Aluminium induced changes in the morphology of the quiescent centre, proximal meristem and growth region of the root of Zea mays

Treatment of the primary root with 8 mg dm-3 Al altered the pattern of cell growth so that with 48–h treatment, the root apex was no longer an organized, cytologically heterogeneous complex. An increase in mean cell volume was first observed with 2–h treatment, primarily in the cells of the quiescent centre where cell enlargement was associated with the removal of the constraint to quiescence by Al and in the mid-cortex 1–2 mm from the root tip where growth stimulation was associated with a departure from osmotic equilibrium. An initial increase in the osmotic potential of the root cell sap arising from Al treatment (1 h) was not maintained and subsequent decreases in the osmotic potential coincided with changes in the cell growth direction. Stress patterns arising from the anisotropic growth response of cortical cells with 20–h root exposure to Al were associated with the collapse of the conducting tissue of the stele and disintegration of the outer cells of the root. Advanced vacuolation of the cells of the root apex, first observed with 5–h treatment, was a feature of Al-stressed roots. Alteration to the nuclear structure in cells of the proximal meristem involving the degree of chromatin condensation and structure of the nucleolus was considered indicative of senescence. S. Afr. J. Bot. 1985, 51: 355–362Behandeling van die primêre wortel met 8 mg dm−3 Al het die patroon van selgroei verander, sodat die wortelpunt na blootstelling van 48 h nie meer ‘n sitologies-georganiseerde geheel was nie. ‘n Toename in die gemiddelde selvolume is na blootstelling van 2 h waargeneem, hoofsaaklik in die selle van die rustende gebied, waar die vergroting van selle geassosieer is met die opheffing van die rustoestand, en in die sentrale korteks, 1–2 mm vanaf die wortelpunt, waar die aanvangsgroei gestimuleer is deur ‘n afwyking vanaf osmotiese ewewig. Die aanvanklike verhoging in die osmotiese potensiaal van die selsap, as gevolg van die Al-behandeling (1 h), is nie volgehou nie; die verlaging in osmotiese potensiaal het ooreengestem met veranderings in die rigting van selgroei. Spanningspatrone wat voortspruit uit die anisotropiese groeireaksie van die korteksselle na 20 h se blootstelling van die wortel aan Al, is geassosieer met die ineenstorting van die geleidingsweefsel van die stele en die disintegrasie van die wortels se epidermis. Gevorderde vakuolering van die wortelpuntselle wat vir die eerste keer opgemerk is met die 5 h-blootstelling is ‘n kenmerk van wortels wat aan Al blootgestel is. Veranderings in die kernstruktuur van die selle van die proksimale meristeem ten opsigte van die graad van chromatienkondensering in die struktuur van die nukleolus is beskou as aanduidings van veroudering. S.-Afr. Tydskr. Plantk. 1985, 51: 355–36

Soil chemistry in South Africa: A quarter century of progress and prospects for the future

[No abstract available]Articl

Declining soil quality in South Africa: Effects of land use on soil organic matter and surface crusting

Soil conservation in South Africa has historically focussed on preventing soil erosion. Effective maintenance of the soil resource requires, in addition to erosion control, an understanding of how land use practices affect more subtle indicators of soil quality. This review outlines how land use in South Africa can rapidly result in a marked reduction in soil organic matter (SOM) content and a greater tendency of soils to crust. Removal of a cover of vegetation whether by ploughing, grazing or burning tends to reduce SOM due to reduced organic matter inputs and enhanced soil microbe activity. Loss of SOM, particularly from the top few centimetres of soil (named here the pedoderm) has a disproportionately large effect on soil infiltrability and nutrient supply. The mineralogy of the clay fraction also has great bearing on the response of soil to land use effects. The unexpected role of quartz in soil dispersion and crusting in South Africa has only recently been unveiled. Apart from SOM effects, land use can lead to subtle changes in soil chemistry. Plantation forestry has resulted in an increase in soil nitrate in many areas, possibly due to greater mineralisation under forests than grasslands. Annual burning in the Kruger National Park bushveld has been shown to increase clay dispersibility and crusting of the pedoderm, which was ascribed to a reduction in electrical conductivity and SOM as well as an increase in the exchangeable sodium percentage. Soil quality is a multifaceted concept. One aspect stands out, however, as critical and that is the conservation and replenishment of nitrogen which is all important for retaining humus and maintaining soil quality.Revie

Soil carbon and nitrogen in five contrasting biomes of South Africa exposed to different land uses

Stocks of soil C to a depth of 50 cm in untransformed, indigenous veld ranged from 21 t ha-1 in karoo to 168 t ha-1 in thicket and stocks of N ranged from 3.4 t ha-1 in karoo to 12.8 t ha-1 in grassland. Mean soil C in thicket (5.6%, 0-10 cm) was approximately five times greater than expected for a semi-arid region. Removal of vegetation due to cultivation, grazing or burning reduced soil C and N at all sites. Soil C under intact thicket was greater than at sites degraded by goats (71 vs 40 t ha-1, 0-10 cm). Restoration of thicket could potentially sequester ∼40 t C ha-1. The sale of this sequestered carbon to the international market may make restoration of thousands of hectares of degraded thicket financially feasible. Soil C under plant cover was greater than in exposed soil in renosterveld (28 vs 15 t ha-1 and in karoo (7 vs 5 t ha-1). Parent material was also related to soil C content. In grassland, soil C was greater in dolerite-derived than sandstone-derived soils (54 vs 27 t ha-1); and in bushveld it was greater in basalt-derived than granite-derived soils (28 vs 14 t ha-1 in unburnt plots). Annual burning in bushveld reduced soil C, particularly at the surface. Soil C in the 0-1 cm layer of unburnt plots was 2 to 3 times greater than in burnt plots.Articl

