Interstock-Mediated Graft Incompatibility: Insights into Photosynthetic Pigments, Carbohydrates, Antioxidant Defense Systems, and Hormones Response Mechanisms in Citrus

Interstock, located between rootstock and scion, plays a critical role in determining graft compatibility. This study aimed to elucidate the physiological mechanisms mediated by interstock in graft compatibility by comparing various leaf and root system parameters between compatible and incompatible graft combinations. These parameters included growth parameters, photosynthetic pigments, carbohydrates, antioxidant enzyme systems, and hormones. The study found that both PG (‘Yuanxiaochun’/‘Ponkan’/‘<i>Trifoliate orange</i>’) and JJ (‘Yuanxiaochun’/‘Kumquat’/‘<i>Trifoliate orange</i>’) treatments exhibited a noticeable phenomenon of “small feet” (scion diameter exceeding interstock), indicating mild graft incompatibility. Compared to grafted compatibility groups, chlorophyll content in PG and JJ treatments leaves was significantly reduced, particularly in carotenoids (Car). Additionally, PG and JJ treatments leaves showed lower levels of total soluble sugars, fructose, sucrose, gibberellin A4, zeatin-Riboside, and N6-(delta2-isopentenyl) adenosine, as well as catalase (CAT) activity. In contrast, peroxidase (POD) activity, glucose, soluble proteins, hydrogen peroxide (H₂O₂), malondialdehyde (MDA), aminocyclopropane carboxylic acid, and abscisic acid content were higher. In roots, PG and JJ treatments had elevated starch, sucrose, jasmonic acid, and jasmonic acid-isoleucine content, but showed lower levels of total soluble sugars, MDA, indole-3-acetic acid, and abscisic acid. Comprehensive analysis revealed that total soluble sugar content in both leaves and roots under PG and JJ treatments were reduced. These findings offer valuable insights into enhancing citrus grafting practices, particularly by guiding the selection of compatible rootstock-scion combinations. By elucidating the physiological mechanisms underlying graft compatibility, this research enables researchers and growers to refine grafting strategies, thereby improving citrus grafting success rates.