Soil Collection and Pot Preparation: Two pot experiments were carried out at Net-house of the Department of Soil Science, Bangladesh Agricultural University (BAU), Mymensingh. Soils were collected from the Soil Science Field Laboratory, BAU. Equal size plastic pots were prepared with 8 kg soils. The soil was silt loam having pH 6.15, electrical conductivity (EC) 0.17 dS/m, exchangeable Na 0.35 meq/100 g soil, total N 0.11% and organic matter 1.90%.
Plant materials and treatments: Two hybrid maize varieties viz. BARI Hybrid Maize-5 and Hybrid Maize Pacific-987 were used as plant materials. Nine treatment combinations viz. control (no NaCl or proline), 25 mM NaCl, 25 mM NaCl+25 mM proline, 25 mM NaCl+50 mM proline, 25 mM NaCl+100 mM proline, 50 mM NaCl, 50 mM NaCl+25 mM proline, 50 mM NaCl+50 mM proline, 50 mM NaCl+100 mM proline were used for the two maize varieties. Three maize seeds were sown into each pot. One plant was kept in each pot and others were uprooted after emergence. The pure salt (NaCl) was used for developing salinity. Maize plants were exposed to different concentrations of NaCl at vegetative stage (6 weeks after sowing). On the same day, different doses of proline containing 0.1% Tween-20 were sprayed on the leaves at a volume of 25 mL per plant as per treatment. Similarly, proline was applied at tasseling stage as per treatment. The experiment was laid out in a completely randomized design with four replications.
Management practices, crop harvesting and data recording: Normal water was used as irrigation. Fertilization and other management practices were performed as and when required. The crops were harvested at full maturity. Root growth, plant dry matter, yield attributes and grain yield were recorded. Chlorophyll contents were measured from green leaves. N, P, K, S and Na contents were measured from grain and straw samples. Determination of chlorophyll content: Chlorophyll contents were measured according to Porra et al. (1989). Fifteen days after first proline application,
healthy green leaf was detached from the plants. An aliquot amount of leaf was suspended in 10 mL of 80% acetone, mixed well and kept at room temperature in the dark for 7 days. The supernatant was collected after centrifugation at 5000 rpm and the absorbance was recorded at 645 and 663 nm in a spectrophotometer.
Chemical analysis of plant and soil samples: Soil pH, EC, exchangeable Na, total N and organic carbon content were measured using standard methods. Grain and straw samples were analyzed for N, P, K, S and Na contents using the methods described by Khanam et al. (2001).
Statistical analysis: Data were analyzed statistically using analysis of variance with the help of software package MSTAT-C. The significant differences between mean values were compared by Duncan’s Multiple Range Test. Differences at P<0.05 were considered significant.