Soil potassium–magnesium imbalance induces magnesium‐deficiency chlorosis in macadamia leaves: impacts on microstructure, photosynthesis, yield, quality, and fatty acid profile

BACKGROUND Magnesium deficiency is a common symptom of chlorosis in macadamia plantations in southern China, particularly in Yunnan, which restricts their sustainable production and processing quality. This study clarified its negative effects on leaf physiology and fruit quality. RESULTS Magnesium‐deficient plants exhibited interveinal chlorosis in lower leaves, disordered mesophyll cells, weakened chlorophyll autofluorescence, and 45.52–70.08% lower leaf magnesium content ( P < 0.05), hindering chlorophyll biosynthesis. Chloroplasts were fewer in number, and ultrastructurally exhibited reduced grana lamellae, increased plastoglobuli, and severe grana degradation. Net photosynthetic rate ( P n ), stomatal conductance ( g s ), and transpiration rate ( T r ) declined significantly, while intercellular CO 2 concentration ( C i ) increased by 16.13%. magnesium deficiency reduced single‐plant yield by 49.90% (5.10 vs. 10.18 kg plant −1 ), fruit transverse diameter by 7.63%, kernel weight by 18.15%, and total fatty acid content by 2.57%; palmitoleic acid and linolenic acid decreased by 8.17% and 18.18%, while arachidonic acid and erucic acid increased by 27.21% and 30.00%, respectively ( P < 0.05). In magnesium‐deficient areas, soil available potassium was 49.30% higher (200.69 vs. 134.42 mg kg −1 ) and showed a significant negative correlation with leaf magnesium content. CONCLUSION In conclusion, high‐potassium–low‐magnesium soil is closely associated with magnesium deficiency in macadamia. Such nutrient imbalance may impair photosynthetic carbon assimilation and the conversion from carbohydrates to fatty acids, thereby reducing the yield and quality of macadamia. Optimizing the potassium‐to‐magnesium application ratio is therefore recommended to support the sustainable development of the macadamia industry.