EIACP Centre on Plants and Pollution

Cadmium and Plants

Introduction

Cadmium (Cd)is a heavy metal toxic to all organisms. The accumulation of Cd in biotic systems as a consequence of human activities is becoming a major environmental problem. The application of sewage sludge, city waste, and Cd-containing fertilizers causes the increase of Cd content in soils. Cd is easily taken up by plants and then enters the food chain, resulting in a serious health issue for humans. For instance, people will suffer from renal tubular disease if they consume rice with relatively high Cd content. Therefore, addition of toxic heavy metals into soil and transference of these metals into the food chain have been a matter of concern to people in arable soils. The presence of excessive amount of Cd in soil causes many toxic symptoms in plants, such as reduction of growth, especially root growth, disturbances in mineral nutrition and carbohydrate metabolism, and may therefore strongly reduce biomass production. The reduction of biomass by Cd toxicity could be the direct consequence of the inhibition of chlorophyll synthesis and photosynthesis.

Source: Jing et al 2005 ,Journal of Zhejiang University Science

Effects of Cd exposure to plants at different levels in photosynthetic machinery

• (a) Cd uptake in cells through plasma membrane transporters.
• (b) Alteration in organisation of oxygen evolving and light harvesting complexes, Cd also binds with QB pocket thus slows down electron flow from QA to QB.
• (c) Incorporation of Cd in chlorophyll molecule.

Source: https://as-botanicalstudies.springeropen.com

The effect of cadmium stress on the rate of photosynthesis in the first leaf Of barley plants

Cultivar	Variant (mg Cd.l-1)	A (mg CO2.m-2.s-1)	%
Obzor	0 3 6	0.2235 ± 0.0192 0.2064 ± 0.0246 0.1955 ± 0.0227	100 92 87
Hemus	0 3 6	0.2366 ± 0.0935 0.2376 ± 0.0136 0.1888 ± 0.0128	100 100 80

Source: http://www.bio21.bas.bg/ipp/gapbfiles

Effect of Cd on ROS generation

Generally, heavy metals cause oxidative damage to plants, either directly or indirectly through reactive oxygen species (ROS) formation. Certain heavy metals such as copper and iron can be toxic through their participation in redox cycles like Fenton and/or Haber-Weiss reactions. In contrast, Cd is a non-redox metal unable to perform single electron transfer reactions, and does not produce ROS such as the superoxide anion (O2 •–), singlet oxygen (1O2), hydrogen peroxide (H2O2), and hydroxyl radical (OH•), but generates oxidative stress by interfering with the antioxidant defence system. Cd inhibits the photoactivation of photosystem 2 (PS2) by inhibiting electron transfer. Thus, Cd could lead to the generation of ROS indirectly by production of a disturbance in the chloroplasts. In addition, other reports suggested that Cd may stimulate the production of ROS in the mitochondrial electron transfer chain.

Source: Tuan et al 2012 ,Turkish Journal of botany

Effects of Cd as a potent inhibitor of photosynthesis

Photosynthesis inhibition may be attributed to diminished chlorophyll biosynthesis, interrupted O2 - evolving reactions of PSII and altered electron flow around PSI and PSII. Cd hampers Calvin cycle by slowing down activity of various enzymes hence resulting in decreased photosynthesis. Cd has also been known to show inhibitory effect on various enzymes such as ribulose-1, 5-biphosphate carboxylase oxygenase, phosphoenolpyruvate carboxylase, aldolase, fructose-6-phosphate kinase, fructose-1, 6-bisphosphatase, NADP+-glyceraldehyde-3-phosphate dehydrogenase and carbonic anhydrase. Stomatal closure due to entry of Cd into the guard cells in competition to Ca+2 and reduction in stomata count per unit area are also characteristic symptoms of Cd stress resulting in lesser conductance to CO2 which consequently leads to overall inhibition of photosynthesis.

Source: Pooja Parmar at al. (2016), Structural and functional alterations in photosynthetic apparatus of plants under cadmium stressBotanical Studies An International Journal

Effect of Cd on mineral nutrition

It has been reported that uptake, transport, and subsequent distribution of nutrient elements by the plants can be affected by the presence of Cd ions. In general, Cd has been shown to interfere with the uptake, transport, and use of several elements (Ca, Mg, P, and K) and water by plants. Cd also reduced the absorption of nitrate and its transport from roots to shoots, by inhibiting nitrate reductase activity in the shoots.

It should be mentioned that several plant nutrients have many direct as well as indirect effects on Cd availability and toxicity. Direct effects include decreased Cd solubility in soil by favouring precipitation and adsorption, competition between Cd and plant nutrients for the same membrane transporters , and Cd sequestration in the vegetative parts to avoid its accumulation in the grain/edible parts .Indirect effects include dilution of Cd concentration by increasing plant biomass and alleviation of physiological stress.

