Antioxidant enzymes activity and biochemical components of borage (Borago officinalis) in response to water stress and humic acid treatment

The effect of water stress on plants is complex and can be studied from different aspects. In this regard, studying the physiological reactions can be an effective and helpful tool in identifying the drought tolerance. In order to evaluate the effect of water stress and foliar application of humic acid on photosynthetic pigments, antioxidant enzymes and biochemical compoounds (proline and soluble carbohydrates) of borage (Borago officinalis), a field experiment was conducted as a split plot design with three replications in 2012 at Agricultural Research Station, University of Zabol. Water stress levels included 90 percent of field capacity (W1) as the control, 70 percent of field capacity (W2) and 50 percent of field capacity (W3) as the main plots and four levels of humic acid foliar application including control or without humic application (H1), H2=1.5, H3=3 and H4=4.5 liter of humic acid in 1000 L water. Analysis of variance results showed that water stress had a significant effect on chlorophyll a, b, soluble carbohydrate, proline content and antioxidant enzymes, ascorbate peroxidase (APX) and gaycol peroxidase (GPX) in green leaves of borage plants. By increasing levels of water stress from control to W3, the carbohydrate content as well as APX and GPX activities increased and the amounts of chlorophyll a and b decreased. Meanwhile the increased concentration of proline only happened until W2 treatment. Humic acid treatment at different levels of water stress had a significant and positive influence on all of the above studied factors except CAT enzyme, chlorophyll b and carotenoid pigments. The maximum effect of humic acid at the most severe water stress level (W3) was obtained by foliar application of 1.5 and 3 liter of humic acid and this treatment had the most impact on the osmotic adjustment, photosynthesis pigments and APX antioxidant enzymes, which was very effective in drought tolerance.