Arbuscular Mycorrhiza Rhizophagus irregularis and Silicon Modulate Growth, Proline Biosynthesis and Yield in Cajanus cajan L. Millsp. (pigeonpea) Genotypes Under Cadmium and Zinc Stress

Cd is a non-essential metal and is hazardous for plant growth. Although Zn is regarded as an essential element, its elevated levels are highly toxic for plants. Silicon (Si) fertilization and Arbuscular mycorrhiza (AM) have been recognized in alleviating phytotoxicity of heavy metals. Benefits of Si are limited in legumes because they are low Si accumulators. Recently, reports have indicated increased Si uptake in a few plants through mycorrhization. The present work investigated the roles of Si and/or Rhizophagus irregularis on growth, nutrient acquisition, proline biosynthesis and productivity of two pigeonpea genotypes (metal tolerant—PUSA 2002, metal sensitive—PUSA 991) under Cd (25, 50 mg kg−1), Zn (600, 1000 mg kg−1) stress. Plants were subjected to potassium silicate (300 mg kg−1) alone and in combination with R. irregularis. Cd and Zn stress hampered growth, chlorophylls, nutrients uptake and productivity with more negative effects in PUSA 991 than PUSA 2002. AM improved root biomass and nutrient status (P, N, Mg, Fe), whereas Si was more effective in increasing shoot biomass, K and Ca under both heavy metals. Moreover, AM boosted proline biosynthesis by enhancing glutamate dehydrogenase and pyrroline-5-carboxylate synthetase activities along with a decline in proline dehydrogenase activity under heavy metal stress. Application of AM to Si-amended soils was highly beneficial in modulating proline biosynthesis and further improving growth, nutrient, water status and yield with complete amelioration at Zn600, 1000 as well as Cd25 in PUSA 2002 due to its better ability for mycorrhizal symbiosis. Highly positive impacts of combined applications (+Si+AM) could be correlated to mycorrhiza-mediated enhanced Si uptake and resultant metal tolerance.