Sumnmiiary. Ammonium toxicity resulted in morphological modifications of tomato leaf chloroplasts. The chloroplasts, which are normally flattened around the protoplast periphery, became ellipsoidally rounded and dispersed through the protoplasm. The first apparent effect of plastid degradation was development of many vesicles from the fretwork. Later the grana lamellae swelled, and some disappeared. Eventually, distinct grana could not be detected.Ammonium accumulation, chlorophyll loss, and photosynthetic decrease occurred simultaneously. Initial changes in these processes preceded the detection of modifications of fine structure; however, each continued with further breakdown of the chloroplasts.In many higher plants prolonged application of ammonium as a source of nitrogen leads to serious physiological and morphological -disorders resulting in chlorosis, restricted growth, and in some cases death (4,5,10,29,30 morpho'ogical changes. The symptoms of ammoniumii toxicity suggest that morphological changes mighlt occur in the chloroplasts. Chlorosis of plants treatecl with ammonium has been reported to occur in conjunct.on with an imbalance between the soluble and insoluble nitrogen compounds (4). Chlorophyll synthesis has been shown to be dependent on protesn synthesis (16). Within the leaves of higher plants, the majority of proteins are chloroplastic (19. 35) and exist in a state of turnover. The (proteins of chloroplasts are found in the stroma and in the co;viplex system of lamellar memnbranes of lipoprotein (31). The organization of the lamel]ar network of the chloroplast is dependent on the cellular environment of the leaf (7,14,20,24), particularly the nutrient composition. It was felt that an ultrastructural examination of the clhloroplasts of plants treated with ammonium would provide an indication of the morphological aspects of ammonium toxicity and permit further correlation between morphological features and physiological symptoms of ammonium toxicity. Materials and MethodsTomato plants (Lycopcrsicon escdlenthm Mill., cv. Heinz 1350) were chosen for this study because they deve'op ammloniumn toxicity symptoms rapidly and sequentaliv. Seecds were sown in flats of soil, and the seedlings were grown for 5)weeks in a greenhouse. The vcung plants were then tranisplanted into 8-inch l'astic pots containiing a 1:1 mixture of coarse and f:ne silica sand ancl were treated with Hoagland's.

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The relationship between water stress and rates of net photosynthesis, respiration, and transpiration was determined for four Abies species native to Canada. Net photosynthesis, after an initial optimum rate, declined in three phases as water stress increased, viz. a rapid linear rate of decline, a second more gradual reduction and, finally, a steady rate of zero net photosynthesis. Of the four species, photosynthesis of A. grandis was affected the least by water stress, whereas that of A. balsamea was affected the most. Respiration declined at about the same water stress as photosynthesis, but was only reduced to between 40–75% of the maximum rate, depending on the species. Transpiration declined at similar water stresses to those of photosynthesis and after an initial decline, continued at between 10–30% of the maximum rate at water stresses up to 35 bars.