Flowering competence after exposure to naturally fluctuating winter temperatures in a perennial grass, Deschampsia caespitosa (L.) Beauv.

Tussocks of three clones of the perennial grass Deschampsia caespitosa were exposed to natural temperatures, for successive periods with increments of one week, throughout each season in four consecutive autumn/winter seasons. After exposure they were returned to a warm, illuminated glasshouse and the incidence, intensity and timing of flowering were recorded.

The year-to-year variations in exposure time requirement for full flowering were very highly correlated (r2 > 0.98) with mean air temperature in the period between mid-September and early December. The relationship with the natural logarithm of accumulated air temperature was similarly good. Daily mean air temperature was also demonstrated to be a good indicator of daily mean tussock temperature (r2 = 0.91), in a series of measurements in the autumn of 1976.

More prolonged exposure increased the proportion of apices in a tussock which flowered, but the maximum value at the end of a whole winter was only about 15 per cent. The cumulative vernalization effects of naturally fluctuating temperature regimes are analysed and the significance of these findings for the maintenance of perenniality in clonal grasses is considered.