Study on Distribution Pattern of Pb among Plant Parts at Selected Growth Stages of Roadside Grown Wheat and Maize at Kadawa, Kano State

Abstract

Distribution pattern of Pb in plant tissues (leaf, stem and root) at selected growth stages (germination or seedling stage (15 days), tillering stage (30 days), shooting/booting stage (45 days), heading( maize) and earing (wheat) stage( 60 days), flowering stage( 75 days) and ripening stage (90 days) of two varieties each of wheat- Triticum aestivum L. var. Pavon-76 (SU 1) and Siettecerros (SU 2) and maize – Zea mays L. var. TZEE-Y (yellow maize)(SU 3) and Zea mays everta L. (popcorn) (SU 4) were investigated at Kadawa, Kano State of northern Nigeria. Doruwa Salau (S1) with an average daily traffic density of 19,288 comprises of SU 1 and SU 3 as the experimental sites perpendicular to the Kano-Zaria Highway while SU 2 and SU 4 with a traffic density of 3 are the control sites within the Irrigation Research Station (IRS) (S2). They were selected based on traffic density and distances. Distances from the experimental and control sites to the Kano-Zaria highway is 345m and 2438.28m respectively. Soil and plant samples were collected for one growing harmattan season (December 2008 through to April 2009).  Prior to sowing, triplicate soil samples were collected at a depth of 25cm for physico-chemical and Pb level analyses. Triplicates of both the representative plants and corresponding soil samples were collected at a depth of 25cm at the selected growth stages after sowing. Double-beam AAS was used in determining the Pb levels. Results reveals that plant Pb in each of the two varieties of wheat and maize were reportedly higher than the soil Pb indicating atmospheric inputs. Plants differ in their uptake of heavy metals and their subsequent distribution within plant organs. Pb level was highest in the leaves at SU 1 and SU 3 and in the roots at SU 2 and SU 4 indicating atmospheric inputs and deposition on the soil respectively. Plant Pb was highest at SU 1 at the 30 days growth stage than at all the sampling units and other growth stages respectively indicating proximity to the highway and dilution of the trace metal levels as growth progresses. Soil Pb before sowing was highest (160 mg/kg) at SU 3 and lowest at (40mg/kg) at SU 1 and SU 4 respectively, which suggests that Pb levels originated from vehicular emissions due to absence of industrial, residential and commercial activities. Individual ANOVA for each sampling units was highly significant for plant parts at SU 2 only and combined ANOVA was highly significant (p=0.01 and 0.05) for sampling units or varieties, growth stages, plant parts and interaction of plant parts, growth stages and sampling units, which remarks that plant parts, growth stages and distances as well as their interactions had a great influence on the distribution of Pb in the crops. Pb (2 mg/kg) levels exceeded the WHO permissible limits posing a potential health risks to food crops, livestock and humans.