Water stress improves whole-canopy water use efficiency and berry composition of cv. Sangiovese (Vitis vinifera L.) grapevines grafted on the new drought-tolerant rootstock M4

•Cv. Sangiovese behaviour on SO4 and M4 was compared.•A water deficit at 50% actual water use was imposed for 25 days across veraison.•Stressed M4 shower higher canopy water use efficiency (+22%) than SO4.•Despite similar yield decrease, stressed M4 had higher °Brix and total anthocyanins than SO4.•Differential gene regulation in the two genotypes for color synthesis is postulated. Testing of new rootstocks for drought tolerance targets traditionally rain-fed districts where supplemental irrigation is more frequently needed due to the pressures of global warming. A seasonal evaluation of whole-canopy gas exchange, water-use efficiency (WUEc), yield components and compositional traits of Vitis vinifera cv. Sangiovese grafted to the new drought-tolerant genotype M4 against the commercial SO4 stock is reported. The experiment was conducted in 2015 on twelve four-year-old, fruiting potted Sangiovese grapevines grafted on M4 and SO4 stocks and assigned to SO4-WW (well-watered), SO4-WS (water-stressed), M4-WW and M4-WS treatments. Water deficit was imposed pre-veraison by reducing water supply to 50% of whole-canopy demand derived from concurrent measurements of transpiration in WW and maintained until three weeks after veraison prior to full rewatering. While WUEc was unchanged in WS-SO4 as compared to WW-SO4, WUEc in WS-M4 increased by 22% vs. WW-M4 over the whole water deficit period and such gain was partially maintained upon rewatering. Higher WUEc in WS-M4 resulted from an ability to maintain canopy photosynthesis similar to WS-SO4 at a reduced water use. Although yield per vine was similarly reduced in the two WS treatments (about 1kg less than WW), overall grape composition was improved in WS-M4 and worsened in WS-SO4 when compared to the WW controls. Total soluble solids (°Brix) rose by 11% in WS-M4 vs. the respective control, whereas in WS-S04 there was a slight decrease (−0.6 Brix). Most notably, anthocyanins accumulation was largely limited in vines grafted on SO4 (−45% vs. WW-SO4 when given on a concentration basis), while in M4-grafted plants berry pigmentation slightly improved vs. WW (+12.5%). Hypothesis is made that grafting onto different stocks can trigger differential gene regulation under water stress and high temperatures leading to different sensitivity in synthesis and/or degradation of already formed anthocyanins.