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Qualitative study of the effects of Cadmium ion on the morphology, physiology and protein profile of… Guo, Yinuo (Yvette) Jun 1, 2012

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UBC Social Ecological Economic Development Studies (SEEDS) Student Report         Qualitative study of the effects of Cadmium ion on the morphology, physiology and protein profile of Lolium perenne (Perennial Ryegrass) and Trifolium hybridum (Alsike Clover)  Yinuo (Yvette) Guo University of British Columbia BIOL 448 June 1, 2012         Disclaimer: “UBC SEEDS provides students with the opportunity to share the findings of their studies, as well as their opinions, conclusions and recommendations with the UBC community. The reader should bear in mind that this is a student project/report and is not an official document of UBC. Furthermore readers should bear in mind that these reports may not reflect the current status of activities at UBC. We urge you to contact the research persons mentioned in a report or the SEEDS Coordinator about the current status of the subject matter of a project/report”. [Academic work for this project was completed on 2010/April 30]  1  UBC Social Ecological Economic Development Studies (SEEDS) Student Report    Qualitative study of the effects of Cadmium ion on the morphology, physiology and protein profile of Lolium perenne (Perennial Ryegrass) and Trifolium hybridum (Alsike Clover)  Yinuo (Yvette) Guo    University of British Columbia UBC SEEDS Project June 1, 2012 Disclaimer: UBC SEEDS provides students with the opportunity to share the findings of their studies, as well as their opinions, conclusions and recommendations with the UBC community. The reader should bear in mind that this is a student project/report and is not an official document of UBC. Furthermore readers should bear in mind that these reports may not reflect the current status of activities at UBC. We urge you to contact the research persons mentioned in a report or the SEEDS Coordinator about the current status of the subject matter of a project/report.    2  Qualitative study of the effects of Cadmium ion on the morphology, physiology and protein profile of Lolium perenne (Perennial Ryegrass) and Trifolium hybridum (Alsike Clover)    UBC SEEDS RESEARCH PROJECT  YINUO (YVETTE) GUO            June 1, 2012 Research Supervisor: DR. SANTOKH SINGH  Department of Botany Faculty of Science University of British Columbia   3  	Abstract 	 Roadside	plants	are	exposed	to	heavy	metal	ions	every	year	in	the	Lower	Mainland.	On	an	average	rainy	day,	roadside	groundwater	may	contain	potentially	toxic	cadmium	concentrations	as	well	as	other	heavy	metals.	Does	this	pose	severe	problems	for	plant	growth	and	survival?	The	current	study	investigated	the	physiological	symptoms	of	Ryegrass	(L.	perenne)	and	Clover	(T.	hybridum)	in	response	to	cadmium	stress.	Using	plate	culturing	and	hydroponic	growth	medium,	we	found	that	both	plant	species	in	high	cadmium	concentration	(1000µM)	experienced	1)	reduction	in	the	morphological	development	of	seedling	and	young	plants,	and	2)	reduction	in	metabolism	(e.g.	photosynthesis	rate).	These	findings	shed	light	on	the	tolerance	limits	of	roadside	plants	and	their	sustainable	growth	in	heavy‐metal	prone	environments.			Introduction  One	of	the	recourses	of	industrialization	is	the	effect	of	chemical	leaching	on	the	environment.	Mining	processes	can	be	a	major	source	of	mineral	leaching,	which	not	only	can	significantly	affect	the	chemical	and	structural	composition	of	the	environment	impacted	but	also	the	animals	and	plants	that	reside	there(Laws,	2000).	