Page 214 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 203

Cadmium Uptake

Plant roots readily absorb cadmium from soil if it is available and translocate it to
the aboveground parts. Cadmium concentrations are typically higher in the plant
leaves than in fruits or storage organs, indicating that cadmium is highly mobile in
plant tissues. The uptake of Cd increases proportionally to increasing soil Cd levels.
A linear increase in uptake of Cd has been observed in different greenhouse studies
and fi eld trials (Brown et al. 1998 ; Kadar 1995 ). The Cd transfer factor (TF) is the
ratio of Cd concentration in the plant to that in the soil. Generally, leafy vegetables
have higher Cd concentrations and TF than storage organs or fruits. The TF concept
suggests that plant Cd can be properly predicted from soil Cd. However, the TF
varies with soil properties. Field surveys carried out in different parts of the world
showed that soil Cd usually explains less than 20 % of the variability of crop Cd
(Eriksson et al. 1996 ). It is often observed that Cd concentration in soil solution or
Cd concentrations in neutral salt extracts of soil (NH 4 NO 3 , NaNO 3 , or CaCl 2
extracts) are better predictors for crop Cd than total soil Cd (McLaughlin et al. 2000 ).
This indicates that Cd availability is linked with Cd mobility. However, mobility
and plant availability do not always go hand in hand. Soil pH has a negative rela-
tionship with Cd uptake. On the other hand, soil salinity and lime-induced zinc

deficiency enhance uptake of Cd by plants (Smolders 2001 ). Some agricultural

crops usually contain high Cd concentrations. Durum wheat, sunflower kernels, and
fl ax have been identifi ed as high Cd crops compared to spring wheat, barley, corn,
or oats (Li et al. 1994 ).

Lead Uptake

There are various complexes of lead in soil, and only a small fraction of the lead
present in these complexes in the soil solution are phyto-available. It has no essen-
tial function in plants; still, lead is absorbed by them mainly through the roots
from soil solution and enters the food chain. Roots absorb lead via the apoplastic
2+
pathway or via Ca permeable channels. The factors that affect lead availability
and lead uptake by plants are speciation of lead, soil pH, soil particle size, cation
exchange capacity, root surface area, root exudation, and degree of mycorrhizal
transpiration. After uptake, lead primarily accumulates in root cells, because of
the blockage by Casparian strips within the endodermis. Lead is also trapped by
the negative charges that exist on roots cell walls. Excessive lead accumulation
in plant tissue impairs various morphological, physiological, and biochemical
functions in plants, either directly or indirectly, and induces a range of deleterious
effects. It causes phytotoxicity by changing cell membrane permeability, by react-
ing with active groups of different enzymes involved in plant metabolism and by
reacting with the phosphate groups of ADP or ATP, and by replacing essential ions
(Pourrut et al. 2011 ).