Carbohydrate Translocation in Sugar Beet Petioles in Relation to Petiolar Respiration and Adenosine 5′-Triphosphate

Sugar beets (Beta vulgaris L) infected with the Beet Yellows Virus exhibit lower rates of net photosynthesis at light saturation than do healthy plants. These Pn reductions were correlated with increases in leaf resistance to water vapor loss. Theoretical analyses demonstrated that, although the leaf resistance to water vapor loss increases could account for a major part of the net photosynthesis decreases, some other aspect of leaf functioning also was debilitated by infection. Both the levels and the activities of ribulose-l, 5-diP carboxylase were less on a leaf area basis in extracts from infected leaves than from healthy ones. Soluble carbohydrates accumulate in Beet Yellows Virus-infected leaves, but inhibiting translocation in several ways provided no evidence in support of the hypothesis that the accumulation of photosynthates in leaves has a direct, short term, feed-back effect upon the photosynthetic rate.Sugar beets (Beta vulgaris L.) exhibit reductions in net photosynthesis at light saturation when infected with the Beet Yellows Virus (6). The physiological explanation for this Pn8 reduction has practical significance in that (a) it might be useful in selecting genotypes for tolerance to the virus (4), and (b) it may also provide an evaluation of the physical and biochemical factors that control net photosynthesis at light saturation in healthy plants. It has already been shown that differences in photorespiration (estimated from the inhibition by oxygen of CO2 uptake in normal air) and differences in respiration (estimated from rates of CO2 evolution in the dark) could not account for the reduction in infected leaf Pn (6). There are indications that the physical resistance to the entry of CO2 into the leaf may be greater in infected plants 1This investigation was partially supported by a grant from a cooperative agreement with the United States Department of (6, 21), and it has also been reported that soluble and insoluble carbohydrates accumulate to high levels in leaves of BYVinfected sugar beets (18, 23). Thus, it was possible that the frequently postulated feed-back effect of photosynthate accumulation in leaves on photosynthesis might also be present. No unequivocal, direct evidence has been reported which supports this postulate (15), yet it is a potentially important aspect of plant functioning. If a feed-back mechanism of this type does exist, then photosynthetic rates may reflect translocation capacity, growth, and sink activity, as well as climatic factors and the inherent photosynthetic capabilities of both chloroplast and leaf. MATERIALS AND METHODSA uniform hybrid of Beta vulgaris L. (MS NB1 X NB4) was grown in pots in a mixture of peat moss and sand in different environments. The plants were thinned for uniformity to one plant per pot, and half the plants were inoculated with the Brawley strain of BYV supplied by C. W. Bennett, United States Department of Agriculture, Salinas, California. Four plant replicates were used in most of the treatments. In experiments I and II, the p...

Section: Discussionmentioning

confidence: 53%

An Explanation for the Difference in Photosynthetic Capabilities of Healthy and Beet Yellows Virus-infected Sugar Beets (Beta vulgaris L.)

Hall

Loomis

1972

“…The steady-state labeling system was similar to that described by Coulson et al (3). Both the level of radioactivity in the gas stream and the level of CO2 could be monitored.…”

mentioning

confidence: 80%

Partitioning of ¹⁴C-Photosynthate, and Long Distance Translocation of Amino Acids in Preflowering and Flowering, Nodulated and Nonnodulated Soybeans

