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Trials for frost protection on blackcurrants using concentrations of a high-molecular-weight surfactant, Teric 12A23B, are described. In a preliminary frost-simulation trial, the optimum concentration for frost protection was 0.25% with a decline in effectiveness at higher concentrations. A similar result was obtained in a field trial. Differential thermal analysis was used to determine the freezing-point of treated flowers, and these followed the same trends as the fruit set data, with protection down to −6.25°C. Yields were not affected significantly in the field trial, but the frosts were relatively mild and overall damage was minimal. A second field trial to compare multiple applications of the protectant showed no advantage of 2 or 3 applications compared to 1 application at flower emergence, and a significant disadvantage with a fourth spray. Overall, the results reinforce the feasibility of chemical frost protection of blackcurrants. It appears to offer protection similar to overhead sprinkling with a single application of a readily available spray material.

DEPEG, a chemical with known frost-protective properties (a cryoprotectant), and 5 other high-molecular-weight surfactant chemicals were first tested for phytotoxicity on blackcurrant leaves and subsequently for cryoprotectant activity on blackcurrant flowers. Within the concentration-range tested (0.5–8% w/v), there was no visible damage. Four chemicals caused a decrease in leaf water potential at low concentrations. Higher concentrations had no such effect. These 4 chemicals were then tested, at the concentrations which caused changes in water potential, for cryoprotectant activity on flowering blackcurrant bushes subjected to spring frost, in a commercial plantation. Teric 12A23B- and DEPEG-treated plants gave significant increases in fruit set and yield following relatively mild spring frost conditions. Nuclear magnetic resonance spectra were determined for treated flowers at 0 and −2°C. The proportion of free unfrozen water at −2°C compared with 0°C was significantly higher than the control treatments for the chemicals which gave the highest yields. The results are discussed in terms of a possible relationship between the ability of the chemicals to induce changes in plant water characteristics and cryoprotective effects.

Potted blackcurrant plants subjected to moisture stress following harvest produced 50% less total fruit yield in the following season than plants which received regular irrigation. The factors which contributed to this effect were a reduction in the numbers of productive buds, flowers per bud and fruit set. An increase in seed number per fruit in response to the stress suggested that reserve carbohydrates accumulated during the autumn may have been the controlling factor in fruit set. Bud-burst in the stressed plants was both early and uneven, radial growth was reduced, and there was no evidence of starch accumulation at the onset of dormancy. All plants received the same irrigation treatment in the spring following treatment, and there was no significant difference in fruit size except for a few positions at the distal end of the strig. Fruit on stressed plants in this case were the smallest.

Treatment of 2-year-old blackcurrant bushes, at the early grape stage of flowering, with a 0.5% v/v spray of DEPEG prevented abscission of flowers following exposure to a simulated frost of −2°C for 3 hours. Untreated bushes subjected to the same low-temperature treatment suffered a 25% reduction in fruit set.