Freezing Tolerance of Field- and Growth Chamber- Acclimated Zoysiagrass Hybrids

Freezing tolerance in zoysiagrass (Zoysia spp.) is an important trait that influences its geographical adaptation in the transition zone. The objectives of this study were to evaluate freezing tolerance of six fine-textured interspecific hybrids (DALZ 1304–1309) and an ecotype (DALZ 1301) with medium-fine texture in comparison to Z. japonica (Steud.) cv. Meyer and to determine the relationship of LT₅₀ (lethal temperature; i.e., the temperature at which 50% of plants die due to injury resulting from low temperature) from plants acclimated to freezing temperatures under field and controlled environmental conditions. Six interspecific hybrids used in the study were derived from crosses between fine-textured species Z. minima (Colenso) Zoltov. or Z. pauciflora (Mez.) crossed to Z. matrella (L.) Merr. Two independent field experiments (FE-1 and FE-2) and two growth chamber cold-acclimation experiments (GC-1 and GC-2) were conducted in 2014 and 2015. The cold-acclimated samples were subjected to six freezing treatments (-3, -5, -7, -9, -11, -13°C) in a freeze chamber for 1 h each. Treated samples were then thawed and placed under optimum growing conditions in a greenhouse to allow for 5 wk of recovery. Greenup was visually rated on a 1 to 9 scale (1 = brown; 9 = completely green; 5 = acceptable). Nonlinear regression analysis was used to determine LT₅₀ of each genotype. Our results show high correlation of freezing tolerance (R² = 0.718–0.767) between the LT₅₀ of field-acclimated and growth chamber–acclimated samples. For field-acclimated plants, mean LT₅₀ ranged from -5.5°C (DALZ 1307) to -9.3°C (Meyer). For growth chamber–acclimated plants, mean LT₅₀ ranged from -1.6°C (DALZ 1304) to -9.9°C (Meyer) in GC-1 and from -4.8°C (DALZ 1304) to -9.4°C (Meyer) for GC-2. Of the experimental genotypes tested in this study, the LT₅₀ of DALZ 1301 was statistically similar to Meyer, whereas all interspecific hybrids exhibited higher LT₅₀ values than Meyer. These results suggest a limited potential of selected Z. minima, Z. pauciflora, and Z. matrella lines to impart freezing tolerance and possibly direct future breeding effort toward using cold-tolerant Z. japonica germplasm crossed to these finer-textured species to develop fine-textured and cold-hardy cultivars for the transition zone.