By Sigridur Dalmannsdottir, Ingerd Skow Hofgaard, Siri Ahlskog, and Trygve S. Aamlid, Norwegian Institute of Bioeconomy Research
Light is a crucial factor during cold acclimation. Yet, our understanding of how light intensity impacts freezing tolerance and snow mold resistance in grasses is limited. This study aimed to explore this relationship.
Our investigation took place in the phytotron in Tromsø, Norway under fully controlled climatic conditions. We tested two varieties of turf-type perennial ryegrass (Lolium perenne), ‘Fagerlin’, a Norwegian variety, and ‘GreenPlanet’, an American variety. Seedlings were grown for four weeks at 18°C and 24 h light (200-250 µmol/m2s-1). During this pre-acclimation period, seedlings were trimmed to 7 cm weekly. The grasses were then cold acclimated for six weeks at two different light intensities (100 µmol/m2s-1 versus 450 µmol/m2s-1, 12 h photoperiod) using Light Emitting Diodes (LED). Temperatures were 12°C for the first 1 ½ weeks, 6°C for the next 1 ½ weeks and finally 2°C for 3 weeks. Plants were not mowed during cold acclimation (Figure 1). Next, plants were either inoculated with a snow mold pathogen (Microdochium nivale) or exposed to various temperatures to test freezing tolerance. We hypothesized that stronger light intensity during cold acclimation would increase both the tolerance to freezing and the resistance to snow mold in perennial ryegrass.
The freezing test was performed by placing crown segments of trimmed seedlings (3 cm shoot length and 1 cm root length) in plastic boxes covered with fine, moist sand in a programable freezer with a temperature sensor in each box. The boxes were then frozen to pre-determined temperatures (-12, -14, -16, -18, -20, -22, -24°C). After thawing at 2°C, the seedlings were planted in a potting soil (Figure 2); after three weeks, survival was determined.
We observed a negative, though not significant, impact of high light intensities during cold acclimation on freezing tolerance in the Norwegian cultivar ‘Fagerlin’. In contrast, the freezing tolerance of the American cultivar ‘GreenPlanet’ did not differ between plants which had been exposed for different light intensities (Figure 3). Thus, the results so far do not support our hypothesis. It is possible that the higher light intensity was too high to be exploited by the plants at such low temperatures as used in our experiments. Instead, the higher light intensity may have caused photoinhibition in ‘Fagerlin’.
Regarding the effect of light intensity on snow mold resistance, results are yet to be revealed as the inoculated plants are still incubated under an artificial snow cover at 2°C (Figure 4).
