Accumulation of free proline in sorghum (Sorghum bicolor) pollen

Suboptimal temperatures cause grain yield reductions generally associated with low seed-set in sorghum [Sorghum bicolor (L.) Moench]. Low proline concentration in pollen has been associated with cold-induced male sterility. This study was done to (i) determine free proline concentrations in grain sorghum pollen after low temperature exposure, (ii) determine the relationship between seed-set and proline concentration in pollen, and (iii) determine the association of proline concentration in pollen with grain yield in sorghums of different maturity. Eleven sorghum genotypes, grown in a greenhouse at 27/22 °C (day/night) until the early boot stage were transferred to 20/10 °C in growth chamber until physiological maturity. Concentration of proline in pollen was determined at anthesis and related to seed number and grain yield. Proline concentrations in pollen increased from 19 to 36 μg∙mg−1 fresh wt. to 30 to 49 μg∙mg−1 fresh wt. at low temperatures. Seed number and grain yield were reduced by low temperature in most of the genotypes. Percent seed-set in male sterile lines was significantly correlated (r = 0.45**) with free proline concentrations in the pollen from 13 fertile hybrids grown under cold or control conditions. Higher concentrations of proline in pollen under cool and also control temperatures seemingly contribute to greater pollen viability in grain sorghum. In 21 sorghum hybrids grown under control temperatures, seed number per panicle was found correlated (r = 0.42*) to free proline concentrations in pollen. Genotypic differences in pollen proline concentration were not associated with differences between maturity groups. Keywords: Sorghum bicolor (L.) Moench, pollen, free proline, low temperatures, seed-set, grain yield.