Assertion (A): Gross primary productivity is the amount of organic matter available for consumption by heterotrophs.

Reason (R): Net primary productivity (NPP) is calculated by subtracting the energy used by plants for respiration (R) from the gross primary productivity (GPP).

A is false, but R is true.

The correct answer is Option (4)- A is false, but R is true.

Primary production refers to the amount of biomass or organic matter produced per unit area over a given time period by plants during photosynthesis.

Primary production can be further divided into two components: gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity represents the total rate of organic matter production during photosynthesis in an ecosystem. However, a significant portion of this energy is utilized by plants for their own respiration needs.

Net primary productivity, on the other hand, is the amount of organic matter available for consumption by heterotrophs, such as herbivores and decomposers. It is calculated by subtracting the energy used by plants for respiration (R) from the gross primary productivity (GPP):

GPP - R = NPP.