Extracellular invertase is the key enzyme of an apoplasmic phloem unloading pathway and catalyses the hydrolytic cleavage of the transport sugar sucrose released into the apoplast. This mechanism contributes to long-distance assimilate transport, provides the substrate to sustain heterotrophic growth and generates metabolic signals known to effect various processes of primary metabolism and defence responses. The essential function of extracellular invertase for supplying carbohydrates to sink organs was demonstrated by the finding that antisense repression of an anther-specific isoenzyme provides an efficient method for metabolic engineering of male sterility. The regulation of extracellular invertase by all classes of phytohormones indicates an essential link between the molecular mechanism of phytohormone action and primary metabolism. The up-regulation of extracellular invertase appears to be a common response to various biotic and abiotic stress-related stimuli such as pathogen infection and salt stress, in addition to specific stress-related reactions. Based on the observed co-ordinated regulation of source/sink relations and defence responses by sugars and stress-related stimuli, the identified activation of distinct subsets of MAP kinases provides a mechanism for signal integration and distribution within such complex networks. Sucrose derivatives not synthesized by higher plants, such as turanose, were shown to elicit responses distinctly different from metabolizable sugars and are rather perceived as stress-related stimuli.

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The salt tolerance of the commercial F1 tomato hybrid (Lycopersicon esculentum Mill) Radja has been agronomically and physiologically evaluated under greenhouse conditions, using a control (nutrient solution), a moderate (70 mM NaC1 added to the nutrient solution) and a high salt level (140 mM NaC1), applied for 130 days.The results show that Radja is a Na +-excluder genotype, tolerant to moderate salinity. Fruit yield was reduced by 16% and 60% and the shoot biomass by 30% and more than 75% under moderate and high salinities, respectively. At 90 days of salt treatment (DST), the mature leaves feeding the 4th truss at fruiting accumulated little Na + (178 mmol kg-l DW). At this time, the sucrose concentration in these leaves even increased with moderate salinity and the amino acid proline was not accumulated under salt conditions as compared to control. At 130 DST, Na + was accumulated mainly by the roots in proportion to the salt level applied, while in leaves appreciable amounts were found only at high salinity (452 mmol kg-I DW). In the leaves, CI-was always accumulated in proportion to the salt level and in a very much greater amounts than Na + (until 1640 mmol kg-I DW). The sucrose content was reduced in all plants by salinity, and was distributed preferentially toward the distal stem and peduncle of a truss at fruiting under moderate salinity, and toward the basal stem and root at high salinity. Moreover, proline was accumulated in different organs of the plant only at high salinity, coinciding with Na + accumulation in leaves. Attempts are made to find a clear relationship between physiological behaviour triggered by stress and the agronomical behaviour, in order to assess the validity of physiological traits used for salt-tolerance selection and breeding in tomato.

Soluble sugar content has been studied in relation to sucrose metabolism in the hexose-accumulating cultivated tomato Lycopersicon esculentum Mill, the wild relative species Lycopersicon cheesmanii Riley, in the sucrose-accumulating wild relative species Lycopersicon chmielewskii Rick, Kesicky, Fobes & Holle. and in two hexose-accumulating interspecific F1 hybrids (L. esculentum × L. cheesmanii; L. esculentum × L. chmielewskii), cultivated under two irrigation regimes (control: EC = 2.1 and saline: EC = 8.4 dS m–1). Under control conditions the total soluble sugar content (as hexose equivalents) in the ripe fruits of L. cheesmanii was 3-fold higher than in L. esculentum, while L. chmielewskii and both F1 hybrids contained twice as much as the cultivar. With the exception of L. esculentum × L. cheesmanii, salinity increased the sugar content by 1.3 (wild species) and 1.7 times (cultivar and L. esculentum × L. chmielewskii) with respect to control fruits. Wild germplasm or salinity provided two different mechanisms for the increases in fruit sugar content. The hexoses accumulated in ripe fruits were strongly influenced by those accumulated at the start of ripening, but the hydrolysed starch before start of ripening only partially explained the final hexose levels and especially the increase under salinity. The early cell wall acid invertase and the late neutral invertase activities appeared to be related to the amount of hexoses accumulated in ripe fruits. However, no metabolic parameter was positively related to the amount of sugar accumulated (including sucrose). The major differences between genotypes appeared in ripe fruits, in which up to 50% of the total amount of sugars accumulated in the wild species (mainly in L. cheesmanii) and hybrids cannot be explained by the sugars accumulated and the starch hydrolysed before the start of ripening stage. As a consequence, the higher fruit quality of the wild species compared with L. esculentum may depend more on the continuation of sucrose import during ripening than on osmotic or metabolic particularities such as the hexose / sucrose-accumulator character or specific enzyme activities.

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SummaryCytokinins and cell wall invertase are positive players in regulating fruit sink strength, growth, and yield under salinity as components of the same signalling cascade establishing and developing sink organs.

The different growth responses under salinity in relation to the carbon partitioning and sucrose metabolism in both sink and source organs have been studied in a salt‐tolerant (cv. Pera) and in a salt‐sensitive (cv. Volgogradskij) tomato genotype (Lycopersicon esculentum Mill.). After 3 weeks of salinization, the plant dry weight was reduced by 12–34% in cv. Pera and by 45–58% in cv. Volgogradskij. Photosynthesis was positively correlated to plant growth in cv. Pera but not in cv. Volgogradskij. In this salt‐sensitive genotype, both photosynthesis and growth were negatively correlated with fructose, glucose and sucrose accumulation in both mature and young leaves, suggesting a blockage in their use for growth. The transient accumulation of sucrose and hexoses in the young leaves of cv. Pera was linked to increases in all soluble sucrolytic activities, mainly acid invertase (EC 3.2.1.25) and sucrose synthase (EC 2.4.1.13), which was related to sink activity and growth capacity. The sucrose‐phosphate synthase activity (EC 2.4.1.14) was related to the ability of mature leaves to regulate assimilate production, accumulation and export. The salt‐tolerant cv. Pera accumulated a higher amount of total carbohydrates, but cv. Volgogradskij showed the highest soluble fraction under salinity. The carbohydrate availability and the photosynthetic rate do not seem to be the first limiting factors for plant growth under saline conditions, but the different behavior observed in both genotypes concerning the distribution and use of photoassimilates could help to explain their different salt‐tolerance degrees.