Abstract
BackgroundBoron (B) is an essential micronutrient and necessary to plants for their proper growth. The concentration of B in soil and plant is greatly critical for the growth and productivity. Even at the low amount available in soils, B can be extremely toxic to plants especially in semi-arid and arid environments. In the present study, the effects of high B concentrations (2.5, 5, 10, 20 and 30 mg/kg) were elucidated on two important essential oil-bearing plants; Mentha arvensis and Cymbopogon flexuosus which are aromatic and antimicrobial herbs having well-known medicinal values.ResultsApplication of different concentrations of B showed growth inhibitory effects on plant as evident by shoot and root lengths, fresh and dry weights of shoot in the studied plants. Treatments of B also reduced the total chlorophyll and carotenoid content, chlorophyll fluorescence and reduced the activities of carbonic anhydrase and nitrate reductase enzymes. Moreover, B stress considerably increased the proline content and lipid peroxide content as compared to control. The activities of antioxidant enzymes like catalase, peroxidase and superoxide dismutase were also significantly increased under B stress. The content of essential oil of M. arvensis and C. flexuosus increased at 2.5 mg/kg of B and decreased with further increase in concentrations of B.ConclusionThe findings of present work suggest that increasing concentrations of B inhibited growth and photosynthetic pigments, increased oxidative damage and activities of antioxidant enzymes; however, a mild stress of B increased essential oil production in M. arvensis and C. flexuosus plants.
Highlights
Boron (B) is an essential micronutrient and necessary to plants for their proper growth
At 30 mg/kg B, shoot length was decreased by 46.9% in M. arvensis and 84.5% in C. flexuosus, root length was decreased by 59.8% and 82.5% in M. arvensis and C. flexuosus, respectively, as compared to control (Table 1)
The inhibition of shoot fresh weight by 67.3% in M. arvensis and 50.7% in C. flexuosus was noticed while shoot dry weight was significantly decreased by 71.6% and 56.8% in case of M. arvensis and C. flexuosus, respectively, as compared to control (Table 1)
Summary
Boron (B) is an essential micronutrient and necessary to plants for their proper growth. Boron plays significant role in several processes that are necessary for the growth of higher plants such as cell wall synthesis, lignification process, auxin metabolism, sugar and carbohydrate metabolism, photosynthesis, nucleic acid metabolism and several enzyme activities [2, 3]. High B levels in the soil, in arid and semi-arid regions inhibit plant growth and decrease crop yield and quality [6]. Increase in the concentration of ROS in response to B toxicity, has been reported in many crops such as apple root stock [17], tomato [18], wheat [19], and barley [20]. Accumulation of hydrogen peroxide (H2O2) and an increase in lipid peroxidation in the cells and tissues caused by B toxicity has been reported in many plants [17,18,19, 21]. Our knowledge of B toxicity effects on essential oil-bearing plants is very limited
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