Drought stress effects on pot marigold (Calendula officinalis L.) revealed by REML and BLUEs analysis

Pot marigold (Calendula officinalis L.) is an ornamental-medicinal plant commonly planted in green spaces, and it has various industrial and medicinal applications. It is widely cultivated in semi-arid and Mediterranean regions, where it often faces drought stress. This research was conducted to evaluate the impact of moderate and severe drought stress during the flowering stage and to determine the drought response mechanisms in pot marigold. Therefore, the pot marigold plants were treated with control (100%), 60% and 30% of field capacity (FC) in the greenhouse, starting from the blooming stage, when usually the decrease in rainfall occurs during its life cycle. The impacts of different levels of stress were evaluated on eight plant water content parameters, seventeen morpho-physiological and phenological traits, and nine biochemical factors. Restricted maximum likelihood (REML) by best linear unbiased estimates (BLUEs) indicated that reducing water supply at moderate stress still led to satisfactory growth of pot marigold, where most of the morpho-physiological and phenological traits, including days from budding to flowering and pollination, flowers number, flower weight and diameter, and plant height, were not significantly affected at 60% of FC in comparison to 100% of FC. However, severe drought stress significantly reduced plant height, flower diameter, and flower weight by 31%, 20%, and 38%, respectively. In addition, the factor analysis demonstrated that pot marigold employs a combination of avoidance and tolerance mechanisms, including root system elongation, adjustment in antioxidant enzymes activity, and chlorophyll and carotenoid content to cope with drought stress. Our results represent a significant advance in understanding the level of tolerance and the responsive mechanisms to drought stress in pot marigold during the most critical stage of its life cycle, when reduced rainfall usually occurs in its cultivation area.