14C-Studies on Apple Trees. V. Translocation of Labelled Compounds from Leaves to Fruit and their Conversion within the Fruit

Following application of ¹⁴CO₂ to fruit spur leaves, the majority of the ¹⁴C absorbed is transfered to the fruit on the same spur, and the total content of ¹⁴C within the leaf-fruit system as a whole remains virtually constant with time. The considerable reduction in activity in the leaves is accounted for mainly by a decrease in the amount of ¹⁴C-sorbitol, although relatively speaking the decrease in ¹⁴C-sucrose is also considerable. The major part of the activity of the sugar fraction in the conducting tissues between blade and fruit (petiol, spur) is found in sorbitol. Immediately following uptake of ¹⁴C yia the leaves a large part of the activity of the sugar fraction in the fruit is found in sorbitol; but this activity is rapidly reduced, accompanied by an increase in sucrose activity, and over longer periods of time increases in particular in glucose and fruclose activity, and in that of methanol insoluble compounds. The changes in activity distribution in the fruit vary with the variety of fruit and the dates within the growing season. By injecting labelled sorbitol directly into the fruit sorbitol is converted into sucrose, glucose and fructose, while injection of labelled sucrose, glucose and fructose has yielded proof of interconversions between these compounds but no measurable amounts of surbitol. After application of ¹⁴CO₂ directly to the outer skin of the fruit considerably less of the activity is found in sorbitol than is the case in leaves following exposure to ¹⁴CO₂. A minor, but significant, translocation of ¹⁴C away from the fruit was found to take place following the application of labelled ¹⁴C compounds to the fruit. The smallness of the respiratory loss of ¹⁴C in the leaf-fruit system is discussed. It is concluded that in apple trees considerable translocation occurs in the form of sorbitol which in the fruits rapidly converted into other compounds.     

14C-Studies on Apple Trees. II. Distribution of Photosynthates from Top and Base Leaves from Extension Shoots

The photosynthates from leaves from extension shoots may be used either for new growth of the shoot itself, or for growth in other parts of the tree. An attempt has been made to elucidate this problem by determining the content of ¹⁴C in long shoots to which ¹⁴CO₂ was applied either through fully developed leaves at the base, or through very young leaves at the apex. In the case of ¹⁴C application at the base, 80 per cent or more of the ¹⁴C initially taken up disappears from the shoot, and only a very minor part is translocated upwards in the shoot. When the leaves at the apex are exposed, on the other hand, 80 per cent of the ¹⁴C absorbed is retained in the leaves and shoot components treated as long as there is still considerable terminal growth taking place, although a small percentage is deposited in the lower parts of the shoot. At the same time, a much higher proportion is incorporated into methanol-insoluble components. As terminal growth decreases, a larger proportion of the ¹⁴C activity of the apex leaves also disappears from the shoot. The distribution of activity between the sorbitol, sucrose, glucose and fructose fractions was not significantly different in young and in fully developed leaves. The ¹⁴C labelling in the sugar fraction was highest for sorbitol, then sucrose, but decreases with time compared to glucose and fructose.     

Differences in sucrose metabolism relative to accumulation of bird-deterrent sucrose levels in fruits of wild and domestic Vaccinium species

We examined variability in sucrose levels and metabolism in ripe fruits of wild and domestic Vaccinium species and in developing fruits of cultivated blueberry (V. ashei and V. corymbosum). The objective was to determine if sufficient variability for fruit sucrose accumulation was present in existing populations to warrant attempts to breed for high-sucrose fruit, which potentially would be less subject to bird predation. Threefold differences in fruit sucrose concentration were found among Vaccinium species, ranging from 19 to 24 mg (g fresh weight)^?1 in V. stamineum and V. arboreum to approximately 7 mg (g fresh weight)^?1 in cultivated blueberry (V. ashei and V. corymbosum) and V. darrowi. Hexose levels were similar among species, ranging from 90 to 110 mg (g fresh weight)^–1, and glucose and fructose were present in equal amounts. Soluble acid invertase (EC 3.2.1.26) activity was negatively correlated with fruit sucrose concentration. There was no apparent correlation between fruit sugar concentration and either sucrose synthase (EC 2.4.1.13) or sucrose phosphate synthase (EC 2.4.1.14) activities, both of which were low for all species studied. Developmental increases in fruit sugar levels of cultivated blueberry followed a pattern similar to that observed in fruit fresh weight accumulation. Hexose concentrations ranged from 6 to 30 mg (g fresh weight)^?1 during the first 60 days after anthesis. Between 60 days and fruit ripening (80 days), hexose levels rose from 30 to 80 mg (g fresh weight)^?1. Sucrose was not detected in fruits until ripening, when low levels were found. Insoluble acid invertase activity was relatively high early in fruit development, decreasing as soluble acid invertase activity increased. Between 60 days and fruit ripening, soluble acid invertase activity increased from 3 to 55 μmol (g fresh weight)^–1 h^–1. Both sucrose synthase and sucrose phosphate synthase activities were low throughout development. The extent of sucrose accumulation in fruits and the degree of variability for this trait among Vaccinium species support the feasibility of developing high sucrose fruits, which would be a potentially valuable addition to current strategies of minimizing crop losses to birds.     

