Physiological and Biochemical Responses of Wheat (Triticum aestivum L.) to Boron under Saline Conditions

High levels of boron (B) and salinity are a serious constraint to crop production around the world. Cropping on saline and B toxic land is restricted by the low tolerance of agricultural crops to these abiotic factors. Prospects for improving B and salt tolerance in wheat can only be made possible by advance research. Frequently, B and salt occur together, however, it is unknown whether the interactions of B and salt increase or decrease the tolerance of a plant to both of these stresses. Low concentration of B is essential to plant growth and may limit the plant growth and development in excess quantity especially under saline conditions. Limited information was available regarding the effect of B on wheat (Triticum aestivum L.) under saline conditions. The present studies were conducted to investigate the interactive effects of salinity and B on growth, yield, physiological and biochemical responses of wheat. Hydroponics and pot studies were conducted at various B levels under normal and saline conditions. First, twelve wheat genotypes were screened out against various levels of salinity (control, 100, 200 mM NaCl) in solution culture. The SARC-I and Sehar-2006 were found as tolerant to salinity whereas, Kohistan-90 and MH-97 were categorized as salt-sensitive wheat genotypes. In a second study, these four wheat genotypes (differing in salt tolerance, selected from study-I) were grown at control, 0.5, 1.0 and 1.5 mM B in hydroponics under non-saline and saline conditions (100 and 200 mM NaCl). The data of physical and biochemical characteristics showed that the decrease in root and shoot fresh and dry weights were more in salt-sensitive (Kohistan-90 and MH-97) than in salt-tolerant (SARC-I and Sehar-2006) wheat genotypes. Salt-tolerant wheat genotypes accumulate less B and Na+, while more K+ in their leaves than the salt-sensitive wheat genotypes. Salinity resulted in higher reduction of shoot growth while B toxicity affected root growth more than the shoots growth. The reduction in plant growth by combined salinity and B toxicity was less than the sum of reduction caused by individual salinity and B toxicity. Salinity reduced B toxic effects and B toxicity reduced salinity stress in wheat plants. Regarding the physiological responses like photosynthetic rate (PR) stomatal conductance (SC) and transpiration rate (TR) were also decreased with salinity and toxic levels of B, and salt-tolerant genotypes showed better response regarding PR, SC and TR than salt-sensitive genotypes. Moreover, the activity of superoxide dismutase (SOD) and catalase (CAT) was found higher in tolerant genotypes than sensitive ones in the presence\ud of individual and combined salinity and B stresses. In a pot study, the effects of various levels of B (control, 2.5, 5 and 7.5 mg kg-1) on the growth, yield, physiological and biochemical processes of two wheat genotypes (differing in salt-tolerance, selected from study-II) in normal as well as saline (electrical conductivity i.e., EC = 10 and 20 dS m-1) soils were investigated. The plant height, straw and grain yields of wheat were increased at lower level of B and decreased at toxic B rates in both normal and salt-affected soils. In comparison with salttolerant SARC-I, the salt-sensitive MH-97 showed more reduction in growth, yield and physiological attributes in the presence of individual as well as combined salinity and B toxicity. Leaf B and K+ concentration decreased under saline conditions, while leaf Na+ concentration increased. Salt-tolerant genotype accumulated less Na+, Cl- and B and more K+ in leaves. The PR, TR and SC decreased, while the activity of SOD and CAT increased with increasing salinity and B stresses either alone or in combination and that were found higher in tolerant genotype as compared to sensitive one. Regarding Pakistani conditions, wheat genotype i.e., SARC-I was proved as most promising one under saline and B toxic conditions and can be directly used by farmers or can be used for the development of more salinity and B tolerant wheat genotypes by the breeders. The impacts of soil salinity and B toxicity on the carbohydrate partitioning, growth and ionic composition of two Australian wheat varieties (i.e., Halberd, salt-tolerant and Westonia, salt-sensitive) were explored, in another glasshouse experiment conducted at Murdoch University, Perth-Australia. The results showed that salt-tolerant variety accumulated more Na+, B and Cl- in their leaf sheath and kept their leaf blades free of these toxic ions than sensitive variety. Water soluble carbohydrates (WSCs; such as glucose, sucrose, fructose and fructans) concentration increased in response to individual and combined stresses of salinity and high B in leaf blade of both tolerant and sensitive varieties but the increase was higher in tolerant variety than the sensitive one. The concentration of WSCs in leaf sheath of salt-tolerant Halberd wheat variety was increased in response to stress conditions but remained low in saltsensitive Westonia. In squat, it can be concluded from the above Ph.D. research work that salinity and B toxicity showed antagonistic relationships for their effects on wheat growth, yield, physiological and biochemical responses. The highly tolerant and low B accumulating wheat genotype can be a wise option for normal and salt-affected B-toxic conditions