Mining	can	very	often	produce	environmental	toxins	such	as	cadmium	(Cd2+),	iron	(Fe),	zinc	(Zn2+),	lead	(Pb2+)	and	copper	(Cu2+	or	Cu4+)	at	a	concentration	suspected	to	be	intolerable	to	many	species	(Laws,	2000;	EPA	2000).	In	humans,	Cd2+	is	not	easily	degraded,	long‐term	exposure	to	which	can	cause	kidney	to	malfunction,	adversely	affect	calcium	metabolism	and	bone	formation,	e.g.	osteoporosis	(Laws,	2000).	For	the	present	study,	the	focus	was	to	determine	at	different	concentrations	of	Cd2+	and	different	lengths	of	exposure	whether	certain	plant	species	commonly	grown	on	the	BC	roadside	exhibit	1)	tolerance	reflected	in	morphology,	transpiration	and	photosynthesis	rates,	and	to	identify	2)	proteins	that	may	be	highly	expressed	as	a	function	of	toxin	tolerance.	It	was	thought	that	due	to	the	rarity	of	cadmium	in	the	natural	environment,	organisms	would	not	be	generally	adapted	to	high	Cd2+	concentrations,	e.g.	no	more	than	1‐10μM	(EPA,	2000).	However,	a	study	on	 4  tumbleweed	Cd2+	tolerance	by	de	la	Rosa	et	al	(2005)	led	to	the	hypothesis	that	some	plants	may	be	fairly	resistant	even	at	higher	concentrations.	The	hope	is	that	if	tolerant	species	are	found,	they	can	be	favoured	to	grow	at	industrial	sites.	Moreover,	species	that	tolerate	internally	high	amounts	of	cadmium	have	the	potential	to	act	as	a	buffering	zone	to	prevent	chemical	leaching	to	nearby	watersheds	(Laws,	2000).	Therefore,	the	potential	exists	for	some	plants	to	play	important	roles	in	bioremediations	especially	considering	the	growing	demand	for	zinc,	which	when	mined	yields	releases	cadmium	as	a	byproduct	that	accounts	for	most	of	the	cadmium	emitted	to	date	(Laws,	2000;	EPA	2000).		Methods and material  I. Plant	species	selection	(see	Figure	1)	Seeds	of	thirteen	(13)	common	roadside	grown	plant	species	near	BC	highways	were	incubated	in	separate	petri	dishes	(n=15)	on	filter	paper	soaked	in	1.5	mL	of	water	or	various	diluted	concentrations	of	cadmium	in	the	form	of	cadmium	chloride.	After	sealing	in	the	moisture	with	parafilm,	one	set	of	the	seeds	were	incubated	in	the	dark	and	another	identical	set	in	the	light	at	constant	room	temperature	for	2‐3	days,	the	normal	time	found	for	germination	of	these	seeds.	The	morphology	(colour,	texture)	of	the	seedlings	was	noted,	in	addition	to	the	roots	and	shoot	lengths	of	each	seedling	being	measured	using	a	computer	tracing	program.	The	results	for	each	petri	dish	were	averaged	from	the	specimen	that	germinated.	With	the	water	treatment	as	a	control,	the	seedlings	that	showed	little/no	change	at	high	concentrations	of	cadmium	were	then	selected	for	the	next	step.		 5  II. From	the	step	I,	it	was	determined	that	Clover	and	Ryegrass	were	the	most	tolerant	among	the	eleven	other	commonly	grown	plant	species	along	BC	highways.	It	was	now	to	determine	at	what	concentration	and	the	length	of	exposure	to	cadmium	do	seedlings	become	intolerant	along	with	the	accompanying	morphological	and	physiological	characteristics	they	exhibit.			Plant	seedlings	(n>3)	that	germinated	successfully	at	high	Cd2+	concentration	were	transferred	from	the	petri	dishes	to	small,	short	boxes	filled	with	either	diluted	nutrient	solutions	(see	lab	book	for	a	full	recipe)	or	nutrient	solutions	mixed	with	various	concentrations	of	Cd2+.	Since	hydroponic	growth	of	Ryegrass	(started	before	Clover)	revealed	no	visible	effect	from	exposure	to	Cd2+	concentration	below	1M,	it	was	assumed	that	10M	Cd2+	would	be	a	reasonable	minimum	concentration	for	Clover.	