Growth responses to water stress and vapour pressure deficit in nectarine

Diel growth dynamic is one of the most significant interest areas of plant-water relations, with practical applications for irrigation scheduling. In the present study, we determined the effects of two irrigation regimes: well-watered (WW) and water-stressed (WS) on diel branch growth dynamic. Dendrometers were placed on main branches of 10-year-old nectarine trees for both treatments, in a commercial orchard. The readings were monitored at ~15 min intervals. The amount of irrigation water was determined using evapotranspiration (ET) values, with full irrigation for WW and 25% water restriction for WS. During this period, the days that were high (1.62 kPa) and low (0.11 kPa) in maximum (max) vapour pressure deficit (VPD) were taken as extreme days. The timing of the maximum diameter increment of the branch in both extreme days did not change notably with irrigation treatment and VPD. In the high VPD day, the maximum diameter of the branch was earlier compared to the low VPD day. However, the timing of minimum diameter showed a difference between water regimes on the low VPD day, with an earlier/later minimum on WW-branch. The higher VPD in WW increased the maximum diel shrinkage in the branch compared to WS. As a whole, our study showed that for a given water regime, the diameter growth of a branch may be strongly altered by VPD.