Low Temperature Treatments Induce an Increase in the Relative Content of Both Linolenic and {lambda}3-Hexadecenoic Acids in Thylakoid Membrane Phosphatidylglycerol of Squash Cotyledons

The effect of low temperatures on the fatty acid compositionof phosphatidylglycerol (PG) in thylakoid membranes, in particularon the ratios of nmol% 16:1(3t) (mg fresh weight)^–1 ofcotyledons and nmol 16:1(3t) (mg chlo rophyll)^–1 weremeasured during squash seedling growth. Plants were germinatedand grown for one day at 30°C then were either kept at 30°C(control plants) or trans ferred to low temperatures (18, 14or 10°C). When plant were transferred from 30°C to lowtemperatures, the increase in fresh weight was gradually limited.The lowe the temperature, the smaller was the fresh weight.In contrast, the relative content of 16:1(3t) and 18:3, as wella the ratios of nmol 16:1(3t) (mg chlorophyll)^–1 and mol%16:1(3t) (mg cotyledon fresh weight)^–1 increased indicatingthat the increase of fresh weight and chlorophyll was mor sensitiveto low temperature than PG desaturation in thyla-koid membranes.Furthermore, low temperatures inducei an increase in 16:1(3t)and 18:3 (the final products of PC synthesis) at the expenseof 16:0 and 18:1 (the initial products of PG synthesis). However,within a range of temperature from 10 to 18°C, the extentof these changes (nmol% of 18:3 or 16:1(3t) per day) was graduallylimited by lower temperatures. We therefore propose that lowtemperature inhibit both fatty acid synthesis and desaturationactivities. However, at low temperatures the fatty acid synthesisis likely to be more strongly inhibited than the desaturationactivities, thus explaining the observed increase in the relativecontent of PG-18:3 and PG-16:l(3t). Results an discussed interms of the mechanism which could be in volved in the metabolismof PG in squash cotyledons. (Received July 5, 1996; Accepted March 10, 1997)     

Effect of linolenate on photosynthesis by intact spinach chloroplasts II. Influence of preillumination of leaves on the inhibition of photosynthesis by linolenate

The inhibitory effect of linolenate on intact spinach chloroplastsdepends on the level of the internal pool of metabolites. Chloroplastsfrom preilluminated leaves or chloroplasts artificially loadedwith 3-phosphoglyceric acid required higher concentrations oforthophosphate for maximal rates of CO₂ dependent O₂ evolutionthan untreated chloroplasts. The loaded chloroplasts were moresensitive to linolenate, and in the presence of linolenate theoptimal phosphate concentration was shifted toward lower values.We propose that the inhibition of photosynthesis by linolenateis due to inhibition of the "phosphate translocator". ¹ Part of this work has been published in the Book of Abstracts,4th International Congress on Photosynthesis, Reading, U.K.,1977, p. 265–266. ² This work is part of a doctoral programme carried out by L.Mv6 Akamba in this laboratory. ³ To whom reprint requests should be adressed. (Received October 14, 1978; )     

Influence of Temperature on Intra- and Interspecific Resource Utilization within a Community of Lepidopteran Maize Stemborers

Competition or facilitation characterises intra- and interspecific interactions within communities of species that utilize the same resources. Temperature is an important factor influencing those interactions and eventual outcomes. The noctuid stemborers, Busseola fusca and Sesamia calamistis and the crambid Chilo partellus attack maize in sub-Saharan Africa. They often occur as a community of interacting species in the same field and plant at all elevations. The influence of temperature on the intra- and interspecific interactions among larvae of these species, was studied using potted maize plants exposed to varying temperatures in a greenhouse and artificial stems kept at different constant temperatures (15°C, 20°C, 25°C and 30°C) in an incubator. The experiments involved single- and multi-species infestation treatments. Survival and relative growth rates of each species were assessed. Both intra- and interspecific competitions were observed among all three species. Interspecific competition was stronger between the noctuids and the crambid than between the two noctuids. Temperature affected both survival and relative growth rates of the three species. Particularly at high temperatures, C. partellus was superior in interspecific interactions shown by higher larval survival and relative growth rates. In contrast, low temperatures favoured survival of B. fusca and S. calamistis but affected the relative growth rates of all three species. Survival and relative growth rates of B. fusca and S. calamistis in interspecific interactions did not differ significantly across temperatures. Temperature increase caused by future climate change is likely to confer an advantage on C. partellus over the noctuids in the utilization of resources (crops).     

Association of glycoproteins and pepsinogen in the secretory granules of fundic epithelial cells isolated from guinea pig stomach

Epithelial cells were isolated from the fundic portion of the guinea pig stomach. Cells were separated by velocity sedimentation at unit gravity in a Ficoll 70 gradient and pooled in three fractions. By morphological and biochemical criteria, each fraction was characterized as a population highly enriched in one of the three main functional types: oxyntic cell; chief cell and mucus-secreting cell. Measure of the pepsinogen content and specific stainings of the secretory granules for light and electron microscopy led to the definition of two types of mucus-secreting cells in nearly equal quantity; mucous cells with smaller secretory granules entirely glycoproteic in nature and muco-peptic cells containing larger heterogeneous secretory granules. These granules were made of a proteic core containing pepsinogen surrounded by a thin membrane and a voluminous cap, both containing carbohydrates. The cap appeared as if built of orderly packed layers of glycoproteins. Secretory granules of chief cells were also surrounded by a membrane containing glycoproteins and occasionally a small glycoproteic cap. Pepsinogen content was estimated to be three times higher in a single chief cell than in a muco-peptic cell.     