Plastic	netting	with	Styrofoam	edges	was	used	to	support	the	seedlings	upright	while	their	roots	were	exposed	to	the	medium	solution.	All	media	were	aerated	using	a	handmade	bubbling	device	with	a	valve	controlled	air	source	(see	Figure	2).	The	incubation	continued	as	long	as	the	seedlings	sustained	a	healthy	appearance.	Leaf	samples	were	frozen	for	protein	analysis	when	changes	were	observed	and	when	possible,	photosynthesis	and	transpiration	rates	were	taken	with	the	LiCOR	machine	as	measures	of	the	overall	physiological	performance	of	each	replicate.	Technically	speaking,	LiCOR	measures	and	records	the	CO2	concentration	and	water	loss,	which	were	later	converted	to	photosynthesis	rate	and	transpiration	rate	with	a	computer	program.			 6  III. Protein	analysis	With	the	frozen	leaf	sample	from	step	II,	standard	SDS	gel	electrophoresis	were	performed	to	detect	if	major	protein	groups	(e.g.	RUBISCO/ATP	Synthase	and	Light	Harvesting	complexes)	increase	or	decrease	with	higher	cadmium	concentration.	These	groups	can	give	us	an	indication	of	whether	cadmium	inhibits	or	stimulates	the	major	biochemical	functions	of	the	plants.	With	proteins	migrated	onto	the	SDS	gel,	Western	blotting	using	the	heat	shock	protein	group	Dehydrin	(AgriSara	™	enzyme)	at	a	ratio	of	1:5000	(vol/vol	protein	versus	5‐10	mL	of	milk	protein	blotting	solution)	was	performed	as	a	biochemical	indicator	of	stress.	It	is	important	to	note	that	the	biochemical	cues	do	not	usually	explain	all	macroscopic	changes	because	there	are	many,	interacting	microscopic	processes.		Limitations	to	the	methods	It	was	difficult	to	keep	the	conditions	for	hydroponic	growth	consistent	throughout	the	experiment,	especially	considering	the	unwanted	growth	of	algae	in	the	nutrient	solution	and	the	periodic	evaporation	of	medium	through	the	atmosphere	via	aeration.	These	difficulties	could	have	led	to	non‐negligible	nutrient	deficiencies,	confounding	the	results	from	both	the	controls	and	the	treatments.	For	the	Western	Blot,	the	samples	were	only	assayed	with	Dehydrin	though	there	could	be	other	proteins	up‐	or	down‐regulated	as	a	result	of	cadmium	stress.	The	rationale	for	testing	with	Dehydrin	was	that	this	family	of	proteins	has	been	linked	to	stress	symptoms	and	heavy	metal	detoxification	in	plants	(Zhang	et	al,	2006).	Jonak	et	al	(2004)	have	demonstrated	the	upregulation	of	certain	mitogen‐activated	protein	kinases	aiding	in	stress	response	in	alfalfa	exposed	to	high	[Cd2+].	However,	another	reason	for	studying	 7  Dehydrin	is	that	it	is	generally	less	characterized	than	other	protein	groups	known	for	their	roles	in	plant	metabolism.	Due	to	time	constraints,	the	hydroponics	experiment	was	not	able	to	be	replicated.	Roots,	shoots	and	metabolic	rates	sampled	at	each	sampling	time	were	based	on	one	replicate	from	each	treatment.	The	results	from	the	current	study	were	therefore	qualitative	only	and	not	statistically	robust.		Results & Discussion 	Perennial	Ryegrass	(Lolium	perenne)		Morphology	After	2	days	of	hydroponic	growth	with	various	concentrations	of	cadmium,	the	Ryegrass	seemed	to	exhibit	no	changes	in	root	or	shoot	length	relative	to	the	control	(Fig.	3).	On	the	8th	day,	however,	plants	treated	at	1000	uM	began	to	show	stunted	growth	(Fig.	3),	yellowing	of	the	shoots	and	lack	of	turgor	as	compared	to	the	control	and	to	other	treatments	at	lower	Cd2+	concentrations.	These	symptoms	are	typical	of	plants	under	heavy	metal	stress,	which	indicates	an	interference	with	the	normal	physiological	functions	(Zhu	et	al,	2005)	Physiology	Photosynthesis	rates	–	on	day	2,	there	was	a	ten‐fold	decrease	in	photosynthesis	rate	from	the	control	for	the	treatment	at	1000	uM	Cd2+	(Fig.	4).	