Factors Affecting Plant Tissue Hyperhydricity at in vitro Culture and Strategies to Resolve it Case Study: (Dianthus caryophyllus L.)

Hyperhydricity is the development of an undesirable physiological and morphological change in plant tissue. In vitrified plants, the number of vascular bundles, stomata and cuticle cells are reduced and mesophyll cells contain massive vacuole. Several factors influence this phenomenon and the most important of these factors include: type of the plant growth regulator, type and concentration of cytokinin, ratio of ammonium to nitrate in the culture medium, type and concentration of agar, ventilation and number of subculturing. Increasing agar concentrations leads to decreasing this phenomenon due to reducing the moisture of culture medium. Increasing concentration of cytokinins and decreasing the ratio of nitrate to ammonium in the medium cause the occurrence of hyperhydricity. Presence of BA in compare to other types of cytokinins and also phytagel in compare to other type of gelling agent enhance the rate of hyperhydricity. Use of Parafilm cap decreases the amount of this phenomenon due to increasing gas exchange and decreasing accumulation of ethylene. The application of bottom cooling systems can reduce the activity of antioxidant enzymes and therefore cause reduction in the amount of hyperhydricity. By considering the reported results, to reduce the amount of hyperhydricity in vitro, using cytokinins with lower concentration, replacing BA with other commonly used cytokinins and if possible use lower concentrations of BA (0.5 mg/l), using Parafilm as vessel closure cap, increasing the concentration of agar to an acceptable level (10 g/l), using bottom cooling, reducing the frequency of sub-culturing as much as possible and acclimating tissue culture plantlets as soon as possible, is recommended.