We previously demonstrated that both trehalose and LEA protein protect plants from damage by drought, salt, and heat. Here, we compared their effectiveness in preserving photosynthetic capacity under those abiotic stresses. Upon dehydration, the Pmax (maximal photosynthetic rate) of $O_2$ evolution decreased similarly in both nontransformants and otsA plants. Contrastingly, Pmax was maintained at a considerably higher level in CaLEA6 plants. However, no significant differences in Chl fluorescence parameters were observed between transformants and nontransformants. Under salinity stress, CaLEA6 plants were also better than otsA plants in terms of their values for Pmax, photochemical efficiency of PSII (Fv/Fm), and photochemical quenching (qP). After heat both otsA and CaLEA6 plants maintained a higher Pmax as well as more favorable Chi fluorescence parameters, although the latter transformant performed slightly better overall. Therefore, despite the comparable effectiveness of trehalose and LEA protein in enhancing tolerance against those abiotic stresses, they confer differential protection in maintaining photosynthetic capacity. Compared with trehalose, the CaLEA6 protein appears to be a more universal and effective agent under those stresses.