Graeme E. Hobson

The constituents of tomato fruit — the influence of environment, nutrition, and genotype

It is proposed to describe briefly the rise in popularity of the tomato during the 20th century to become the worlds fourth most significant fruit, making an important contribution to human nutrition. The grading standards for fruit used in various major tomato-producing countries will be outlined. A major part of the review will be devoted to describing changes in composition during the maturation, ripening, preservation, and storage of good quality tomatoes especially as regards carbohydrates, organic and amino acids, proteins, steroids, pigments, minerals, and the lipids, volatiles, and other minor constituents. A range within which composition should normally fall will be given. Additionally, the effects on composition of environment, cultivar, nutrition, and physiological disorders inter alia will be described. How new growing methods and genetic manipulation could influence the tomato of the future will also be considered.

The Inhibition of Tomato Fruit Ripening by Silver

Mature green tomato fruit, infiltrated with STS (up to 10 μmol) while still attached to the plant, ripened unevenly to give extensive green areas on an otherwise red background. Pericarp wall tissue from the two contrasting areas was analysed for various organic constituents. Both the green and, to a certain extent, the red tissue from treated fruit showed differences from normal in AIS, acidity, and PE activity. PG activity, which usually increases rapidly as tomatoes ripen, was low in the green but not significantly different from normal in the red tissue from STS-treated fruit. TEM examination revealed that electron-dense particles were present in the cell walls of phloem elements in vascular bundles of the green tissue, but these deposits were not found in the red tissue from the same fruit. X-ray microanalysis of the particles suggested that they contained concentrations of silver and sulphur. The results are interpreted as suggesting that silver is affecting those sites in the cell that would subsequently be involved in promoting the synthesis of PG.

Differential Effect of Silver Ions on the Accumulation of Ripening-related mRNAs in Tomato Fruit

Summary The infiltration of silver thiosulphate into intact mature green tomato ( Lycopersicon esculentum Mill.) fruit via the peduncle produced asymmetrical ripening, with areas receiving silver remaining green on an otherwise ripe tomato. Silver-containing areas were inhibited from ripening for 3–4 weeks, but subsequently coloured and softened slowly. Changes in pigments, enzyme activity and gene expression in control and inhibited pericarp were examined over a 7-week period following silver application. Silver delayed the normal ripening-associated increases in lycopene, PG and invertase by 3 weeks. Invertase eventually reached normal levels in silver-treated fruit, but PG and lycopene remained unusually low. Changes in the concentration of specific mRNAs were examined using ripening-related cDNA clones as hybridization probes. The accumulation of some mRNAs was largely unaltered by silver, although the appearance of others was delayed until week 4. The extent of inhibition varied greatly, the PG mRNA being the most severely reduced. The results support the idea that silver acts by blocking ethylene perception. However, the expression of some ripening-related genes occurs independently of ethylene perception or action, whereas others require continuous ethylene above a threshold level for full expression. PG fits into the second category.

Pectic enzymes associated with the softening of tomato fruit

Abstract Some degree of softening of maturing tomato fruit takes place before the usually accepted mechanism for the loss of firmness, involving the sequential action of pectin esterase and polygalacturonase, is operative to any extent. The possibility that pectin and pectic acid transeliminases were implicated, providing an alternative pathway for the breakdown of pectin in the tomato, was investigated. Direct spectrophotometric determination of these enzymes proved unreliable and the thiobarbituric acid test, which should give a specific colour reaction with the products, was negative. It is concluded that transeliminases do not play a significant part in the softening of tomato fruit either prior to or during ripening.

Electrophoretic investigation of enzymes from developing Lycopersicon esculentum fruit

Abstract Acetone powders were prepared from tomato fruit tissue sampled during development and the proteins were separated by polyacrylamide disc gel electrophoresis. The gels were stained to show up general proteins, lipoproteins, glycoproteins and certain enzymes. Minor changes in protein and glycoprotein patterns accompanied development. Most enzymes exhibited more than one active band, with maximum diversity and specific activities usually appearing in extracts from mature green tissue and least with over-ripe tissue. The results support the view that enzyme synthesis accompanies the climacteric respiration rise at the expense of non-metabolic protein.

The effects of alleles at the “Never ripe” locus on the ripening of tomato fruit

Abstract The tomato variety “Potentate” has been modified by the introduction of the dominant allele (Nr) at the “Never ripe” locus. Fruit from the resulting plants ripen and soften much more slowly than control fruit. This behaviour is associated with a much reduced solubilization of the pectic substances and a considerable attenuation in the activity of the pectic enzyme polygalacturonase. There is evidence of incomplete dominance at the “Never ripe” locus. The analyses provide additional evidence for an association between the action of the pectic enzymes and the ripening processes of tomato fruit.

