The environmental conditions necessary for floral initiation and development in North Carolina can be arrested by the onset of an early drop in temperatures in the fall soon after planting. Floating rowcovers were placed on plots of three cultivars of strawberry (Fragaria ×ananassa), `Chandler', `Camarosa' and `Sweet Charlie,' for 2 weeks in the fall to determine if fruit yield could be increased by enhancing additional plant growth. Yields were taken on plots with and without rowcovers the following spring. Temperatures and photosynthetic photon flux were monitored under the rowcovers, as well in adjacent plots that were not covered throughout the year. Daily, minimum, maximum and mean temperatures were generally higher under the rowcovers when ambient temperatures were above 10 °C (50 °F). Rowcovers had no effect on leaf area, root, crown, leaf, flower and fruit dry weight in the first year and a minimal affect in the second year. The application of rowcovers increased total yield and marketable yield by 80 to 100 g/plant (0.18 to 0.22 lb/plant) but did not affect fruit weight. This study suggests that rowcovers can improve yield. However further studies are needed to assess effects of time of initial placement and duration of rowcovers on strawberry yield, growth and development.
The objective of this experiment was to determine the effects that altering the probable source-sink relationships would have on subsequent growth and yield components under field conditions. The balance between vegetative and reproductive growth was altered by imposing light stress (shading) on various growth phases, or removing primocanes, floricanes or fruit. Removal of primocanes significantly increased yield the year of removal. However, if primocane removal coincided with canopy shading, this increase in yield was not achieved. Overall, a significant negative correlation existed between 1991 and 1992 yields. Treatments with high yields in 1991 had low yields in 1992, and visa verca. This evidence-suggests that: 1) primocanes and floricanes are competing for light, not photosynthates during the flowering and fruiting period and 2) altering the balance of vegetative and reproductive growth one year had a significant effect on growth the subsequent year.
Gibberellic Acid (GA3) was applied to `Venus' table grape flower clusters seven days after full bloom at 0, 150 and 300 ppm in 1987 and 1988. For both years, berry, cluster and average individual seed trace weights were not significantly affected by GA3 treatment. Total seed traces/berry were reduced an average of 50%, resulting in one seed trace/berry for GA3 treated clusters compared to two seed traces/berry for the control. Yield was increased in 1988 with the 300 ppm rate.
Stomatal and non-stomatal limitations to photosynthesis were determined for both primocanes and floricanes of “Titan” red raspberry. Limitations to photosynthesis were determined from the relationships between rates of photosynthesis (A), stomatal conductance (g), and the internal CO, concentration (Ci) of the leaf. We generated this data (A, g and Ci) using steady state gas exchange. Calculation of limitations were determined from A/Ci and A/g curves, second order polynomial regression and computer simulation. Using methods developed by Farquhar & Sharkey (1982), stomatal limitation during and after fruiting in both primocane and floricane leaves was approximately 28%. Non-stomatal limitations were determined through computer modeling and expressed as the maximum rates of carboxylation, Vcmax, and of electron transpont, Jmax.
A 2-year study was conducted to investigate the influence of the light environment on source-sink relationships in `Titan' red raspberry. Treatments imposed included flower and cane removal in conjunction with partial or whole canopy shading. Raspberry plants were remarkably resistant to a reduction in carbon supply. Yields and primocane production were maintained even when canopies were shaded. Furthermore, if raspberry plants were prevented from producing a full crop in one year, yields the following year tended to be higher than normal. These data, and other studies demonstrating that raspberry roots are strong carbon sinks, suggest that raspberry plants may rely on stored carbohydrate to mature the current crop of fruits when current photosynthate is inadequate. This trait is characteristic of some perennial species adapted to progressively changing environments, but may not be optimal for horticultural situations where growing conditions are relatively constant from year to year. A large root storage capacity and excessive primocane production likely contribute to the relatively low yields that are typical of this species.
Seasonal changes in growth, mean maximal photosynthetic rates, and the temperature and light response curves of `Titan' red raspberry (Rubus idaeus L.) were obtained from potted plants grown under field conditions. Primocane dry weight accumulation increased steadily at the beginning and the end of the season, but growth slowed midseason during fruiting. The slower midseason dry-weight accumulation rate coincided with an increase in root dry weight. Primocane net assimilation rate (NAR) was highest early in the season. Floricane photosynthetic rates (A) were highest during the fruiting period, while primocane A remained steady throughout the season. Primocane and floricane leaflets displayed a midday depression in A under field conditions, with a partial recovery in the late afternoon. Photosynthetic rates of primocane and floricane leaves were very sensitive to temperature, exhibiting a decline from 15 to 40C. Light-response curves differed depending on cane type and time of year. A temporal convergence of sink demand from fruit, primocanes, and roots occurs when plants experience high temperatures. These factors may account for low red raspberry yield.
Yield performance of primocane-fruiting experimental blackberry (Rubus spp.) cultivars from two breeding programs were evaluated in the southern Appalachian Mountains. The trial consisted of the nine experimental cultivars: NC 533, NC 534, NC 535, NC 537, NC 538, NC 539, APF-27, APF-46, and Prime-Jim®. In addition, rowcovers were applied to split plots in the spring to determine if harvest dates could be advanced. In 2007, five experimental cultivars had comparable high total yields (APF-27, APF-46, NC 535, NC 537, and NC 539), yet only three of these had comparable marketable yields (APF 27, APF-46, and NC 537). In 2008, APF-27 and APF-46 had significantly higher total yield (2234.5 and 2257.3 g/plant, respectively) and marketable yield (1611.2 and 1654.2 g/plant, respectively) than all other experimental cultivars. Rowcover treatments did not increase or decrease either total or marketable yield in any of the experimental cultivars in either year. In addition, rowcovers did not advance the date of 5% or 50% harvest in either year. On the basis of the conditions imposed in these studies, we determined that “APF” selections evaluated in this trial have higher second-year yield than “NC” selections, primocane-fruiting experimental cultivar yields are lower than yields commonly found with floricane-fruiting blackberries, and rowcovers as applied in this study are not a viable option for advancing harvest season of primocane-fruiting blackberries in the southern Appalachian Mountains.
An unprecedented freeze occurred between 4 and 10 Apr. 2007, causing extensive crop loss across a large area of the United States. This event occurred late in the spring and temperatures were unusually low for an extended period. Low-temperature injury on small fruit plants was reported in 21 states. Missouri and Arkansas experienced the highest estimated percentages of crop loss of grape (Vitis spp.), strawberry (Fragraria ×ananassa Duch.), blueberry (Vaccinium spp.), and blackberry (Rubus subgenus Rubus Watson). Kentucky and Tennessee also reported high percentages of small fruit crop loss. Temperatures preceding the freeze event in the affected region were unusually warm and many of the crops were at a more advanced stage of growth than they would have been under more usual conditions. Although frost/freeze warnings were issued, the terminology used by different weather forecasters was inconsistent. Growers used various cold protection methods, but these were generally ineffective because of the stage of plant development and/or the advective nature of the freeze. Actual grape and blueberry crop losses may not be known for several years because of secondary injury to plant tissues from various pathogens.