Department of Plant Sciences

The study examined the concentration of heavy metals (Ni, Zn, Cu, Pb and Cd) in the sewage sludge dump soil and Cassava tuber harvested from cassava plant grown by farmers on the sewage sludge dump site along Owerri-Onitsha road Imo state, Nigeria, a dump site that was subjected to severe and intense sewage sludge dumping for about 2 years, abandoned for about 3 years and then cultivated with cassava plant. Soil and Cassava tubers were sampled and analysed using Atomic Adsorption Spectrophotometer. Concentration of the metals both in the soil and plant were found to be in excess but all, were not within critical concentration range except for Ni and Cu with concentrations within soil critical range (20-70, 50-250 mg kg-1 respectively). In all cases, the concentration were not above the established maximum tolerable limits. Thus, Ni , Zn ,Cu, Pb, and Cd concentrations in the soil were 25.15, 59.7, 68.2, 10.85, and 8.38 mg kg-1 respectively. For cassava plant, heavy metal concentration were 38.25, 138.7, 125.8, 5.50 and 3.60 mg kg-1 respectively. The concentrations were consistent with the findings of previous studies of sewage sludge contaminated soils .

Plant reproduction has higher boron (B) requirements than vegetative growth and generally reproductive plant parts contain more B, but the form and function of the extra B remains unexplained. In this paper we review the literature on B in plant reproductive development and add to it recent results on B distribution and forms in reproductive parts (silks, pollen) and vegetative parts (youngest open leaves) of maize grown in B-buffered solution culture. Cell wall binding of B in the silk and pollen of maize does not explain the higher B concentrations in reproductive than vegetative tissue. Given the rather high proportion of B present in the non-cell wall fraction in pollen and silk, the high B requirement for plant reproduction suggests an additional role for B other than in cell wall formation. The identity of non-cell wall B binding substrates in pollen and carpel tissue awaits further study. The higher sensitivity of plant reproduction to B deficiency is also related to weaker B transport into floral organs, especially where transpiration is suppressed in reproductive plant parts by enclosure of sheaths (e.g. wheat ear) or husks (e.g. maize ear) during the critical stage of development.

Agricultural approaches are suggested to increase micronutrients in cereal grain and then to alleviate human malnutrition. A long-term (1999-2007) field experiment was conducted to investigate the effect of three nitrogen (N) fertilization rates (0, 130 and 300 kg N/ha) on micronutrient density in wheat grain and its milling fractions. There were three N rates in this trail: 0, 130 and 300 kg N/ha. At maturity, grains were harvested and fractionated into flour, shorts and bran for micronutrients, N and protein analysis. The results showed that N fertilization increased iron (Fe), zinc (Zn) and copper (Cu) density in wheat grain compared to the control. Increase of N application rate from 130 to 300 kg N/ha, however, didn't further increase the three micronutrient density in grain. Most micronutrients were accumulated in bran while the lowest micronutrient concentrations were found in the flour. High N application increased Zn and Cu densities in three fractions while for Fe, its density in shorts and bran were increased, not in flour. Manganese (Mn) concentration in grain was not influenced by N application. It is concluded that nitrogen plays an important role in micronutrient accumulation in wheat grain. Proper nitrogen fertilizer management has a potential to enhance both micronutrients and grain protein.

Productivity of mango in India is low at 6.5 t/ha. Among field crops Zn deficiency is the most important disorder and in mango also it was widespread. A survey of the mango orchards of India to identify cause for Zn deficiency indicated that widespread visible zinc deficiencies were noticed in all the four agro-climatic zones in the acid, neutral and high pH alfisols of all the states (pH range 4.8 to pH 7.8). But the analysis of soil indicated adequate DTPA extractable Zn level of 0.75 to 2.2 mg/kg. But about 20% of the orchards showed visible symptoms of Zn deficiency and 40% of the orchards showed leaf Zn in the range of 12 – 17 mg/kg, which is classified as deficient. The high soil Zn level was not reflected in the leaf zinc. A study was carried out to investigate the cause for Zn deficiency in a Zn adequate soil. In pots of 25 litre volume 10 banana plants of Cv. Robusta known for fast growth and nutrient uptake and grafts of Mango Cv. Alphonso known for susceptibility to zinc with 0.75 DTPA Zn were grown for 6 months. After 6 months both banana and mango leaves were analyzed. The banana was healthy with a leaf zinc content of 33 mg/kg whereas Mango had a leaf Zn of 18 mg/kg only. The pH of the rhizosphere soil in banana was 6.2 with a DTPA Zn of 1.2 ppm whereas the pH was 7.0 in Mango with a DTPA Zn of 0.76 ppm. The capacity of banana to acidify the soil to extract native Zn whereas mango is unable to use the same mechanism.

