Effect ofPseudomonas syringaepv.tagetisInfection on Sunflower Leaf Photosynthetic and Ascorbic Acid Relations1

Studies were conducted to assess the role of photosynthesis and dark respiration in maintaining daily foliar ascorbate levels. Seven‐day‐old sunflower plants were inoculated with a tagetitoxin‐producing or nontoxigenic (control) strain ofPseudomonas syringaepv.tagetis. After 10 d, tagetitoxin‐affected leaves were so severely chlorotic that they were white. Tagetitoxin‐affected leaves had 99% less chlorophyll per unit fresh mass than control plant leaves. Electron microscopy of sections of chlorotic (toxin‐affected) sunflower leaf cells revealed that leaf palisade and spongy parenchyma cells possessed few chloroplasts and that those present contained disorganized thylakoids and grana and no apparent starch grains. Rates of photosynthetic CO2assimilation per unit leaf area in toxin‐affected leaves were significantly lower than in control leaves. Compared to leaves of the control plants, the leaves of toxin‐affected plants had significantly less free glucose, and less glucose was incorporated into sucrose and starch during the day. The parenchymatous cells of toxin‐affected leaves had well‐formed mitochondria with distinct internal membrane structure; however, dark respiration of the chlorotic sections of those leaves, monitored as dark CO2evolution, was often significantly lower than in control leaves. Regardless of the time of day that leaves were sampled, the chlorotic leaves of toxin‐affected plants had significantly lower levels of ascorbic acid than was detected in the control plant leaves. Thus, the study implied that carbohydrate products of photosynthetic CO2assimilation such as glucose along with one or more functions of mitochondrial respiration were required for the synthesis of normal levels of ascorbic acid in green leaves. Further, the chlorotic sections of leaves of the toxin‐affected sunflower plants often displayed the ability to maintain a range of 64% to 90% of the total foliar ascorbic acid present in the reduced form, even when the toxin‐affected sunflower plants were exposed to elevated ozone levels (58 μL L−1air) for 16 d. This observation indicates that the recycling of DHA to ASC by the ascorbate‐glutathione cycle enzyme system remains functional in toxin‐affected leaves when exposed to ozone‐mediated oxidative stress.