<p>Macadamia varieties with poor heat resistance are prone to cause leaf yellowing under high-temperature conditions, resulting in reduced yields. However, changes in the oxidative damage and the antioxidant machinery and glyoxalase system regulation during leaf yellowing of macadamia are not completely known. In this study, compared with the normal green leaves, the ultrastructure of chloroplasts was damaged in the yellowing leaves, and the hydrogen peroxide (H₂O₂) distributed widely in their chloroplast stroma, mitochondria and vacuole. During leaf yellowing, the content of methylglyoxal (MG) tended to increase significantly, as well as the levels of dehydroascorbic acid (DHA), glutathione (GSH) and oxidized glutathione (GSSG). Moreover, the activities of dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), and glutathione reductase (GR) were also observed to be enhanced dramatically, while the ascorbic acid (AsA) content and the ascorbate peroxidase (APX) and glutathione peroxidase (GPX) activities were remarkably increased first but then decreased. Furthermore, the ratio of AsA/DHA tended to reduce markedly, but the GSH/GSSG ration had no significant change. In the glyoxalase system, glyoxalase I (GlyI) activity showed a significant downward trend, while glyoxalase II (GlyII) did the opposite. In addition, the key enzymes for GSH synthesis, namely serine acetyltransferase (SAT) and gamma-glutamylcysteine synthetase (γ-ECS), tended to increase their activities significantly, while the critical enzymes for AsA synthesis, namely L-galactono-1,4-lactone dehydrogenase (GalLDH) and D-galacturonic acid reductase (GalUR), were on the contrary. It can be seen from the results that a severe oxidative and MG damages occurred in the yellowing leaves of macadamia, but these injuries were probably alleviated to some extent by activating glyoxalase system, increasing GSH synthesis, and strengthening AsA-GSH cycle.</p>