We subjected bean plants (Phaseolus vulgaris L. ‘Bianco di Bagnasco’) to phosphate deprivation stress and studied the enzymatic activity of some major anti-oxidative enzymes in both leaves and roots extracts. P deprivation provoked a significant decrease of catalase activity in leaves of 14-day-old plants, but no differences were measured after prolonged P deprivation. The enzyme activity in roots was not affected by the withdrawal of P from the culture medium. The deprivation of P from the growing medium provoked an increase in ascorbate peroxidase activity in leaves after 4 days of growth. No differences were detected in roots of +P and –P plants. Glutathione reductase activity decreased in leaves of 14 day-old P-deprived plants in comparison to +P plants. However, after 18 days of growth, the enzymatic activity was similar in both –P and +P plants. The deprivation of P provoked a significant decrease of glutathione reductase activity in roots of both 14 day-old and 18 day-old plants. The activity of dehydroascorbate reductase was increased in both leaves and roots, either after 4 and 8 days, by P withdrawal. The reduced availability of Pi in the plant tissues affected the activity of anti-oxidative enzymes mainly after the first days of deprivation, particularly in leaves. It could be suggested that during this period the plant is adjusting its metabolism to cope with the nutritional stress and that P-deprivation is acting as a mild oxidative stress.

Synthesis and accumulation of storage starch is a main action of endosperm organogenesis. The enzymes for starch synthesis should work in concert, but the regulatory mechanism is largely unknown. Rice SPK, a calcium-dependent protein kinase, is uniquely expressed in endosperm of immature seeds. Antisense-SPK transformants showed lacked accumulation of storage starch resulting in production of watery seeds with a large amount of sucrose. In vitro phosphorylation indicated that SPK specifically phosphorylated a serine residue in sucrose synthase involved in its activity, and thus SPK is a sucrose-synthase kinase. Sucrose synthase catalyzes the reversible conversion of sucrose in the presence of UDP to UDP-glucose and fructose. Recently, the reaction accompanied with ADP is also reported. Since sucrose synthase is regulated by reversible phosphorylation, we determined effect of the phosphorylation on its activity. The basal activity of sucrose synthase accompanied with ADP and ADP-glucose was quite low but apparently detected as well as that with UDP and UDP-glucose. The activity of sucrose cleavage accompanied with ADP was significantly elevated after sucrose synthase was phosphorylated, whereas little alteration was detected on other reactions. Thus, it is suggested that SPK enhances efficiency of sucrose cleavage reaction with ADP of sucrose synthase by its phosphorylation, and promotes the generation of ADP-glucose from sucrose in endosperm. Since storage starch is mainly produced from ADP-glucose, it appears that this reaction greatly contributes the supply of substrate for storage starch biosynthesis.

