The Perilous Journey to Fruitset

You would think that Vitis vinifera grapes have it easy because they are a perfect flower, meaning that they contain both the male and female plant parts that makes self pollination leading to fruitset something much less than a chance occurrence.
The parts of a grape flower is quite interesting and I found a great diagram:

The calyptra or cap is actually the grape flower petals that cover the stamen (male---mnemonic for me is "men") and stigma (female---mnemonic for me is "ma") until anthesis or bloom when the calyptra detaches from the pedicel exposing the stamen and stigma. Bloom usually occurs from 6 to 8 weeks after budbreak. The bloom period is when things can get tricky and conditions must be just right for fruitset to occur:

• Temperature is an important factor in germination and growth of the pollen tube. Fruitset is greatly reduced when temperatures fall below 65^oF or exceed 100^oF
• Cold temperatures, rainfall or high humidity are often associated with incomplete detachment of the calyptras leading to reduced fruitset
• Rain can dilute the stigmatic fluid and interfere with germination of the pollen grains

Given all the conditions above, it's amazing that we actually can have viable grape bunches! Not to mention all the diseases that can occur once those tender buds breakthrough in spring. Nature is amazing!
Here is another look at the grape flower from a previously published post, Cabernet Sauvignon and Chardonnay: Where Do They Come From?

1. Nick K. Dokoozlian, Grape Berry Growth and Development.
2. The illustration is from the UC Davis online Viticulture Course, VID257. Dr. Andrew Walker's Lesson 2---Plant Materials