Introduction: Water deficiency and its result (i.e. drought stress) is one of the main factors limiting agricultural production in tropical and subtropical regions in the world; because it decreases plants’ growth and yield due to a reduction in their photosynthesis activity, transpiration, and stomata conductivity (Farooq et al., 2009). In order to enhance plants’ survival under drought condition, application of anti-transpiration materials is of underlying approaches to reduce the rate of transpiration (Cao et al., 2010). Kaolin, as a white-clay mineral substance, is soluble in water and neutral in terms of chemical activity. Its color reflexes a large majority of solar radiations (Glenn, 2012). Spraying plants with water-suspension kaolin can improve plants’ tolerance towards environmental stresses (Boari et al., 2016). In this regard, it has been reported that white particles settled on the external surfaces of plants causes to reflex sunlight and make a change in angle of sun radiation as well as maintaining temperature balances; and accordingly it reduces stresses’ damages to leaf and fruit at higher temperatures (Glenn, 2012). This research aimed to investigate the effect of foliar application of kaolin on plants’ growth and yield as well as their physiological responses at different irrigation regimes in pepper under the weather of Ilam province.
Materials and Methods: The present study was conducted in a 3×3 factorial arrangement in a randomized complete block (RCB) design with three replicates in Research Greenhouse of Agriculture Faculty of Ilam University in 2019. Kaolin treatments (WP₉₅) included three levels of kaolin concentrations in the provided water solution (0% –water spraying as control, 2.5%, and 5%) which applied in the form of spray to make all surfaces of leaves wet and this was repeated 30 days later. Then, the plants were subjected to different irrigation interval treatments (2, 4 and 6 days) applied with a drip tape irrigation until completing plants’ growth. In the end of experiment (85 days after transplanting seedlings), five plants in each treatment were considered as our samples for measuring the respected treats. 
Results and Discussion: The results showed that an increase in an irrigation interval of pepper reduced growth parameters and yield. In this regard, previous studies demonstrated that drought stress had a negative effect on plants’ growth and yield (Haghighi & Najafi, 2020). Under drought condition, the natural mechanism of plant cells is disrupted, and this leads to emerging oxidative stresses. At this condition, ROS is generated, and this per se intensify the destruction of chlorophyll, proteins, and cellular membranes (Carvalho et al., 2015).
The results also showed that kaolin spray on the pepper’s leaves improved its growth and yield at different levels of irrigation. It has been documented that kaolin, by increasing fruit yield, enhances plants’ yield under drought stress (Cantore et al., 2009). Because of a positive relationship between the speed of fruit growth and fruit temperature, the temperature higher than 30 °C not only increases fruit growth, but also intensifies fruit ripening; although it can reduce fruit weight (Boari et al., 2016). During plant growth, applying kaolin at presence of higher temperature of experimental field (>30 °C) extends the time of fruit growth and accordingly fruit weight. Because of its role on reducing fruit ripening’s process, kaolin paves the way for raising fruit weight under drought condition. A reduction in oxidative damages and an increase in yield of pepper under different levels of irrigation indicate the activation of defensive mechanisms created by external application of kaolin. In this research, applying kaolin significantly increased chlorophyll content as well as antioxidants’ activities of catalase and peroxidase, whereas it reduced MDA accumulation in the stressed plants. In this regard, kaolin was shown to improve antioxidant capacity and to maintain relative water content (RWC) in the stressed plants (Brito et al., 2019). Accumulating protein and hormones in stressed olive is of changes intensified by kaolin resulting in rising olive’s yield under drought stress as compared with the controls. In addition, Kaolin was reported to improve the antioxidants in the stressed plants of different crops (Dinis et al., 2016; Abdallah et al., 2019), and this is in consistent with our findings.
Conclusions: The results of experiment also revealed that application of kaolin somehow reduced the harmful effect of drought stress on pepper. The effectiveness of kaolin on mitigating the harmful effects of drought is associated with its ability in improving antioxidant systems and chlorophyll content, whereas reducing accumulation of MDA in the stressed plants. In conclusion, applying kaolin (especially at concentration of 2.5%) improved pepper’s growth and yield at different regimes of irrigation by affecting on the plants physiological processes.