A	similar	decreasing	pattern	was	expected	on	day	7	but	this	was	not	reflected	in	the	graph.	We	suspected	that	the	aeration	mechanism	in	the	hydroponics	was	not	consistent	throughout	the	experiment	and	thus	occasional	lack	of	O2	in	the	medium	could	have	induced	stress	on	some	of	the	 8  plants,	including	the	control,	resulting	in	photosynthesis	rates	generally	being	low	on	day	7	compared	to	day	2.	Decrease	in	photosynthesis	was	expected	because	plants	under	ionic	stress	tend	to	produce	fewer	stomata	due	to	inhibition	of	guard	cell	precursors	(Zhu	et	al,	2005),	leading	to	decreased	intake	of	CO2,	the	substrate	for	photosynthesis.		Transpiration	rates	–	on	day	2,	there	was	a	two‐three	fold	increase	in	transpiration	in	the	1‐100uM	Cd2+	treatments	compared	to	the	control	(Fig.	5).	Similarly	on	day	7,	there	was	a	two‐fold	increase	in	transpiration	rate	at	0.1uM	and	an	even	larger	increase	(five	fold)	at	1000uM.	This	finding	is	contrary	to	that	of	Leita	et	al	(1995),	who	cited	attributed	to	lower	transpiration	in	soybean	plants	to	stomatal	closure.	However,	this	may	be	due	to	different	plant	used	since	Zhu	et	al	(2005)	found	larger	stomatal	aperture	in	Brassica	juncea,	which	if	found	to	be	the	case	in	this	experiment,	would	explain	in	part	the	heightened	transpiration	in	Ryegrass.		Protein	analysis	For	all	but	one	treatment	(1000	μM),	it	was	possible	to	detect	distinct,	near‐uniform	bands	for	Rubisco	and	ATP	synthase	and	light‐harvesting	complexes	(LHC)	I	&	II	on	the	stained	gel	(Fig.	6).	This	could	explain	the	lack	of	morphological	changes	in	treatments	at	Cd2+	concentrations	under	1000	μM,	but	generally	do	not	reflect	the	changes	in	physiology,	i.e.	decrease	in	photosynthesis	and	rise	in	transpiration	with	increasing	cadmium	concentrations,	the	exception	being	the	“smeared”	or	degraded	gel	pattern	of	plants	that	became	increasingly	stressed	by	high	Cd2+	as	the	treatments	continued	(i.e.	undergoing	proteolysis).	However,	the	lack	of	biochemical	signals	may	instead	suggest	that	cadmium	does	not	directly	antagonize	physiological	functions	directly,	but	rather	more	directly	through	the	closure	of	stomata,	which	lead	indirectly	to	the	observed	physiological	changes	 9  (Zhu	et	al,	2005).	Dehydrin	was	present	in	various	amounts	in	the	controls	and	treatments	(Fig.	7),	which	indicated	that	experimental	conditions	in	general	were	not	properly	controlled	(e.g.	through	unequally	sustained	air	pressure	and	algal	growth	on	nutrient	solution	past	day	2)	and	could	have	resulted	in	plants	functioning	out	of	their	normal	range.		Alsike	Clover	(Trifolium	hybridum)		Morphology	It	was	possible	to	visualize	toxic	effects	of	cadmium	in	the	1000uM	Cd2+	treated	clover	seedlings	on	day	2.	Notably,	the	plants	showed	stunted	growth,	yellowing	and	crispy	texture	on	most	of	its	leaves	as	well	as	lack	of	turgor	throughout	the	plant	(Figs.	8	and	12).	By	the	7th	day,	the	100uM	treatment	had	also	begun	to	exhibit	the	same	effects	(Fig.	8).	Compared	to	Ryegrass,	Clover	appeared	to	be	more	vulnerable	to	the	heavy	metal.	This	could	be	partially	due	to	the	large	surface	area	of	clover	leaves	which	makes	the	plant	more	susceptible	to	functional	decline	which	leads	to	more	visible	signs	of	stress.	Physiology		Photosynthesis	rates	–	a	100%	drop	in	photosynthesis	from	the	control	treatment	in	1000uM	Cd2+	treated	clover	on	day	2	was	found	(Fig.	9).	Though	the	data	are	not	shown	here,	the	decreasing	trend	showed	signs	of	continuation	on	day	7.	