ABSTRACT
Heavy metals (HMs) are in general dened as metals with a specic gravity greater than 5.0 or withhighatomicmass.Often,toxicityisassociatedwiththeirdenition;however,severalHMsare necessaryfortheproperfunctioningandmetabolismoflivingorganisms.Suchelementsessential forplantdevelopmentareCu,Co,Fe,Mn,Mo,Ni,andZn.Onetypeofplantstressesisnutrient deciency,withwhichmostplantscanbarelycope;thereforetheirgrowthandcropproductivity are impaired when the phytoavailability of any of these metals is low. Unfortunately, in several agriculturalelds,essential-metaldeciencyisaseriousconcern,especiallyinthecaseofFeand Zn(Guerinot2000).AnotherplantstressisrepresentedbytheexcessofessentialHMsandbythe presenceofnonessentialmetals.ThesenonessentialHMs(e.g.,Cd,Cr,Hg,Pb)areingeneraltoxic orneutralforplantmetabolism.TheincreasedpresenceofHMsinair,soil,andwaterisalsoa global problem that represents a growing threat to the environment and to humankind and requires immediateattention.TherearehundredsofnaturalandanthropologicsourcesofHMs,including industry,atmosphericdeposition,useofagrochemicals,andwastedisposal.Thepollutantscan enterplantsviavariousmechanismsandinthiswaycaneasilyreachintracellularcompartmentsof plants,suchasplastids.Inthischapter,thestresscausedbyessential-HMdeciencyandbyexcess
26.1 Introduction .......................................................................................................................... 675 26.2Metals Inside Plants .............................................................................................................. 676
26.2.1Metal Transporters .................................................................................................... 676 26.2.2Interactions between Different Metals .....................................................................680
26.3Metals and Plastid Metabolism............................................................................................. 681 26.3.1 Impact of Unbalanced Metals on Nongreen Plastids ............................................... 682 26.3.2Impact of Unbalanced Metals on Photosynthesis ..................................................... 685
