The effects of physical and chemical factors on vegetative and reproductive growth, and on must composition of Vitis vinifera cv. Cabernet Sauvignon

Abstract

Several aspects of water stress, nutrient stress, and other growth manipulation techniques, including root restriction and paclobutrazol, were imposed on fruiting cuttings of Vitis vinifera L. cv. Cabernet Sauvignon to investigate the response of vegetative and reproductive growth, and must composition. A wick watering system was used to supply and control media moisture in most of the water stress experiments and a hydroponic system was adopted in the root restriction and paclobutrazol experiments.

Most vegetative growth parameters responded linearly, and positively, to root volume (150, 450, 900 ml) and inversely to the dosage of paclohutrazol (0, 125, 250 ppm, two applications) but the latter was more effective in controlling vine growth than the former. When a particular factor was having a strong influence on growth suppression, vines responded to the other factor to a much lesser extent. Nutrient stress, at one fifth of the standard concentration, suppressed shoot growth more than did short water stress cycles. More detailed examination of water stress showed the degree of growth suppression was as follows: drying out of the whole root system reduced growth more than partial root zone drying; continuous stress reduced vigour more than alternate stress; early and long stress was better than late and short stress. The effect of partial root zone drying tended to diminish as more roots grew in the moist area. In most cases the shoot (excluding leaves) was the part affected most by all growth manipulation techniques.

Water and nutrient stress which commenced before flowering reduced berry number possibly due to early bunch stem necrosis. Water stress, beginning about three weeks after flowering, reduced the number of berries in one of two years when the stress was severe. Water stress starting from veraison, root restriction and paclobutrazol, did not have significant effect on berry number.

Yield loss occurred when berry number and/or berry size was reduced by the treatments. Paclobutrazol did not significantly decrease yield, but water stress started before veraison, and severe (continuous) stress started from veraison, significantly reduced berry size and therefore final yield. Water stress which significantly reduced shoot growth, but did not affect berry number and size, did not affect yield.

Root restriction increased the harvest index and the low rate of paclobutrazol increased it further. A high rate of paclobutrazol maximized the index but when combined with root restriction, the index decreased. All stress treatments increased the harvest index except: water stress which started before flowering, early and severe continuous water stress, and nutrient stress. In these cases the index decreased.

Degree Brix was increased under nutrient stress and root restriction but decreased with paclobutrazol. Water stress had no effect on °Brix unless the degree of stress was high in which case °Brix was decreased.

Nutrient stress and root restriction had no effect on must pH, but early continuous water stress increased it, while late continuous stress and paclobutrazol decreased it. Water stress which was short (starting from veraison) and did not cause severe stress, did not affect the pH, but if the development of stress was fast and severe, then the pH was decreased.

Water stress decreased anthocyanin/g fresh skin. If berry size was significantly decreased, then anthocyanin/g fresh berry was not affected unless the water stress was severe and prolonged, or berry size was not decreased, in which case anthocyanin/g fresh berry was decreased.

Maturity of berries was very much advanced in short stress cycles. The effect was diminished when the stress cycle was longer and became much delayed in continuous and severe stress. A combination of short stress cycles and high level of nutrients advanced maturity the most. The onset of veraison was not normally affected but the length of maturation was prolonged under severe stress. Paclobutrazol, especially at the high rate (250 ppm x 2) and root restriction advanced maturity. The advancing effect was more pronounced at high root volume and low dosage of paclobutrazol, respectively.

Immature berries wilt much more readily than the mature ones in response to water stress. However they re-hydrated quite rapidly upon re-watering. Stressed ripe berries did not rehydrate after re-watering. The analysis of the berries showed no change in must composition and percentage moisture content 17 hours after re-watering.

Deformability of grape berries increased rapidly as they approached veraison and the skin colour appeared one day before the maximum deformability rate was reached.

From the practical point of view, it is suggested that water stress starting after flowering will maximize berry number and reduce berry size thereby increasing skin to pulp ratio. To advance maturity, stress should be imposed before the onset of veraison with short repeated stress cycles until all berries turn colour. From this point on, mild water stress can be used to control shoot growth without affecting yield and must pH. Root restriction can be used to reduce vegetative growth in the following circumstances: in a high rainfall area, where the water table is high or where soils have a high water holding capacity. It can be used when slightly higher °Brix and/or early maturation of berries is desirable. Although paclobutrazol reduced °Brix at harvest (due to early maturation and harvesting), the effect of decreasing must pH makes it suitable for use in hot arid areas to counteract high pH. It may also prevent vines from severe water stress due to the reduction of water use.