potassium efflux channels participate in mediating release of potassium from guard cells. Anion channels were identified as important controllers of stomatal closing. Anion channels have several major functions in controlling stomatal closing: (a) They allow release of anions, such as chloride and malate from guard cells, which is needed for stomatal closing. (b) Anion channels are activated by signals that cause stomatal closing, for example by intracellular calcium and ABA. The resulting release of negatively charged anions from guard cells results in an electrical shift of the membrane to more positive voltages (depolarization) at the intracellular surface of the guard cell plasma membrane. This electrical depolarization of guard cells leads to activation of the outward potassium channels and the release of potassium through these channels. At least two major types of anion channels have been characterized in the plasma membrane: S-type anion channels and R-type anion channels. Vacuolar ion transport Vacuoles are large intracellular storage organelles in plants cells. In addition to the ion channels in the plasma membrane, vacuolar ion channels have important functions in regulation of stomatal opening and closure because vacuoles can occupy up to 90% of guard cell’s volume. Therefore, a majority of ions are released from vacuoles when stomata are closed. Vascuolar K+ (VK) channels and fast vacuolar channels can mediate K+ release from vacuoles. Vacuolar K+ (VK) channels are activated by elevation in the intracellular calcium concentration. Another type of calcium-activated channel, is the slow vacuolar (SV) channel. SV channels have been shown to function as cation channels that are permeable to Ca2+ ions, but their exact functions are not yet known in plants. Guard cells control gas exchange and ion exchange through opening and closing. K+ is one ion that flows both into and out of the cell, causing a positive charge to develop. Malate is one of the main anions used to counteract this positive charge, and it is moved through the AtALMT6 ion channel. AtALMT6 is an aluminum-activated malate transporter that is found in guard cells, specifically in the vacuoles. This transport channel was found to cause either an influx or efflux of malate depending on the concentrations of calcium. In a study by Meyer et al, patch-clamp experiments were conducted on mesophyll vacuoles from arabidopsis rdr6-11 (WT) and arabidopsis that were overexpressing AtALMT6-GFP. It was found from these experiments that in the WT there were only small currents when calcium