Phenotyping Summer Dormancy in Tall Fescue: A Surrogate Approach.

Tall fescue [Lolium arundinaceum (Schreb.) Darbysh] is a cool-season perennial grass widely grown for forage. There are two major types of germplasm, continental and Mediterranean.  Most Mediterranean germplasm, sought after for heat tolerance and other important traits, exhibit summer dormancy even if the growing conditions are favorable. There is a large variation in the degree of dormancy and phenotyping the trait in the field is lengthy and costly. Understanding the mechanisms and genetic factors underlying summer dormancy in tall fescue can aid in a faster transfer of favorable traits across germplasm pools and speed up the process of cultivar development. The objectives of this study are to develop a fast and low-cost surrogate phenotyping approach for summer dormancy. The environmental cues that trigger dormancy in the mother plant would act similarly on seed dormancy/germination and thus may be used as a surrogate phenotype. Germination tests were conducted under various combinations of photoperiod and temperature extremes with 3 non-dormant and 2 dormant check varieties in growth chamber conditions that simulate summer/fall natural conditions. Six different temperatures: 18°C, 20°C, 22°C, 28°C, 30°C, 32°C, and 34°C and 5 different photoperiods: 0/24, 12/12 14/8, 16/8 and 24/0 hours of light (230 µMol) /dark were tested for each temperature and photoperiod combination. A complete dark treatment (seeds placed in a covered box) was used with each combinations. The data indicates that temperature had a larger effect on germination compared to photoperiod with higher temperatures reducing the germination of the dormant but not the non-dormant checks. 168 accessions were phenotyped for dormancy in field conditions using a combined score of regrowth height and senescence percentage based on Digital Image Analysis. Twenty seven putative dormant and 24 putative non-dormant accessions were selected for seed germination testing and to establish a correlation between the seed germination phenotype and field dormancy. The data showed that seed germination at 20^oC and 34^oC correlates well with field dormancy phenotypes and has potential to be used as proxy for  phenotyping summer dormancy in tall fescue.