Heterologous expression of the apple hexose transporter MdHT2.2 altered sugar concentration with increasing cell wall invertase activity in tomato fruit

Sugar transporters are necessary to transfer hexose from cell wall spaces into parenchyma cells to boost hexose accumulation to high concentrations in fruit. Here, we have identified an apple hexose transporter (HTs), MdHT2.2, located in the plasma membrane, which is highly expressed in mature fruit. In a yeast system, the MdHT2.2 protein exhibited high ¹⁴C‐fructose and ¹⁴C‐glucose transport activity. In transgenic tomato heterologously expressing MdHT2.2, the levels of both fructose and glucose increased significantly in mature fruit, with sugar being unloaded via the apoplastic pathway, but the level of sucrose decreased significantly. Analysis of enzyme activity and the expression of genes related to sugar metabolism and transport revealed greatly up‐regulated expression of SlLIN5, a key gene encoding cell wall invertase (CWINV), as well as increased CWINV activity in tomatoes transformed with MdHT2.2. Moreover, the levels of fructose, glucose and sucrose recovered nearly to those of the wild type in the sllin5‐edited mutant of the MdHT2.2‐expressing lines. However, the overexpression of MdHT2.2 decreased hexose levels and increased sucrose levels in mature leaves and young fruit, suggesting that the response pathway for the apoplastic hexose signal differs among tomato tissues. The present study identifies a new HTs in apple that is able to take up fructose and glucose into cells and confirms that the apoplastic hexose levels regulated by HT controls CWINV activity to alter carbohydrate partitioning and sugar content.     

Effect of zinc nutrition on sucrose biosynthesis in maize

Zinc deficiency caused an accumulation of ¹⁴C into malic acid, sugar phosphates, sugar nucleotides, glucose, fructose, phosphoenolpyruvate, glycine and alanine, whereas the ¹⁴C labelling in sucrose decreased. The activity of sucrose synthetase (EC 2.4.1.13) was unaffected up to the 15th day and thereafter it declined. Severe Zn deficiency reduced the biosynthesis of total protein and sucrose synthetase by 50 and 20%, respectively.     

Sucrose and starch metabolism in carrot (Daucus carota L.) cell suspensions analysed by 13-labelling: indications for a cytosol and a plastid-localized oxidative pentose phosphate pathway

Cells were grown in batch culture on a mixture of 50 mM glucose and fructose as the carbon source; either the glucose or the fructose was [1-¹³C]-labelled. In order to investigate the uptake and conversion of glucose and fructose during long-term labelling experiments in cell suspensions of Daucus carota L., samples were taken every 2 d during a 2 week culture period and sucrose and starch were assayed by means of HPLC and ¹³C-nuclear magnetic resonance. The fructose moieties of sucrose had a lower labelling percentage than the glucose moieties. Oxidative pentose phosphate pathway activity in the cytosol is suggested to be responsible for this loss of label of especially C-1 carbons. A combination of oxidative pentose phosphate pathway activity, a relatively high activity of pathway to sucrose synthesis and a slow equilibration between glucose-6-phosphate and fructose-6-phosphate could explain these results. Starch contained glucose units with a much lower labelling percentage than glucose moieties of sucrose: it was concluded that a second, plastid-localized, oxidative pentose phosphate pathway was responsible for removal of C-1 carbons of the glucosyl units used for synthesis of starch. Redistribution of label from [1-¹³C]-hexoses to [6-¹³C]-hexoses also occurred: 18-45% of the label was found at the C-6 carbons. This is a consequence of cycling between hexose phosphates and those phosphates in the cytosol catalysed by PFP. The results indicate that independent (oxidative pentose phosphate pathway mediated) sugar converting cycles exist in the cytosol and plastid.Key words: Daucus carotaL., cell suspensions, carbon-13 nuclear magnetic resonance, ¹³C-NMR, carbohydrate cycling, oxidative pentose phosphate pathway, plastid.      

