Determining how much energy a plant takes in?
Just a speculation for whether diamond berries could exist or not. This is not an answer for "what is the energy uptake rate of plants" because the actual question tends to be "whether a plant can gather enough energy to form diamonds". For energy uptake rate of plants please see MCM's answer.

Even though enthalpy of formation is same as heat required for destruction, burning a diamond is much easier than creating it (I presume it is easier to generate high temperature compared to high pressure). Natural diamond requires both high pressure and temperature to form. Synthetic diamonds are made by chemical vapour deposition; that also requires high temperature. Such high energies cannot be produced by any known organism (requires devices like arc furnace), and moreover at these temperatures the biological structure will disintegrate.

However there is one possibility of diamond berries; the berries would not be solid diamond though. A plant could be growing in an area where there are a lot of nanodiamonds (meteorite rocks) which could be bioaccumulated in the berries. Living organisms do accumulate nanoparticles [1,2]. There have been studies on biological application and effects of nanodiamonds too [3,4]. However these would not look lustrous like bigger diamonds. Xylem vessels in trees have a mean diameter of 30-40µm; it may be possible that the plant can take up bigger sized diamond particles. Density of diamond is ~3g/cm3 — three times that of water and it would require thrice the force required for water to get diamond to the destination. 

For a spherical diamond of diameter 20µm:

Cross sectional Area = 314.15 µm2
Volume = 3351 µm3
Mass = 3×3351×10-12 ≅ 10-8 g
Gravitational force on the diamond = 9.8×10-8 ≅ 10-7 N
Pressure required to counteract gravity = 10-7/314.15×10-12 ≅ 3.18×102 Pa


However, I am not sure if that much of pressure can be generated within a vessel and whether small particles can be lifted like this.

There is another possibility that bigger diamonds are encapsulated by living tissue (not the fruits of course).