Transpiration	rates	‐	there	was	a	slight	to	negligible	decline	in	transpiration	of	all	treatments	on	day	2	(Fig.	10)	and	cadmium	seem	to	have	resulted	in	more	toxic	effects	on	Clover	as	more	time	passed	(day	7	data	incomplete,	not	shown).	Altogether,	the	rates	for	Clover	were	much	lower	than	those	of	the	Ryegrass,	which	is	counter‐intuitive	since	the	large	surface	area	should	favour	higher	metabolic	rates.	Aside	from	potentially	stressful	conditions	caused	by	the	algae	in	the	Clover	hydroponics,	we	 10  suspected	there	may	have	been	errors	in	calibration	of	the	LiCOR,	which	tended	to	underestimate	rates	as	the	CO2	filters	became	congested	over	time.	This	would	have	caused	an	overall	lack	of	physiological	stress	signal	expressed.	This	is	another	important	reason	that	the	experiment	needs	to	be	replicated	in	the	future	to	confirm	the	current	findings.		Protein	analysis	Unlike	the	Ryegrass,	even	though	there	was	no	appreciable	difference	in	protein	patterns	between	the	control	and	the	treatments	from	0.1‐100μM	Cd2+	in	both	days	2	and	,	the	1000	uM	treatment	appeared	to	be	affected	since	the	gel	lane	had	a	smear	pattern	that	is	usually	accompanied	by	protein	degradation	of	a	dying	plant,	which	corresponds	to	morphological	symptoms	of	stress	described	earlier.	While	the	general	protein	pattern	did	not	indicate	a	difference	between	the	control	and	a	stressed	cadmium	treatment,	Dehydrin	was	found	to	be	highly	expressed	in	the	1000μM	treatment,	suggesting	that	part	of	the	plant’s	stress	response	can	be	traced	to	the	upregulation	of	Dehydrin.	On	day	7,	however,	only	a	smear	of	Dehydrin	was	found	in	the	100μM	sample,	possibly	resulting	from	proteolysis.			Synthesis	It	appears	that	Ryegrass	(L.	perenne)	is	generally	very	tolerant	of	high	Cd2+	concentrations,	only	showing	major	signs	of	stress	at	1000μM,	a	concentration	that	is	two	to	four	magnitudes	higher	than	what	is	generally	found	at	industrial	sites	(Peralta‐Videa	et	al,	2009;	EPA,	2000).	Clover	(T.	hybridum)	exhibited	fairly	high	tolerance	to	cadmium,	though	not	as	much	as	L.	perenne	in	terms	of	morphology	and	photosynthesis.	The	differences	in	stress	responses,	especially	in	morphology	between	the	two	plants	may	be	attributed	to	differential	uptake	by	leaves	(de	la	Rosa,	2005),	possibly	more	controlled	in	 11  Ryegrass	than	it	is	in	Clover.	Or,	as	was	mentioned	earlier,	the	morphology	of	the	plants	could	also	be	a	factor	by	relating	our	results	to	Haghiri	(1973)	where	the	relative	sensitivity	of	soybean	(a	trifoliate)	to	Cd2+	was	found	to	be	higher	than	in	wheat	(a	grass	species).	T.	hybridum	appears	to	be	resistant	over	extended	periods	of	time	at	10uM	Cd2+but	not	at	higher	concentrations	(Fig.	12).	T.	hybridum’s	response	to	cadmium	is	also	time	and	concentration	dependent,	which	was	reflected	in	the	presence	of	Dehydrin.	Where	Dehydrin	was	not	detected	(due	to	absence	of	expression	or	to	proteolysis)	in	Ryegrass,	it	was	in	Clover,	suggesting	that	Dehydrin	can	play	a	significant	in	mediating	the	stress	response	of	these	plants	to	cadmium.	Among	the	very	few	studies	available	examining	the	relationship	between	Dehydrin	and	heavy‐metal	stress,	Zhang	et	al	(2005)	and	Xu	et	al	(2008)	have	proposed	that	the	found	Dehydrin‐like	proteins	(two	specific	proteins	cloned	by	the	authors)	seek	out	oxidative	derivatives	harmful	to	the	plant	that	is	under	the	stress	of	heavy	metals.	Kruger	et	al	(2002,	cited	in	Xu	et	al,	2008),	however,	found	heavy‐metal	binding	Dehydrins.	There	appears	to	be	diverse	but	important	roles	of	Dehydrin	in	heavy‐metal	tolerance	of	plants.			  