Handbook for Irrigation of Wine Grapes in South Africa

Chapter 6

Grapevine water status

6.1 INTRODUCTION Plants need to absorb water stored in the soil to allow sustainable physiological functioning. When plants experience water constraints, it will have negative effects on a number of plant physiological processes. However, the level of water constraints where these processes begin to respond negatively differs substantially (Fig. 6.1). For example, cell growth and wall synthesis that determine root, shoot and fruit growth, are more sensitive to water constraints than sugar accumulation. In practice, this means that a grapevine will show strong vegetative growth and produce large berries if it is well watered. On the other hand, less irrigation, i.e. more water constraints, might be necessary to enhance sugar accumulation. In some plants, stomatal opening and CO 2 assimilation that are prerequisites for photosynthesis, i.e. one of the most important physiological processes that depends on water (Fig. 6.2), can be relatively insensitive to water constraints. The foregoing implies that subjecting grapevines to water constraints will prevent excessive vegetative growth, produce smaller berries and enhance sugar accumulation without compromising on photosynthesis. This is of particular importance if high quality wine production is the objective. Figure 6.1

Sensitivity to stress

Very sensitive

Relatively insensitive

Tissue osmotic potential required to affect process

Process Cell growth Wall synthesis Protein synthesis Protochlorophyl formation

0 MPa

-1 MPa

-2 MPa

Nitrate reductase ABA accumulation Cytokinin level Stomatal opening CO 2 assimilation Respiration Proline accumulation Sugar accumulation

FIGURE 6.1. Schematic illustration of the level of water constraints that will induce negative effects on plant physiological processes (adapted from Hsiao, 1973).

146 CHAPTER 6 – GRAPEVINE WATER STATUS