PHOSPHOROUS MANAGEMENT FOR BIOFORTIFICATION OF ZINC IN MAIZE (ZEA MAYS L.) GROWN ON CALCAREOUS

Abstract

Zinc (Zn) and phosphorus (P) deficiencies are widespread in most of the alkaline calcareous soils of Pakistan. These both caused the significant reduction in their bioavailability and uptake by the plants from the soil. Most of the applied Zn and P are adsorbed on soil colloidal surfaces short after their applications in the soil. As these both are essential plant nutrients and interact with each other in soil–plant systems. Therefore, such interactions may cause deficiency of one of the nutrients interacting with each other if interactions are antagonistic. However, papers about positive interaction between Zn–P are also present, but the exact mechanism is still unknown. Excessive P fertilizer to the soil decreased plant bioavailable Zn from the soil. There is the dire need to increase the Zn concentration in maize grain by managing P fertilization. Maize (Zea mays L.) is generally low in bioavailable Zn; agronomic biofortification with Zn is an easy and rapid solution to combat Zn deficiency problems in humans. For this purpose, series of experiments were conducted focusing on these major issues: 1) fixation and retention capacities of Zn and P in different textured soils; 2) Efficiencies of Zn and P applied to open–pollinated and hybrid maize cultivars; 3) phosphorous management to increase Zn bioavailability in maize grain; 4) changes in phytate and Zn human bioavailability over different Zn and P fertilization rates; 5) better Zn fertilization approach for maximum plant growth, yield and grain Zn density; 6) to improve nutritional status of maize grain. In first experiment, different levels of Zn (0, 4, 12 mg kg−1 soil) and P (0, 40, 120 mg kg−1 soil) were applied in all possible combinations to three differently textured soils (clay, loam and loamy sand) in plastic lined pots. The soils were incubated and soil samples were extracted at different time intervals (20, 40, 60, 90 and 120 days) at 25+5 °C. Adsorption of Zn and P in these soils was measured by AB–DTPA method. Adsorption of Zn and P depended on their applied rate, inherent soil physico–chemical properties and time. Percent adsorption of added Zn and P increased with increasing clay content and this adsorption behavior of soil was best described by Michaelis-Menten equation. Most of the fixation of Zn and P took place within 65–75 days after their addition in soils. Labile Zn in soil was reduced by P application but labile P was not as significantly affected by Zn application. In second experiment, a study was conducted to check the interactive effects of different levels of Zn (0, 9 mg kg-1 soil) and P (0, 40 mg kg-1 soil) on growth, mineral acquisition and nutrient efficiencies of 04 maize cultivars (DK–6142, P1543, Neelam and Afghoi) grown in pots. Maize cultivars significantly differed for growth, nutrient uptake and nutrient efficiencies in all treatments. Combined Zn+P fertilization significantly increased dry matter production, nutrient acquisition and xiii efficiencies as compared with unfertilized control. Agronomic, physiological and recovery efficiencies of P and Zn were increased in Neelam, Afghoi and DK-6142 if the other nutrient was supplied to soil. Afghoi and DK–6142 were more responsive in agronomic, physiological, nutrient and apparent Zn and P recovery efficiency than other varieties. With the application of the other nutrient to P1543, physiological Zn and P efficiencies were decreased, agronomic Zn and P efficiencies remained almost unchanged and recovery efficiency of Zn and P was increased. On average, Neelam, Afghoi and DK-6142 were more efficient for the applied nutrients than P1543. Efficiency of nutrients did not relate to open–pollinated or hybrid cultivars rather it varied with genetic makeup which ultimately effect fertilizer use efficiency. In third trial, a field experiment was conducted to investigate the interactive effect of Zn (0, 16 kg ha–1) and P (0 and 60 kg ha–1) on growth, yield and grain Zn concentration of two best maize genotypes obtained from the previous experiment i.e. Neelam (open–pollinated) and DK–6142 (hybrid) grown in field. Growth and yield of both maize genotypes were increased significantly (P≤0.05) by the application of Zn and P treatments compared with control, but Zn+P was more effective than their sole application. As compared to control, combined application of Zn+P increased grain Zn and P concentrations by 52% and 32%, respectively, averaged for the two genotypes. Sole application of P decreased grain Zn concentration and Zn bioavailability by 10% and 11% respectively, over unfertilized control. Application of P and Zn particularly in combination decreased the grain [phytate] : [Zn] molar ratio and increased the estimated human Zn bioavailability in grains based on a trivariate model of Zn absorption in both maize genotypes. In last experiment, different Zn fertilization approaches were used for maize grain agronomic biofortification and human Zn bioavailability in maize grain. Zinc was applied in pots to best Zn responsive maize cultivar DK–6142 as foliar spray (0.5% w/v Zn sprayed 25 days after sowing and 0.25% w/v at tasseling stage), surface broadcasting (16 kg Zn ha–1), subsurface banding (16 kg Zn ha–1 at the depth of 15 cm), surface broadcasting + foliar and subsurface banding + foliar in comparison to an unfertilized control. As compared to unfertilized control, all treatments significantly (P≤0.05) increased growth, yield and nutritional attributes in maize grain. Grain Zn and protein concentrations were correlated with each other and ranged from 22.3 to 41.9 mg kg−1 and 9 to 12 %, respectively. Zinc fertilization also significantly reduced grain phytate and increased grain Zn concentration. Zinc fertilization, especially broadcasting and subsurface banding combined with foliar spray decreased grain [phytate] : [Zn] molar ratio to 28 and 21 and increased Zn bioavailability by trivariate model of Zn absorption to 2.04 to 2.40, xiv respectively. Therefore, Zn biofortified maize grain by proper P fertilization was suggested for human consumption to overcome malnutrition problems in humans. As a whole, this project supply adequate Zn concentration in maize grain by proper P fertilization and their importance to grain yield and nutritional quality of maize grain under suitable application approaches.