The quick answer to the question above is "nobody knows for sure"! However, a study starting in Norway aims to find out.

The researchers will give 120 volunteers either three kiwi fruit to eat each day, or an apple a day for eight weeks. During this time, both the volunteers' systolic (top number) and diastolic (bottom number) blood pressure readings will be taken and compared.

The study seeks to find out if kiwi fruits are more effective than apples in helping to lower blood pressure.

The doctors are keen to carry out this research because of the results of an earlier study. In the Oslo Antioxidant Study, they found that people who ate three kiwi fruits a day had a significant reduction in their blood pressure levels.

However, they do not know how kiwi fruit had this effect, so they would like to compare its actions to those of apple to see if the blood pressure lowering is general effect of all types of fruit or a specific effect caused by a molecule found in kiwi fruit.

Reference

Study about to begin: The Effect of Kiwi on Blood Pressure, Endothelial Function, Antioxidant Capacity and Gene Expression. Ullevaal University Hospital, University of Oslo. Trial details available at: http://clinicaltrials.gov/ct2/show/NCT00948363