Zoysiagrass (Zoysia spp.) accessions differ in nitrogen cycling during a simulated seasonal senescence

Zoysiagrass is a valuable low maintenance warm-season turfgrass. Nitrogen management is one of the most important management practices that affect colour retention and winter hardiness. However, the N cycling during seasonal senescence and its interaction with low temperature acclimation are not well-understood. The objective of this experiment was to understand the initiation of senescence as affected by environmental signals and the contribution of chlorophyll and protein degradation to N metabolites. Two commercial cultivars (‘Meyer’ and ‘Zenith’), seven wild accessions of Z. japonica, one wild accession of Z. matrella, and one wild accession of Z. pacifica from China were tested. Mature plants were exposed to four sequential environmental conditions to simulate fall shortening light photoperiods and decreasing temperatures as low as 0°C. Major N containing compounds and key enzymes in N metabolism during the senescence and low temperature acclimation were measured. The results showed that accessions of zoysiagrass from low latitudes had higher chlorophyll retention and lower N recycle (low soluble protein and NH₄-N) than those from higher latitudes in a simulated late fall, indicating that different mechanisms were responsible for the senescence and low temperature acclimation. In general, chlorophyll and soluble protein concentration decreased during the senescence which led to an increase of total free amino acids, especially glutamine, prior to the low temperature dormancy. The changes in N metabolism and the N uptake were also reflected in the decrease of NO₃-N and increase of NH₄-N, as well as relevant enzyme activities in the key steps. The signal and regulation mechanism in N cycling during the seasonal senescence and acclimation requires further investigations.