12  Figures			Figure	1.	Plate	culturing	experiment	with	Ryegrass	(above)	and	Clover	(below)	seedlings	incubated	in	various	concentrations	of	Cd2+	in	light	and	dark	conditions.		Figure	2.	Hydroponics	setup	of	Clover	seedlings	grown	at	various	concentrations	of	Cd2+.		Figure	3.	Ryegrass	morphology	(roots	and	shoots)	subject	to	2	days	(left)	and	8	days	(right)	of	cadmium	treatments	(concentrations	indicated	above).	 13  -0.200.20.40.60.81Control 0.1µM 1µM 10µM 100µM 1000µMTreatment concentration (µM of Cd)Photosynthesis rate(µmol/m2/s)Day 2 Day 7	Figure	4.	Photosynthesis	rates	of	Ryegrass	treated	at	various	concentrations	on	days	2	and	7.		00.511.522.53Control 0.1µM 1µM 10µM 100µM 1000µMTreatment concentration (µM of Cd)Transpiration rate (gH20/m2/min)Day 2 Day 7	Figure	5.	Transpiration	rates	of	Ryegrass	treated	at	various	concentrations	on	days	2	and	7.		Figure	6.	Day	8	Ryegrass	SDS	electrophoresis	gel	showing	degradative	protein	profile	at	1000μM	Cd2+	 14  	 	Figure	7.	Day	2	Ryegrass	Dehydrin	profile	(Western	blotting)	at	various	Cd2+	concentrations	and	accompanying	morphology.		Figure	8.	Clover	morphology	(roots	and	shoots)	subject	to	2	days	(left)	and	7	days	(right)	of	cadmium	treatments	(concentrations	indicated	above).	-0.00500.0050.010.0150.020.0250.03Control 10µM 100µM 1000µMTreatment concentration (µM Cd)Photosynthesis rate (µmol/m2/s)Day 2	Figure	9.	Photosynthesis	rate	of	Clover	treated	at	various	concentrations	on	day	2;	day	7	data	not	available.	 15  00.0050.010.0150.020.0250.030.0350.040.045Control 10µM 100µM 1000µMTreatment concentration (µM of Cd)Transpiration rate (gH2O/m2/min)Day 2	Figure	10.	Transpiration	rate	of	Clover	treated	at	various	concentrations	on	day	2;	day	7	data	not	available.		 		Figure	11.	Top:	Day	2	and	7	SDS	electrophoresis	gel	(left	to	right)	of	Clover	treated	at	various	concentrations	of	Cd2+.	Arrows	indicate	protein	groups	specified	in	Figure	#.	Bottom:	accompanying	Dehydrin	profile.		 	Figure	12.	Clover	hydroponics	revealing	morphological	differences	between	different	treatments	of	Cd2+	(control	at	top	left,	[Cd2+]	increase	is	seen	counterclockwise.				 16  Conclusions and future direction  The	highest	concentrations	detected	in	both	Clover	and	Ryegrass	in	this	study	are	more	elevated	than	the	typical	Cd2+	levels	reported	at	industrial	sites,	usually	not	more	than	the	10μM	range	(Peralta‐Videa,	2009;	EPA,	2000),	and	near	highways	(usually	0.04‐0.1µM).	Considering	the	results	of	this	study,	it	is	reasonably	likely	that	both	Clover	and	Ryegrass	have	the	ability	to	germinate	and	survive	for	some	time	at	higher	than	usual	concentration.	It	is	worth	noting	that	the	highest	concentrations	of	heavy	metal	usually	occur	in	stormy	seasons	when	the	mineral	leachates	are	the	most	easily	flushed	out	intothe	surrounding	biological	environment	but	this	is	expected	to	last	a	few	days	(Laws,	2000).	In	the	case	of	Clover,	tolerance	to	high	but	short‐lived	flushes	of	Cd2+	seems	quite	possible.	All	in	all,	heavy	metal	resistance	demonstrates	in	both	Clover	and	Ryegrass	would	make	them	competitive	in	Cd2+‐prone	environment	in	addition	to	their	being	fast	germinating	species.		 It	is	still	difficult	to	extrapolate	our	result	to	the	natural	soil	environment	where	many	factors	can	impact	cadmium	toxicity.	For	instance,	it	is	possible	that	cadmium	ions	(positively	charged)	are	adsorbed	by	the	negatively	charged	soil	particles,	which	makes	Cd2+	more	bioavailable	to	plants.	In	addition,	some	studies	have	found	cadmium	to	be	co‐transported	with	iron	and	zinc	under	certain	circumstances,	which	leads	to	increased	Cd2+	uptake	(Peralta‐Videa	et	al,	2009;	de	la	Rosa	et	al,	2005).	