Freeze tolerance of Zoysia matrella (L.) Merrill as affected by late-season nitrogen application, and changes in carbohydrates during cold acclimation

This study was conducted with the aim of (1) assessing the development of freeze tolerance (LT50) in “Zeon” zoysiagrass under the effect of two different levels (15 and 30 g m−2) of late-season nitrogen applications, and (2) evaluating the dynamics of nonstructural carbohydrates composition during cold acclimation and overwintering. A combined approach with natural acclimation in the field, followed by monthly controlled exposure to sub-freezing temperatures, was employed to evaluate LT50. Fall color of “Zeon” zoysiagrass was improved by nitrogen applications, the latter extending the green period by more than one month. In October, the higher nitrogen treatment caused significantly less cold hardiness (LT50=−10.0°C) compared with the other treatments, but was beneficial to freeze tolerance in March. The controls reached cold hardiness in November, 1 month earlier than the N treatments, in agreement with the color retention data. Controls also showed a more linear freeze tolerance during the experimental period. Significant variations in total soluble sugars occurred in zoysiagrass tissues during the acclimation and de-acclimation process. In particular, in leaves and, partially, in stems, total soluble sugars (TSS) levels were associated with LT50, while sprigs and roots did not show any significant association between LT50 and TSS. Alteration of metabolism was more evident in the 30-g N treatment, in particular as regards the storage sink.