Genetic Analysis of Lettuce Seed Thermoinhibition

Seed thermoinhibition (inhibition of germination at warm temperatures) can reduce seedling emergence and stand establishment of cultivated lettuce (Lactuca sativa L.) when soil temperatures are warm (>25°C) at planting. Genetic variation for high temperature germination tolerance exists among accessions of lettuce and related wild species. Seeds of a L. serriola accession (UC96US23) germinated 100% up to 37°C, while seeds of L. sativa ‘Salinas’ were completely inhibited from germinating at temperatures above 31°C. A recombinant inbred line population, developed from a cross between ‘Salinas’ and UC96US23, was analyzed for germination capacity at high temperatures. A major quantitative trait loci (QTL) for high temperature germination (Htg6.1) and additional QTL having smaller effects were identified. Near-isogenic lines confirmed the effect of Htg6.1 and are being utilized for further fine-mapping of the locus. Candidate genes associated with seed dormancy have been mapped to test for co-localization with thermoinhibition QTL. Expression patterns of candidate genes, particularly those associated with gibberellin and abscisic acid synthesis and metabolism, have also been analyzed in relation to genotype and germination temperature. Combined genetic and molecular analyses hold promise for elucidating the physiological regulation of thermoinhibition and for developing lettuce cultivars with enhanced stand establishment of warm temperature plantings. INTRODUCTION Rapid and uniform germination immediately after planting is essential for optimal stand establishment of annual crop plants. Thermoinhibition (inhibition of seed germination by high temperature) in lettuce (Lactuca sativa L.) can delay or prevent germination, resulting in reduced field emergence and stand establishment (Valdes et al., 1985). Thermoinhibitory conditions often occur in major winter lettuce production areas where temperatures permissive of germination are exceeded during late summer and fall plantings. Delays in germination and emergence can diminish yield (Cantliffe et al., 1981) or require multiple harvests and create other management problems through nonuniformity of crop development, resulting in losses in quality and profitability (Benjamin, 1990). Thermoinhibition is moderated by physiological and environmental factors including light, temperature, hormones, and physical constraints on seed germination (Fielding et al., 1992; Cantliffe et al., 2000). Thermoinhibitory temperature is determined by cultivar and environment, cultivars being divided into thermosensitive types (in general, failure to germinate ≥28°C) and thermotolerant types (germination above 90% at ≤36°C) (Gray, 1975; Thompson et al., 1979; Nascimento et al., 2000). Seed maturation at warmer temperatures has been shown to expand upper temperature limits and increase ethylene production during subsequent seed germination (Sung et al., 1998; Kozarewa et al., 2006). Activities of enzymes responsible for weakening the endosperm membrane enclosing the embryo and allowing radicle emergence at higher temperatures have been shown to be affected by abscisic acid (ABA), gibberellin (GA) and ethylene (Dutta et al., 1994; Nascimento et al., 2000; Gonai et al., 2004). Thermotolerance can be increased by exposure to ethylene, GA, and red light or reduced by ABA (Saini et al., 1986; Dutta and Bradford, 1994). Germination at high temperature can be promoted by the application of Proc. IV IS on Seed, Transplant and Stand Establishment