XATUHAL INHIBITORS OF THE HILL REACTION 283 



Discussion 



Blinks : You are inclined to rule out effects due to osmotic properties of the 



plastid? 



Clendenning : In seli'cting species for this study, \vc included chloroplast sources 

 of \vi(k'l\- (litfcrciit osmotic pressures, ranging from aciuatic plants to tree leaves 

 containing over 50% solids. Over this extreme range of cellular osmotic pressures, 

 the chloroplasts' capacity for water i)hotolysis in vitro was consistently improved 

 by the use of SO^c Carbowax 4U0U. 



Bishop : What about the relative stability of chloroplasts at room temperature 

 in sucrose and Carbowax solutions? 



Clendenning : In our experience, chloroplasts isolated and tested in 30% Carbo- 

 wax 4U00 at limiting chloroplast concentrations have shown higher photochemical 

 activity than in 0.5 M sucrose, this effect of the Carbowax becoming stronger as 

 the reaction temperature is raised. I think that the main way in which the Carbo- 

 wax acts is by stabilizing the chloroplasts after isolation, and that it does so by 

 surrounding the plastids with precipitated cytoplasm. It should be pointed out 

 that 30% Carbowax 4000 precipitates egg albumin from aqueous solutions. 



Granick: I was interested in why the chloroplast can't be preserved with su- 

 crose, if this really is an osmotic effect. Don't you think it may also be an effect of 

 the size of the molecules? 



Blinks : Precisely that. Sucrose penetrates and the polyethylene glycol does not. 

 A redistribution of the phycoerythrin occurs with red algal plastids suspended in 

 sucrose solutions but not in concentrated polyethylene glycol solutions. 



Arnon : In working with particles such as mitochondria or chloroplasts what 

 criterion should w^e use for distinguishing between physiological and nonphysio- 

 logical substances, excepting substances such as versene which are admittedly non- 

 physiological? Dr. Jacobs included magnesium in the nonphysiological ones. 

 WTiat is your criterion for saying this? 



Jacobs : A good set of criteria is hard to find. For instance, in the intact system 

 there is no magnesium requirement. Under certain conditions it will show up. 

 Since magnesium is such a good physiological substance, it is assumed that it is 

 an actual physiological requirement. But as far as these other things go— like, say, 

 manganese — unfortunately, we have no criteria. 



Brown : Operationally, there seems to be no distinction that one can make. 



Arnon : That is the point. 



Gaffron : Is sodium chloride not a case in point? A plant grown without chloride 

 preserves its capacity to photosynthesize and yet it is known that the chloroplast 

 does better if sodium chloride is added. 



Arnon: Yes, I am very glad that we were cautious in stating that the non- 

 essentiality of chloride for photosynthesis was based on the assumption that chlo- 

 ride is not essential for intact green cells. Recent evidence by my colleagues at 

 Berkeley suggests that chloride may have to be added to the list of essential 

 elements for higher plants. Thus the original proposal of Warburg that sodium 

 chloride may be a cofactor in photosynthesis is not ruled out by our argument 

 made at that time. 



