Fine Structure in Detached, Senescing Wheat Leaves

The quantity of RNA in the ribosomal fraction of the first leaf of cucumber (Cucumis sativus) increases during growth, reaches a maximum before the final fresh weight is attained, and then decreases. The main changes are in the free ribosome fraction, the quantity of membrane-bound ribosomes remaining about constant. Few 65.5S chloroplast ribosomes are present in small leaves; however, they increase in quantity rapidly during growth and form about half of the ribosomes present in the mature fully green leaf. The cytoplasmic ribosomes have a sedimentation coefficient of 77.6S. Ribonuclease-sensitive polysomes were present in leaves of all ages except possibly the very oldest. The proportion of ribosomes in polysome form decreases during growth and then remains roughly constant during senescence. Following maturation of the leaf, the rate of incorporation of 'P into ribosomal-fraction RNA begins to decline. This decline could account for the loss of ribosomes during the early stages of senescence. The possibility that leaf ribonuclease might be responsible for the final, more rapid loss of RNA, is discussed.The quantity of protein in a leaf increases during growth but then, at the time of senescence, it declines, although the evidence suggests that the leaf has not completely lost the power to synthesize protein (14). In view of their role in protein synthesis it was thought that a study of leaf ribosomes might throw some light on the mechanism controlling protein metabolism in leaves.Two classes of ribosome are found in leaf tissues, 70S in the chloroplasts and 80S in the cytoplasm (8). Most studies have been made with young tissues, and little is known about the changes that occur with aging. A decrease in the proportion of polysomes during growth has been indicated in Chinese cabbage leaves (6) and maize shoots (10), and an electron microscope study revealed the presence of polysome-like clusters in aging cucumber cotyledons (5). An over-all loss of ribosomes during aging has been observed in several plant tissues both by electron microscopy (1, 4,5,13) and from biochemical studies (6,15) Preparation of Ribosomes and Polysomes. The leaves (10 to 40 g) were rapidly washed, blotted dry, and weighed; then they were cooled and ground by pestle and mortar with enough acid-washed sand to facilitate grinding and with one to two times their volume of tris-magnesium buffer containing 0.05 M tris, pH 7.8, and 0.01 M magnesium acetate. The low molarity of the grinding medium allowed chloroplast lysis. The homogenate was squeezed through four layers of muslin, and the material in the muslin was extracted three times with fresh buffer and the extracts combined. This preparation and all subsequent handling of extract or ribosomes were made at about 0 C unless otherwise stated. Two types of preparations were made from the extract. Differential Centrifugation. To determine the quantity of free and membrane-bound ribosomes, the extract was divided into halves, and Triton X-100 in tris-magnesium buffer was added to one o...

Section: Introductionmentioning

confidence: 99%

“…A decrease in the proportion of polysomes during growth has been indicated in Chinese cabbage leaves (6) and maize shoots (10), and an electron microscope study revealed the presence of polysome-like clusters in aging cucumber cotyledons (5). An over-all loss of ribosomes during aging has been observed in several plant tissues both by electron microscopy (1,4,5,13) and from biochemical studies (6,15) 16 hr of artificial light (about 1000 ft-c). The temperature varied between 15 to 25 C in winter and 20 to 35 C in summer.…”

mentioning

confidence: 96%

Ribosomes and Polysomes in Cucumber Leaves during Growth and Senescence

Eilam¹,

Butler²,

Simon³

1971

“…The bean cotyledon mitochondria were isolated on day 1, day 4, and day 7. Figure 7 shows that the swelling of mitochondria on day 1 and day 7 (24). Opik (21) also found that maximal activity of bean cotyledon mitochondria was from the cotyledons at the age of 36 hr after planting and then it gradually decreased.…”

Section: Materiais and Methodsmentioning

confidence: 88%

Changes in the Respiratory, Enzymatic, and Swelling and Contraction Properties of Mitochondria from Colytedons of Phaseolus vulgaris L. during Germination

