Effects of exogenous polyamines and their synthesis inhibitors on the growth and development of blueberry fruits.

Polyamines play crucial roles in fleshy fruit development, yet their regulatory effects on the morphological and cytological growth of blueberry (Vaccinium uliginosum L.) remain unclear. This study investigated the effects of exogenous polyamines-spermine (SPM), putrescine (PUT), and spermidine (SPD)-and their synthesis inhibitors (DArg and MGBG) at 0.1 mmol/L on blueberry fruit development. Morphological parameters and cytological microstructures (using paraffin sectioning) of the exocarp, mesocarp, and endocarp were quantitatively analyzed. Results showed that exogenous polyamines significantly promoted fruit growth without altering the typical double-sigmoid growth curve. Notably, SPM application exerted the most profound effect, significantly increasing the longitudinal diameter, transverse diameter, and single fruit weight by 8.23%, 11.29%, and 29.79%, respectively, compared to the control. Microscopic analysis revealed that SPM primarily facilitated fruit enlargement by synergistically promoting cell expansion (increasing cell area and layer thickness) in the exocarp, mesocarp, and endocarp, particularly during the late rapid growth phase. Conversely, polyamine synthesis inhibitors delayed early-stage growth but triggered late-stage compensatory expansion. Path analysis confirmed that flesh cell expansion, rather than cell division, was the dominant factor driving final fruit weight. These findings provide critical cytological evidence for understanding polyamine-mediated regulation and highlight SPM as a potential growth-promoting regulator in blueberry cultivation.