Heat and drought stress impair nitrogen uptake and allocation in maize by reshaping root morpho-physiology

Global climate change is increasing the frequency of concurrent heat and drought events, highlighting the urgent need to elucidate their synergistic effects on crop root function and nutrient uptake. We subjected maize plants to normal condition (CK), heat stress (H), drought stress (D), and combined heat and drought stress (HD) at the 12 th fully expanded leaf stage (V12) for five days. The root-to-shoot ratio decreased under H but increased under D. Both single stresses induced “low-cost” root anatomical changes. Under HD, however, these anatomical alterations were most pronounced. They coincided with the strongest oxidative damage, the greatest suppression of root respiration, and the most severe cellular energy deficit. This energy limitation downregulated key nitrogen assimilation enzymes (NR, GS, and GOGAT) and impaired the compensatory upregulation of glutamate dehydrogenase observed under single stresses. Consequently, root nitrogen uptake efficiency declined by 9.0%, 10.4%, and 18.0% under H, D, and HD, respectively. Total plant nitrogen accumulation was lowest under HD, with nitrogen allocation increasingly skewed toward the shoot. Grain yield also lowest under HD. Collectively, these findings demonstrate that combined heat and drought cause oxidative damage, which in turn worsens the energy deficit in roots and suppresses nitrogen assimilation, thereby reducing nitrogen acquisition efficiency in maize.