Malhotra

Spencer

1970

Mitochondria isolated from cotyledons of germinating wax beans (Phaseolus vulgaris L.) showed fairly good respiratory control on days 1 and 2 after planting. The respiratory control was completely lost from days 3 to 5. During this period mitochondria were shown to be very leaky, losing about 88% of their total nicotinamide adenine dinucleotide to the suspending medium in a short time. The respiratory control was partially recovered by day 7, after which it completely disappeared again. By the use of differential centrifugation, the mitochondria were divided into subfractions by sequential centrifugation: 10,000g for 5 minutes, 25,000g for 5 minutes, and 40,000g for 5 minutes. The 10,000g subfraction was responsible for the recovery of mitochondrial activity (respiratory control value, adenosine diphosphate to oxygen ratio, and rate of oxygen utilization), on day 7. Activities of succinate dehydrogenase, cytochrome oxidase, pyruvate dehydrogenase, and isocitrate dehydrogenase from different mitochondrial subfractions of aging cotyledons were determined. In general, the enzyme activities, adenosine diphosphate to oxygen ratios, and the ability of mitochondria to swell and contract followed the same pattern as for respiratory control.Previous research has shown that the cotyledons of germinating seedlings and other plant tissues go through many biochemical changes at the subcellular level during aging (1,4,12,16,21,25,28). Lado and Schwendimann (16) showed that in castor bean endosperm, mitochondrial proteins and enzyme activities decrease rapidly during seed maturation, while other workers (1) observed a rapid increase during the earliest part of germination. Opik (21) studied the changes in respiration rate and mitochondrial activity in bean cotyledons. She found that while oxygen uptake of the intact cotyledons rose to a peak between the 3rd and 5th days of germination, mitochondrial activity decreased from 36 hr onwards and in most of the storage cells mitochondrial cristae had become swollen and matrix had darkened by the 3rd day. Since the mitochondria are highly important as centers of energy production, one would expect changes in the physiological state of cotyledons to be reflected in the mitochondrial activity. This report concerns the changes in the bean cotyledon mitochondrial activity with aging. The mitochondrial activity is measured by the ability of mitochondria to oxidize different substrates and to swell and contract, and by the activities of various mitochondrial enzymes. MATERIAIS AND METHODSThe bean seeds (Phaseolus vulgaris L. var. Kinghorn wax) were soaked in running tap water for 4 hr and then planted in vermiculite (horticulture grade). The seedlings were grown in the dark at 27 C. The cotyledons from the germinating seedlings were removed at the required age, washed with deionized water, and used as the source of mitochondria. Mitochondria were prepared from about 50 g of cotyledons by the following method: the cotyledons were gently crushed with a mortar and pestle for 90 sec in...

“…Many recent studies have demonstrated that several different factors such as mineral or vitamin deficiencies (7,18), treatment with unnatural pyrimidine (11,22), treatment with herbicides (2, 10), senescence (17), and genetic alternation (3,15,21) can cause similar aberrant ultrastructural changes in the chloroplast structure. It is evident from our results that 6706 also disrupts the chloroplast morphology and development.…”

Section: Discussionmentioning

confidence: 99%

Chloroplast Development in 4-Chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3 (2H)-pyridazinone (Sandoz 6706)-treated Wheat Seedlings

Bartels

Hyde

1970

The first visible effect following the application of sublethal doses of 4-chloro-5-(dimethylamino)-2-(a ,a,a-trifluoro-mtolyl)-3(2H)-pyridazinone (hereafter referred to as 6706, manufacturer's code number) to germinating wheat grains was the formation of albinistic seedlings. These seedlings grew and developed as well as untreated seedlings for 9 days. Hilton et al. (12) studied the effect of 6706 on established green leaves and meristematic tissues of several different species. They found that Hill reaction and CO2 fixation were inhibited in green leaves and that chloroplast development was blocked in embryonic cells.The action of 6706 on wheat seedlings grown under high light intensities appears to be similar to that of 3-amino-s-triazole (4) and of 3,4-dichlorobenzylmethylcarbamate (5). These herbicides cause the loss of grana-fret membranes and chloroplast ribosomes. Burns et al. (9) suggested that the mode of action of these herbicides was the inhibition of carotenoid synthesis. We investigated the effect of 6706 on the ultrastructure, ribosome, and pigment composition of the aberrant plastids of wheat seedlings which were grown in dark only, light only, or dark followed by light.MATERILIS AND METHODS Wheat seedlings (Triticum vulgare L. var. Maricopa) were germinated and grown in Petri dishes (15 grains per dish) containing either 10 ml of 0.1 mi 6706 or distilled water, under one of the following conditions: (a) 6 days in light at 1 ft-c, or 1500 ft-c, 16-hr photoperiod, 21 C; (b) 6 days in darkness except for exposure to dim green light during watering, 23 C; or (c) 6 days in darkness followed by 1, 4, or 12 hr of light. The light intensity used during greening was either 1 ft-c, 75 ft-c, or 1500 ft-c, 23 C. Following germination, plants were watered with distilled water. On the 6th day shoots were harvested, weighed, and prepared for either ultrastructural, sedimentation, or pigment studies.Sedimentation and ultrastructural studies were performed as discribed in an earlier publication (5) except that (a) grinding media used to isolate ribosomes contained 4% Triton X-100 (Rohm and Haas, Philadelphia) and (b) fixing solutions used in electron microscope preparations contained 0.01 M cacodylate-HCI buffer (pH 7.2) instead of phosphate buffer.