Incontrovertibly, a reasonable number of morphological indices influence Irish potato (Solanum tuberosum Lin.) yield. More often than not, some of these parameters (e. g. plant height, collar girth, leaf count) contribute positively to potato yield while a very few number of them (e. g. stem count stand-1) negatively affect the yield in both rain-fed and irrigated cropping seasons under alley cropping. Thus, this experiment was conducted to know the most influential (external morphological) growth parameter that determines potato yield. The experiment was a completely randomized block design consisting of five treatments and three replicates for 5 cropping seasons within 3 years. Results from analysis of variance indicated significant differences among some morphological parameters on yield (at P<0.01). Correlation/ regression analyses indicated that collar girth had the highest positive correlation with yield (r = 0.954, R2 = 0.910, P < 0.01) in rain-fed cropping seasons while leaf count recorded the highest positive correlation with yield during the irrigated cropping seasons (r = 0.958, R2 = 0.913, P < 0.01) and in all the combined cropping seasons (from the simple correlation matrix, bi-variate /multiple Correlation/ regression analyses). Therefore, it was confirmed that leaf count was the most influential/main determinant of potato yield under alley cropping (an agroforestry system). Keywords: morphological parameters, alley cropping, determinant, potato yield
