Background and Aims
Hybanthus austrocaledonicus (Violaceae) is a Ni hyperaccumulator endemic to New Caledonia. One of the specimens stored at the local herbarium had a strip of bark with a remarkably green phloem tissue attached to the sheet containing over 4 Wt% Ni. This study aimed to collect field samples from the original H. austrocaledonicus locality to confirm the nature of the green “nickel-rich phloem” in this taxon and to systematically assess the occurrence of Ni hyperaccumulation in Hybanthus austrocaledonicus and H. caledonicus populations.
Methods
X-ray fluorescence spectroscopy (XRF) scanning of all collections of the genus Hybanthus (236 specimens) was undertaken at the Herbarium of New Caledonia to reveal incidences of nickel accumulation in populations of Hybanthus austrocaledonicus and H. caledonicus. In parallel, micro-analytical investigations were performed via Synchrotron X-ray fluorescence microscopy (XFM) and Scanning electron microscopy with X-ray microanalysis (SEM-EDS)
Key Results
The extensive scanning demonstrated that Ni hyperaccumulation is not a characteristic common to all populations in the endemic Hybanthus species. Synchrotron X-ray fluorescence microscopy (XFM) revealed that nickel was exclusively concentrated in the epidermal cells of the leaf blade and petiole, conforming with the majority of (tropical) Ni hyperaccumulator plants studied to date. Scanning electron microscopy with X-ray microanalysis (SEM-EDS) of freeze-dried and frozen-hydrated samples revealed the presence of dense solid deposits in the phloem bundles that contained >8 Wt% nickel.
Conclusions
The occurrence of extremely nickel-rich green-coloured phloem tissues appears to be a characteristic feature of tropical nickel hyperaccumulator plants.
Cadmium (Cd)-mineralized soils occur in many nations. When these soils are noncalcareous, crops and especially leafy vegetables such as lettuce ( L.) and spinach ( L.) may accumulate levels of Cd in their edible portions that exceed international standards. Vegetable crops grown on Lockwood loam from Monterey County, CA, absorb an excessive amount of Cd into their edible portions. Agronomic or genetic management alternatives are needed to allow the use of these otherwise highly productive soils for spinach. Previous research has shown that zinc (Zn) fertilizer plus limestone incorporation or biosolids compost and sorbent oxide amendments can reduce spinach and lettuce Cd accumulation. We tested combinations of biosolids compost (10%), Mn, Zn, and limestone (5%) on Cd phytoavailability. Manganese sulfate (in the absence of limestone) caused minor pH reduction, which increased the Cd level in spinach. The addition of ZnCO+ZnSO inhibited Cd accumulation, as did biosolids compost, but much greater reductions were achieved when soil pH was raised with limestone to prevent the acidification from the addition of compost or Zn salts. Cadmium accumulation was suppressed below international guidelines limits when combinations of compost+Zn+limestone or compost+Zn+Mn+limestone were applied, highlighting the complexity of processes limiting Cd phytoavailability.
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