Effect of cytokinin and auxin application on double cropping performance in Vitis vinifera: preliminary findings

Abstract

Double cropping is a novel technique, driven by the extension of the growing season caused by global warming. Previous field studies have shown that feasibility of double cropping in Vitis vinifera cultivars grown in a temperate climate relies on bud forcing efficiency. Key factors include a prompt unlocking of the apical dormant buds which should also have reached sufficient fruitfulness. While it can be assumed that bud dormancy release and subsequent shoot development are influenced by hormonal balance and correlative inhibition, this study hypothesized that foliar applications of exogenous cytokinin (CK) and auxin (NAA) could modulate these processes, potentially affecting bud forcing dynamics and shoot fruitfulness. Forcing treatments consisted of trimming primary shoots at the eighth node and removing all lateral shoots, whereas main leaves and inflorescence developed until the trimming cut were retained. Five treatments were tested: unforced control (UC), control + cytokinin (CCK), Forcing (FR), forcing + cytokinin (FCK), forcing + auxin application (FNAA).

Results showed that among the forcing treatments, FCK achieved the highest bud forcing success rate (106%), expressed as the ratio between forced to primary shoots, followed by F (94%) and FNAA (21%). Primary yield was similar across all treatments (2.74 kg), whereas total yield (primary + forced crop) was highest in FCK (out of a total of 4.78 kg, 2.02 kg were forced grapes), followed by FR (3.62 kg with 1.09 kg of forced grapes) and with no forced crop in FNAA (2.68 kg). Single leaf assimilation (A) rates, measured on 5th to 8th node, were higher in forced leaves (10.9 in FR and 11.2 μmol/m2/s in FCK vines) than primary leaves, which ranged from 6.3 in FNAA to 7.9 in UC, and to 8.3 μmol/m2/s in CCK vines. The harvest of the forced crop was delayed by 47 days compared to the primary crop. Despite cooler late season conditions, temperatures remained adequate for the maturation of forced grapes. They exhibited higher total soluble solids (22.0 in FR and 21.4 °Brix in FCK vines) compared to their primary grapes (18.1 and 19.8 °Brix, respectively) coupled with a significantly higher titratable acidity (10.0 in FR and 11.1 g/L in FCK vines) than primary grapes (4.9 g/L, as average). The study indicates that in Vitis vinifera, exogenous CK and NAA can respectively enhance or impair dormant bud forcing, with CK promoting the feasibility of double cropping in the same vine under temperate climates.