Moderate salinity stress and its effect on water conservation capacity of in vitro plants of Persian walnut during desiccation

Long term exposure of tissue culture plants to unusual environmental conditions during in vitro production (e.g. high relative humidity, low CO2 concentration during the photoperiod, high ethylene concentration and restricted air movement) induce poor development of leaves (e.g. thin epicuticular wax formation) and disturb normal function of sotmata, which result in high fatality of plantlets after transfer to ex vitro conditions. Osmotic adjustment can improve stomatal response. Therefore the objectives of this study were to investigate the effect of salinity especially moderate salinity stress on reducing uncontrollable water loss in micropropagated walnut explants during ex vitro desiccation.

This experiment was carried out as a completely randomized design (CRD) with four replications. To study the effects of salinity stress, three different concentrations of NaCl (0, 5 and 10 ds.m-1) were used. After stress period, stomatal morphology, transpiration rate, RWC, morphological (e.g. shoot length, specific leaf area, salt injury index, leaf mass area and leaf succulence) and physiological traits (e.g. chlorophyll content, osmotic potential (ψs), proline and glycinebetaine content) were evaluated in micropropagated walnut explants.

According to the results of the present experiment, low concentration of salinity (5 ds.m-1 NaCl) caused a decrease in transpiration rate and an increase in RWC of the leaves of walnut explants during desiccation. Some stomatal traits such as stomata length and width, pore length and pore aperture were improved and led to the closure of the stomata. Salt injury index, proline and glycine betaine increased with increasing salinity stress and osmotic potential (ψs) in the leaves of plants that were grown in salty medium were significantly reduced. Although no significant differences were observed in shoot length, salt stress reduced specific leaf area (SLA), chlorophyll content and increased LMA and LS in 5 and 10 ds.m-1 NaCl concentrations when compared with the control plants.

Due to the role of salinity stress in osmotic adjustment, low concentrations of salinity can increase ex vitro water conservation capacity of in-vitro plants during transplantation and acclimatization through changes in stomatal morphology and improvement of water relations. According to the results of this study, 5 ds.m-1 concentration of NaCl, had less stressful effect on micropropagated shoots of walnut and reduced transpiration and increased RWC when compared with 10 ds.m-1 concentration of salinity. In conclusion, moderate salinity stress increased water conservation capacity and decreased water loss during desiccation of in-vitro explants of Persian walnut, which can have a positive effect on in vitro adaptation of walnut plantlets before transfer to ex vitro condition.