Changes in Endogenous Carotenoids, Flavonoids, and Phenolics of Drought-Stressed Broccoli Seedlings After Ascorbic Acid Preconditioning

Abstract

Drought is an abiotic disturbance that reduces photosynthesis, plant growth, and crop yield. Ascorbic acid (AsA) was utilized as a seed preconditioning agent to assist broccoli (Brassica oleracea var. italica) in resisting drought. However, the precise mechanism by which AsA improves seedlings' development remains unknown. One hypothesis is that AsA works via antioxidant mechanisms and reduces oxidative stress. This study aims to confirm the effect of varied concentrations of AsA (control, 0 ppm, 1 ppm, or 10 ppm) on seedling growth and changes in the antioxidant status of broccoli seedlings under regular watering or drought stress. AsA increased shoot dry mass, leaf area, net photosynthesis, and water use efficiency in watered and drought-stressed seedlings. AsA significantly (p < 0.001) increased carotenoid content in watered and drought-stressed seedlings by approximately 27% and 111%, respectively. Drought increased chlorophyll b, flavonoids, phenolics, ascorbate, and hydrogen peroxide production in control seedlings, but either had no effect or less effect on plants preconditioned with 10 ppm AsA. There was no improvement in reactive oxygen species scavenging in AsA-preconditioned seedlings compared to the control. The absence or reduction in biochemical indicators of stress suggests that preconditioned broccoli seedlings do not perceive stress the same as control seedlings. In conclusion, the consistent increase in carotenoid concentration suggests that carotenoids play some role in the preconditioning response, though the exact mechanism remains unknown.

Keywords: Brassica oleracea; antioxidant; ascorbate; chlorophyll; photosynthesis; reactive oxygen species; water stress.

Figure 1

(a) Shoot dry biomass of broccoli seedlings; (b) leaf area of broccoli seedlings. Each figure compares four seed preconditioning treatments in both watered and drought conditions. Bars represent an average of 8 replicates for each treatment combination. Bars with different letters are significantly different based on Tukey’s multiple means comparison at 5% significance.

Figure 2

(a) Net photosynthesis (Pn); (b) evapotranspiration (E); (c) stomatal conductance (Gs); (d) water use efficiency (WUE) of broccoli seedlings. Each figure compares four seed preconditioning treatments in both watered and drought conditions. Bars represent an average of 8 replicates for each treatment combination. Bars with different letters are significantly different based on Tukey’s multiple means comparison at 5% significance.

Figure 3

(a) Hydrogen peroxide production and (b) ROS scavenging in broccoli seedlings in 4 seed preconditioning treatments in broccoli seedlings that were watered or exposed to drought. Bars represent an average of 8 replicates for each treatment combination. Bars with different letters are significantly different based on Tukey’s multiple means comparison at 5% significance.

Figure 4

(a) Principal component score plot, with points identified by treatment. Shaded areas indicate clusters of points associated with drought or watered conditions; (b) principal component loading plot. Pn = net photosynthesis, E = transpiration, Gs = stomatal conductance, WUE = water use efficiency, Chl a and b = chlorophyll a and b, and ROS = reactive oxygen species.