Effects of Priming and Endosperm Integrity on Seed Germination Rates of Tomato Genotypes

Abstract

The bases of differences in germination rates (GRg = inverse of time to germination [tg] of percentage g) among three cold/salt-tolerant tomato (Lycopersicon esculentum Mill.) accessions (PI 341988, PI 120256, and PI 174263) and one cold/salt-sensitive tomato cultivar (T5) were investigated. The effects of seed priming (6 d imbibition in aerated −1.2 MPa polyethylene glycol solution at 20 °C followed by redrying) and of removing the endosperm/testa cap covering the radicle on the temperature sensitivity of GRg, and the interaction of these treatments with genotypes, were also examined. GRg decreased linearly with decreasing temperature for all genotypes and seed treatments. The minimum or base temperatures for germination (Tb) varied by ≤ 1 °C among the tomato lines, so genotypic differences in GRg were due to differing thermal time requirements for germination. The mean thermal time requirement (θ¯T=(T-Tb)t50) for germination of T5 seeds was 22% and 19% greater than that of PI 341988 and PI 120256 seeds, respectively, but only 9% greater than that of PI 174263 seeds. Seed priming did not lower Tb of any genotype, but significantly reduced θ¯T by 24, 49, 41, and 49% in T5, PI 341988, PI 120256, and PI 174263, respectively, indicating that priming increased the rate at which the seeds progressed towards germination when T>Tb, but did not lower the minimum temperature at which germination could occur. Priming increased the GRg of T5 seeds to equal or exceed those of control (non-primed) seeds of the cold/salt-tolerant genotypes at any T>Tb, but the PI lines exhibited an even greater response to priming. Times to germination within each seed lot were normally distributed on a logarithmic scale. Priming increased the variance in tg within a seed lot when compared to control seeds. However, the variation in thermal time for germination between the 10th and 90th percentiles of the seed population (θT(10–90)) was relatively unaffected by priming due to the reduction in θ¯T in primed seeds. Removing the endosperm cap and testa opposite the radicle tip decreased θ¯T almost 6-fold and and reduced Tb by 5 °C in T5 and PI 341988, implicating processes in the endosperm/testa as the limiting factors in germination at suboptimal temperatures.