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60 p r a c t i c a l w i n e r y & v i n e ya r d J U ly 2 0 1 4 g r a p e g r o w i n g severe water stress and become vulnerable to sudden heat waves. Irrigating at 100% ET c may not be sufficient to release water stress, as that may only maintain the cur- rent levels of soil moisture and water stress. Monitoring soil/vine water status during this process would be useful. With partial root-zone drying (PRD), irrigation is applied alternately to two spatially separated root-zones. 23 There- fore one-half of the roots are water- stressed, while the other half are not. Similar to RDI, ABA is produced in the water-stressed roots, and it modifies g s and shoot growth. 23 Different from RDI, vine remains relatively stable, owing to the continuing water supply from the irrigated half of root-zone. It also has been reported that final yield and berry size are less affected by PRD, compared with RDI. 10 Because water stress is applied over space, timing is less critical for PRD. Due to the adequate water supply from the irrigated root- zone, PRD is more forgiving of error. The dry and wet parts of the root-zone need to be alternated in order to stimulate continuous production of stress signals and avoid root death. Depending on the soil and weather conditions, PRD could be alternated every five to 14 days. 10,11 The similarities and differences between RDI and PRD are summarized in Table II. Properly managed deficit irrigation (either RDI or PRD) can control canopy growth, improve berry quality and substantially save irrigation water. 11 Close and accurate monitoring of soil/vine water status is necessary for precise management. Match-making: Design irrigation strategies according to your goals The fundamentals and principles dis- cussed above could serve as a basis to design an irrigation strategy. Yet more importantly, an irrigation strategy should be tailored according to specific vineyard conditions and production goals. Many wine grape varieties respond dif- ferently to water stress due to their isohy- dricity. Isohydricity refers to the ability of a vine to maintain a certain level of water status during water stress, through control of stomata. Near-isohydric varieties (such as Grenache) tend to react "pessimistically" to water stress by decreasing stomatal aperture, decreasing gs and E, and there- fore maintaining a (relatively) stable vine . On the other hand, near-anisohydric varieties (such as Syrah and Chardon- nay) behave "optimistically" to water stress, and therefore maintain g s and E by decreasing vine . The complex mechanisms for isohydric and anisohydric behaviors have attracted extensive research interests. 24 Due to the difference in varietal responses, g s and E of near-anisohydric varieties or leaf of near-isohydric varieties are not as sensi- tive to changes in soil compared with their counterparts. 20 Therefore, knowing the isohydric nature of varieties is impor- tant when choosing parameters of vine water status to aid in irrigation schedul- ing. More research is required to identify the isohydricity of more varieties and to solve disputes in previous reports. 10 Owing to the nature of white winemak- ing, small berry size is not necessarily advantageous for white grape produc- tion. Additionally, high phenolic content (especially flavonols) in white grapes caused by water stress is responsible for bitterness and unstable color. 9,25 There- fore, an irrigation schedule designed for red varieties (especially if it aims to reduce berry size) should not be applied to white varieties. Moreover, based on desired characteris- Irriga on scheduling approach Evapotranspira on Soil water status Vine water status Pros Easy to track, if weather Direct measurements Direct assessments sta on data available. of soil water availability; of vine responses; easy to calculate integrate irriga on amount; environmental and prac cal. soil effects. Cons Need to be adjusted May require extensive Most parameters locally (varie es, soil monitoring due to soil require high technical types, vineyard layout, heterogeneity; require support and an mesoclimates) by knowledge of soil types. established threshold modifying crop to calculate irriga on coefficient. amount; require repeated monitoring. table I. Summary of the pros and cons of different irrigation scheduling approaches.