Regulation of Photosystem I electron flow activity by phosphatidylglycerol in thylakoid membranes as revealed by phospholipase treatment

Thylakoid membranes were treated by potato lipolytic acyl hydrolase, phospholipases A₂ from pancreas and snake venom, and by phospholipase C from Bacillus cereus under various conditions. The changes in the uncoupled rates of electron transport through Photosystem I (PS I) and in lipid composition were followed during these treatments. Pancreatic phospholipase A₂ which destroyed all phospholipids in thylakoid membranes stimulated the NADP⁺ reduction supported by reduced 2,6-dichlorophenolindophenol. This stimulation concerned only the dark but not the light reactions of this pathway. The main site of action of pancreatic phospholipase A₂ may be located on the donor side of PS I; the hydrolysis of phospholipids at this site caused an increased ability of reduced 2,6-dichlorophenolindophenol and ascorbate alone to feed electrons into PS I. A second site may be located on the acceptor side of PS I, probably between the primary acceptor and the ferredoxin system. When thylakoid membranes were first preincubated with or without lipolytic acyl hydrolase at 30°C (pH 8), the NADP⁺ photoreduction was inhibited whilst the methyl viologen-mediated O₂ uptake was stimulated. A subsequent addition of pancreatic phospholipase A₂ (which had the same hydrolysis rates for phosphatidylglycerol but not for phosphatidylcholine) further stimulated the O₂ uptake and restored NADP⁺ photoreduction. The extent of this stimulation, which depended on the presence of lipolytic acyl hydrolase, was ascribed partly to the hydrolysis of the phospholipids and partly to the generation of their lyso derivatives but not to the release of free fatty acids. On the contrary, phospholipase C which destroyed only phosphatidylcholine failed to restore this activity. It is suggested that phosphatidylglycerol is the only phospholipid associated with thylakoid membrane structures supporting PS I activities and that this lipid may play a physiological role in the regulation of these activities.     

Effect of linolenate on photosynthesis by intact spinach chloroplasts I. Inhibition of orthophosphate uptake and of 3-P-glyceraldehyde efflux across the chloroplast envelope

Linolenic acid (C_18:3) inhibited photosynthesis by intact spinachchloroplasts. This inhibition was due neither to a lack of NADPHin chloroplasts nor to a direct inhibition of the enzyme activitiesin the Calvin-Benson cycle. Linolenate inhibited CO₂ fixationand oxygen evolution more effectively than NADP⁺ photoreductionbut did not inhibit the activity of several key enzymes of theCalvin cycle. Linolenate inhibited phosphate influx and 3-phosphoglyceraldehydeefflux across the chloroplast envelope. A hypothesis explainingthe inhibition of photosynthesis by linolenate is presented. ¹ This work is part of a doctoral program which is carried outby L. Mv? Akamba in this laboratory. (Received October 14, 1978; )     

Transmembrane distribution of phospholipids and their involvement in electron transport,as revealed by phospholipase A2 treatment of spinach thylakoids

Thylakoid membranes were treated with either pancreatic or snake venom phospholipase A₂, and the residual phospholipid content of these membranes was determined and compared to the rates of Photosystem II and/or Photosystem I electron transports. The hydrolysis curves of both phosphatidylglycerol and phosphatidylcholine displayed a first, rapid phase which was almost temperature-insensitive, followed by a second, slower phase which depended strongly on the temperature. When pancreatic phospholipase A₂ had access either to the outer face or to both faces of the thylakoid membrane, either only part of or all the phospholipids, respectively, could be hydrolysed. These results were interpreted as indicating an asymmetric distribution of phospholipids across the thylakoid membrane, phosphatidylglycerol and phosphatidylcholine being preferentially located in the outer and the inner layer, respectively. When acting on uncoupled thylakoid membranes, phospholipase A₂ exerted an inhibitory effect on Photosystem II activity and a stimulatory effect on Photosystem I activity. The involvement of phosphatidylcholine and of phosphatidylglycerol in electron transport activities of Photosystem II and of Photosystem I are discussed with special reference to the role of the external and internal pools of these phospholipids.     

Rapid analysis of membrane lipids using a combination of thin-layer chromatography and scanning of photographic negatives

A simple method is proposed for the analysis of the distribution and changes in membrane lipids subjected to different treatments (lipolytic, aging, etc.). This technique involves only one-thin layer chromatographic step followed by a scanning of the photographic negative of the charred thin layer. This method is time saving, inexpensive and does not require the technical skill usually demanded in lipidology. The precision of this method is compared with that obtained with the classical TLC-GLC method: its variability is roughly twice that of the TLC-GC method.