COMPARING WATER USE EFFICIENCY OF APPLES AND GRAPES - PHYSIOLOGICAL AND MORPHOLOGICAL ASPECTS

Abstract

Water use efficiency (WUE) is of critical importance for crop production with dwindling water resources in a warming climate. WUE is carbon uptake, dry matter accumulation or yield per unit of water lost or applied as irrigation. It can be expressed from organ-level gas exchange to whole plant or crop dry matter gain per unit of water use. Leaf stomatal behavior and leaf and canopy morphological aspects of WUE are examined by comparing apples that have a good WUE to grapevines that generally have lower WUE. Stomatal regulation is critical to WUE as it affects both CO2 and water exchange, especially in response to VPD and coupling to photo-synthesis. Apple stomata in the field are normally highly coupled to the photo-synthesis, avoiding excessive opening, and maintaining very good WUE. Grapes tend to open stomates more than needed unless water stressed. Unless water-stressed, grape stomata normally will open more than appropriate for best WUE. With whole canopies, WUE may be improved by reducing the amount of heavily shaded leaf area since shade reduces photosynthesis more than transpiration. Leaf size effects on leaf heating may be important to WUE as large entire leaves heat up when stomata close, increasing the leaf-to-air VPD partially and counterbalancing the stomatal closure. This is especially noticeable in large-leaf grapevines compared to smaller-leaf apples. Potential integrated methods of improving WUE of grapes may need to consider more finely-divided leaf forms and physiological mechanisms that more tightly couple stomatal conductance to photosynthesis.