Study the effects of superabsorbent on root characteristics and yield of mung bean (Vigna radiata L.)

Water is an important economic resource in many parts of the world, especially in arid and semi-arid reigns. Also, drought stress is a major constraint for crop production in arid and semiarid regions, such as Iran. Drought stress affects different aspect of plant growth, through a series of morphological and physiological changes. Application of some soil conditioners, like superabsorbent polymers, could be effective for absorbing seasonal rain and suitable source of water for plant growth during dry seasons. These polymers can reserve different amounts of water in itself and so increases the soil ability of water storing and preserving and approve plant growth under water deficiency. Also, they can improve plant growth indices and increase its growth by changing the soil structure and improving the environmental conditions of plant growth. This experiment was conducted in order to study the effects of superabsorbent and irrigation period on root characteristics and yield of mung bean.
 

The field experiment was conducted at two locations during summer in 2015. First location was Khorramabad which located at 33.46° N, 48.33° E and has moderate and semi-humid climate with average annual rainfall of 504.3 mm. The second location was Kuhdasht that located at 33.52° N, 47.61° E and has semi-dry climate with average annual rainfall of 390.4 mm. The field experiment was carried out as a split-plot base on randomized complete block design with four replications. The treatments included irrigation period at three levels (5, 10, and 15 day) as main plots and four levels of superabsorbent polymer Aquasorb (control, 100, 200 and 300 Kg ha-1) as subplot. Finally, at the end of growth period, 5 plants was used to measure mung bean traits included root volume, root dry matter, the weight ratio of shoot to root stem dry weight, total root area, total root length and grain yield. The volume and length of root was measured using the method of Atkinson (Atkinson, 2000). ANOVA and Duncan's Multiple Range Test were performed to determine and analyze the significant difference and means comparison respectively at P<0.05. SAS-9.1 software was used to do statistical analysis. Also, OriginPro 9.1 software was applied to draw figures.
 

The results of variance analysis showed that location and irrigation had a significant effect on all the evaluated properties. Also, superabsorbent had a significant effect on all the evaluated properties (except the weight ratio of shoot to root). In addition, interaction effects of superabsorbent and irrigation and superabsorbent and location were just significant on grain yield (p<0.05). Interaction effects of superabsorbent and irrigation was also significant on water use efficiency (WUE) (p<0.05). Interaction effects of location and irrigation were significant on attributes such as the weight ratio of shoot to root, grain yield (p<0.01) and total root area (p<0.05). Results indicated that drought stress reduced root volume and root dry weight and increased total root length traits and root dry weight, increasing the total length of the roots so that the lowest root volume (3.11 cm3 Plant-1) and root dry weight (0.93 g. Plant-1) and the highest total root length (9.1 cm Plant-1) was recorded at 5-day irrigation period. However, all these traits and grain yield were increased by applying the super superabsorbent polymer. The highest root volume (3.62 cm3 Plant-1), root dry weight (1.1 g Plant-1) and total root length (9.8 cm Plant-1) were obtained at 200 kg superabsorbent per ha. The highest WUE (1.12 Kg grain yield m-3) was obtained at 200 Kg superabsorbent per ha and 15-day irrigation period and the lowest (0.31 Kg grain yield m-3) was recorded at control and 5-day irrigation. Also, across irrigation periods and locations, the difference between the highest (200 Kg superabsorbent per ha and 10-day irrigation period) and lowest (300 Kg superabsorbent per ha and 15-day irrigation period) grain yield was 800 Kg per ha. Actually by increasing superabsorbent per ha, grain yield was increased but it improved until a certain level of superabsorbent and after that grain yield was reduced. Result showed that although the interaction of irrigation period and superabsorbent was not significant on root morphological characteristics, effects of superabsorbent was significant on root morphological characteristics and these traits of mung bean were improved by applying superabsorbent.
 

Generally, results revealed that the Aqasorb superabsorbent hydrogel can improve the morphological characteristics of mung bean root and can reduce the negative impact of the high periods of irrigation on grain yield and WUE of mung bean. Thus, applying superabsorbent that has the ability of absorbing considerable amount of water, caused improving in physical conditions of soil and affected the plants response to water stress indirectly and can help plants in water shortage conditions. Also, the amount of optimal application of Aqasorb superabsorbent is 200 Kg ha-1 and using more than 200 Kg superabsorbent per ha (especially in the high periods of irrigation) will reduce grain yield, WUE and the morphological characteristics of mung bean root and will increase the cost of production.