Glutamate oxaloacetate transaminase activity in developing Lycopersicon esculentum fruit

Abstract Glutamate oxaloacetate transaminase (GOT) occurs in both the mitochondrial and cytoplasmic fractions from tomato fruit tissue. Changes in activity of the enzyme from fruit at selected developmental stages have been followed. The combined activity fell from the immature green stage to the full red condition whilst the proportion in the mitochondria reached a peak in green-orange fruit. The activity of cytoplasmic, but not mitochondrial, GOT was stimulated by the addition of pyridoxal-5-phosphate. In the green areas of fruit showing blotchy ripening, the combined activity was equivalent to that in normal immature green fruit but with a much higher proportion of the activity in the mitochondria. Mitochondrial GOT could constitute a system in ripening tomato fruit whereby the accumulation of inhibitory concentrations of oxaloacetate affecting the oxidation of succinate and malate might be controlled.

The oxidation of malate by mitochondria from normal and abnormal tomato fruit

Abstract An improved method for the isolation of mitochondria from the outer walls of tomato fruit during maturation and ripening has been developed and the oxidation of malate by the particles so obtained has been studied. When particles from fruit showing some external red colour were tested, especially in the presence of thiamine pyrophosphate or sodium glutamate (plus pyridoxal phosphate), either of which increased the rate of malate oxidation considerably, a short period of inhibited oxidation interposed between the ADP-stimulated rate (State 3) and the true ADP-limited rate (State 4) was found. Mitochondria from both the green and the red areas of non-uniformly ripened (“blotchy”) fruit behaved similarly. In any set of conditions the extent of the inhibition was proportional to the amount of ADP available to the mitochondria. It is suggested that the inhibition is due to an accumulation of oxaloacetate and that the addition of any substance capable of lowering its concentration increases the rate of malate oxidation.

Salt-stress induces partial ripening of the nor tomato mutant but expression of only some ripening-related genes

Summary Tomato plants homozygous for the character non-ripening (nor) when grown in normal nutritional conditions produce fruit that change colour at maturity only very slowly, in a typical non-climacteric fruit behaviour pattern. However, by subjecting the plants to salinity stress, at the end of development the fruit redden and partially ripen. Wild type cv. Ailsa Craig plants and an isogenic selection containing the alleles nor/nor were grown in deep solution culture at conductivity levels of either 3 or 8 mS cm -1 (mS) in the first season, and 3, 9 or 15 mS in a subsequent year. Various physiological and chemical parameters concerning fruit development were measured. As salt-stress was increased, the average fruit weight decreased along with the firmness index, while the percentage dry matter, the tomato colour index (TCI), acidity, sugar levels and K + ion concentrations in the fruit sap increased. Na + ion concentrations reached a plateau at moderate salinities and then decreased at even higher levels. No activity for the pectic enzyme polygalacturonase (PG) was detected in any nor fruit, whereas salt-stress induced high levels of invertase activity. Total cellular RNA was extracted from both wild type and nor fruit from plants grown normally, and from nor fruit of plants grown at high salinity. The expression of four ripening-related genes was examined using cDNA probes. One mRNA, corresponding to clone pTOM4, was at similar levels in 3 mS nor and wild type fruit, whilst two, corresponding to pTOM5 and pTOM13, were present in substantially reduced levels in nor tomatoes compared to parental lines. No mRNA was detected in nor fruit corresponding to pTOM6, a cDNA clone for PG, which is abundant in normal ripe tomatoes. Salt-stress on nor plants had no apparent effect on pTOM4, pTOM6 or pTOM13 mRNA levels in the fruit, but the mRNA for pTOM5 was greatly induced, in parallel with colour development. The results emphasize that there is strict control of development at the physiological, enzymic and gene expression levels, but high salinity can partially overcome the nor lesion without the involvement of PG.

Ethylene and the Control of Tomato Fruit Ripening

The sequence of events collectively known as ripening is the single most dramatic event in the life of climacteric fruit. It is quite clear that in the tomato, ribonucleic acid (1) and protein synthesis (see 2) are involved in the process, and a great deal of study has gone into an elucidation of the sequence of changes, especially in terms of precise alterations in enzymic components of various fruits (3–7). While specific proteins play a part in promoting ripening, it is possible that increased turnover, activation and transfer of proteins across membranes can best explain the process, possibly with ethylene being concerned with the initiation and co-ordination of many of the separate events.