Fig. S1 Steepness was employed to describe the tip-focused Ca2+ gradient.

Fig. S2 Effects of S-nitroso-N-acetylpenicillamine (SNAP), 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) and Nω-nitro-L-arginine (L-NNA) on the morphology of Pinus bungeana pollen tubes after treatment for 24 h.

Video S1 The relative position of the electrode and pollen tube tip in the net Ca2+ flux experiment using the scanning ion-selective electrode technique (SIET).

Video S2 Hepes buffer (the buffer for Calcium Green-1 indicator) was microinjected into normally growing pollen tubes as a control injection to determine whether injection affected pollen tube growth.

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