Background. Kok-saghyz is an emerging promising natural rubber crop plant, but there is still no economical and convenient seed-quality grading or screening method for production.

Materials and methods. We took accession k-445 of kok-saghyz from Russia as the study object, using four sieves with different meshes to screen its seeds into four grades, and studied the effects of seed size on seed germination, plant growth, and yield. The study was carried out in 2020–2022 under the conditions of a field experiment at the experiment station in Harbin (45.592729°N, 126.581668°E).

Results. It was established that sieves No. 28 and No. 30 with mesh sizes of 0.83 mm and 0.60 mm retained about 90% of the total seed mass. The germination capacity of seeds selected with sieves No. 28 and No. 30 was significantly higher compared to seeds selected with sieves No. 32 and No. 35 with mesh sizes of 0.50 mm and 0.43 mm. No significant differences in germination energy or germination uniformity were found among seeds of different fractions. The duration of growth stages of kok-saghyz plants sorted with sieves having different mesh sizes generally increased as the seed size decreased (with increasing sieve number) and was prolonged by one to four days. With an increase in the sieve number, rubber yield in kok-saghyz plants gradually decreased.

Conclusions. Thus, in production practice, sieves No. 30 and No. 35 can be directly used to improve operational efficiency by separating kok-saghyz seeds into two fractions. The use of sieves No. 30 and No. 35 for two-fraction seed sorting is a convenient and cost-effective method for grading and selecting kok-saghyz seeds according to quality.