Abstract: A field experiment was conducted to determine the influence of magnesium (Mg), boron (B) and silicon (Si) availabilities on stomatal morphology, stomatal conductance and transpiration rate of Rastali. The experiment was done at Universiti Putra Malaysia under field condition arranged in split-split plot design with four blocks. The main factor was NPK fertilizer recommended by Department of Agriculture Malaysia (DOA) and United Plantation Berhad (UPB). The sub-factors were applications and non-application (as control) of kieserite (Mg), boric acid (B) and sodium silicate (Si). The sub-sub factors were the four Rastali accessions, namely, R08, R62, R34 and R12. In particular, the Berangan accession was selected as the control treatment. The observations were carried out on the stomatal morphology, stomatal conductance and transpiration rate. Data were analyzed with Analysis of Variance (ANOVA) at 5% level and continued with the Least Significant Differences (LSD) if significant. The relationship patterns among the parameters were determined using regression analysis. The findings showed that high-dose applications of NPK fertilizer enhanced stomatal length and width (abaxial and adaxial), stomatal conductance and transpiration rate, with or without Mg, B and Si. Similarly, in low doses, NPK fertilizers added with Mg, B and Si significantly enhanced stomatal length and width, stomatal conductance and transpiration rate. On the contrary, stomatal density (abaxial and adaxial) and ratio were not influenced by NPK fertilizer, Mg-B-Si or their combinations. Moreover, stomatal density and ratio were found to be different among the cultivars (Berangan and Rastali), but similar within the Rastali accessions. In particular, Berangan had a high stomatal density (abaxial and adaxial) compared Rastali, so stomatal ratio became smaller. There were linear relationships between stomatal width, stomatal conductance and transpiration rate, whereby wider stomata were shown to have resulted in higher conductance and transpiration rate.