Definition - What does Petiolar Sinus mean?
The petiolar sinus is an anatomical part of the grapevine found on the leaves which ampelographers use to identify varietals of grape vines. It includes the empty spaces surrounding the stem from the leaf. How narrow or how wide the sinuses are, will determine the grape variety.
WineFrog explains Petiolar Sinus
When scientists or viticulturists need to identify grapevines, they can look at the petiolar sinus by looking at a flat leaf, starting where the stem of the leaf ends at the base of the leaf, it is the first region where the veins of the leaf meet the stem.
Specifically, the petiole sinus is the area of empty space between the stem and the leaf tissue.
Depending on the grape variety, it can take on a v-shape, u-shape and vary in width from being open to close or overlapping or naked.