Soil samples (from Czech and German long term field experiments) were used for estimating soil phosphorus (P) fractions. More then 100 topsoil (0-30 cm) samples from different fertilizing treatments were taken. These were analysed for P in soil solution (PCaCl2), exchangeable sorbed P (Pex) and bioavailable P. The methods used were 0,01M CaCl2 extracts, anion exchange (AE) membranes, Doppel-Lactat (PDL) and Mehlich 3 (PM3). Other fractions analysed were total inorganic (Pin), total (PM-tot) and organic (Porg) P estimated with Marks (1977) fractionation, P sorbed on Fe and Al (PFeAl) and residual P (Par). Comparison of medians appeared to be better for evaluating extraction abilities. Phosphorus fractions increased in the following order: [Par =100%]; PCaCl2 (0.1%) < Pex (10%) < PDL (12%) < PM3 (17%) < Pin (25%) < Porg (38%) < PM-tot (60%) < PFeAl (62%). Low amounts of Pin, Porg and PM-tot did not verified the applicability of Marks fractionation for the set of studied soils. Close correlations at α<0.001 were found for all methods for estimating the fractions of bioavailable phosphates (PCaCl2, Pex, PDL a PM3). Statistically significant relations were observed between Pin with Par, PM-tot and PFeAl.

Iron is essential for most living organisms and is required for normal plant growth. Plants induce iron utilization systems under conditions of low iron availability, but the molecular mechanisms of this gene regulation system remain largely unknown. We identified the rice transcription factor IDEF1, which specifically binds the iron-deficiency-responsive cis-acting element IDE1. IDEF1 belongs to an uncharacterized branch of the plant-specific transcription factor family ABI3/VP1 and efficiently binds to the CATGC sequence within IDE1. IDEF1 transcripts are constitutively present in rice roots and leaves. Transgenic tobacco plants expressing IDEF1 under the control of the constitutive cauliflower mosaic virus 35S promoter transactivate IDE1-mediated expression only in iron-deficient roots. Transgenic rice plants expressing IDEF1 under the control of the iron-deficiency-inducible IDS2 promoter tolerate iron deficiency in hydroponic culture and calcareous soil. Conversely, transgenic rice plants with repressed IDEF1 expression are susceptible to early stage iron deficiency in hydroponic culture. Expression analysis of these transgenic plants revealed that IDEF1 positively regulates iron-deficiency-induced genes, including the ferrous iron transporter gene OsIRT1 and the iron-deficiency-induced transcription factor gene OsIRO2. These data suggest the presence of a sequential gene regulatory network that functions via novel cis element/trans factor interactions to promote the iron-deficiency response.

No-till can increase soil microbial activity, which can affect phosphorus availability. The use of some crops in rotation can also modify P availability, and it has been evidenced that brachiaria may enhance soil P availability. The activity of acid phosphatases and the changes in soil P as affected by Brachiaria were studied in a pot experiment, in two tropical soils. Brachiaria ruziziensis was grown for 84 days in 8 L pots. At harvest, brachiaria response to P rates and soil P forms were evaluated. Dry matter production and P accumulation by Brachiaria were increased with P doses. Fertilizer addition increased microbial P, but decreased the activity of acid phosphatases in the soil. Brachiaria modified some forms of inorganic and organic P in soil, but the changes were not enough to support the nutrition of the subsequent crop.

While most studies have tried to assess phosphorus bioavailability based on phosphorus availability determined in the bulk soil, we question this approach which does not account for changes of P availability that occur in the rhizosphere. In this study, we combined the extraction of soil inorganic phosphorus (P) with CaCl2 or water and geochemical modelling in order to unravel the variations of P availability in the bulk soil and rhizosphere of durum wheat (Triticum turgidum durum L.) over a range of soil pH (pH ~ 4 to 9) chemically modified from initial pH. For this purpose, mechanistic models were used to simulate the adsorption of cations and anions by soil constituents with an additive approach. The geochemical codes accurately reproduced the concentration of extracted P by CaCl2 and water for both bulk soil and rhizosphere (RMSE = 5.3 and 10.6 µg.kg-1 for bulk and rhizosphere soil respectively). In rhizosphere, the effect of pH on P availability obtained by both extraction methods exhibited two different trends, which contrasted with that on P bioavailability. Phosphorus uptake by durum wheat mainly explained the difference of P availability between the rhizosphere and bulk soil for CaCl2 extraction. For water extraction, Ca uptake as well as P uptake need to be accounted to explain the observed difference of P availability between rhizosphere and bulk soil. Our results highlight the extra benefits of mechanistic models to clearly understand how P availability can change in the rhizosphere, in order to better predict P bioavailability to crops.