The effect of atmospheric and soil conditions on the grapevine water status

[摘要] ENGLISH ABSTRACT: Due to the extraordinary drought resistance of the grapevine, viticulture withoutirrigation in the winter rainfall coastal areas of South Africa is a feasible andcommonly used practice. Wine quality is largely determined by the quality of thegrapes from which it is made. Grapevine physiology is affected both directly andindirectly by water stress, which may vary according to soil type and prevailingatmospheric conditions. The water status of the grapevine can affect grapecomposition profoundly, either directly or indirectly, in either a positive or negativeway, depending on the degree as well as the duration of water stress. There arethree important factors involved in the development of water stress, namely thetranspiration rate, the rate of water movement from the soil to the roots, and therelationship of soil water potential to leaf water potential. All three these factors areaffected by atmospheric and/or soil conditions.In warm winelands such as South Africa (Western Cape), with a mediterraneanclimate which is characterised by a hot, dry summer period, the most importantcharacteristic of soil is its ability to supply sufficient water to the grapevine during theentire growing season. Leaf water potential (Ψl) has gained wide acceptance as afundamental measure of grapevine water status, and has been widely applied inviticultural research. Shortly before dawn, Ψl approaches equilibrium with soil waterpotential and reaches a maximum daily value.The study formed an integral part of a comprehensive, multi-disciplinary researchproject (ARC Infruitec-Nietvoorbij Project No. WW13/01) on the effect of soil andclimate on wine quality, which commenced in 1993 and will be completed in 2004.This study was conducted during the 2002/03 growing season in two Sauvignonblanc vineyards situated at Helshoogte and Papegaaiberg, both in the Stellenboschdistrict, approximately nine kilometres apart. Two experiment plots, representingcontrasting soil types in terms of soil water regime, were selected in each vineyard.At Helshoogte the two soils represented the Tukulu and Hutton forms, and the soilsat Papegaaiberg were of the Avalon and Tukulu forms.The aim of this study was to determine the effect of atmospheric conditions andsoil water status on the level of water stress in the grapevines for each soil at eachlocality, as well as the effect of grapevine water stress on yield and wine quality. Thiswas done by determining and comparing the soil water status, soil water holdingcapacity of the soils and the evapotranspiration of the grapevines on the two differentsoils, at each of the two localities differing in mesoclimate and topography. Theatmospheric conditions at the two localities during the 2002/03 season were alsodetermined and compared to the long-term average atmospheric conditions, and thelevel of water stress of grapevines on each soil at each locality was measured. During the 2002/03 growing season, atmospheric conditions were relatively warmand dry in comparison to the long-term averages of previous seasons. Theseconditions accentuated the effects of certain soil properties that may not comeforward during wetter, normal seasons.The usually wet Tukulu soil at Helshoogte was drier than expected during the2002/03 season compared to the Hutton soil. Due to more vigorous growth on theTukulu soil, grapevines extracted more soil water early in the season, leading to alow soil water matric potential and more water stress in the grapevines. Due to thehigher vigour, resulting in more canopy shading, and more water stress, the dominantaroma in wines from the Tukulu soil was fresh vegetative. The Hutton soil maintainedconsistency with regards to both yield and wine quality compared to previousseasons. On the other hand the Tukulu soil supported a higher yield, but with lowerthan normal wine quality.The Avalon soil at Papegaaiberg maintained the highest soil water potentialtowards the end of the season, probably due to capillary supplementation from thesub-soil. Grapevines on the Tukulu soil at Papegaaiberg experienced much higherwater stress than ones on the other three soils, especially during the later part of theseason. This could be ascribed to a combination of factors, the most important beingthe severe soil compaction at a shallow depth, seriously limiting rooting depth androot distribution, which is detrimental to grapevine performance.Both the soil water status and atmospheric conditions played important roles indetermining the amount of water stress that the grapevines experienced at differentstages. The air temperature and vapour pressure deficit throughout the season wereconsistently lower at Helshoogte, the cooler terroir, compared to Papegaaiberg, thewarmer terroir. At flowering, Ψl was lower for grapevines at Helshoogte than atPapegaaiberg, showing that diurnal grapevine water status was primarily controlledby soil water content. The difference in grapevine water status between the twoterroirs gradually diminished until it was reversed during the post harvest period whenΨl in grapevines at Papegaaiberg tended to be lower compared to those atHelshoogte. The relatively low pre-dawn Ψl at Helshoogte indicated that thegrapevines were subjected to excessive water stress resulting from the low soil watercontent. However, grapevines at Helshoogte did not suffer material water stress (i.e.Ψl < -1.20 MPa) during the warmest part of the day, suggesting that partial stomatalclosure prevented the development of excessive water stress in the grapevines.This suggests that low pre-dawn Ψl values do not necessarily imply thatgrapevines will experience more water stress over the warmer part of the day, or visaversa. This does not rule out the possibility that side-effects of partial stomatal closure, such as reduced photosynthesis, can have negative effects on grapevinefunctioning in general. These results also suggest that measurement of diurnal Ψlcycles at various phenological stages is required to understand and quantify terroireffects on grapevine water status.

[发布机构] Stellenbosch University