ZINC TOXICITY SYMPTOM DEVELOPMENT AND PARTITIONING OF BIOMASS AND ZINC IN PEANUT PLANTS

High soil zinc (Zn) concentrations can cause Zn toxicity in peanuts (Arachis hypogaea L.), which decreases productivity and can be fatal to the plants. The objectives of this study were 1) to determine the optimal sampling time and plant part for diagnosis of Zn toxicity in peanuts, 2) to relate toxicity symptoms to plant Zn concentrations and calcium:zinc (Ca:Zn) ratios, and 3) to model the distribution of Zn and biomass into plant parts in relation to Zn concentration in the whole plant. A greenhouse study utilized four soils (Lakeland sand, Tifton loamy sand, Greenville sandy clay loam, and Greenville sandy clay) with Zn applications of 0, 10, 20, and 40 mg Zn/kg soil. Plants were sampled for analysis of nutrient concentrations, and Zn toxicity ratings were recorded biweekly. Toxicity symptoms became visible 4-8 weeks after planting, with stunting appearing at four weeks, horizontal leaf growth and leaflet folding at six weeks, and stem splitting at eight weeks. Optimal sampling time for diagnosis of Zn toxicity using plant Zn concentrations in peanuts was 6-10 weeks after planting. Zinc toxicity ratings were more highly correlated with plant Zn concentration in stems (r = 0.84) than leaves (r = 0.79). However, the Zn concentration in the total aboveground plant had a correlation coefficient (r = 0.83) almost as high as for the stems alone and is more convenient to measure. Zinc toxicity symptoms occurred with Zn concentration in plant shoots >240 mg/kg, and Ca:Zn ratios <35. Increases in total plant Zn concentration were partitioned into peanut stems more than into leaves. Zinc toxicity also reduced stem biomass accumulation to a greater degree than leaf biomass.