Treatment with calcium chloride enhances water deficit stress tolerance in Viola (Viola cornuta)

Water deficit stress can reduce the postproduction shelf life and marketability of floriculture crops. To alleviate the damage by water deficiency, plants need to limit transpirational water loss by inducing stomatal closure. Osmotic stress induces stomatal closure like the response to water deficit stress. It could be used as a convenient tool to enhance water deficit stress tolerance by reducing water loss. The objective of this research was to investigate whether osmotic treatment with a high concentration of chemical solutions could trigger a response to osmotic stress so that stomatal closure can be induced, resulting in enhanced water deficit stress tolerance in viola (Viola cornuta 'Sorbet XP Yellow'). Osmotic treatments with CaCl₂, Ca(NO₃)₂, NaCl, NaNO₃, BaCl₂, Ba(NO₃)₂, and mannitol were applied at the osmotic potentials (c_S) of L1.3 and L2.0 MPa. Chemical treatments [except Ca(NO₃)₂, NaCl, and mannitol] helped to delay wilting and gave a longer shelf life, up to 5.2 days over that of the control, 2.5 days. However, leaf necrosis was observed on the violas treated with NaCl, NaNO₃, BaCl₂, Ba(NO₃)₂, and mannitol. CaCl₂ was the most effective agent in delaying wilting under water deficit stress in viola without leaf necrosis. Compared with the control, violas treated with CaCl₂ at 200 and 300 mM showed an increase in shelf life by 2.6 and 1.2 days, respectively. Stomatal conductance (g_S) was reduced within 4 hours after treatment with CaCl₂ compared with that of control violas. Leaf relative water content (RWC) of control violas was dramatically reduced 3 days after treatment and fell below 50% on day 4, while CaCl₂-treated violas maintained higher leaf RWC (70% to 81%) during the water deficit period. These results indicated that osmotic treatment with the high concentration of CaCl₂ caused stomatal closure, resulting in a reduction of water loss and an extension of shelf life under water deficit stress in viola.