Methods are described for quantifying an amount of natural rubber in a plant from a sample of the plant by obtaining a T1 spin-lattice relaxation rate or T2 spin-spin relaxation rate from a NMR apparatus, such as a low-field NMR apparatus operating at a magnetic field strength of 2 T or less. The NMR relaxation rates obtained from the plant sample are compared to reference relaxation data or plots. The plots can represent the weight percent polyisoprene versus T1 spin-lattice relaxation rate or T2 spin-spin relaxation rate. The reference relaxation data or plots are generated from testing the same type of plant as the sample.