The effect of irrigation intervals on growth, physiological and biochemical indices of coneflower (Echinaceae purpurea (L.) Monch) under humic acid foliar application

Drought is one of the most important factors limiting the growth of plants around the world and the most common environmental stress. Considering the serious risk of drought and water scarcity, the use of appropriate methods to improve the negative effects of drought has attracted special interest. Recently, different types of organic acids have been widely used to improve the quality and quantity of crops and horticultural products. Due to the hormonal compounds, extremely low amounts of organic acids have significant effects on the improvement of physical, chemical and biological properties of the soil and the production and quality of agricultural products. In this regard, an experiment was conducted to investigate the effect of irrigation intervals and the foliar application of humic acid on some morphophysiological and biochemical characteristics of coneflower.

This experiment was conducted in the 2016-17 crop year with split plot arrangement in a randomized complete block design with three replications at the research farm of the Baharan non-profit organization in Gorgan. The treatments were included four levels of irrigation intervals (including the irrigation every one, three, six and nine days) and four levels of humic acid foliar application (including 450, 300, 150 and 0 mg/L). The evaluated traits included fresh and dry weight of plant, fresh and dry weight of root, proline, relative water content (RWC) of leaves, antioxidant activity, total phenol and soluble sugar. After planting, growing and harvesting, the measurement of proline growth performance was done by the Bates method, total phenol by the Folin-Ciocalteu method, carotenoid by the Barnes method, and soluble sugar by the Omokolo method.

The results showed that the increased levels of irrigation intervals caused a significant decrease in plant height, root length, fresh weight of plant, and fresh and dry weight of root, while the highest dry weight of the plant (9.41 g per plant) and root length (10.40 cm) was observed from the second and third levels of irrigation interval, respectively. Also, the effect of irrigation interval at the highest level resulted in 29.88% reduction in the relative water content of leaf and 69.29% increase in proline compared with the control. The level of antioxidant activity and soluble sugar was increased with the irrigation intervals, and at the third level of irrigation (irrigation every six days), it was at most 71.39 and 43.56 mg/mL, respectively. Humic acid treatment had a significant effect on all of the measured traits except carotenoid, and the effect reached to the maximum level at 450 mg/L. In the conditions of irrigation every nine days, the foliar application with 450 mg/L resulted in a 0.58% increase in proline compared with not to use it in this condition. The maximum level of antioxidant activity (82.67 mg/mL) from the third level of irrigation interval and application of 150 mg/L of humic acid and the highest level of soluble sugar (53.56 mg/g) from the third level of irrigation interval and application 300 mg/L of humic acid was observed.

The results of this study showed that increasing the irrigation interval significantly reduced the morphological traits. In contrast, some traits such as proline, antioxidant activity, total phenol, and soluble sugar were increased. However, humic acid protects the coneflower plants from the drought stress and reduces the drought damage. In general, the results of this study introduced the foliar application of 300 mg/L of humic and every 9 days irrigiration as the best treatment, economically. Because with less water and humic acid can be reached to the same methabolites yield that higher levels of them.