Sink Metabolism in Tomato Fruit : II. Phloem Unloading and Sugar Uptake

Analysis of [³H]-(fructosyl)-sucrose translocation in tomato (Lycopersicon esculentum Mill.) indicates that phloem unloading in the fruit occurs, at least in part, to the apoplast followed by extracellular hydrolysis. Apoplastic sucrose, glucose, and fructose concentrations were estimated as 1 to 7, 12 to 49, and 8 to 63 millimolar, respectively in the tomato fruit pericarp tissue. Hexose concentrations were at least four-fold greater than sucrose at all developmental stages. Short-term uptake of [¹⁴C]sucrose, -glucose, and -fructose in tomato pericarp disks showed first order kinetics over the physiologically relevant concentration range. The uptake rate of [¹⁴C]-(glucosyl)-1′-fluorosucrose was identical to the rate of [¹⁴C]sucrose uptake, suggesting sucrose may be taken up directly without prior extracellular hydrolysis. Short-term uptake of all three sugars was insensitive to 10 micromolar carbonyl cyanide m-chlorophenylhydrazone and to 10 micromolar p-chloromercuribenzene sulfonic acid. However, long-term accumulation of glucose was sensitive to carbonyl cyanide m-chlorophenylhydrazone. Together these results suggest that although sucrose is at least partially hydrolyzed in the apoplast, sucrose may enter the metabolic carbohydrate pool directly. In addition, sugar uptake across the plasma membrane does not appear to be energy dependent, suggesting that sugar accumulation in the tomato fruit is driven by subsequent intracellular metabolism and/or active uptake at the tonoplast.     

Control of the hexose content of potato tubers

The amounts of glucose and fructose in a range of harvested tubers of Solanum tuberosum were compared with the labelling of these hexoses by [U-¹⁴C]sucrose supplied to the tubers. Hexose content varied. Fructose was more heavily labelled than glucose. There was no correlation between the amounts of glucose and fructose in the tuber and their labelling. The maximum catalytic activities of α-glucan phosphorylase, acid invertase, alkaline invertase, sucrose synthase, α-amylase and β-amylase in tubers stored for 17 weeks at 5° and at 10° were estimated. The values showed no clear correlation with hexose content, but provided sound evidence that starch breakdown was phosphorolytic. It is suggested that the amounts of glucose and fructose in mature harvested tubers may be determined more by the partitioning of the translocated sucrose during the development of the tubers than by the metabolism of the harvested tuber.     

Seasonal differences in the pattern of assimilate movement in branches of Coffea arabica L

At Ruiru, Kenya, ¹⁴CO₂ was fed to single leaves at different distances from the apex on branches of mature fruiting and non-fruiting trees of Coffea arabica on six occasions from the Short Rains (November) 1966 to the Long Rains (April-May) 1967. The location of ¹⁴C-labelled assimilates in the treated branches was determined 26 h later by autoradiography. The direction of movement of labelled assimilates indicated large seasonal differences in the relative sink strengths of the shoot tip, fruit and trunk-root systems. On vegetative trees the sink strength of trunk-roots was much smaller, as compared with the shoots, at the beginning of the Long Rains than at the end of the previous Short Rains or in the intervening dry season. Assimilate use by growing fruits did not alter the pattern of distribution of assimilates to the other sinks at the end of the Short Rains, but it did restrict assimilate movement to both shoot tips and trunk-roots at the_ beginning of the Long Rains. In the dry season, virtually all assimilates were utilized by growing fruits when these were present. Vegetative secondary shoots provided assimilates to growing fruits and trunk-roots at the end of the Short Rains and in the dry season. Some practical implications are noted.     

