Most plant species are adapted to a relatively narrow range of ambient temperatures. Temperatures that are above the preferred range elicit a variety of responses at tissue, cellular and sub-cellular level. One of the earliest responses is post-translational modification by SUMOylation and phosphorylation; modulating heat stress (HS)-induced gene expression, metabolism adaptation and metabolite synthesis to improve plant tolerance to the heat stress. In this study we used a combination of three-dimensional gel electrophoresis fractionation, Western-blot analysis, difference in gel electrophoresis (DIGE) technology and mass spectrometry to identify those proteins that first showed changes in SUMOylation and phosphorylation in potato leaves exposed to sublethal HS. We found that these early response proteins are mostly involved in photosynthesis, or defence mechanisms, or energy metabolism. The relationship between the fractional change in SUMOylated proteoforms and in serine- phosphorylated proteoforms was not constant, but varied between proteins. The physiological significance of these differences needs further investigation.