Citrus rootstocks physiological and anatomical response to heat stress

Citrus is a globally grown fruit crop because of the nutritional benefits. Climate change drastically affects plant physiology, morphology, and anatomy. Physiological alterations within plants allow them to adapt to changing environments, however, the degree and quantification of these modifications differs with varieties. Ten exotic and local citrus rootstocks were evaluated based on morphology, physiology, and anatomical response to different heat stress conditions to identify best suitable rootstocks in climate change scenarios. Brazilian sour orange and Gadha dahi showed heat stress tolerance through maintaining plant growth, retention of root and shoot moisture contents, higher photosynthetic pigments and leaf water potential. Higher moisture contents of root and shoot were recorded (66.00 ± 1.1% and 73.00 ± 0.94%) in Brazilian sour orange at moderate levels of stress respectively. Which significantly reduces in Rangpur poona nucellar to (33.33 ± 0.6% and 32.67 ± 1.45%) when the level of stress was intensified. Under moderate and severe stress conditions the lowest chlorophyll a content was recorded in Kirrumakki nucellar (2.20 ± 0.05 mg/g) and Brazilian sour orange (1.32 ± 0.05 mg/g) respectively, due to reduced photosynthetic pigment, thylakoid membrane damage and alteration of chloroplast. Higher H₂O₂ and proline production were indication of tolerant rootstock to heat stress in Brazilian sour orange and Gadha dahi. Two highly tolerant (Brazilian sour orange and Gadha dahi) and two highly sensitive rootstocks (Rangpur poona nucellar and Sunki Bentake) from the experiment were selected for stem and leaf anatomical observations. Brazilian sour orange under heat stress was characterized with thicker cell epidermis which reduce the transpiration and evaporation losses in addition to having higher vascular bundle, xylem, and phloem length. These anatomical modifications ensured the transport of water and nutrients across the plant body which helped in maintaining plant vigor and growth. This study provides valuable knowledge in tolerant citrus rootstock to mitigate global warming.