Photooxidation in interveinal tissues of magnesium-deficient leaves is associated with sugar accumulation

Magnesium (Mg) deficiency often leads to leaf photooxidation, typically manifested as interveinal chlorosis. This symptom arises from an imbalance between light energy absorption and its utilization, resulting in excessive excitation energy, reactive oxygen species (ROS) accumulation and subsequent photodamage. However, the spatial heterogeneity of Mg deficiency responses within a leaf remains poorly understood, particularly the physiological basis for the greater severity of photooxidation in interveinal tissues compared with vein-adjacent areas. Here, a hydroponic culture system was employed to investigate tissue-specific responses of rapeseed (Brassica napus L.) leaves to varying Mg supply levels. Mg deficiency significantly inhibited leaf growth, with symptoms increasing as leaves aged. Interveinal tissues exhibited pronounced chlorosis, elevated ROS accumulation, and a sharp decline in photosynthetic capacity relative to vein-adjacent tissues. Despite comparable Mg concentrations between the two leaf regions, interveinal tissues accumulated significantly higher levels of soluble sugars under Mg deficiency, suggesting that localized carbohydrate accumulation may contribute to their greater susceptibility to oxidative stress. To further test this hypothesis, a complementary experiment showed that foliar application of fructose markedly aggravated chlorosis in Mg-deficient leaves, providing direct evidence that leaf yellowing is closely associated with carbohydrate accumulation. To elucidate the underlying mechanisms, further analyses revealed that Mg deficiency suppressed the expression of sugar transporter genes, reduced Mg²⁺-ATPase activity, and decreased vascular bundle area, with these effects being particularly pronounced in interveinal tissues. These changes likely impair phloem loading and carbohydrate translocation, leading to localized sugar retention in interveinal regions. Collectively, our results indicate that Mg deficiency disrupts sugar transport and vascular function, resulting in carbohydrate accumulation that intensifies photooxidative stress and ultimately drives the development of interveinal chlorosis.