|Irrigation technique for sustainable viticulture and premium quality grapes
Water resources in South Africa are limited and regarded as scarce on a global scale. The situation is worsened by population growth and the demands of a vibrant economy. According to current patterns of utilisation, South Africa will reach its limits of economically usable, land-based fresh water around the year 2030 (Basson, 1997). Since we can no longer rely on developing another water resource we are compelled to re-evaluate our current strategies of water use. Water, although renewable, is a finite resource that is distributed unevenly both geographically and through time. Therefore, its use and distribution will become more and more expensive as the demand grows in competing sectors.
Water use in South Africa is dominated by irrigated agriculture, representing about 54% in 1997, while domestic and urban use of water constituted 11%, mining 8% and afforestation 8% (Basson, 1997). By far the dominant growth in water requirements is foreseen in the domestic and industrial sectors, largely driven by population growth and industrialisation. The total requirements for water in these sectors are estimated to grow at roughly 3% per annum. A survey done in July 2001 by the Department of Water Affairs and Forestry showed that only 51% of all South Africans, about 23 million people, have access to free basic water. Supplying more people with free basic water and a continually growing urban sector will undoubtedly put pressure on agricultural sectors like the wine industry, dominated by irrigation practices, to use less water. This will become increasingly difficult as the number of hectares under grapevines continues to increase at the current rate (Figure 1). What is required is a conscious and strategic change in the use and conservation of our most precious resource.
Increased water use efficiency
Vineyard water use efficiency (crop produced per unit water applied; WUE) is a critical issue for the expansion and sustainability of irrigated viticulture. Rainfall aside, WUE is an outcome of the efficiency with which irrigation water reaches the grapevine and the efficiency with which water is transpired in fixing carbon. Grapevine transpiration efficiency (W) is defined as W=A/T, where A = carbon assimilation for dry matter production, and T = transpiration. Vineyard WUE can therefore be improved by either cultivar selection with an inherently higher W or the use of irrigation water saving technology. Apart from the obvious cost benefits, using less irrigation water is also an environmentally sound practice. Not only does it save water, it reduces the amount of pollution of our rivers and underground aquifers. Excessive irrigation beyond the effective root zone causes the loss of costly fertilizers and minerals that leach into our waterways, damaging our basic ecosystem. Over the years this could turn a fresh, renewable water resource into a contaminated or saline water source. Irrigation with saline water not only impacts negatively on plant growth, crop yield and quality but breaks down soil structure, changing soil characteristics and pH. Excessive irrigation on soils with layered characteristics where drainage is a problem may lead to the formation of a watertable within the rooting zone. When a watertable forms within the rooting zone it leads to poorly aerated conditions for root respiration and function. Extended periods of saturation due to a rising watertable during the growing season may cause a loss of yield and grape quality due to the lack of oxygen and the production of toxins in the roots. A watertable may also cause capillary movement of water and minerals to the surface where it causes soil salinisation (van Zyl, 1981).
The PRD irrigation system
PRD requires the frequent irrigation of approximately half of the root system while the other half is left to dry (Figure 2). After a certain period of time the ‘wet’ and ‘dry’ zones are alternated, allowing the former ‘wet’ zone to dry while the ‘dry’ zone is irrigated (Dry and Loveys, 1999). The adjustment of a normal one dripper line per vine row to a two dripper line per vine row with offset drippers that can be operated independently can achieve the desired wetting pattern. PRD irrigation can start when normal irrigation commences and depending on type of soil and climatic conditions, the alternation of ‘wet’ and ‘dry’ zones would typically occur on a ten to fifteen day cycle.
The PRD system is thought to rely on hormonal signals originating from the roots in response to low soil water potentials within the ‘dry’ zone. Much evidence has been accumulated that drying roots are the origin of abscisic acid (ABA), which is involved in regulating stomatal aperture (Zhang and Davies, 1990; Davies et al., 1994). Normally, the closure of stomata in response to drying soil conditions serves to protect leaf tissue from excessive loss of moisture, thereby conserving water by reducing transpiration. In the PRD system the vine is given a false sense of water stress, because one root zone is constantly exposed to low soil water potentials, producing ABA and sending a signal to the above ground organs. The observed effects of ABA in above ground organs due to PRD are a reduction in shoot growth and partial stomatal closure (Dry and Loveys, 1999). Without alternating the ‘wet’ and ‘dry’ sides, i.e. wetting only one side of the vine while the other side continues to dry out, has shown that the hormonal signal diminishes and stomatal conductance and shoot growth rate will start to recover after a certain period of time.
Monitoring the PRD system
The irrigation regimes implemented during the PRD period are shown in Figure 3 (measured by the Enviroscan soil moisture sensor system). Enviroscan – probes were installed on either side of the vine within the wetting zones. The soil water content of the ‘wet’ zone was never allowed to fall below a certain soil water content (Refill point 1) to avoid any water deficits. The PRD cycle was achieved by switching the wetting zones as soon as the soil water content in the ‘dry’ zone reached refill point 2. Refill point 2 is an arbitrary value where the slope of the graph of the soil water content in the ‘dry’ zone flattens to indicate a low rate of soil drying.
