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Trace Elements in Abiotic and Biotic Environments
show that some leaves accumulated a higher proportion of Li (e.g., celery, 6.6 mg/kg)
than do storage roots or bulbs. Nevertheless, a higher Li content is very often reported
for roots. The ratio of root to top for Li in ryegrass is 4.4, whereas for white clover
it is 20. This may suggest that a difference in plant tolerance to Li concentration
is related mainly to mechanisms of biological barriers in root tissues. However, Li
is concentrated in above-earth parts of plants rather straight from aerial sources.
This is especially noticeable in plants growing in industrial regions; for example, the
leaves:roots ratio for Li in dandelion from a rural region is 0.8, and from an industrial
region is 5.0 (Kabata-Pendias 2011).
Although Li is not known to be an essential plant nutrient, there is evidence that
Li can affect plant growth and development. However, stimulating effects of several Li salts, reported by various authors, have never been confirmed. The observed
stimulation may also be related to the influence of other factors, including secondary
effects of anions associated with Li. There have been reported antagonistic effects
from Rb and possible Zn, whereas synergistic effects from Fe and Mn. Calcium
inhibits Li uptake by plants, whereas the addition of lime to high-Li soil may reduce
toxic effects of this element. Although Li is not known yet as an essential plant nutrient, there are some evidences that Li can affect plant growth and may play some
metabolic function in halophytes.
Increased Li contents in soil can be toxic to some plants. Citrus trees are probably
the most susceptible to an excess Li, and their growth in salt-enriched soils can be
significantly reduced due to high Li contents. In high-Li soils, damage to root tips,
injured root growth, and chlorotic and necrotic spots on leaves have been observed
in corn. Threshold concentrations of Li in plants are variable; for example, moderate
to severe toxic effects of 4–40 mg Li/kg in citrus leaves were reported. The most
resistant to a high-Li concentration are plants of the Solanaceae family, which may
accumulate Li even above 1000 mg/kg.
25.6 HUMANS
Lithium content in human body (70 kg) is estimated on 7 mg, of which in the
blood 4 μg/L, in the bones and tissues 1300 and 24 μg/kg, respectively (Emsley
2011), and according to Zaichik et al. (2011) its content in the rib bones is about
70 μg/kg. Lithium is absorbed efficiently in the intestine and is excreted mainly
in the urine. The biochemical mechanisms of Li action appear to be multifactorial and are intercorrelated with the functions of several enzymes, hormones, and vitamins, as well as with growth and transforming factors. Lithium
is thought to stabilize serotonin transmission in the nervous system, in addition
to influencing Na transportation. It may even increase lymphocytic (white blood
cell) proliferation, and depress the suppressor cell activity, thus strengthening
the immune system. Lithium may act by altering the distribution of electrolytes
within the brain.