Morphophysiological and phenological responses of wheat genotypes to zinc oxide nanoparticles under drought stress

Drought at grain filling is a major limitation for wheat production in arid and semi-arid regions. Zinc oxide nanoparticles (ZnO NPs) have been proposed as a tool to improve drought tolerance, but their performance across different wheat genotypes and water regimes is still unclear. In this pot experiment, five wheat lines (MS-1-14c-67, MS-13-17C-210, MS-14-14C-172, MS-15-17C-139 and MS-16-14C-143) were exposed at the grain filling stage to six treatments: well-watered control (T0), terminal drought (T1), 10 and 20 ppm ZnO under well-watered conditions (T2, T3), and 10 and 20 ppm ZnO combined with drought (T4, T5). Morphological, phenological and physiological traits, including biomass components, grain yield, grain filling percentage, relative water content (RWC), membrane stability index (MSI) and gas exchange parameters, were recorded and analyzed using two-way ANOVA. Drought alone significantly reduced shoot and spike biomass, thousand grain weight, RWC and MSI, and shortened the grain filling period from 43.7 to 28.1% of the crop life cycle. Foliar ZnO under non-stress (T2, T3) enhanced biomass, increased net photosynthesis above the control and raised MSI, with 10 ppm ZnO producing the highest mean grain yield and 20 ppm ZnO giving the greatest MSI. Under drought, ZnO partly restored RWC (to around 78% in ZnO treatments) and MSI and increased grain filling to 50–52% of the life cycle, indicating improved use of the post-anthesis period. Across treatments, MS-14-14C-172 and MS-16-14C-143 showed the highest biomass and yield with relatively stable physiological status, whereas MS-1-14c-67 produced lower biomass but maintained higher pigment and reserve levels. Overall, moderate foliar doses of ZnO NPs alleviated terminal drought mainly by supporting tissue water status, membrane integrity and grain filling, and identified MS-14-14C-172 and MS-16-14C-143 as promising lines for ZnO-mediated drought management in wheat.