Evaluation of photosynthetic and biochemical characteristics of oilseed rape under drought stress and MoO3 nanoparticles application

Oilseed rape (Brassica napus L.) is one of the most critical oilseed plants ranked third in oil production after soybeans and palm. Drought stress, especially at the end of the growing season, is the most critical environmental stress affecting the growth, development, and production of crop plants. The development of practical strategies for increasing drought tolerance is crucial to sustaining crop production, particularly in arid and semi-arid regions. The utilization of micronutrients such as molybdenum is one of the best strategies to increase drought stress tolerance in plants. This micronutrient is one of the most valuable and practical approaches for improving plant growth and plant adaptation under drought-stress conditions. The present study aimed to investigate the effect of foliar application of molybdenum oxide nanoparticles on the photosynthetic indices and biochemical traits in oilseed rape under end-season drought stress conditions.

To investigate the effect of molybdenum oxide nanoparticles on the photosynthetic indices, some biochemical traits, and grain yield of winter oilseed rape under the end-season drought stress, a split-plot experiment based on randomized complete block design (RCBD) was conducted with three replications at a research farm station of the University of Mohaghegh Ardabili in 2018-2019. The experimental treatments included irrigation (regular irrigation until the end of the season (control) and omitting irrigation from the flowering stage) as the main plots and molybdenum oxide nanoparticles foliar application (0 (control), 25, and 50 mg.L-1) as sub-plots. Drought stress was applied through irrigation stopping at the flowering stage (50% flowering). One week after the second spraying, the chlorophyll fluorescence including minimum fluorescence (F0), maximum fluorescence (Fm), variable fluorescence (Fv), and maximum quantum efficiency of photosystem II (Fv/Fm) was measured. Furthermore, chlorophyll pigments (chlorophyll a, chlorophyll b, and total chlorophyll), carotenoid, and proline content, and the activity of antioxidant enzymes (peroxidase and catalase) were measured in the leaf samples of treated and control plants. Analysis of variance and comparison of means was carried out using SAS 9.1 software. The means were compared using Duncan’s Multiple Range (DMRT) tests at the 5% probability level.

The results showed that irrigation and foliar application of molybdenum oxide nanoparticles treatment significantly influenced the photosynthetic and biochemical properties of winter oilseed rape. Furthermore, F0, Fv/Fm, proline content, and grain yield were significantly influenced by irrigation × molybdenum oxide nanoparticles interaction. The end-season drought stress significantly decreased the chlorophyll a, b, and total chlorophyll, carotenoid, Fm, Fv, Fv/Fm, and grain yield and increased F0, the activity of peroxidase and catalase enzymes, and the proline content in winter oilseed rape leave. Chlorophyll pigments, Fm, Fv, Fv/Fm, proline content, and peroxidase and catalase enzyme activity in oilseed rape leaves were increased with the application of different concentrations of molybdenum oxide nanoparticles. Foliar spraying with 25 mg.L-1 molybdenum oxide nanoparticles caused a significant increase in the content of the photosynthetic pigment, including chlorophyll a, b, total chlorophyll, and carotenoid content (about 25.81, 24.88, 25.57, and 17.78 present, respectively) as compared to the control treatment. Under regular irrigation and drought stress conditions, molybdenum oxide nanoparticles improved the maximum quantum efficiency of photosystem II and grain yield by decreasing the F0 (about 11.37%) and increasing the Fv (about 7.22%). The highest activity of the catalase and peroxidase enzymes was obtained with the application of 25 mg.L-1 molybdenum oxide nanoparticles, which was significantly higher than that of the control. Under drought stress conditions, proline content in the foliar sprayed plants with 25 and 50 mg.L-1 of molybdenum oxide nanoparticles was significantly higher than that of the control plants.

The results of the present study showed that end-season drought stress significantly reduced the photosynthetic pigments and chlorophyll fluorescence including Fm, Fv, and Fm/Fv, and increased F0 in oilseed rape plants. Molybdenum oxide nanoparticle reduces oxidative damage of drought stress, by improving the enzymatic (activity of catalase and peroxidase enzymes) and non-enzymatic (metabolites) antioxidant systems. Furthermore, molybdenum oxide nanoparticles increased the content of the photosynthetic pigment and decreased the minimum fluorescence (F0) of chlorophyll under drought stress, which ultimately significantly increase the maximum quantum efficiency of photosystem II. Generally, although the foliar application of different concentrations of molybdenum oxide nanoparticles reduced the damage caused by the end-season drought stress oilseed rape, the highest increase in the photosynthetic and biochemical traits was obtained with 25 mg.L-1 molybdenum oxide nanoparticles. Therefore, the application of 25 mg.L-1 molybdenum oxide nanoparticles on oilseed rape plants can be used for reducing the destructive effects of drought stress and increasing crop tolerance to end-season drought stress.