The contrasting Fe2+ and Fe3`chelating properties of the synthetic chelators ethylenediaminedi(o-hydroxyphenylacetate) (EDDHA) and 4, 7-di(4-phenylsulfonate)-1,

Sunimarv. Stemii exu(lates of sunflower, soybean, cucunmber, and( tomiiato were analyzed for citrate and iron content. Generally, the lowest iron treatments given at decapitation were associated with lowest citrate levels in the exudate an(l intermediate treatments witlh the highest citrate levels. Citrate reaohed 6.2 > l()-4 M in sunlflower exudate. 7.3 X 10-4 M in sovbean, 9.4 X 10-4 M in cucuimber, an(l 1.8 x 10-4 AI in tomato. Some of these values represent a (loublling or tripl-inig of citrate whenl comlpare(d to the lowest ones obtaine(l. High Fe depresse(d the citrate content oif exu(late in some cases.As iron was raisec! in the nutrienit, the inicreases of iron in the exudate wvere proportionately greater than those of citrate. The highest Fe treatnments led to exaggerated uptake of iron by suniflower. In some cases iron was tranislocate(d in miiolar excess of citrate. A time-response experiment with tomato showed a rapid flooding of iron into the roots but a relativelv slows release of iron into xylem exudate.Similar electrophoretic patterns of iron were obtained despite changes in citrate anld iron with time. Exudates from sunflowver, cucumber. and tomato gave iron concenitrations in the positioni of iron-citrate.In the previous paper (2) it was shown that precul,ture had proniounced effects oln the iron and citrate content of sunflower exudate. But despite these

,S11 oiIiiiNV. Plant culture. exudlate sami-pling, all(l analytical metlho(ds designle(d to ascertaim the formi of iron translocated are presented.Restorati,on of ireni to sunifloxwer plants precultured at (liffereint Fe levels resulted in exu(late iroin conicenitratioils raliginig fromil 0.2 to 31 X 10-°Nt. Citrate was froml1 3 to 89 X 10 0--WI. Iron and citrate were highest in exudates from iron-leficient plants. In the present study, iron was supplied to sunflower roots, and exudates were used to sample translocated ironi. The quantities of iron inivolvedl in other phases (root absorption and accumulation ) have been coInsi(lere(l mostly in relation to translocation.Restorati,on of iron to plants precul,tured at different ironi levels resu;lts in wide variations of this nietal in the leaves (8,14) and stemii exudates (3,14, 15). \NVhen iron is supplied as a synthetic chelate, the roots of sutf'lower (13) and soybean (14) absorb the metal but leave Imlost of the chelating agent in the nutrient rediuni.In the present study, sunflower plants precultured at different iron levels were given similar treatments at (lecapitatioin. Assays were carried out to determine quanitities of citrate and iron in the exudates. The primary objective was to determine the form of iron translocated. A brief report of this work is published (11).In later experiments ( 12) the iron treatments were reversed. Planits were preculture(d similarly but given (lifferent iron levels at decapitation. The purpose was to (letermine the effect of iron supplied in the exudlation period on the citrate and iron content of the exudates. Attention has also been given to the changes in conicentrationi of citrate and(I iron that occtir (luring exudationi. Materials and MethodsA-iutr-ienit S'olutioui. h'lle stan(lardl niutrienit wvas a modified Steinberg solution (10. 14). Elements were approximately onie-fifth the Steinberg conicentratioll. excelpt for lower phosphorus. TI'he nutrienit conitaine(l (mn.g/li-ter') 51 Ca. 6.6 Mig. 59