Serendipity in Research – Hollow Heart in Watermelon

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Much of research is tedium, the repetitive collection of data. A number of years ago, I had a conversation with a colleague about the value of being in the field to collect data. She stated that while most of the activities were tedious, she prefers to be there in person when data is collected. This is because of the unexpected,new observations that are often made by a trained eye. She termed it the serendipity factor.

The definition of serendipity is a “fortunate happenstance” or “pleasant surprise”. This is exactly what has happened with the discoveries that the University of Delaware vegetable program has made regarding hollow heart disorder in seedless watermelon.

If you look at any reference up to 2 years ago, the stated cause for hollow heart in seedless watermelon was a stress related change in the growing environment, excess nitrogen, or wide fluctuations in water uptake by the plant. However, no watermelon researcher was able to replicate hollow heart by stressing plants, by giving excess nitrogen, or by fluctuating water.

On hearing a watermelon researcher several years ago observe that seedless watermelon hollow heart is often found in stray melons away from a pollen source, and another researcher postulate that plant hormones might be involved I set up some simple side experiments to look for hollow heart.

To make a long story short, these side projects have led to the discovery that hollow heart can be induced by limiting pollen. Seedless fruit will set, but will not develop properly under reduced pollen levels and hollow heart incidence will be increased dramatically.

Back to the idea of serendipity; we just had a “fortunate happenstance” occur this year with striking results. We had planted 4 blocks of seedless watermelons with no pollenizers to do some hand pollination research.  We separated this trial from another trial with pollenizers by 50 feet, feeling this was adequate to avoid stray pollen. We did not have the time to do the hand pollination research and just recently decided to kill the plots with non-selective herbicide.

Lo and behold, once the vines were killed back, we found that one of the varieties that was nearest to the other trial 50’ away had some fruit set. There were no other pollenizers or stray pollen sources nearby. I decided to cut these melons to look for hollow heart. Nearly all had severe hollow heart, and showed where development of the fruit occurred and where it did not.

HHFig1

In the picture above, note how the fruit developed normal size, but when cut, the fruit tissue showed extreme developmental abnormalities. The rind developed normally. The septa of the fruit developed, but not completely. The locules did not fill around the ovules (seedless watermelon ovules do not fertilize, even though there is a fertilization-like event during pollination). This resulted in severe hollow heart.

HHFig2

In the picture above, the septum was more developed and the locules filled more completely, making the hollow heart less severe.

HHFig3

In the picture above, the locules were more filled around the ovules but not completely and the septa were not completely developed leading to the triangular shaped hollow heart in the middle of the fruit. Note that hollow heart was already present in this immature fruit. We find hollow heart in immature fruit, suggesting that hollow heart occurs very early in fruit development.