33 2 Mineral Nutrition of Plants 



both the chloroplasts and the cytoplasmic fluid showed no chloride in 

 either, as would be expected in plants grown without chloride. The 

 chloroplasts, even without washing, showed only feeble oxygen evolu- 

 tion. Unlike the experience of Warburg and Liittgens, the addition of 

 the cytoplasmic fluid failed to reactivate them, but it was already noted 

 that our cytoplasmic fluid contained no chloride. On the other hand, 

 we fully substantiated the finding of Warburg and Liittgens that the 

 addition of chloride brought about the activation of chloroplasts, giving 

 us stoichiometric yields of oxygen in relation to the oxidant used. The 

 effect of chloride on the course of oxygen evolution by illuminated 

 chloroplasts is shown in Figure I. We also confirmed the findings of 

 these authors with regard to the influence of other anions on oxygen 

 evolution (Figure i). Bromide had an activating effect about equal with 

 chloride; nitrate and iodide were much less effective; and sulfate, phos- 

 phate, thiocyanate, and acetate were without effect. 



How should these results be interpreted? The intact plant is able to 

 carry on normal photosynthesis without chloride, as judged by its ex- 

 cellent growth despite the absence of this ion either in the nutrient 

 medium or in the leaf tissue. Yet when chloroplasts are isolated from 

 the same plant, they require chloride for the vigorous progress of the 

 photochemical reaction. One explanation would be that chloride acts 

 in the leaf as a micronutrient and that minute amounts of chloride 

 which would escape detection by the usual chemical analysis are never- 

 theless present in the nutrient medium as an impurity and find their 

 way to the leaf. This explanation, however, although it cannot be ruled 

 out entirely, is rendered unlikely by the data presented in Figure 2. 

 In this chart the rate of oxygen evolution by illuminated chloroplasts 

 (QoJ) 1S plotted against chloride concentration. It will be seen that, 

 whereas small additions of chloride brought about appreciable activa- 

 tion, a fairly high concentration, around 0.007 A/, * s required for full 

 activation. This is in agreement with the value of M/150 potassium 

 chloride reported by Warburg and Liittgens (§2) as necessary for full 

 activation in their experiments. Such relatively high concentrations of 

 chloride are not uncommon in soil-grown plants, but there is strong 

 evidence from these and numerous other experiments that plants can 

 make excellent growth without the presence of measurable amounts of 



