Intraspecific differences in the response of perennial ryegrass (Lolium perenne L.) to drought

Perennial ryegrass (Lolium perenne L.) has traditionally been the grass species of choice for most New Zealand regions and is currently an important component of our highest-producing pastures. However perennial ryegrass is drought-sensitive and recent climate change projections for increased drought frequency and intensity in New Zealand have raised industry concern about the limitations of perennial ryegrass, particularly for northern and eastern regions. One possible solution is the introgression of wild-type germplasm that possess morphological and physiological attributes which improve drought tolerance, into high-producing commercial cultivars of perennial ryegrass. However there has been little research on perennial ryegrass in New Zealand that includes morphological and physiological drought response mechanisms. The objective of the current study was to investigate the responses to drought, of two bred perennial ryegrass cultivars; Grasslands Samson and Cropmark Seeds Kai; and two ecotypes, one of Norwegian origin and the other from Tunisia. This research was carried out in a glasshouse at Lincoln University, Canterbury during the winter of 2016. Under favourable conditions, the bred populations out-performed the ecotypes, producing 43% greater shoot biomass over the whole experiment than the ecotypes. The drought treatment reduced the shoot biomass of all populations, resulting in comparable shoot biomass production under drought among the four populations. However the Norwegian ecotype, in relation to its well-watered plants, was reduced by 58% by drought, showing a smaller reduction than Kai (-70%). Leaf DM (dry matter) production and LER (leaf extension rate) were both reduced by drought treatment in all populations. The mean LER and leaf DM production of the Tunisian population under the second drought by -58% and -23% respectively was a smaller reduction than that experienced by Kai (-70% and -64% respectively). The mean LER of the Norwegian population was reduced to the largest degree in the first drought cycle (-85%). The root to shoot DM ratio was increased in Kai, Samson and the Tunisian population under drought and to the greatest degree in the Tunisian population (+88%). However the root to shoot DM ratio of the Norwegian population remained unaffected by the drought treatment. The RWC (relative water content) across all populations was halved by the drought treatment, from a mean RWC of 92% in the well-watered plants to 47% under drought. However, the Norwegian population was able to maintain a greater RWC (60%) than that of Kai in the drought treatment (35%). The solute potential was reduced by drought in Kai (-145%), Samson (-139%) and the Tunisian population (-92%), but was unchanged in the population from Norway, indicating that it did not osmotically adjust. There were no differences in the chlorophyll concentration or leaf temperature between the four populations under drought. The Tunisian population was the only endophyte-infected population of the four. Taken together, this study identified intraspecific differences in key morphological and physiological drought responses of perennial ryegrass. The ecotypes showed signs of early drought tolerance, in contrast to the bred populations. Furthermore, there were differences in the drought tolerance strategies between the ecotypes, where the Norwegian population reduced leaf growth and preserved resources, and the Tunisian population increased its root to shoot DM ratio and was able to maintain LER to a greater extent than the other populations under drought as a result. The presence of endophyte in the Tunisian population likely contributed to its drought tolerance strategy. These findings provide functional information that can be used in future breeding initiatives towards improving drought tolerance in perennial ryegrass.