Introduction. Consumers and processors value tomatoes with high fruit sugarcontent; however, most breeding and cultural practices negatively impact this trait. Wildtomato species can accumulate two- to three-fold more fruit sugar than cultivars and areproving to be valuable both as a source of high-sugar loci to broaden thegenetic base of currently produced cultivars, and as research material to understand thistrait. Synthesis. While cutting-edge genomic approaches have taught us muchabout fruit phenotypes, it is still important to assess fruit enzyme activities andmetabolic fluxes in lines with contrasting fruit sugar accumulation. These metabolicfunctions are closest to the ripe fruit sugar trait. In this review, we focus ourattention on the biochemical pathways, especially starch biosynthesis, that may influencetomato fruit sugars. We try where possible to put this information into a physiologicalcontext because together they influence yield. We compare and contrast sugar metabolism incultivars and wild tomato species and identify factors that may influence differences intheir fruit size. Conclusion. Although difficult, we show that it is possibleto develop fruit with high horticultural yield and use the breeding line ‘Solara’ as anexample. In addition, we suggest avenues of further investigation to understand theregulation and control of fruit carbohydrate content.

Introduction. Drought stress is the major factor affecting growth,development and production of walnut trees. In Iran, approximately 33 Mha of land isaffected by salinization and drought stress. Finding genetic resources tolerant to droughtstress at different growth stages is important for such semi-arid regions. Our aim was tounderstand better the adaptive mechanisms that enable different genotypes of walnutpopulation to survive under drought stress, and to provide some useful clues for walnuttree breeding toward improved drought tolerance with utilization of existingdrought-tolerant genetic resources. Materials and methods. To study themechanism(s) involved in drought tolerance of some Persian walnut genotypes, droughtstress was induced using polyethylene glycol-6000 to produce water potentials of 0 Mpa(control), –0.10 MPa, –0.50 MPa, –0.75 MPa, –1.00 MPa, –1.50 MPa and –2.00 MPa. The amountof proline and soluble sugar accumulation in four walnut genotypes(‘Panegine20’, ‘Lara’, ‘Serr’ and ‘Chandler’) were determined after beingexposed to the various water potential levels. Results. The rates of seedgermination in all genotypes were significantly reduced by low external water potentials.Plants exposed to water stress had a higher amount of soluble sugars in roots and shootsof tolerant genotypes (‘Panegine20’ and ‘Chandler’) and a lower amount ofstarch in their tissues. These results imply the important roles of soluble sugars assolutes conferring resistance to drought in these genotypes. The free proline levels werealso increased in response to drought stress. They were higher in drought-tolerantgenotypes than in sensitive ones (‘Lara’ and ‘Serr’). Proline increased more in shootsthan in roots. However, the soluble sugar and starch fluctuations were higher in theroots. Conclusion. Our results support a direct correlation between thedegree of drought stress and proline content. As a consequence, proline concentrationscould be used as a biochemical marker of drought stress level in walnut plants.