Glucofructosan metabolism in Cichorium intybus roots

A 10-fold purification of sucrose sucrose fructosyl transferase from Cichorium intybus roots was achieved by ammonium sulphate fractionation and DEAE-cellulose column chromatography. The energy of activation for this enzyme was ca 48 kJ/mol sucrose. Sucrose sucrose fructosyl transferase and invertase were prominent during early months of growth. Evidence obtained from: (1) the changes in carbohydrate composition at monthly intervals; (2) comparative studies on fructosyl transferase and invertase at different stages of root growth; and (3) incubation studies with [¹⁴C]glucose, [¹⁴C]fructose and [¹⁴C]sucrose revealed that, during the later stages of root growth, fructosan hydrolase is responsible for fructosan hydrolysis. No evidence for the direct transfer of fructose from sucrose to high M_r glucofructosans was obtained.     

Seasonal CO2 exchange patterns of developing peach (Prunus persica) fruits in response to temperature, light and CO2 concentration

CO₂ exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m^?2S^?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO₂ evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO₂ (g dry weight)^?1 S^?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m^?2 S^?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O₁₀ value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO₂ exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO₂ concentrations via CO₂ refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m^?2 S^?1. Young peach fruits responded to increasing ambient CO₂ concentrations with decreasing net CO₂ exchange rates in light, but more mature fruits did not respond to increases in ambient CO₂. Fruit CO₂ exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO₂ concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO₂ response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO₂1^?2. In CO₂ free air, fruit photosynthesis was dependent on CO₂ refixation since CO₂ uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO₂-free air and 370 μl CO₂ 1^?1 indicated that young peach fruits were apparently able to take up CO₂ from the external atmosphere. CO₂ uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO₂ refixation.     

14C-Studies on Apple Trees. IV. Photosynthate Consumption in Fruits in Relation to the Leaf-Fruit Ratio and to the Leaf-Fruit Position

The photosynthate consumption in apple fruits in relation to the leaf/fruit ratio was studied in sections of branches of the Graasten and Golden Delicious varieties by exposure to ¹⁴CO₂during July and August. A significant, negative correlation was found between the fixation of ¹⁴C in the fruits in terms of percent of the total amount of ¹⁴C absorbed and the leaf/fruit ratio of the branch sections. The leaf area required for the saturation of one fruit was found to be ca. 190 and 230 cm² (14 and 17 leaves) in the case of Golden Delicious, and ca. 400 and 670 cm² (25 and 42 leaves) for Graasten in July and August, respectively, calculated under conditions of large leaf areas per fruit. In such cases a fairly good, reverse proportionality exists between the ¹⁴C fixation in the fruit in terms of percent and the leaf area expressed in multiples of saturation area. At low leaf/fruit ratios, however, the actual saturation area is found to be lower than the theoretically computed one. The translocation of the ¹⁴C assimilated in the leaves of the extension shoots or of spurs without fruits to fruits on other spurs was on the whole promoted by a decreasing leaf/fruit ratio in the parts in question; similarly the greatest transport took place on the side where the leaf/fruit ratio was lowest. The fixation was often greatest in the fruit closest to the treated leaves, hut in a number of cases the value was higher for the second closest or even further removed fruits. In this connection the importance of the size of the fruit and the vascular connections is discussed.     

Conversions of Imported 14C-Glucose and Sucrose Synthesis in Bai-lan Melon Fruits

