Grow a plant in a pot. Let this be the system. Everything that enters the system is measured.
Would the system weigh the same throughout? If not, there will be energy taking part in the experiment.
if this measuring procedure is a big problem..lets check whether energy is converted to mass and that results in building up mass (ie growth)!!
Your example depends on classical physics, where mass is a conserved quantity. In classical dimensions, much larger than h_bar the only connection of mass with energy is in calculating the kinetic energy of the classical objects.
In plants there are chemical reactions that lead to growth, and if one could capture all matter interacting with the plant, the mass of the system would the sum of the mass of the constituent elements.
For small dimensions commensurate to h_bar, and velocities close to the velocity of light one is in a quantum mechanical system where energy and mass are connected through special relativity:
It is true that chemical reactions are basically quantum mechanical but the energies involved are very small and the differences in mass in chemical reactions cannot be measured. It is only at the level of nuclear physics transitions that the special relativity effects are detectable.
The growth of plants uses energy, kinetic and potential, but mass is a conserved quantity at this classical dimensions system. Mass missing means evaporation or other chemical processes, not the conversion of mass to energy.
Energy from the sun is used in the same way energy from chemical bonds are used in humans. While we don't use it to create mass, we use it to bond mass together - we bond mass gained from food and plants from soil.
while this is extremely simplistic, I hope it helps.
Even in classical physics energy is converted to mass. The energy plants get is converted to mass(in the form of chemical energy). This mass can then turn into sum amount of energy. It is just that the you need a lot of energy to get even a small amount of mass. In classical physics high energy interactions are really rare so the amount of extra mass created by energy is negligible and can be excluded. This is the reason why mass can be treated as a conserved quantity.
