Hollow Heart in Watermelon | Amazing Images.
You might assume that this watermelon here has been carved by hand,
but actually is completely natural. This strange condition is known as
“hollow heart”. Hollow heart can happen due to poor pollination
causing the watermelon to develop cracks internally. Cutting a
watermelon, affected with hollow heart, across the body will reveal a
well defined structure that may surprise those who aren’t familiar with
the condition. Hollow heart is not a disease and the internal cracking
has no negative impact on the watermelon’s taste or quality, and is
entirely safe to consume. 
Photo credit
Gordon Johnson, the Extension Fruit & Vegetable Specialist from the University of Delaware, explains:
Photo credit
Photo credit
Photo credit
Photo credit
Photo credit
Photo credit
Gordon Johnson, the Extension Fruit & Vegetable Specialist from the University of Delaware, explains:
In the past, the cause for hollow heart was thought to be related to rapid growth of the fruit where the rind expanded faster than the internal flesh leading to separation of the three internal fruit compartments and an open area between. Excess nitrogen and over-watering along with favorable growing conditions were implicated in higher incidence of hollow heart.
There is growing evidence that hollow heart is not directly tied to nitrogen and water management but is related to pollination and weather conditions during pollination. Plant hormones are thought to be important in this effect. Several researchers have found no increase in hollow heart with increases in nitrogen; even in varieties know to have hollow heart problems. It is thought that with inadequate pollination, there is reduced release of the plant hormone that controls the development of storage tissue leading to hollow heart.
Photo credit
Photo credit
Photo credit
Photo credit
Photo credit