The conversions of incorporated 14C-glucose in fruit flesh and in seeds were investigated at different stages of fruit development. In addition, the biochemical mechanism of sucrose synthesis in fruits of Bai-Lan melon were also studied. The results were summarized as follows: In fruit flesh and in seeds at young fruit stage, more than half of the incorporated 14C was found in the dilute acid hydrolyzable and non-hydrolyzable fractions, both of which represent the structural elements. While in the fruits approaching maturity (42 days old), a relatively small amount of 14C associated with the structural elements was found. The contributions of 14C of fractions hydrolyzed and non-hydrolyzed by dilute acid to the total radioactivity were reduced to 18% and 32%, respectively, in fruit flesh and in seeds. The results of identification of soluable sugar by using paper chromatography indicated that, the 14C was only associated with fructose after infiltrating the young fruit slices with 14C-glucose, but the 14C was predominately incorporated into sucrose at later stage of fruit development. The above results of 14C labeling experiments suggest that the pattern of metabolism is changed with fruit development. The greater part of metabolites is used in synthesis of structural elements which is necessary for growth and related processes at early stage of fruit developrnent. However, as the fruit reaches full size. the demand for carbon used in structural tissue is reduced. At this time, the direction of the enzymatic reactions changed in favor of sucrose synthesis. The activity of sucrose synthesis in young fruits was rather low when various substrates were supplied. It was possible that the enzymes related to sucrose synthesis were absent in young fruits of Bai-Lan melon. The activity of sucrose synthesis in fruits at later stage of development increased by about 5-fold of that at early stage. The higher activity of sucrose synthesis was observed when UDPG+F-6-P were supplied as substrates. It is shown that the sucrose in Bai- Lan melon fruits may be mainly synthesized by sucrose phosphate synthetase catalyzing the reaction between UDPG and F-6-P to yield sucrose-P. The biochemical mechanism of sucrose synthesis in Bai-Lan melon fruits is briefly discussed.     

Sucrose phosphate synthase and other sucrose metabolizing enzymes in fruits of various species

Recent reports have suggested that sucrose phosphate synthase (EC 2.4.1.14), a key enzyme in sucrose biosynthesis in photosynthetic “source” tissues, may also be important in some sucrose accumulating “sink” tissues. These experiments were conducted to determine if sucrose phosphate synthase is involved in sucrose accumulation in fruits of several species. Peach (Prunus persica NCT 516) and strawberry (Fragaria x ananassa cv. Chandler) fruits were harvested directly from the plant at various stages of fruit development. Kiwi (Actinidia chinensis), papaya (Carica papaya), pineapple (Ananas comosus) and mango (Mangifera indica) were sampled in postharvest storage over a period of several days. Carbohydrate concentrations and activities of sucrose phosphate synthase, sucrose synthase (EC 2.4.1.13), and acid and neutral invertases (EC 3.2.1.26) were measured. All fruits contained significant activities of sucrose phosphate synthase. Moreover, in fruits from all species except pineapple and papaya, there was an increase in sucrose phosphate synthase activity associated with the accumulation of sucrose in situ. The increase in sucrose concentration in peaches was also associated with an increase in sucrose synthase activity and, in strawberries, with increased activity of both sucrose synthase and neutral invertase. The hexose pools in all fruits were comprised of equimolar concentrations of fructose and glucose, except in the mango. In mango, the fructose to glucose ratio increased from 2 to 41 during ripening as sucrose concentration more than doubled. The results of this study indicate that activities of the sucrose metabolizing enzymes, including sucrose phosphate synthase, within the fruit itself, are important in determining the soluble sugar content of fruits of many species. This appears to be true for fruits which sweeten from a starch reserve and in fruits from sorbitol translocating species, raffinose saccharide translocating species, and sucrose translocating species.     

14C-Studies on Apple Trees. I. The Effect of the Fruit on the Translocation and Distribution of Photosynthates

The presence of fruits affects the translocation and distribution of photosynthates from apple leaves to other organs of the tree. An attempt has been made to study the relationship in greater detail by following the distribution of ¹⁴C introduced in the form of ¹⁴CO₂ on shoots with and without fruits, respectively. Determinations of the ¹⁴C-content were made on different parts of the shoot sampled at varying intervals after the introduction of the tracer. The ^l4C-labelling and the content of sorbitol and sugar in the leaves were determined by means of paper chromatography. A total of nearly 90 per cent of the ¹⁴C taken up by the leaves can be transferred to a fruit close by, the majority during the first 4 to 5 days following the addition of the ¹⁴C. The content of ¹⁴C in the leaves is reduced more rapidly in shoots with fruits than in those without. Young leaves retain more of the added ¹⁴C than do fully developed ones. The greatest changes with time are found in the methanol-soluble ¹⁴C-compounds. Immediately following application, the leaves contain 58 to 80 per cent of the ¹⁴C added in sorbitol, 7 to 9 per cent in sucrose, and 1 to 4 per cent in the form of glucose. Within 5 days after the introduction of ¹⁴C the amount of ¹⁴C-sorbitol is reduced very considerably, while in certain cases the amount of ¹⁴C-glucose increases. The ¹⁴C-sorbitol content was higher in leaves from shoots without fruits than in those from fruit-bearing shoots, and this applied also to the total contents of sorbitol and of glucose.     