This	study	did	not	investigate	whether	cadmium	absorption	by	Clover	and	Ryegrass	roots	was	indeed	happening	and	if	so,	whether	either/both	plants	have	a	threshold	for	its	intake.	The	strong	tolerance	of	Ryegrass	to	high	Cd2+	concentration	suggest	that	cadmium	may	be	taken	up	only	past	a	certain	concentration	or	it	may	have	a	very	high	threshold	of	accumulation.	By	contrast,	Clover	exhibits	limited	tolerance	to	cadmium,	suggesting	a	Cd2+	is	readily	metabolized.	The	 17  possibilities	of	Cd2+	binding	and	compartmentalization	cannot	be	ruled	out	either.	These	considerations	can	be	helpful	for	future	studies,	which	may	be	interested	in	establishing	whether	certain	plant	species	as	particularly	strong	absorbants	for	cadmium	while	being	resistant	to	it,	thus	capable	of	acting	as	buffer	zones	to	prevent	heavy	metal	runoffs	to	watersheds.	From	the	viewpoint	of	physiological	studies,	it	would	be	insightful	to	determine	the	fate	of	the	cadmium	ion	once	in	the	plant,	i.e.	whether	it	is	transported	to	different	organs,	stored	or	metabolized.	A	relevant	implication	of	this	is	that	studies	have	shown	Cd2+	to	be	selectively	accumulated	in	different	plant	organs,	which	while	not	phytotoxic	at	this	level,	can	be	bioaccumulated	or	pose	significant	health	risks	to	humans	and	mammals	(Peralta‐Videa	et	al,	2009,	EPA	2000;	Haghiri,	1973).	 Acknowledgments   I	would	like	to	thank	Dr.	Santokh	Singh	for	advice	and	support	throughout	this	project,	and	Mr.	Jarnail	Mahroke	and	for	technical	help.		Thank	you	to	Brenda	Sawada	from	UBC	SEEDS	for	coordinating	this	project,	and	to	Kelly	Coulson	for	supplying	seeds	of	various	grasses.   18  References 	De	la	Rosa,	G.,	Martinez‐Martinez,	A.,	Pelayo,	H.,	Peralta‐Videa,	J.	R.,	Sanchez‐Salcido,	B.	And		Gardea‐Torresdey,	J.	L.	2005.	Production	of	low‐molecular	weight	thiols	as	a	response	to		cadmium	uptake	by	tumbleweed	(Salsola	kali).	Plant	Physiology	and	Biochemistry	43:		491‐498.		EPA.	2000.	2000	update	of	ambient	water	quality	criteria	for	cadmium.	Prepared	by	Great	Lakes		Environmental	Centre.	For	the	U.S.	Environmental	Protection	Agency	Office	of	Water.		Haghiri,	F.	1973.	Cadmium	Uptake	by	Plants.	J.	Environ.	Qual	2	:	93‐96.		Jonak,	C.,	Nakagami,	H.	and	Hirt,	H.	2004.	Heavy	Metal	Stress.	Activation	of	Distinct		Mitogen‐Activated	Protein	Kinase	Pathways	by	Copper	and	Cadmium.	Plant	Physiology	136:	3276‐3283.	Laws,	A.	2000.	Aquatic	Pollution.	3rd	Edition.	USA:	John	Wiley	&	Sons	Inc.		Leita,	L.,	Marchiol,	L.,	Martin,	M.,	Peressotti,	A.,	Vedove,	G.	D.	and	Zerbi,	G.	1995.	Transpiration		Dynamics	in	Cadmium‐Treated	Soybean.	(Glycine	max	L.)	Plants.	Agronomy	and	Crop		Science	175:	153‐156.	Peralta‐Videa,	J.	R.,	Lopez,	M.	L.,	Narayan,	M.,	Saupe,	G.	and	Gardea‐Torresdey,	J.	2009.	The		biochemistry	of	environmental	heavy	metal	uptake	by	plants:	Implications	for	the	food		chain.	Biochemistry	&	Cell	Biology	41:	1665‐1677.	Xu,	J.,	Zhang,	Y.	X.,	Wei,	W.,	Han,	L.,	Guan,	Z.	Q.,	Wang,	Z.	and	Chai,	T.	Y.	2008.	BjDHNs	Confer		Heavy‐metal	Tolerance	in	Plants.	Mol.	Biotechnol	38:	91‐98.	Zhang,	Y.,	Li,	J.,	Yu,	F.,	Cong,	L.,	Wang,	L.,	Burkard,	G.	and	Chai,	T.	2006.	Cloning	and	Expression		Analysis	of	Skn‐Type	Dehydrin	Gene	From	Bean	in	Response	to	Heavy	Metals.	Molecular		Biotechnology	32:	205‐217.	Zhu,	R.,	Macfie,	S.	H.	and	Ding,	Z.	2005.	Cadmium‐induced	plant	stress	investigated	by	scanning		electrochemical	microscopy.	Journal	of	Experimental	Botany	56:	2831‐2838.			

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