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.
Douglas D. Archbold
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.
Douglas D. Archbold
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.
Douglas D. Archbold
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.
Douglas D. Archbold
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.
Rumphan Koslanund and Douglas. D. Archbold
Strawberry cultivars grown for “pre-picked” markets need to maintain quality during short-term postharvest storage in contrast to those destined for “U-pick” harvest. However, very little information is available on berry quality during postharvest storage of cultivars grown in matted-row culture in eastern North America. To determine how rapidly berry quality may change and identify cultivars best-suited for pre-picked markets, the postharvest performance of 16 cultivars grown in matted rows was compared. Berries were sampled at harvest, after 3 days of 4 °C storage within sealed plastic bags, and after 3 subsequent days at 20 °C. Quality traits assessed included fruit firmness, color, titratable acidity, pH, soluble solids, and percent weight loss. At harvest, berry quality varied by cultivar and from early to late harvest dates. Berry quality changed very little during 4 °C storage. During the subsequent 20 °C storage, berry quality traits changed more for some cultivars than others. In particular, soft fruit at harvest and/or a rapid decline in berry firmness indicated that several cultivars were not suited for short-term storage. Based on the cumulative data, several cultivars can be identified as better suited for pre-picked markets.
Douglas D. Archbold and Lailiang Cheng
Plants of Fragaria chiloensis cv. RCP-37 were grown with their root system split between two separate containers. Water was withheld from one container of each pair (drought side), while the other was subirrigated. Control plants were subirrigated in both containers. Over several days, a drought-side leaf exhibited reductions in stomatal conductance (gs) and transpiration rate (T), while a subirrigated side leaf showed no change in either parameter. However, foliar water relations components (water, osmotic, and pressure potential) did not differ between the two leaves. The leaf on the subirrigated side exhibited gs, T, and water relations components similar to leaves on control plants. The abscisic acid (ABA) content of xylem exudate, collected from a stolon emerging from the axils of the measured leaves, was highest from the drought side and was negatively correlated to gs and T at some sampling dates. A root-derived drought stress signal, perhaps ABA, although other factors cannot be discounted, was limited within the plant to the drought side, even though water relations components indicated that water from the subirrigated side was allocated to all parts of the plant.
Baolin Zhang and Douglas D. Archbold
Plants of F. chiloensis cv. BSP14 (FC) and F. virginiana cv. NCC85-13V (FV) were stressed until wilting, then watered for 2 days prior to measurement. Diurnal measurements of leaf conductance and water relations were conducted. Leaf conductance of stressed FC plants was generally lower, than that of controls at most times, but there wee no difference between the two in FV. Leaf conductance and transpiration rates had not fully recovered to pre-stress levels within this recovery period, Leaf wafer potential declined from predawn to midday, more in stressed than control plants of both species. Leaf osmotic potential averaged 0.4 and 0.2 MPa lower in stressed than control FC and FV plants, respectively, Greater differences occurred at midday than predawn. Leaf pressure potential of stressed plants was higher predawn than midday, 1.4 vs. 0.7 MPa, in FC; it was not different for FV at most times. The difference in water relations between these two species may be explained by a greater residual effect from the osmotic adjustment in FC es compared to FV that occurred during prior water deficit stress.
Hermen Malik and Douglas D. Archbold
The potential for plant growth regulator (PGR) manipulation of `Chester Thornless' blackberry (fibus spp.) primocane growth was evaluated. PGR treatments included combinations of soil-applied uniconazole at 1, 5, 25, and 125 mg/plant and GA, foliar-applied one or two times at 100 ppm 3 and 4 weeks after a 25-mg/plant uniconazole application. Also, GA and BA were applied at 100 ppm alone or in combination one, two, or three times. Increasing rates of uniconazole reduced primocane length, leaflet count, and leaf, cane, and root dry weights. GA, applications reduced primocane length and increased branch elongation but failed to reverse the effects of uniconazole at 25 mg/plant, except those on branch length, leaflet count, and primocane dry weight. Only applications of BA + GA, increased both branch production and elongation and dry weights of some component tissues, while BA alone generally had no effects. Chemical names used: (E)-1-(p-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-penten-3-ol (uniconazole); N-(phenylmethyl)-1H-purin-6-amine (benzyladenine, BA); gibberellic acid (GA).