灵武长枣果实质膜和液泡膜H~+-ATPase和H~+-PPase活性与果实糖分积累

以不同发育时期灵武长枣(Ziziphus jujuba cv.Lingwuchangzao)的果实为材料,通过测定与分析果肉组织中细胞质膜、液泡膜H+-ATPase和H+-PPase活性、果实糖分含量变化,研究了灵武长枣果实质膜、液泡膜H+-ATPase和H+-PPase活性与糖积累特性的关系。结果表明:(1)果实第二次快速生长期之前主要积累葡萄糖和果糖,之后果实迅速积累蔗糖,葡萄糖和果糖含量则逐渐下降,成熟期果实主要积累蔗糖。(2)在果实发育的缓慢生长期S1,质膜H+-ATPase活性最低;第一次快速生长期,质膜H+-ATPase活性最高;缓慢生长期S2,其活性降低;第二次快速生长期,质膜H+-ATPase活性升至次高;完熟期,质膜H+-ATPase活性下降幅度较大。(3)在果实发育过程中,液泡膜H+-ATPase和H+-PPase活性的变化趋势相似。缓慢生长期S1,液泡膜H+-ATPase和H+-PPase活性较低;从缓慢生长期S1至第一次快速生长期缓慢下降至最低;从第一次快速生长期开始,液泡膜H+-ATPase和H+-PPase活性呈现为逐渐增高的变化趋势;除第二次快速生长期以外,液泡膜H+-PPase活性始终高于H+-ATPase。由此推测,质膜H+-ATPase和液泡膜H+-ATPase、H+-PPase对灵武长枣果实糖分的跨膜次级转运起到重要的调控作用。     

Properties of proton and sugar transport at the tonoplast of tomato (Lycopersicon esculentum) fruit

Tonoplast vesicles were isolated from tomato (Lycopersicon esculentum Mill.) fruit pericarp and purified on a discontinuous sucrose gradient. ATPase activity was inhibited by nitrate and bafilomycin A₁ but was insensitive to vanadate and azide. PPase hydrolytic activity was inhibited by NaF but was insensitive to nitrate, bafilomycin A₁ vanadate and azide. Kimetic studies of PPase activity gave an apparent K_m, for PP₃ of 18 μM. Identical distributions of bafilomycin- and NO₃-sensitive ATPase activities within continuous sucrose density gradients, confirmed that bafilomycin-sensitive ATPase activity is a suitable marker for the tonoplast. By comparing the distribution of bafilomycin-sensitive ATPase activity with that of PPase activity, it was possible to locate the PPase enzyme exclusively at the tonoplast. The apparent density of the tonoplast did not change during fruit development. Measurements of tonoplast PPase and ATPase activities during fruit development over a 35-day period revealed an 80% reduction in PPase specific activity and a small decrease in ATPase specific activity. ATP- and PP₁-dependent ΔpH generation was measured by the quenching of quinacrine fluorescence in tonoplast vesicles prepared on a discontinuous Dextran gradient. No H⁺ efflux was detected on the addition of sucrose to energized vesicles. Therefore a H⁺/sucrose antiport may not be the mechanism of sucrose uptake at the tomato fruit tonoplast. Similar results were obtained with glucose, fructose and sorbitol. The lack of ATP (or PP₁) stimulation of [¹⁴C]-sucrose uptake also suggested that an antiport was not involved. Initial uptake rates of radiolabelled glucose and fructose were almost double that for sucrose. The inhibition of hexose uptake by p-chloromercuribenzene sulphonate (PCMBS) implicated the involvement of a carrier. Therefore storage of hexose in the tomato fruit vacuole and maintenance of a downhill sucrose concentration gradient into sink cells is likely to be regulated by the activity of sucrose metabolizing enzymes, rather than by energy-requiring uptake mechanisms at the tonoplast.     

