Since NIR is not absorbed by any pigments within a plant, it travels through most of the leaf and interacts with the spongy mesophyll cells. This interaction causes about half of the energy to be reflected and the other half to be transmitted through the leaf. In plants with turged and healthy mesophyll cell walls and in dense canopies, more NIR energy will be reflected and less transmitted. This cell wall/air space interaction within these cells causes healthy vegetation to reflect most of NIR. (https://extension.usu.edu/nasa/htm/on-target/near-infrared-tutorial)

http://fas.org/irp/imint/docs/rst/Sect3/Sect3_1.html

Hi Alex

I cannot see your answer under my question, although I was notified by RG about your response; however, under your profile I found your answer; yes, you describe in detail the theoretical background of my expectation; but my question is on an empirical confirmation; has the reflectance of white flowers been measured? On the ground, I have measured the reflectance of grasslands containing colourful flowers, but without substantial amounts of white flowers.

regards 

Hein, 

I think you can find an answer in database of NIR pics for region with massive flowering of white color. Some support can be found for yellow rape flowers in paper http://www.degruyter.com/view/j/quageo.2011.30.issue-1/v10117-011-0007-z/v10117-011-0007-z.xml. I think that yellow/pink or blue color of flowers  will not change situation. 

Thanks for helpful reference, Alex