PERSUADING kids to eat fruit and vegetables may have just got harder.

It turns out the produce you buy in the shops is still alive and all those blueberries and zucchinis are still responding to the time of day, scientists say.

However, the discovery in the journal Current Biology suggests there is a healthy upside to knowing that.

It could spell changes to the way produce is harvested and stored that increase the food's nutritional value, says one of the authors, Janet Braam of Rice University in the US.

"Vegetables and fruits, even after harvest, can respond to light signals and consequently change their biology in ways that may affect health value and insect resistance," she said.

"Perhaps we should be storing our vegetables and fruits under light-dark cycles and timing when to cook and eat them to enhance their health value."

Plants are made up of many separate parts that can continue to metabolise and survive more or less independently, at least for some time after harvest.

The study showed that post-harvest vegetables and fruits continued to perceive light so their biological clocks kept on ticking.

The plants responded to the light by altering levels of chemicals that protect them from being eaten by insects and other herbivores.

Some of those same phytochemicals also have anti-cancer effects when people eat them.

"It may be of interest to harvest crops and freeze or otherwise preserve them at specific times of day, when nutrients and valuable phytochemicals are at their peak," Ms Braam says.

The researchers made their initial discovery with cabbages - then lettuce, spinach, zucchini, sweet potatoes, carrots and blueberries.

Fruits and veggies subjected to light-dark cycles at the right times suffered less insect damage.

Originally published as Harvested fruit and veg 'still alive'