Alleviation of low-irrigation effects on chickpea (Cicer arietinum L.) using spermidine

Chickpea is one of the pulse crops and its protein percent is about 22 to 24 percent. Therefore it plays an important nutritional role in human being diet. All plants, including chickpea, experience physiological changes and decreased growth while facing water deficit. Change in photosynthetic electron transport under low-irrigation conditions is one of the factors causing formation of reactive oxygen species (free radicals) including oxygen peroxide, super oxide and hydroxyl (oxidative stress). These free radicals are mainly produced in chloroplast, mitochondrion, and peroxisomes which could inflict destructive and harmful effects on plant cells. In stressed plants, the endogenous polyamine compounds are increased which is mainly a defensive response of plant to oxidative stress. Polyamines are aliphatic hydrocarbons with low molecular weight, straight chain of 3 to 15 carbons, which arginine and ornithine amino acids are their precursor. There are two major biosynthetic pathways for putrescine including ornithine decarboxylase and arginine decarboxylase. After putrescine synthesis, the larger polyamines (spermidine) are synthesized. This process is catalyzed by spermidine synthase through consecutively adding aminopropyl groups to putrescine. This experiment aimed to study the possibility of decreasing harmful effects of low-irrigation on growth and yield of chickpea through spermidine spraying.

This study was carried out as split plot based on complete blocks design with three replications in research farm of Shahrood University of Technology in 2016 in which the chickpea was sown in June as a secondary planting. The experimental treatments were irrigation regimes (distributed in main plots) in three levels (control (the conventional irrigation; 7-day interval irrigation), 10-day interval irrigation, and 13-day interval irrigation) and spermidine spraying in three levels (control (spraying of water on plant), concentration of 0.3 and 0.6 mM) at 4-leaf, flowering and milky stages. The studied traits were biomass, grain yield, number of pods per plant, grain protein percent, chlorophyll content and activities of catalase and guaiacol peroxidase enzymes.

The results indicated that under 13-day interval irrigation conditions, the activity of guaiacol peroxidase decreased with increasing concentration of spermidine (About 0.07 µM/min.g fw for 0.1 mM increase in spermidine concentration). No significant difference was found between zero (control) and 0.6 mM in terms of catalase enzyme; these spermidine levels appeared to have the highest activity of catalase enzyme. Under 10-day interval irrigation conditions, the catalase activity amount was statistically similar to its activity under control conditions. However, under 13-day interval irrigation conditions, the activity of mentioned enzyme was almost 80% higher than control. The linear decrease of guaiacol peroxidase activity with increasing spermidine concentration under 13-day interval irrigation conditions and also decrease in catalase enzyme activity under spraying plant with 0.3 mM spermidine may confirms the previous reports regarding the capability of polyamine compounds in direct elimination of free radicals and promoting stability and conserving the membrane. The experimental evidences have indicated that the application of exogenous potrisine has increased the polyamines amount in thylakoid membranes; these results have also been repeated by application of exogenous spermidine. Under 13-day interval irrigation conditions, the relative leaf chlorophyll content appeared to be linearly increased with enhancing the spermidine concentration (One Spad value for 0.1 mM increase in spermidine concentration). In each irrigation regimes, the number of pod per plant was proportionally increased with increasing spermidine concentration (Averagely, about one pod per plant for 0.1 mM increase in spermidine concentration). In each irrigation regimes, the biomass and grain yield were proportionally increased with increasing spermidine concentration (For 0.1 mM increase in spermidine concentration, the average increase in biomas and grain yield was about 72 and 31 Kg/ha, respectively), which proves that spermidine alleviates the harmful effects of low-irrigation. The 0.6 mM spermidine could alleviate the harmful impacts of 10- and 13-day irrigation regimes on grain yield by 20% and 34%, respectively. Proportional to spermidine concentration, grain protein content got increased (About one percent for 0.1 mM increase in spermidine concentration). So that for 0.3 and 0.6 mM spermidine concentrations, the grain protein content was higher than control by 3% and 6%, respectively.

The results indicated that imposing low-irrigation stress affected all measured traits. The application of spermidine polyamine at three stages of 4-loaf, flowering and milky stages caused some changes in activity of antioxidant enzymes catalase and guaiacol peroxidase. These changes took place along with increasing chlorophyll content and number of pod per plant (decreasing flower abortion and enhancing fertile flowers). The changes in the mentioned traits were some of the reasons for spermidine-resulted alleviating the negative effects of low-irrigation on growth and grain yield of chickpea. The 0.6 mM spermidine was found to be the best treatment level for both drought stress and no drought stress conditions.