Effects of Putrescine and Biofertilizers on Content of Na+ and K+ Root and Shoots, Stomatal Conductance, Leaf Area Index, and Yield of Wheat (Triticum aestivum L.) under Salinity Stress

Salinity is one of the major constraints to wheat growth, which hampers production, causing yield loss in arid and semi-arid regions. Reductions in growth resulting from high salinity are because of both osmotic stress, inducing a water deficit, and the effects of excess Na+ and Cl– ions on critical biochemical processes. Salt stress induces a significant reduction in photosynthesis through the reduction of leaf area and photosynthetic pigments. Several strategies have been developed to decrease the toxic effects caused by high salinity on plant growth. Among them, the use of plant growth-promoting rhizobacteria (PGPR) such as Pseudomonas and Mycorrhiza play an important role in yield improvement. Many studies have been published on the beneficial effects of bacterial inoculation on plant physiology and growth under salt stress. One of the common hypotheses employed in most of the studies conducted under salinity stress was the lowering of ethylene level by the ACC-deaminase activities of PGPR and improved plant growth and yield under salinity stress.
It was reported that the application of Pseudomonas spp. improved plant growth by decreasing the uptake of Na+ and increasing the activities of antioxidant enzymes under salinity stress. The selective uptake of K+ as opposed to Na+ is considered one of the important physiological mechanisms contributing to salt tolerance in many plant species. Inoculation with PGPR significantly decreased Na+ uptake and increased K+ content and enhanced levels of K+ that could be to mitigate oxidative stress imposed by higher salinity. Some researchers have reported that PGPR species like Azotobacter and Pseudomonas increased the growth and biomass of canola (Brassica napus L.) under salinity stress.
A Better understanding of wheat physiological responses under salinity may help in programs in which the objective is to improve the grain yield under salinity stress. Therefore, this study aimed to evaluate the physiological, stomata conductance, along with root and shoot Na+/ K+ ratios) of wheat to cycocel and PGPR application under salinity stress.

A factorial experiment was conducted based on a randomized complete block design with three replications at the research greenhouse of the Faculty of Agriculture and Natural Resources, the University of Mohaghegh Ardabili in 2018. The experimental factors included salinity at four levels (no-salinity as control, salinity 40, 80, and 120 mM NaCl based salinity), application of biofertilizers at four levels (no biofertilizers as control, mycorrhiza application, application of both Pseudomonas and Flavobacterim, application of mycorrhiza with Pseudomonas and Flavobacterim) and putrescine foliar application at three levels (foliar application with water as control, foliar application of 0.5 and 1 mM putrescine). Mycorrhiza fungi were purchased from the Zist Fanavar Turan Institute and soils were treated based on the manufacturer’s protocol of 20 g of inoculums per m2 of soil. For inoculation, seeds were coated with gum Arabic as an adhesive and rolled into the suspension of bacteria until uniformly coated. The strains and cell densities of microorganisms used as PGPR in this experiment were 107 colony-forming units (CFU). Humidity ranged from 60-65%. The wheat cultivar "Gascogne" was used in the experiment. The optimum density of cultivar "Gaskogen" is 400 seeds m-2, so forty seeds of wheat were sown in each pot at a depth of 4 cm deep. The pots were immediately irrigated after planting. Nano putrescine zinc oxide powder was added to deionized water and was placed on ultrasonic equipment (100 w and 40 kHz) on a shaker for making a better solution. Foliar application with nano putrescine oxide was done in two stages of period growth (pre and post-4 booting stage).

The results showed that with increasing salinity, potassium content, stomata conductance, and leaf area index decreased but the application of putrescine and biofertilizers increased these traits. At the highest salinity level (120 mM), there was a decrease of 24.94 and 21.57% respectively in Na+ root and shoots in the application of mycorrhiza with Pseudomonas and Flavobacterim and foliar application of 1 mM putrescine in comparison with no application biofertilizer and putrescine in same salinity level. At the highest salinity level, application of mycorrhiza with Pseudomonas and Flavobacterim and foliar application of 1 mM putrescine increased K+  root (47.76%), shoots (21.66%) and grain yield (28.57%) in comparison with no application biofertilizer and putrescine in same salinity level. It seems that the application of biofertilizers and putrescine can increase the grain yield of wheat under salinity stress due to improve stomata conductance and leaf area index.