Net carbon exchange in grapevine canopies responds rapidly to timing and extent of regulated deficit irrigation.

Whole-canopy net CO2 exchange (NCEC) was measured near key stages of fruit development in grapevines (Vitis vinifera L. cv. Cabernet Sauvignon) that were managed under three approaches to regulated deficit irrigation (RDI): (1) standard practice (RDIS), or weekly replacement of 60-70% of estimated evapotranspiration for well watered grapevines; (2) early additional deficit (RDIE), or one-half of RDIS applied between fruit set and the onset of ripening (veraison), followed by RDIS; and (3) RDIS followed by late additional deficit (RDIL), or one-half of RDIS applied between veraison and harvest. Summed between fruit set and harvest, nearly 40% less irrigation was applied to RDIE vines and ~20% less to RDIL vines than to those continuously under RDIS. After ~5 weeks of additional deficit, NCEC in RDIE vines was 43-46% less per day than in RDIS vines. After RDIL vines had been under additional water deficit for ~3 weeks, NCEC was ~33% less per day than in RDIS vines. Instantaneous rates of NCEC responded rapidly to irrigation delivery and elapsed time between irrigation sets. Concurrent single-leaf measurements (NCEL) reflected the relative differences in NCEC between irrigation treatments, and were linearly associated with NCEC (r2=0.61). Despite halving the water applied under commercial RDI, mid-day stomatal conductance values in RDIE and RDIL of ~50-125mmolm-2s-1 indicated that the additional deficit imposed only moderate water stress. There was no effect of additional deficit on yield or berry maturity.