This paper presents a performance analysis of an adaptive modulation (AM) scheme based on a simultaneous saving and transmitting (SST) protocol for wireless energy harvesting (EH) in multiple-input multiple-output (MIMO) channels. The transmitter extracts the energy from its environment through various sources. We assume that the harvested energy follows a gamma distribution and the MIMO channel is a Rayleigh flat fading process. A transmit antenna selection (TAS) scheme with a maximal-ratio combining (MRC) receiver are used. We consider a constant power variable discrete-rate AM strategy which adjusts the modulation mode according to the signal-to-noise ratio (SNR) at the MRC output. Analytical expressions for the outage probability (OP), the average spectral efficiency (ASE), and the bit error rate (BER) are derived. Numerical results illustrate the impact of EH on the AM scheme performance and prove an efficient agreement between the theoretical analysis and the Monte Carlo simulations.