Pollen-based screening of soybean genotypes for high temperatures

Soybean [Glycine max (L.) Merr.] reproduction is sensitive to temperatures > 35 degrees C. Two studies were conducted to determine temperature effects on soybean pollen germination (PG) and to detect genotypic differences. Pollen collected from 44 genotypes (Maturity Groups III to VI) grown outdoors was subjected to in vitro temperatures from 15 to 50 degrees C at 5 degrees C intervals. Genotypes differed significantly for in vitro PG percentage (mean of 81%) and tube length (mean of 437 mu m). Mean cardinal temperatures (T-min T-opt and T-max) were 13.2, 30.2, and 47.2 degrees C for PG and 12.1, 36.1, and 47.0 degrees C for pollen tube growth. Genotypes differed for leaf cell membrane thermostability (CMTS), but CMTS did not correlate with pollen parameters. Cumulative temperature response index, CTRI (unitless), of each genotype calculated as the sum of eight individual stress responses (ISRs) derived from maximum PG, maximum pollen tube length (PTL), and the maximum (T-max), minimum (T-min), and optimum (T-opt) temperatures for PG and for PTLs was used to group genotypes for temperature tolerance. Heat-tolerant genotype (DG 5630RR) was less sensitive to high temperature (38/30 degrees C) compared with heat-intermediate (PI 471938) and heat-sensitive (Stalwart 111) genotypes that had deformed pollen, with reduced apertures and collumellae heads. Hence, pollen can be used as a screening tool for heat tolerance. Most sensitive to temperature was D88-5320 with a CTRI of 6.8, while AG 4403RR was most tolerant with a CTRI of 7.5. Elevated [CO2] did not modify reproductive parameters or CTRI. The study also revealed that heat tolerance of vegetative tissue had little or no relationship with the heat tolerance of reproductive tissue. Maturity groups lacked a specific trend for tolerance to high temperature. The identified high temperature-tolerant genotypes and temperature-dependent pollen response functions might be useful in soybean breeding and modeling programs, respectively.