Sucrose metabolism and fruit growth in parthenocarpic vs seeded blueberry (Vaccinium ashei) fruits

Although fruit set and development are induced by applications of gibberellins, final fruit weight of gibberellin-induced parthenocarpic fruit is often less than that of pollinated fruit. We examined changes in the activities of sucrose-metabolizing enzymes and sugar accumulation in developing fruits of cultivated blueberry (Vaccinium ashei Reade) and their correlation with fruit growth upon pollination or exogenous applications of gibberellic acid (GA₃). The objective was to determine if differences in fruit growth could be attributed to differences in enzyme activities and subsequent sugar accumulation in fruits. The fruit development period of GA₃-treated fruits was 15 days longer than that of pollinated fruits. At maturity, GA₃-treated fruit accumulated an average of 180 mg dry weight while pollinated fruit accumulated 390 mg dry weight. Dry weight accumulation in nonpollinated fruits was negligible and these fruits abscised by 45 days after bloom (DAB). The total carbon (C) cost (dry weight C + respiratory C) for fruit development was 109 and 244 mg C fruit^-1 for GA₃-treated and pollinated fruits, respectively. Hexose concentration increased to 100 mg (g fresh weight)^-1 at ripening in both GA₃-treated and pollinated fruits. Nonpollinated fruits reached a maximum hexose concentration at 45 DAB. Sucrose phosphate synthase (EC 2.4.1.14) and sucrose synthase (EC 2.4.1.13) activities reached a maximum of ≤5.0 μmol (g fresh weight)^-1 h^-1 in both GA₃-treated and pollinated fruits. Soluble acid invertase (EC 3.2.1.26) activity increased to about 60 μmol (g fresh weight)^-1 h^-1 in both GA₃-treated and pollinated fruits at ripening, while in nonpollinated fruits, a maximum soluble acid invertase activity of 0.12 μmol (g fresh weight)^-1 h^-1 was measured at 24 DAB. Insoluble acid invertase activity declined during the early stages of fruit growth and remained relatively low throughout fruit development. Neutral invertase activity was low throughout development, increasing to 5 μmol (g fresh weight)^-1 h^-1 at ripening in GA₃-treated and pollinated fruits. Our studies demonstrate that blueberry fruit development does not appear to be limited by sucrose metabolizing enzyme activity and/or the ability to accumulate sugars in either GA₃-treated or pollinated fruits.     

The Effect of Temperature and Carbon Metabolism on Sucrose Uptake by Detached Tomato Fruits

The effect of temperature on sucrose uptake, and changes inlevels of starch, hexoses and sucrose in detached tomato fruitswas used to investigate the role of the sink in regulation ofcarbon import. Sucrose uptake was lower at 5 °C and greaterat 40 °C than at 25 °C. Conversion of radioactive componentsto starch was lower at both 5 °C and 40 °C than at 25°C, while the levels of non-radioactive starch was similarat all three temperatures. There was a depletion of glucoseand fructose in fruits at 40 °C. Uptake of sucrose froman agar medium by detached tomato fruits was negatively correlatedwith initial sucrose content of the fruit. The results indicatethat carbon import by tomato fruits is largely determined bysucrose levels which can be affected by metabolic activity. Lycopersicon esculentum L., tomato, fruit, sucrose uptake, temperature, carbon metabolism     

The effect of a post-harvest heat-treatment on the storage behaviour of Cox's Orange Pippin apple fruits

Samples from a wide range of commercially grown Cox's Orange Pippin apple fruits were immersed in water at 45° C. for 10 min. to control rotting due to Gloeosporium spp. In two seasons the incidence of rotting following storage for 19 weeks at 3·3° C. (38° F.) was reduced to about 40% of the level in the untreated samples. However, there was a marked increase in the incidence of breakdown and core flush in the treated fruit, particularly when stored in a controlled atmosphere of 5% CO₂, 3 % O₂. A similar but less severe increase in the incidence of breakdown was also recorded in the second season in treated fruit stored in 2% O₂ (with continuous removal of CO₂). The treated fruit tended to develop a more yellow ground colour and softened more rapidly during storage in 5% CO₂, 3% O₂. It is concluded that the treatment advances fruit senescence and that the increased incidence of physiological disorders is likely to limit the use of this method for the control of Gloeosporium rotting in English-grown Cox apples.