Eight weeks after anthesis: a critical fruit developmental stage for tissue-border establishment in oil palm tenera fruit.

Oil palm (Elaeis guineensis Jacq.) is the most productive oil crop, contributing to approximately 35% of the global vegetable oil production. The tenera fruit form, derived from dura × pisifera crosses, is planted for its superior mesocarp oil yield due to thicker mesocarp and thinner endocarp fruit characteristics. In this study, histology analyses were conducted to characterize the developmental progression in tenera fruit. Cytological and morphological analyses revealed that fruit tissue compartmentation commence as early as 3 weeks after anthesis (WAA), with the pericarp distinctly differentiating into exocarp, mesocarp, and endocarp layers that enclose the seed coat and endosperm. The endocarp layer becomes prominent at 4 WAA and reaches an average maximum thickness of 1.1 ± 0.55 mm until full ripening at 23 WAA. Sclereid secondary cell wall lignification is initiated at 8 WAA, coincides with the onset of endosperm cellularization and a significant increase in endocarp cell density. The expression of SHELL gene showed a developmental stage-dependent pattern with highest expression recorded at 12 WAA, aligning with the phase of active lignification detected histologically. Collectively, these findings identify 8 WAA as a critical developmental stage that defines seed size, endocarp lignification, shell thickness, endosperm cellularization, and mesocarp capacity for oil accumulation in tenera fruit. These coordinated cytological and molecular changes suggest a shared transcriptional regulation of tissue-border formation and cell wall biogenesis in oil palm fruit. This study provides a foundation for transcriptomic investigations into the molecular control of fruit tissue differentiation and size control, contributing to strategies to enhance oil yield, seed quality and fruit architecture in elite tenera palms.