**9. Arsenic and plant physiology**

Arsenic may limit rice growth and development [1]. Finnegan and Chen [7] and Sharma et al. [8] each reviewed the plant physiology of arsenic on plant growth and development. These authors discussed evidence that arsenite and arsenate are taken up by root cells, but arsenate is rapidly reduced to arsenite. Cellular disruption may be caused by both arsenite and arsenate; however, the mechanisms are distinctly different.

Arsenite is dithiol reactive and readily binds and potentially inactivates selective cysteine containing enzymes and dithiol co-factors. As(III) enters root cells via aquaporin (nodulin26-like intrinsic proteins) and xylem export to stems may occur. Arsenite may bind from one to three sulfhydryl groups, influencing the physiologic behavior of transcription factors, signal transduction proteins, proteolytic proteins, metabolic enzymes, redox regulatory enzymes, and structural proteins. The binding of As(III) to thiols may constitute the main detoxification pathways [7, 8]. Arsenate may replace Pi in critical biochemical reactions: (i) glycolysis, (ii) oxidative phosphorylation, (iii) phospholipid metabolism, (iv) DNA and RNA metabolism, and (v) cellular signaling [7, 8]. Both arsenite and arsenate may increase oxidative stress by inducing the production of reactive oxygen species; that is, the production of superoxide (O2• − ), hydroxyl radical (•OH), and peroxide (H2O2). Glutathione (a tripeptide with linkage between the carboxyl group of the glutamate side-chain and cysteine) is an antioxidant that assists in preventing reactive oxygen species from disrupting cellular function. Ascorbate may also limit reactive oxygen species damage [7, 8].

Other metabolic consequences of arsenic include: (i) chloroplast shape irregularities and reduction of chlorophyll content, (ii) altered carbohydrate metabolism involving sucrose and starch, (iii) reduced micronutrient uptake, (iv) altered ATP synthesis, (v) altered stomatal conductance, (vi) altered lipid metabolism and the integrity of cellular membranes [8]. Belefant-Miller and Beaty [53] observed the plant distribution of arsenic in rice plants might influence "straighthead". Yan et al. [54] identified soil arsenic bioavailability is associated with "straighthead" disorder in rice. Lim et al. [55] reviewed the effect of arsenic compounds on plant growth. In a subsequent review, Kofronova et al. [56] focused on arsenic physiology in hyperaccumulating plants and documented the following research outcomes: (i) arsenic interfered with basic cellular metabolism, including carbohydrate metabolism in

*An Emerging Global Understanding of Arsenic in Rice (*Oryza sativa*) and Agronomic Practices… DOI: http://dx.doi.org/10.5772/intechopen.105500*

photosynthesis, (ii) arsenite and arsenate were xylem transported, (iii) arsenate reduction was associated with arsenate reductase and arsenite interacted with glutathione for passage into the cell's vacuole, (iv) arsenate interfered with cell wall physiology, decreased ribulose-1,5 biphosphate carboxylase/oxygenase, and competed with phosphorus in oxidative phosphorylation. Arsenite interfered with hormonal physiology and restricted pigment system II and chlorophyll functioning.

In a greenhouse project, Jung et al. [57] amended soil at arsenic rates of 0 (untreated check), 25, 50 and 75 mg As kg−1. The 50 mg As kg−1 amended level inhibited shoot growth. Chauhan et al. [58] observed that the presence of increased heavy metal activity and arsenic availability reduced the activity of key soil enzymes, suggesting that bacterial diversity and microbial functioning were impaired.
