Effect of Nano Silicon and Plant Growth-Promoting Rhizobacteria on Biomass, Nodulation and Some Physiological Traits of Grasspea (Lathyrus sativus L.)

Grasspea (Lathyrus sativus L.) is one of the most important forage crops in the world. It contains 12 to 20% protein. Silicon (Si) existing in the Earth’s crust is classified as the most abundant element after oxygen. Although silicon is not considered an essential element for plant growth, but a number of studies have reported that it as an important factor in plants that plays an important role in the resistance mechanisms of plants against environmental stress. Also, it plays a crucial physiological role in photosynthetic rate and chlorophyll content. One of the most effective factors in increasing the Grasspea biomass is seed inoculation with plant growth-promoting rhizobacteria (PGPR). Some of the benefits provided by PGPR are the ability to produce gibberellic acid, cytokinins and ethylene, N2 fixation, solubilization of mineral phosphates and other nutrients. Numerous studies have shown a substantial increase in dry matter accumulation via inoculation with PGPR. Some researchers reported that seed inoculation with PGPR enhanced relative water content and photochemical efficiency of PSII lathyrus under water limitations. Therefore, the aim of this study was to evaluate the effects of nano silicon and seed inoculation with plant growth-promoting rhizobacteria on biomass, nodulation and some physiological traits of Grasspea.

In order to study the effect of nano silicon and plant growth-promoting rhizobacteria on biomass, nodulation and some physiological traits of Grasspea, a factorial experiment was conducted based on randomized complete block design with three replications in research farm of University of Mohaghegh Ardabili in 2021. Treatment were included application of plant growth-promoting rhizobacteria at four levels (no inoculation as control, seed inoculation with Azosprillum lipoferum strain OF, Psedomonas putida strain 186, both application of Azospirillium and Pseudomonas) and nano silicon foliar application at four levels (foliar application with water as control, foliar application 25, 50 and 75 mg.L-1 nano silicon). The area is located at 38° 15ʹ N latitude and 48° 20ʹ E longitude with an elevation of 1350 m above mean sea level. Climatically, the area is situated in the wet zone with cool winter and hot summer. For inoculation, seeds were coated with gum Arabic as an adhesive and rolled into the suspension of bacteria until uniformly coated. Seeds were inoculated with plant growth promoting rhizobacteria (PGPR) at the rate of approximately 1 × 107 colony forming units (CFU) mg-1 just before planting. Foliar application of nano silicon was conducted in two stages of vegetative growth. Nano silicon powder added to deionized water and was placed on ultra sonic equipment (100 W and 40 kHz) on a shaker for better solution. At the Flowering stage, the leaves of plants were selected to measure the stem and leaf protein, chlorophyll index, RWC (relative water content), quantum yield, stomatal conductance and EC (electrical conductivity). RWC was calculated based on method of Kostopoulou et al. (2010). Chlorophyll Index was calculated by chlorophyll meter (SPAD-502; Konica Minolta Sensing, Inc., Japan). The Quantum yield of leaves was calculated with fluorometer (chlorophyll fluorometer; Optic Science-OS-30 USA). Stomata conductance was measured with a porometer system (Porometer AP4, Delta-T Devices Ltd., Cambridge, UK) according to the instructions in its manual. Leaf electrical conductivity (EC) values were measured at room temperature of 23±1°C using an electrical-conductivity meter. Analysis of variance and mean comparisons were performed using SAS 9.1 computer software packages. The main effects and interactions were tested using the least significant difference (LSD) test at the 0.05 probability level.

The results showed that both application of Azospirillium and Pseudomonas and foliar application of 75 mg.L-1 nano silicon increased root weight and volume (40.4 and 41.9%), number of active nodules (81.2%), percentage of active nodules (33.2%), nodule dry weight (37.4%), chlorophyll index (46%), relative water content (46.3%), stomatal conductance (34.6%) and quantum yield (34.1%) in comparison with no application of PGPR and nano silicon. Also, the highest leaf and stem protein (23.37 and 12.66%) and total biomass (37.7 %) were obtained in both application of Azospirillium and Pseudomonas and foliar application of 75 mg.L-1 nano silicon in comparison with no application of PGPR and nano silicon.

It seems that application of PGPR and foliar application of nano silicon can increase biomass of Lathyrus sativus due to nodulation and  improving  physiological traits.