Housley¹,

Schrader²,

Miller³

et al. 1979

The influence of stage of development (preflowering versus flowering) in nodulated and nonnodulated soybeans (Glycine max IL.I Merr. cv. Wells) on partitioning of "C into assimilates following exposure of a soybean leaf to "CO2 by both steady-state and pulse-labeling techniques was studied.Blades on the second fully expanded leaf from the stem apex were exposed to "CO2. Radioactive assimilates were extracted from source leaf blades, petioles, and stems (both the path up and path down from source leaf), were separated into neutral (sugars), basic (amino acids), and acidic (organic acids, sugar phosphates) fractions by ion exchange chromatography. The basic fraction was further resolved using thin layer chromatography and the percentage of radioactivity recovered in each amino acid was determined.The distribution of radioactivity in the neutral, basic, and acidic fractions of the source leaf blades was significantly different from that of the transport path (petiole and stems). About 70% of the radioactivity in source leaf blades was recovered in the neutral fraction, whereas about 90% of the recovered radioactivity in the path was in the neutral fraction. "4C-Aminoacids constituted 8 to 17% and 2 to 7% of the recovered radioactivity in source leaves and paths, respectively. Recovered 14C in organic acids ranged from 13 to 20% and 2 to 7% in source leaves and paths, respectively.Partitioning of "4C-assimilates among the neutral, basic, and acidic fractions was not affected by the presence of nodules or flowers. However, when steady-state labeling was compared to pulse labeling, a significantly lower percentage of 14C was recovered in the neutral fraction with a concomitant increase in the basic fraction. Asparagine-arginine, serine, glutamate, y-aminobutyrate-alanine, and aspartate accounted for 69 to 85% of the recovered radioactivity in the basic fraction from the various treatments. 14CICSerine was significantly higher in pulse-labeling experiments, whereas glutamtate and proline were higher with steady-state labeling. 114CISerine was significantly higher in nonnodulated plants than in nodulated plants, whereas y-aminobutyrate-alanine was significantly higher in preflowering plants as compared to flowering plants. ' 94The transport of photosynthetically derived amino acids in soybean has not been extensively studied. The amino acids transported could differ in nonnodulated and nodulated soybeans, or perhaps with plant maturation. Plants without nodules are dependent on nitrate reduction as the sole source of reduced nitrogen, whereas plants with nodules can derive reduced nitrogen from both nitrate reduction and nitrogen fixation (14). During ontogeny, the amount of nitrogen reduced through these two processes varies (8,9), and may in fact be related to observed changes in amino acids found in soybean stem exudates (23). The relationship of these changes to photosynthesis is unknown because amino acids coupled to CO2 assimilation were not distinguishable in earlier studies.Tagging amino acids through...

“…The translocation system is generally divided into three structurally and physiologically distinct regions: the source, path, and sink regions. Recent structural and physiological studies (5,10,17,18) have indicated that the path region is relatively passive in the translocation process, in that the driving force for transport resides in metabolism-dependent processes operating in the source and sink regions.From a mechanistic viewpoint, the major emphasis in the phloem transport literature has been toward elucidating the role of the path at the expense of source and sink metabolism, particularly at the cellular level. Recent studies by Geiger and co-workers (12, 13) have investigated events operating in the source leaf of Beta vulgaris during the loading of sucrose into the phloem prior to translocation.…”

mentioning

confidence: 99%

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confidence: 99%

Sucrose Hydrolysis in Relation to Phloem Translocation in Beta vulgaris

Giaquinta

1977

Asymmetically labeled sucrose, 14C(fructosyl)sucrose, was used to determine whether sucrose undergoes extracellar hydrolysis duing phloem trandocation in the sugar beet, Beta vulgaris. In addition, the metabolsm of variom sugars accmulated and trasocated was determined in varios gions of the plant. These proceses were studied in detached regions as well as in the intact, trnlocating plant in the source leaf, along the trslocation path, and in a rapidly growing sink leaf nd storage beet. Tbe data show that, unlike sucrose accumulation into the sink tisse of sugarcane, ucrose is neither hydrolzyed prior to phloem loading or during tansit, nor is it extrcellularty hydrolyzed during accumulation into sink leaves or the storage beet.Although it is well established that photosynthetically derived assimilates are transported between source and sink regions along a gradient of hydrostatic pressure, relatively little is known about the cellular and metabolic correlates of the translocation process. The translocation system is generally divided into three structurally and physiologically distinct regions: the source, path, and sink regions. Recent structural and physiological studies (5,10,17,18) have indicated that the path region is relatively passive in the translocation process, in that the driving force for transport resides in metabolism-dependent processes operating in the source and sink regions.From a mechanistic viewpoint, the major emphasis in the phloem transport literature has been toward elucidating the role of the path at the expense of source and sink metabolism, particularly at the cellular level. Recent studies by Geiger and co-workers (12, 13) have investigated events operating in the source leaf of Beta vulgaris during the loading of sucrose into the phloem prior to translocation. What has emerged from these studies and from the more recent studies by Giaquinta (14,15 (19).Hydrolysis of sucrose to hexoses by a free space invertase prior to accumulation quite convincingly occurs in sugarcane tissue. The paucity of information on this subject in other species makes it difficult to determine whether these events constitute a general mechanism of sink metabolism of translocate or whether they are characteristic for the sugarcane system.The literature on the role of extracellular invertase in phloem translocation in both source and sink regions is controversial. Part of the discrepancy and variability probably arises from the observation that invertase activity can be induced in some tissues by physical dissection or prolonged washing (8). Another possibility is that extracellular invertase as related to translocation is not universally present in various species and that sucrose need not be extracellularly hydrolyzed prior to membrane transport into the sink tissue.The form in which sucrose is transported across cell membranes during phloem translocation is important in elucidating the nature and selectivity of membrane carriers involved in phloem loading and unloading of sugars in the source and s...