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  • Author or Editor: D.D. Archbold x
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Abstract

Strawberry (Fragaria × ananassa Duch. cv. Tristar) fruit explants and tissue disks were used for short term in vitro studies of [14C] sucrose import. An aqueous feeding solution of 50 mm Mes, 10 mm CaCl2, 10 mm EDTA, and 600 mm [14C] sucrose at pH 5.5 resulted in maximum accumulation of sucrose by fruit expiants during 2 hr. Young fruit exhibit the highest sink activity (sucrose imported per mg fruit dry weight), a value that declined as fruit aged. Sink strength (total sucrose accumulated per fruit) was positively correlated to fruit dry weight at 13 to 15 days after pollination. Inclusion of abscisic acid (ABA) in the incubation solution of fruit expiants and cortex disks increased sucrose accumulation 45% and 37%, respectively, above control values.

Open Access

Following June drop, apple fruit growth depends on sorbitol import as the primary source of carbon. Sorbitol dehydrogenase plays a key role in carbohydrate metabolism by conversion of sorbitol to fructose, which then enters the general carbohydrate pool. Blocking the pathway and eliminating the source of sorbitol to the fruit by girdling the stem and defoliation after June drop resulted in a decline and eventual cessation of fruit growth. The fruit did not abscise however. Fruit sorbitol and starch levels declined while the fructose, glucose, and sucrose pools did not change. SDH activity declined to low levels and was not detectable in many fruit. The decline in SDH activity was evident 1 week after applying the treatments. A few fruit that resumed growth, presumably after the vascular connection was re-established across the girdle, exhibited normal SDH activity. Feeding sorbitol to whole fruit in vitro via the cut stem raised SDH activity in some fruit, although it was still below control levels.

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Plants of a diverse collection of Fragaria clones from a range of native habitats representing F. chiloensis, F. virginiana, F. virginiana glauca, and F. vesca, were grown in a controlled environment at one of three day/night temperatures, 15/15, 23/15, or 31/15°C. Relative growth rate (RGR) and net assimilation rate (NAR) were estimated from plant leaf areas and total dry weights. At 23/15°C, the species mean RGR and NAR values were comparable although clones within species exhibited significant variation. At 15/15 and 31/15°C, RGR and NAR for species were lower than at 23/15°C. At 31/15°C, chiloensis and vesca mean values were reduced more than the others, to less than 50% the 23/15°C values. Also, NAR declined most for chiloensis, to 45% the 23/15°C value. At 15/15°C, virginiana had much higher RGR and NAR values than the other species, and its NAR mean value was greater than at 23/15°C. Although the species means would suggest that there are interspecific differences in temperature response, intraspecific variability was also large. Thus, classifying Fragaria species by temperature response may be an over-generalization.

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Maintenance of positive cell turgor is an essential factor in cell, and fruit, expansion. Since apple fruit partition carbohydrates between the starch and soluble pools to maintain turgor, variation among cultivars in this osmoregulatory aspect may play an important role in defining cultivar-specific fruit growth rates. Cultivar-specific apple fruit growth rates were determined over a 6 week period following June drop during 2 seasons. Fruit water relations parameters and carbohydrate levels were also measured. Although cultivar differences were evident, generally, fruit absolute growth rate increased, relative growth rate (RGR) declined, water potential and osmotic potential declined, and turgor potential increased as the season progressed. Soluble carbohydrate levels increased over 6 weeks, while starch levels fluctuated. Soluble carbohydrates contributed 50 to 90% of the osmotic potential. RGR was not correlated to either turgor potential or the relative allocation of carbohydrates between the soluble and starch pools. Thus, although positive turgor was maintained, factors other than turgor per se determine fruit growth rate.

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Absolute and relative fruit growth rates (AGR and RGR) of 5 cultivars were calculated from the oven-dry weights of fruits harvested periodically throughout the growing season. Both AGR and RGR were higher for larger fruit of different cultivars with similar days to maturity, and for summer- versus fall-ripening cultivars. Seasonal variability in AGR and RGR was observed, Apple fruit cortex disks were incubated in 14C-sorbitol solutions in vitro to determine if uptake rates at the cellular level varied between cultivars. Rates of sorbitol accumulation, expressed es μg sorbitol per mg dry weight cortex tissue, declined as the season progressed. Within a cuitivar, uptake rates were not relatad to fruit size, nor were differences found between cortex tissue samples from competing fruit on a spur. Sorbitol uptake rates were significantly lower for the more slowly-growing cultivar. The osmotic potential of the expressed cortex sap, sampled on several dates, was consistently lower for the more rapidly-growing cultivar. Thus, inherent differences in fruit growth rates among cultivars may be due to variation in regulation of osmotic potential.