Source: http://journals.tubitak.gov.tr/botany/issues/bot-13-37-1/bot-37-1-1-1112-16.pdf

Cd effect on plant growth

Cadmium (Cd), a toxic element, is dispersed in the natural and agricultural environments mainly through human activities and has a long biological half-life. It is one of the nonessential heavy metals, toxic to flora and fauna, which is easily taken up by plant roots and translocated to the aerial plant parts. Cadmium accumulation reduces photosynthesis, disturbs plant-water relations and the uptake and translocation of nutrients, and results in visible injury symptoms or plant death. Cadmium is known to cause a burst of reactive oxygen species (ROS) in plant tissues, leading to the development of secondary oxidative stress that may damage photosynthetic pigments and other bio-molecules such as lipids, proteins and nucleic acids. It causes leakage of electrolytes via lipid peroxidation, a decrease in the AsA and GSH contents and alteration in activities of antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase.

Source: http://www.ipipotash.org

Cadmium Effects on Enzymes

Heavy metals are important environmental pollutants and their toxicity is a problem of increasing significance for ecological, evolutionary, nutritional, and environmental reasons. Plants posses homeostatic cellular mechanisms to regulate the concentration of metal ions inside the cell to minimize the potential damage that could result from the exposure to nonessential metal ions including mechanisms of cadmium homeostasis, uptake, transport and accumulation are evaluated. The role of the cell wall, the plasma membrane and the mycorrhizas, as the main barriers against cadmium entrance to the cell, as well as some aspects related to phytochelatin-based sequestration and compartmentalization processes are also reviewed. Cadmium-induced oxidative stress was also considered as one of the most studied topics of cadmium toxicity.

Source: www.google.co.in

Defence mechanisms against Cd in plants

The mechanisms leading to heavy metal tolerance can be divided into avoidance strategies and tolerance strategies. Avoidance leads to limitation of Cd uptake. Plant tolerance mechanisms include accumulation and storing of Cd by binding it to amino acids, proteins, and peptides .Other mechanisms that plants have developed to cope with damage caused by Cd are related to some stress signalling molecules, such as salicylic acid, jasmonic acid, nitric oxide, and ethylene. All these compounds were induced by Cd treatment, which suggests that they are involved in cell response to Cd toxicity .Many plants survive, grow, and develop in Cd-polluted soils even in high concentrations of Cd. Investigations showed that some of these plants exhibit a hypertolerant capacity of their organelles and tissues. Strategies to cope with Cd toxicity involve the uptake and the distribution of Cd, defined as “hyperaccumulation”. On the other hand, some plants increased cleaning up of the ROS by antioxidants to protect cells and tissues from destruction. Thus, the mechanism of Cd tolerance in plants can include both antioxidant defence and/or hyperaccumulation defence.

Source: http://journals.tubitak.gov.tr/botany/issues/bot-13-37-1/bot-37-1-1-1112-16.pdf

Long-distance Transport of Heavy Metals

Figure: Modified from Mendoza-Cózatl et al., 2011

Distribution of heavy metals between roots and shoots is a dynamic process orchestrated by several plasma membrane transporters, metal-chelating molecules, xylem-loading/unloading, and phloem-loading/unloading processes. Root-to-shoot transport of metals occurs mainly through the xylem; however, due to the limited transpiration rate within reproductive tissues, the xylem plays only a minor role in allocating nutrients into the seeds. Phloem transport, on the other hand, plays a key role in delivering nutrients, including metals, to developing seeds. Our lab investigates the isolation, identification, and characterization of phloem-specific transporters of trace metals (essential and toxic) using the reference plant Arabidopsis thaliana.

Source: http://ipg.missouri.edu

List of sensitive plants of cadmium contamination

S.No.	Sensitive plants	Metal Contamination	References
1	Arobiodopsis hallerii	Cadmium	Kupper et al. (2000)
2	Pelargonium sp.	Cadmium	Dan et al. (2002)
3	Arabidopsis halleri	Cadmium	Berts and Meerts et al.(2003)
4	Arabis gammifera	Cadmium	Kubota and Takenka (2003)
5	Pistia stratiotes	Cadmium	Odjegba and fasidi (2004)
6	Sedum alfredii	Cadmium	Xiong et al. (2004)
7	Thlapsi caerulescens	Cadmium	Banasova and Horak (2008)
8	Tamarix smyrnesis	Cadmium	Mnausaki et al. (2008)
9	P. griffithii	Cadmium	Hu et al. (2009)
10	Brassica napus	Cadmium	Selvam and wong (2008)
11	Arbidopsis thaliana	Cadmium	Saraswat and Rai (2009)
12	Rorippa globosa	Cadmium	Sun et al. (2010)

Source: Metal hyperaccumulation in plants : A review focusing on Phytoremediation technology, Journal of Environment science and technology.

Accumulation tendency for cadmium in various crop samples

Cadmium accumulation tendency
Low	Midium	High
White Cabbage, Brussels Sprouts	Kale, Red Cabbage	Mangel, Head Lettuce
Broccoli, Kohlarbi	Savoy Cabbage, Chives	Spinach, Parsley
Potatoes, Radishes	Lovage, Carrots	Dill, Leaf Celery
Zucchini, Beans	Radishes, Red Beets	Celeriac, Rhubarb
Apples, Pears	Leeks, Wheat
Plums, Maize
Oat, Barley
Ray

Source: Senate Department of Urban Development and the Environment