Journal of Crop Production and Processing

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Tousi P, Esfahani M, Hosseini Chaleshtori M. Evaluation of the Effect of Paclobutrazol Foliar application under Cold Stress Conditions on Amount of Soluble Carbohydrates and Rice Yield cv Hashemi. Journal of Crop Production and Processing 2025; 15 (2) :75-94
URL: http://jcpp.iut.ac.ir/article-1-3364-en.html

Extended Abstract
Introduction: Rice is a cold-sensitive crop plant, hence, low temperatures reduce its yield. Environmental stresses, especially cold stress, affect the morphological and physiological characteristics of plants and are considered the most significant limiting factors for rice growth and production in areas where this crop is cultivated. The use of growth regulators to promote or restrict plant production has attracted the attention of plant science researchers. Paclobutrazol, commercially known as Bonzi (PBZ), is one of these plant growth regulators and is considered one of the most important and widely used compounds in the triazole group. It acts as an inhibitor of gibberellin synthesis by affecting its biosynthesis pathway and is utilized in various methods to control vegetative growth in plants. Research results have shown that osmotic protectants significantly increase tolerance to low temperature stress in rice and can increase root growth and shoot development by up to 50% under low temperature stress conditions. Low temperatures can affect the developmental stages and photosynthesis of rice, and by reducing the amount of carbohydrates, they may reduce plant growth and, consequently, grain yield. The use of growth regulators to develop or limit plant production is of interest to plant science researchers.

Materials and Methods: Experiment was carried out in a factorial form in a randomized complete block design with 3 replications in the 2023-2024 crop year at the Rice Research Institute of Iran, Rasht, north of Iran. The first factor included the concentration of paclobutrazol at five levels (0, 5, 10, 15, and 20 g/400 L ha equaling to 0, 0.125, 0.25, 0.375 and 0.5%) and the second factor was the date of seeding in the nursery (March 6 and 16, April 4 and 14) with the common nursery method. After seed germination in the nursery, taking notes and reaching the 2-3-leaf stage, foliar application of paclobutrazol at the desired concentrations was carried out in the early hours of the day using appropriate sprayers. The distance between plots and replicates was one meter. The distance between rows was 20 cm, the distance between plants on the same row was 20 cm, and three seedlings were planted at each hole. The traits studied included plant height, main stem diameter, grain yield, fertile tillers/plant, grains/panicle, 1000-grain weight, panicle length, chlorophyll content, soluble carbohydrates concentration, and antioxidant enzymes activity. The Sheligl method was used to measure the amount of soluble carbohydrates.

Results and Discussion: The results showed that there was a significant differences between seed sowing times in the nursery, foliar application concentrations, and the interaction effect in terms of all studied traits. The results showed that the highest grain yield (2.896 kg/4m2), grains/spike (198), 1000-grain weight (32.8 g), fertile tillers/plant (32), plant height (174 cm), stem diameter (7.8 mm), panicle length (39 cm), the amount of soluble carbohydrates (2.89 mg g-1 dry weight) and the antioxidative enzymes activity were obtained when seeds sown on March 6 with 0.5% of paclobutrazol. The lowest amount of the studied traits were observed in the control treatment when seeds sown on 6 and 16 March in the nursery. Early nursery preparation in March, combined with the presence of snow and cold, appears to alleviate the effects of low-temperature stress through paclobutrazol spraying. This treatment increases the levels of antioxidant enzymes, reducing the adverse effects of low temperatures. As a result, it promotes seed germination and enhances the seedlings' adaptability to cold conditions. With seed sowing times in the nursery on March compared to April, the production of soluble carbohydrates in the rice plants increased, which is probably due to increased plant growth and height, tillering, and maximum use of environmental conditions, soil, and water during the growth period, and increased photoassimilates production. The highest amount of non-structural carbohydrates was observed upon sowing in the nursery on March 15. The ability to produce carbohydrates by plant green tissues and their accumulation in other organs varied depending on the time of seeding and concentration of foliar-applied paclobutrazol.

Conclusion: According to the results of the present experiment, sowing times in the nursery on March 6 and spraying with 0.5% of paclobutrazol achieved the highest grain yield, yield components, plant height, stem diameter, and soluble carbohydrates content. In fact, paclobutrazol spray treatments improved the effects of low-temperature stress, enhancing seedling vigor. After transplanting to the main field, it increased tillering and plant height, leading to larger stem diameter, carbohydrate reserves, enhanced antioxidant enzyme activity, grain filling and plant resistance to stress, ultimately compensating for the yield losses caused by cold stress.