Incubator Initiative – Are boron levels in sandy soils adequate to ensure reliable fruit set and would the application of boron have a beneficial effect?

Abstract

Optimum nutrition at flowering and fruit set is crucial for grapevines to attain desired reproductive capacity. Boron (B) is reported to be involved in many aspects of plant reproductive physiology. This project aimed to investigate the effect of B as a foliar application at three concentrations (0.025%, 0.05%, 0.1%) and two phenological stages (EL-16, EL-23) on fruit-set of grapevines in the Riverland region in South Australia. A Verdelho block with a history of poor fruit set and documented foliar and soil B deficiency was used for this study. The foliar B application did not increase B concentrations either in petioles or in inflorescences in alignment with the treatments. Fruit set percentage, coulure index and millerandage index were not significantly improved following a single season of foliar treatments. Soil B concentration was highly variable across the trial block, and likely masked the effect of the foliar B treatments. Moreover, vine Fe, Mg2+ and Ca2+ were deficient. Additionally, hail and storm events around flowering and fruit set period likely impacted the development of the reproductive tissues. Drawbacks in conducting nutrition trials for a single season have been identified and recommendations have been made.

Summary

Foliar boron (B) application during floral development and anthesis has been proven effective at promoting flowering and increasing fruit set and yield in a variety of perennial tree crops. Observation by industry in the Riverland region found a positive response to the application of foliar boron at flowering in some crops such as almonds, where the tissue B concentration was marginal. Soil B availability is influenced by many factors such as soil texture, pH, soil moisture, and interactions with other nutrients. Many of the Riverland soils are low in B, which can be attributed to its sandy texture and the consequent leaching of B into the lower layers of soil. This project investigated the effect of foliar B applications in the form of boric acid at two critical phenological stages on (a) fruit set and yield parameters and (b) concentrations of B and other mineral elements in inflorescences and petioles at three sampling times of grapevine cultivar Verdelho grown in the Riverland region. The aim was to determine when and at what concentration B application will be most effective at improving fruit set and yield.

Soil sampling pointed towards variable B concentration across the block, and minimal impact of foliar B spray on fruit set and yield parameters. Despite foliar application, B concentration in the flowers and petioles was not significantly increased compared to the control, indicating that the applied B was not efficiently and/or uniformly absorbed by the plants, or if it was absorbed, potentially translocated to organs other than inflorescences and petioles. This suggests that higher boric acid concentration in the spray may be required or, alternatively, the addition of an adjuvant may be necessary to improve the efficiency of uptake. The 2021-22 season was a particularly harsh season with storms and hail incidents during the flowering and fruit set period in the Riverland Region. Canopy damage likely had an adverse effect on fruit set, complicating interpretation. This suggests the requirement for trials over multiple seasons to not only deal with the complex dynamics of internal nutrient mobilisation and partitioning within the vine, but also the additional complexity of environmental conditions.