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  • Author or Editor: Douglas D. Archbold x
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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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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.

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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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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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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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Total N and fertilizer N (FN) recovery and use by June-bearing `Redchief' strawberry (Fragaria ×ananassa Duch.) and day-neutral `Tribute' grown in matted-row beds were studied over l-year periods. Fertilizer N was field-applied as NH NO at planting in June or September, and all plants were harvested from bed sections in late autumn (November) and at the completion of the spring harvest (June). Distribution patterns of vegetative biomass were similar in both cultivars, with leaf tissue comprising the bulk of the vegetative dry weight per plant at both sampling dates. The fall and spring fruit crops each contributed >40% of the total biomass per plant. Total N accumulation from soil N and FN increased as total biomass increased. Due in part to the additional biomass of the fall fruit crop, `Tribute' recovered 38% more total N per unit bed area than `Redchief'. Over 30% of the fall N total in `Tribute' and the spring N total in both cultivars was partitioned to the fruit. In both cultivars, greater recovery of FN applied in September that at planting time was observed by the postharvest sampling date. However, `Tribute' recovered only 14.2% of the FN applied in September, much less than the amount recovered by `Redchief' during the same interval, implying a diminished ability to absorb FN during fruiting. In all vegetative tissues, soluble reduced N (SRN) was consistently less than insoluble reduced N (IRN) in November and June. Consistent seasonal trends in SRN and IRN values were not evident in any tissue except roots, where SRN content declined from November to June. Allocation of FN to the SRN and IRN pools was related to FN application date, cropping pattern, and total biomass of the component tissue. In both cultivars, the FN content was greater in the IRN than the SRN pool and leaf IRN was the single largest vegetative sink for FN. Fruit N concentration was constant for most of the fall `Tribute' harvest period and declined in both cultivars during spring harvest. The spring `Tribute' fruit crop received more FN from the September than the planting application, while the fall crop exhibited the opposite pattern, suggesting the fruit crop receives more storage than newly absorbed FN. The accumulation of FN in the root SRN pool in November and its depletion through the spring harvest reveals that root SRN plays a significant role in the temporary seasonal storage and internal cycling of N remobilized during spring growth.

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As the primary nutrient applied to and used by strawberry, N allocation and cycling within the plant may play an important role in determining plant vigor and productivity. Our objectives were to determine 1) how N availability and fruit production affect N and fertilizer N (FN) partitioning among and within the vegetative tissues of `Tribute' strawberry (Fragaria ×ananassa Duch.) and 2) if the root N pool is temporary storage N. Plants were fed 15N-depleted NH4NO3 (0.001 atom percent 15N) for the initial 8 weeks, then were grown for 12 weeks with or without NH4NO3 with a natural 15N abundance (0.366 atom percent 15N), and were maintained vegetative or allowed to fruit. The vegetative tissues were sampled at 6 and 12 weeks. Neither N availability or fruiting had consistent effects on dry mass (DM) across all tissues at 6 or 12 weeks. At 6 weeks, the total N content of all tissues except the roots were higher with continuous N than with no N. Nitrogen availability was the dominant treatment effect on all plants at 12 weeks; continuous N increased leaflet, petiole, and total vegetative DM and total N of all tissues. Insoluble reduced N (IRN) was the major N pool within all tissues at 6 and 12 weeks regardless of treatment. Fruiting inhibited root growth and N accumulation at 6 weeks but had little effect at 12 weeks. The roots were a strong dry matter and N sink from 6 to 12 weeks. The FN pools, from the 15N-depleted FN supplied during the initial 8 weeks, exhibited changes similar to those of total N in plants not receiving N, in contrast to plants receiving continuous N where total leaflet and petiole N content increased while FN content declined. Total FN per plant declined nearly 26% over 12 weeks; the decline was greater in plants receiving N continuously than in those not receiving N, but the magnitude of the decline was not affected by fruiting. Increasing atom percent 15N values, primarily in plants receiving continuous N after the initial 8 weeks of receiving 15N-depleted FN, indicated that N cycling occurred through all tissues and N pools, proportionally more in the soluble reduced N pool but quantitatively more in the IRN pool. The root N pool was not a “temporary” N storage site available for re-allocation to other tissues, although N cycling through it was evident. Rather, leaflet N was primarily remobilized to other tissues.

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The effect of dehydration stress on membrane competence among and within Fragaria species was evaluated using index of injury, I d , and tissue ionic conductance, g Ti. Single accessions of F. chiloensis ssp. lucida Duch., F. virginiana ssp. glauca (S. Watson) Staudt, F. virginiana ssp. virginiana Duch., F. ×ananassa Duch., and F. vesca L. were used to study interspecific variation. Leaf thickness and total electrolyte content were greatest for the F. chiloensis ssp. lucida accession and least for the F. virginiana ssp. glauca accession, but foliar electrolyte concentration did not vary across accessions. The g Ti values were >5-fold higher from 0 to 2 hours than for other intervals, declining over time. Significant differences in g Ti and I d values were only evident at 2 and 4 hours within stress levels, and increased as stress level increased. While the F. chiloensis ssp. lucida accession exhibited low g Ti values at 70% relative water content (RWC), it showed greater relative membrane injury than the other species expressed as g Ti , I d , or the ratio of stress g Ti to control g Ti as dehydration level increased. Although the F. virginiana ssp. glauca accession had the highest g Ti values, even at 100% RWC, its relative injury as stress level increased was not as great as that of the F. chiloensis accession. In a second experiment, intraspecific variation was examined using four accessions each of F. chiloensis and of F. virginiana which were dehydrated to 50% RWC. The species mean g Ti , I d , and g Ti ratio values at 2 and 4 hours for the F. chiloensis accessions were lower than those for the F. virginiana accessions, but significant intraspecific variation was also observed. In spite of the differences between species means, the evidence of intraspecific variation indicates that not all accessions of a species exhibit similar drought responses (i.e., membrane competence). Due to the consistent conclusions derived from using either g Ti or I d after 2 or 4 hours of incubation for characterization of membrane competence, g Ti and I d were comparable techniques for identification of potential drought tolerance in Fragaria.

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