Vine Biology and Physiology
What this domain tests
Vine Biology and Physiology examines the grapevine as a managed plant: its taxonomy, annual growth cycle, berry development, hormone signals, source–sink relations, water use and the physiological reason behind vineyard decisions. The exam expects you to connect a field observation, such as delayed veraison or poor fruit set, to a mechanism rather than to a list.
Growth cycle and berry development
Cultivated grapevines belong to Vitis vinifera subsp. vinifera; most commercial vines are grafted onto rootstocks from American Vitis species or hybrids. The annual cycle runs from budbreak through flowering, fruit set, veraison and maturity to leaf fall and dormancy. Berry growth is double-sigmoid: a rapid cell-division phase, a lag phase, then a second phase of cell expansion and sugar accumulation. Ripeness is not a single event: sugar rises, organic acids fall, pH rises, tannins polymerise and aromas develop.
Hormones, source and sink
Auxins and gibberellins dominate early berry growth; abscisic acid and ethylene are central at veraison. The canopy is the source and the clusters are sinks. A high crop load relative to leaf area slows sugar accumulation and delays maturity, while excessive vigour diverts assimilates to shoot tips and laterals. Balanced pruning and canopy management aim at a leaf-area-to-fruit ratio that can ripen the crop.
Water relations and photosynthesis
Stomata regulate water loss and CO2 entry. Mild water deficit can improve fruit quality by limiting berry size and vegetative growth, but severe stress closes stomata, reduces photosynthesis and can cause sunburn or raisining. Rootzone depth, soil texture and evaporative demand determine how quickly stress develops.
Worked example
A sample rises from 18 to 22 °Brix in 27 days. Since 1 °Brix is approximately 10 g/L sugar, the increase is about 40 g/L, or 40 ÷ 27 ≈ 1.5 g/L per day. If the target is 23 °Brix, about 7 more days are needed at that rate, but pH and acid must also be checked because sugar accumulation can slow before phenolic maturity.
Traps that cost marks
Do not treat °Brix alone as maturity. Do not confuse budbreak with flowering, or veraison with harvest ripeness. Remember that rootstocks affect scion vigour, nutrition and water uptake, not just pest resistance. Avoid claiming that all water stress improves quality; timing and severity matter.
Key points
- Grape berry growth follows a double-sigmoid curve.
- Veraison marks colour change and the shift to sugar accumulation.
- Source–sink balance controls ripening speed and vine balance.
- ABA and ethylene are key ripening-related signals.
- Water deficit can be useful or damaging depending on timing and severity.