Relationships between soil particle size fractions and infiltrability

The influence of particle size fractions on infiltrability was investigated in soils sampled across Namibia and western South Africa. Infiltrability was determined using a laboratory technique calibrated with rainfall simulation, which measures the passage of a suspension of soil particles through a packed soil column. Water-dis-persible soil particle size fractions were determined using a high definition digital laser particle size analyser. Total (calgon-dispersed) particle size fractions were determined by hydrometer. Dispersion of soil particles resulting in crust formation on the soil surface appeared to be a main mechanism reducing infiltrability. Water-dispersible clay and fine silt determined by laser analyser showed higher correlation with infiltrability (r =-0.43 for clay and-0.47 for fine silt) than total clay and fine silt determined by hydrometer (r2 =-0.30 and-0.28, respectively). Clay, fine silt, coarse silt, very fine sand and fine sand fractions (<120 μm) showed a probable plasmic role in soil crusts. At a content of these fractions > 5% infiltrability was inevitably restrained. The 120-200 μm fraction showed no clear relationship with infiltrability. It played either a plasmic or skeletal role, depending on its ratio to the <120 μm and >200 μm fractions. Fine, medium and coarse sand fractions (>200 μm) showed a probable skeletal role in soil crusts, I.e. forming pores that enhanced infiltrability. At levels >50% of these fractions, infiltrability was potentially maximal. This potentially maximal infiltrability was also explained by the concomitant decrease in plasmic fraction content with an increase of the skeletal fraction.Articl

Effects of sodium sulphate, sodium chloride and manganese sulphate on kikuyu (Pennisetum clandestinum) growth and ion uptake

Irrigation with saline water is increasingly practised yet an understanding of plant response to different salt types remains largely elusive. Our study investigated the effects of irrigation with simulated effluent containing three salts on the growth and composition of a common South African pasture grass. We treated pots of kikuyu (Pennisetum clandestinum Hochst. ex Chiov.) in a controlled environment with solutions of sodium sulphate (Na2SO4) or chloride (NaCl) at saturated soil solution electrical conductivity (EC) of 0, 5, 10 and 20 dS m-1, combined with 0, 900 or 1800 mg kg-1 Mn as MnSO4. Kikuyu yields in the Na2SO4 treatments were significantly lower than in corresponding NaCl treatments. This could be explained by increased ion imbalance and osmotic stress as a result of higher ionic strength and a greater concentration of neutral ion pairs in the Na2SO4 system at similar EC. An apparent Na2SO4-induced Ca deficiency was attributed to suppressed Ca2+ activity through ion pairing at high ionic strengths. Under the conditions of this experiment ionic strength appeared to be more effective than EC as a measure of salinity stress when comparing the effects of SO4 and Cl on growth of kikuyu.Articl

Acidic iron oxide waste as a conditioner for calcareous soils

A fine-grained iron oxide (hematite, (α-Fe2O3), containing 1.2% hydrochloric acid by mass, is the main by-product of an acid recovery plant at Saldanha Bay. The alkalinity of calcareous soils in this semi-arid region causes plant deficiencies in many trace elements, including iron. The use of the acidic waste to decrease the pH of two sandy soils (one non-calcareous and the other containing 1.9% CaCO3) was evaluated in a pot trial. No significant differences were observed in the yield of young wheat in either soil treated with 0, 5 or 25 g iron oxide per kg soil. An application rate of 50 g/kg, however, suppressed wheat growth in both soils by about 40%. This was interpreted as being due to soil salinity in both soils (electrical conductivity, ECe, increased to about 10 dS m-1) and to acidification of the non-calcareous soil to pHKCI4.04 (the calcareous soil remained buffered at pH 7).Articl

Prediction of the soil-depth salinity-trend in a vineyard after sustained irrigation with saline water

Remote sensing combined with an ability to look deeper than the soil surface is currently high in demand. This study was conducted through scaling down the amount of soil data from a saline irrigation water experiment to see if one can still capture the essential soil salinity depth trends within the data, to a level that can enhance the ability of remote methods. A saline irrigation experiment with 6 water qualities was conducted for 8 years on 1.2 ha of vineyard land near Robertson in the Western Cape Province of South Africa. Soil water was sampled at regular intervals at 5 depths between 0.15 and 1.2 m with suction cup lysimeters at a fixed time following each irrigation. Electrical conductivity of the soil water (ECsw) was determined after sampling. Data collected over the full 8-year period were investigated for depth trends in ECsw, seeking trend lines with lowest polynomial order that were still significantly predict the salinity profile. At all treatment levels a first order polynomial equation, fitted to the salinity profiles, significantly predicted the salinity trends. The ECsw value at only two depths could therefore be used to calculate total salt accumulation and soil water quality below the root zone. The implication is that considerable value can be obtained from minimal measurements both in estimating salt accumulation in the soil profile and predicting water quality in return flow from saline irrigation.Grapevines Soil salinity profiles Suction cup lysimetry Salt accumulation Sustainable irrigation Trend surface analysis