Wax begonia (Begonia x semperflorens-cultorum Hort.) plants often suffer from high temperature stress during hot seasons in Taiwan. Since salicylic acid (SA) has proved to enhance heat tolerance in many plants, this study evaluates whether exogenous SA applications could alleviate high temperature stress of wax begonias. Plug seedlings of wax begonia 'Super Olympia' were treated with 25, 100, 400, 800, with 1600 mu M SA before 55 degrees C, for 2 h of high temperature stress. Results indicated that 25-400 mu M SA enhanced heat tolerance by reducing the value of Relative Injury (RI) and Malondialdehyde (MDA), whereas 800, 1600 mu M SA had adverse effects. Transplanted plants of wax begonia 'Super Olympia' were treated with 25, 100, 4007 mu M SA before 35 degrees C, for 16 days of high temperature stress. Results showed that 25-400 AM SA enhanced heat tolerance by maintaining a higher value of Chlorophyll Meter Reading (CMR), reducing leaf thickness and the number of red spotted leaves. Especially, 100 mu M SA treatment had most significant improvement among 25-400 mu M SA treatments. The thermo protection period of 100 mu M SA maintained significantly for 5 days, based on the lower value of F0, RI, and MDA, and the higher value of Fm and Fv/Fm. Applying times up to 2 or 3 times extended protective effect significantly, based on reducing the value of RI and MDA, and increasing plant height and number of flowers. Either medium drench or foliar spray treatments improved heat tolerance of wax begonias. Medium drench treatments produced better thermo protection based on the results of higher Fm and Fv/Fm value. In conclusion, salicylic acid pretreatments with satisfactory concentrations between 25 to 400 mu M proved to enhance heat tolerance in the wax begonia. Results recommend medium drench with 100 mu M SA per week, up to 2-3 times as the most optimal application of SA.
