No abstract

The effect of aluminium (Al) phytotoxicity on the growth of soybean (Glycine max (L.) Merr.) cv. Forrest in nutrient solutions (pH 4.25) and its alleviation by organic anions has been investigated. Both 100 and 500 µM Al caused significant reduction in root and shoot lengths and root dry weights over 15 days. Citrate, oxalate, tartrate, malate, malonate and lactate at 1000 µM and humic acid at 1 g L 1 were used to alleviate phytotoxicity caused by the presence of 500 µM Al. The effectiveness of the organic anions in alleviating Al phytotoxicity, measured by the change in root length, was in the order oxalate > citrate Ϸ tartrate Ϸ malate > malonate >> lactate. Humic acid at 1 g L 1 proved almost as effective as 1000 µM citrate. When the remaining fraction of phytotoxic aluminium was measured by the aluminon method and the results combined with calculated values obtained from stability constant data, the order of effectiveness of complexation was citrate > oxalate > tartrate Ϸ malate Ϸ malonate >> lactate. It was concluded that benefits other than from simple complexation of phytotoxic aluminium were provided by the organic anions. Three of the anions were also used at different concentrations; these were citrate and oxalate (at 250, 500 and 1000 µM) and humic acid (at 0.01, 0.1 and 1.0 g L 1 ). For citrate and oxalate, it was found that the concentration of the organic anion must be at least that of the Al to reduce phytotoxicity significantly, and twice as much oxalate as citrate was required to achieve the same reduction in phytotoxicity.

This review compares and contrasts analytical techniques for the measurement of total soil organic carbon (TOC). Soil TOC is seen to be a highly important health and quality indicator for soils, as well as having the potential to sequester atmospheric carbon. Definition of the form of organic carbon measured by a given method is vital to the selection of appropriate methodology, as well as the understanding of what exactly is being measured. Historically, studies of TOC have ranged from basic measures, such as colour and gravimetric analyses, to dry and wet oxidation techniques. In more recent times, various spectroscopic techniques and the application of remote or mobile approaches have gained prominence. The different techniques, even the oldest ones, may have their place in current research depending on research needs, the available time, budget and access to wider resources. This review provides an overview of the various methods, highlights advantages, limitations and research opportunities and provides an indication of what the method actually measures so that meaningful comparisons can be made.

To determine if an increased aluminium (Al) sensitivity is the result of greater accumulation of Al in root apices, the quantity and distribution of Al in root apex cells of Danthonia linkii Kunth and Microlaena stipoides (Labill.) R.Br. was investigated by X-ray microanalysis. Seedlings were grown in nutrient solution with 0, 185 and 370 µM total Al for 24 h after which the terminal 5 mm of the roots were excised, rapidly frozen and embedded using freeze-substitution. Elemental distribution maps of root apices showed that Al had accumulated in the nuclei of root cap and meristematic cells in Al-stressed roots of D. linkii but not in M. stipoides. Al appeared to be co-localised with phosphorous (P) in the nuclei of these cells. Quantitation of Al revealed that Al-stressed root apex cells of D. linkiiaccumulated significantly more Al than M. stipoides. Exposure of D. linkii roots to Al resulted in substantial increases in the P content of root apex cells, and a significant positive correlation was found between Al and P in both root cap and meristematic cells. Analysis of intracellular structures showed that the majority of Al had accumulated in the nuclei of cells. A significant positive correlation was found between Al and P in the nuclei, but not in the cytoplasm or cell walls. No positive correlation was found between Al and P in root apex cells of M. stipoides.

Inhibition of net root elongation and patterns of hematoxylin staining were used to assess relative tolerance to phytotoxic Al in Danthonia linkii Kunth and Microlaena stipoides (Labill.) R.Br. According to net root elongation, M. stipoides is significantly more tolerant of phytotoxic Al than D. linkii. In nutrient solutions with Al concentrations of 370 µM and higher, root elongation is stopped in D. linkii after 24 h while in M. stipoides root elongation is maintained at 60–70% of control rates over 72 h. After removal of Al-stress, root growth in M. stipoides from all Al-treatments recovered to be at or above control growth after 72 h. In D. linkii, root elongation in plants exposed to Al levels that caused a reduction in growth (<370 µM), but not complete cessation, recovered after removal of Al stress. Greater intensities of hematoxylin staining were seen in Al-stressed root tips of D. linkii compared to M. stipoides, suggesting that inhibition of root elongation is associated with increased accumulation of Al in root tips. Roots of M. stipoides seedlings exposed to all Al-treatments showed a short band of intensely stained tissue, correlating with the position of the root apex at the exact point of initial Al- exposure. New root growth after this band did not stain with hematoxylin, indicating activation of a mechanism of Al-exclusion in roots of M. stipoides.