Choose the correct equation from the following options to show correc…

The correct answer is Option (4) → GPP-R = NPP

Gross primary productivity (GPP) is all the organic matter that plants make in photosynthesis. Plants use a part of it in their own respiration (R). What is left is the net primary productivity (NPP), which is the food available to the herbivores and decomposers.

Option (1) GPP = R / (NPP)² — Incorrect. Productivities are amounts of matter made per unit area per year. They are added or subtracted, not divided or squared.

Option (2) GPP = 2NPP+R — Incorrect. GPP is NPP plus R, not twice NPP plus R. The extra NPP has no meaning.

Option (3) GPP= NPP-R — Incorrect. This makes GPP smaller than NPP. In fact GPP is always the bigger value, because NPP is what remains after respiration.

Option (4) GPP-R = NPP — Correct. Gross primary productivity (GPP) is the total rate of making organic matter in photosynthesis. Part of it is used up in respiration (R); what is left is net primary productivity: GPP − R = NPP.

NCERT Extract:
Productivity – “Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g m^–2) or energy (kcal m^–2). The rate of biomass production is called productivity. It is expressed in terms of gm^–2 yr^–1 or (kcal m^–2) yr^–1 to compare the productivity of different ecosystems. It can be divided into gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).
GPP – R = NPP
Net primary productivity is the available biomass for the consumption to heterotrophs (herbiviores and decomposers).”