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Over 4 years, using estimates of fruit dry weight derived from diameter measurements in situ, cultivar variation in apple fruit relative growth rate (RGR) in the period following June drop was evident. These differences diminished as the season progressed however. Using estimates of dry weight per cell, fruit cell absolute growth rate increased over time and RGR showed no clear pattern in contrast to the RGR of whole fruit. There were no cultivar differences in carbohydrate allocation among the soluble, starch, and remaining ethanol-insoluble, non-hydrolyzable pools irrespective of cultivar RGR. The storage carbohydrate pool comprised an increasing fraction of the total dry weight over time with the starch pool comprising 10 to 25% of the storage carbohydrate, varying with season and cultivar. Neither fruit competition within a cluster nor post-June drop thinning altered fruit RGR or carbohydrate allocation patterns when compared to fruit thinned post-bloom.

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Absolute and relative fruit growth rates (AGR and RGR) of apple (Malus domestics Borkh.) were calculated from the fruit dry weights of several cultivars harvested periodically following June drop during 1988-90. AGRs were constant or varied slightly, and RGRs generally declined as the season progressed. Generally, both AGR and RGR values were higher for relatively large fruit of several cultivars with similar days to maturity, e.g., `McIntosh' vs. `Jonathan' and for summervs. fall-ripening cultivars, e.g., `Stayman' vs. others. An exception was observed in 1990, when `Golden Delicious' exhibited a higher AGR but lower RGR than `Rome Beauty', yet ripened 1 month earlier. `Golden Delicious' AGR and RGR values were lower for both fruit of a pair on a spur than the values for a single fruit on a spur, and the dominant fruit of the pair exhibited higher growth rates than the inferior fruit. Rates of sorbitol accumulation (SAR) by cortex disks incubated in 14C-labeled sorbitol solutions in vitro declined as the season progressed. Within a cultivar, SARS were not related to fruit size, nor were differences found between cortex disks from competing fruit on a spur, although SARS were higher for both competing fruit on a spur as compared to that of a single fruit per spur. Due to a positive correlation between RGR and SAR values, the SAR of cortex cells may be regulated in such a manner as to be a physiological constraint on fruit sink strength and growth rate.

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Significant seasonal fluctuations in the soluble and insoluble reduced N pools of strawberry roots and crowns suggest seasonal alterations in the activities of the enzymes responsible for nitrogen assimilation. The activities of glutamine synthetase (GS), NADH- and ferredoxin-glutamate synthase (NADH- and Fd-GOGAT), and glutamate dehydrogenase (GDH) were assayed from extracts of `Allstar' strawberry roots sampled at intervals over 12 months from matted row beds. GS activity was high during late winter and declined 16-fold by the summer and fall. Although Fd-GOGAT activity was not detected, NADH-GOGAT activity exhibited over 5-fold higher levels during bloom and fruiting than at other times of the year. GDH activity fluctuated over 8-fold during the year with the highest levels occurring during the spring growth flush and after bed renovation. The seasonal changes in enzyme activities corresponded to significant events in plant development and indicate involvement of these enzymes in N assimilation and cycling in strawberry.

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The activity of sorbitol dehydrogenase (SDH) may be the primary determinant of apple fruit carbohydrate accumulation, perhaps defining cultivar-specific fruit growth rates. During two intervals following June drop, fruit SDH activity of 5 cultivars was assayed along with measurements of fruit relative growth rate (RGR) and total soluble carbohydrate (TSC) and sorbitol levels of fruit cortex tissue. SDH activity varied over 2-fold among cultivars in interval 1 and nearly 7-fold in interval 2. SDH activity was not correlated to fruit RGR or cortex TSC level, but it was negatively correlated to cortex sorbitol level. Defoliation and girdling to restrict sorbitol import slowed fruit growth and reduced SDH activity to nondetectable levels. Growth resumption, presumably following phloem re-establishment in the girdled zone, was accompanied by substantial SDH activity. SDH activity appears to be cultivar-specific and may be influenced by substrate, i.e., sorbitol, availability. While SDH likely plays a key role in apple fruit carbohydrate accumulation, its activity alone may not determine cultivar-specific fruit growth rate.

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