The effect of cadmium stress on photosynthetic pigments and secondary metabolites in borage (Borago officinalis L.)

Heavy metals are one of the most important abiotic stresses which can have detrimental effects on the growth, metabolic pathways, and physiological and biochemical characteristics of plants. Today, the accumulation of heavy metals in agricultural lands has an increasing trend that can affect the production and quality of agricultural products as well as human health. Among heavy metals, cadmium (Cd) is one of the most important worldwide environmental pollutants. It can rapidly be taken up by plants and accumulates in plant tissues, and easily enter the food chain; so this heavy metal is a serious threat to humans, animals, plants, and environmental sustainability. Secondary metabolites play vital protective and adaptive roles in plants in response to biotic and abiotic stresses. In this study, the effect of cadmium stress on the physiological characteristics and secondary metabolite production, and cadmium accumulation in borage (Borago officinalis L.) was investigated under hydroponic conditions.

Borage seeds were germinated in Petri dishes, and the 5–6 cm seedlings were then transferred to hydroponic containers containing half of the Hoagland nutrient solution with continuous aeration. The cultures were maintained in a growth chamber with 16 hours of light and 25±2 °C. Cadmium treatments were applied at five levels (0, 81, 162, 243, and 324 μM cadmium) using cadmium nitrate (Cd (NO3)2 .4H2O) at the 6-7 leaves stage. European borage seedlings were sampled at five-time intervals (12, 24, 48, 72, and 108 hours after cadmium stress) treatments and cadmium content, physiological characteristics, and secondary metabolites of leaf samples were measured.

The results showed that the amount of photosynthetic pigments, chlorophyll index (SPAD), chlorophyll fluorescence, and secondary metabolites in borage leaves were significantly influenced by cadmium stress. With increasing cadmium concentration and exposure duration, the absorption and accumulation of cadmium in borage leaves increased significantly. Cadmium stress reduced the amount of chlorophyll a and b, total chlorophyll, and carotenoids at all sampling times in comparison with the control treatment. The maximal quantum efficiency of photosystem II and the chlorophyll index (SPAD) were decreased with increasing the cadmium concentration and exposure duration so that the lowest value of these traits was observed at 108 hours after treatment with 324μM cadmium. With increasing the cadmium stress severity, the amount of secondary metabolites including anthocyanin, phenol, and total flavonoids and also the amount of soluble sugars were significantly increased in the borage leaves. The highest amount of these metabolites was observed at 108 hours after treatment with 324 μM cadmium.

In general, the results of this study showed that increasing the concentration and duration of cadmium stress negatively influenced plant photosynthesis by reducing the content of photosynthetic pigments and increasing chlorophyll fluorescence. On the other hand, increasing the concentration and duration of cadmium stress, increased the cadmium absorption and accumulation in the borage leaves as well as the amount of secondary metabolites.