Influence of water stress and exogenous cytokinin application on growth and gas exchange of young 'Trevatt' apricot grown under aeroponic system and controlled environment conditions

Uniform two years old apricot (Prunus armeniaca L.) trees cv. 'Trevatt' on plum rootstock were allocated randomly to 8 aeroponic tanks (13 trees tank(-1)) located in two controlled climate rooms set to similar environmental conditions. The experiment was established at 20/18 degrees C day/night temperature regime, this being gradually increased during the first 4 weeks to 28/24 degrees C day/night at the time of starting treatments, this initial period being the acclimation time. Similarly the vapour pressure deficit (VPD) was increased gradually over 10 weeks to 18 mb from 7 mb at the time of establishment and 10 mb at the start of treatments. The objective was to apply a gradually increasing water stress by increasing VPD and limiting watering frequency. All trees in all of the tanks received continuous misting with nutrient solution only during the acclimation period. After the acclimation period (4 weeks) each treatment was located in two diagonally opposite aeroponic tanks within a room, each tank being one block. Treatments were arranged in a randomized complete block design (RCBD). The treatments were: 1) control with continuous misting, 2) 5 ppm BAP with continuous misting, 3) 5 ppm BAP with intermittent misting (BAP+WS) and 4) water stress (WS). The physiological response of trees to applied treatments was monitored using measured characters such as shoot growth, xylem (leaf) water potential (Psi(xylem)), leaf photosynthesis (A) and stomatal conductance (g(s)). In general control and BAP showed greater shoot length throughout the experiment than the 2 water stress treatments. In addition Psi(xylem) of control and BAP was higher than for all the water stress treatments during the experiment. In terms of leaf photosynthesis, shortly after commencing treatments all treatments showed significantly lower A than control (P < 0.05). The level of gs at this time was higher in control than in water stress treatments. At the end of the experiment BAP had significantly higher A than the other treatments (P < 0.05), while showing higher g(s) (P < 0.05) than most other treatments. In conclusion, with a similar initial water deficit level, but in the presence of exogenous root applied BAP, the reduction in (BAP+WS) shoot growth was much less than in the water stress treatment. This suggested that in water stress conditions the presence of BAP (BAP+WS) reduced the negative effects of water stress on vegetative growth. This might be explained by lower gs or A value in water stress treatment, although higher gs on BAP+WS relative to WS treatment led to higher A only early in the treatment.