As illustrated in Figure 3, the ‘wet’ zone of the PRD system was irrigated when the soil water content reached refill point 2. Refill point 2 corresponded closely to the refill point calculated in a normal irrigation regime and therefore the ‘wet’ zone constituted a normal irrigation regime. The rate at which a specific soil dries depends on its characteristics and would therefore determine the length of the PRD cycle. The soil type depicted in Figure 3 is a sandy loam and it is therefore expected that a more sandy soil would have a higher rate of drying and would need a shorter PRD cycle. Conversely a longer PRD cycle may be possible with a heavier soil.
PRD field performance
PRD is an irrigation system developed in grapevines with a consistent feature in the Australian environment that there is usually no significant reduction in yield (Dry et al., 2001) even though the amount of irrigation water has been substantially reduced in comparison to normal irrigation practices, thereby greatly increasing water use efficiency (WUE). In my experiments PRD received 50% less irrigation water than conventional drip irrigation and increased the WUE by about 85% (Table 1). PRD has the effect of controlling excessive vegetative growth in grapevines, leading to a reduced canopy density and better vine balance with decreased costs of maintenance (Dry et al., 2001). Dense canopies generated in vines with excess vigour produce more bunch shading and difficulties in disease control due to poor spray penetration. Shading experiments done by Archer & Strauss (1989) indicate the potential negative effects caused by dense canopies in excessively vigorous vines. A negative correlation was found between the degree of shading and wine quality of Cabernet Sauvignon. Increased shading caused an increase in potassium concentration, pH and titratable acids while skin colour was also considerably reduced. While other irrigation management techniques such as regulated deficit irrigation (RDI) may reduce vigour, they are often accompanied by a penalty in yield (Goodwin and Jerie, 1992; Dry et al., 2001). PRD reduces canopy density due to a reduction in total leaf area (Figure 4), the result of an effect on both main and lateral shoot growth (Table 1), but mostly on the latter. This PRD effect on vegetative growth is not contributed to just a reduction in irrigation water. Similar experiments on field grown Shiraz where PRD grapevines received the same amount of water as control grapevines showed a similar physiological response (data not shown).
PRD berry characteristics
Experiments under Australian conditions showed no effect of PRD on berry pH and Brix of Cabernet Sauvignon but significantly reduced berry size with minimal effects on crop yield. Smaller berries may have significantly positive effects on berry and wine quality because the skin surface per unit berry weight or volume would be increased. Singleton (1972) found that even a 10% decrease in average berry size without a change in berry composition produced red wine with recognizable and therefore important increases in aroma, colour, tannin and quality. Over the past two years PRD treatments of Cabernet Sauvignon showed minimal effects on berry sugar composition (Table 2) but marked increases in amino acid levels on a per gram fresh weight basis (Table 3). Significant increases were found in amino acids contributing the largest portion of the total amino acid contents in mature berries i.e. proline, arginine, valine and isoleucine. Amino acids are the most readily available source of nitrogen to yeast in musts and the combination of high free amino acids and low total nitrogen content in berries may enhance wine quality (Bena-Tzourou et al., 1999; Hernandez-orte et al., 1999). Musts rich in amino acids not only influence the fermentation process positively but also enrich the volatile components of wines. PRD has been judged to produce wines of higher quality where bunches of PRD vines were much better exposed than control vines (Dry et al., 2000). Cabernet Sauvignon wine made from PRD fruit in the 1996/7 season was rated more highly for aroma characteristics than wine from control fruit. Qualitative changes were found in the anthocyanin pigments of Cabernet Sauvignon brought about by PRD treatment. The concentration of the derivatives of delphinidin, cyanidin and petunidin in berries were increased relatively more than derivatives of malvidin and peonidin. PRD furthermore enhanced the formation of the non-coumarate forms of anthocyanins. Although the impact on wine colour and stability is still unclear it may be associated with better bunch exposure due to a change in canopy microclimate. However, in recent experiments where the difference between relatively open control canopies and PRD canopies were less apparent, there were no apparent effects on fruit quality.
In an arid environment such as South Africa where agriculture needs to be more competitive in its use of natural resources, PRD could be a viable alternative. PRD experiments in the Australian environment using half of the normal amount of irrigation water had no detrimental effect on crop yield, thereby increasing the water use efficiency of vineyards by more than 80%. PRD reduced canopy density by reducing total leaf area with its effect on main and lateral shoot growth, thereby producing vines with better vigour balance and bunch exposure. This was found irrespective of the amount of irrigation water applied. Furthermore, PRD with half the amount of irrigation water as control grapevines significantly reduced berry size with positive ramifications on amino acid content, which may produce wines rated with higher quality.
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About the Author:
Gerhard du Toit, Department of Horticulture, Viticulture & Oenology, University of Adelaide, South Australia, E-mail: firstname.lastname@example.org
Supervisors: Assoc. Prof. Peter Dry (University of Adelaide) and Dr. Brian Loveys (CSIRO Plant Industry)