Insights into seasonal yield variation in subtropical table grape production

Table grape (cv. Menindee Seedless) production from a vineyard in Emerald, Central Queensland, demonstrated a strong alternate bearing pattern in seven consecutive seasons (P < 0.02) and irregular bearing in another seven seasons. Variation in vine yield was 78–90% attributable to the variation in bunch number. A sampling methodology for estimation of vine yield was recommended, and a machine vision-based system for rapid estimation of berry number and size was introduced. Low yields were attributed to low bud fertility, with only one inflorescence primordium (IP) in fruitful buds, and to poor development of IP during hot summers. IP developmental stages stretched across two seasons, with IP initiation and development in season 1 and floral primordia formation on IP in season 2. The hypothesis that IP initiation and survival is related to carbohydrate availability was not supported by the results of a two season sucrose supplementation trial. Total non-structural carbohydrate (TNC) reserves did not recover between flowering and harvest in untreated (control) plants, and decreased in root and trunk tissues from 13.5% and 7.5% w/dw at leaf fall to 7.2% and 3.7% w/dw at flowering, respectively. A continuous low pressure (69 kPa) stem injection system delivered an average of 150 g of sucrose per season into vines estimated to contain 500 g of carbohydrate at flowering but this did not alter the root or trunk TNC reserve content. Tissue δ13C (‰) values were consistent with partitioning of injected sucrose to young shoots. The comparatively low sucrose loading and variability between vines in the field by stem injection was ascribed to necrotic xylem and a decline in hydraulic conductance. Sucrose equivalent to 37% of the total plant dry weight was introduced into shoot cuttings through transpiration flow over four days, however photosynthetic rate is decreased, limiting impact of sucrose injection on TNC status. Hydraulic conductance of shoot cuttings was enhanced about 20% by perfusion with 10 mM KCl compared to water, a result attributed to an effect on pectin hydrogels in pit membranes, but conductance nevertheless had declined by approximately 90% in 72 hours. Gibberellic acid (GA3) applied at earlier stages of flowering (10% and 30% caps-off) increased inflorescence count per vine in the following season by 93% to 122% per vine relative to a control with no GA3 application, justifying a proposed split application strategy. Future research should test the proposal that inter-seasonal yield variation in Menindee Seedless is linked to low bud fruitfulness, triggered by extreme temperatures during IP formation.