, while there was a significant decrease in photosynthetic apparatus, polyphenol oxidase (PPO), and catalase (CAT) under nocturnal chilling stress. Under the chilling, Macrophylla rootstock had the highest concentrations of proline and SOD, whereas it had the lowest MDA and total chlorophyll concentrations. Noticeably, the enzymatic antioxidant profiles were enhanced after being recovered. Moreover, 'Murcott' had the highest LDR, MDA concentration, thickness of lamina, midvein, palisade tissue, spongy tissue, and dimension of midvein vascular bundle. Meanwhile, it had the lowest stomatal conductance (GS) and transpiration rate (E). Similar results were obtained with ' Valencia' genotype. In contrast, the 'Limoneira 8A' genotype under the chilling had the lowest levels of LDR, antioxidant enzymes (POX, CAT, and PPO), thickness of lamina, midvein, palisade tissue, spongy tissue, and dimension of midvein vascular bundle.