A comparison of the effects of environment on inflorescence initiation in nine lines of white clover (Trifolium repensL.)

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Abstract
The flowering behaviour of 9 lines of white clover (Trifolium repens L.), and 3 hybrids between 4 of them, was compared (at Palmerston North) in the natural environment and in a heated glasshouse from March 1977 to November 1979. Lines compared, and their latitudes of origin, were Jygeva-4 (J), latitude 59°N; Kalinin ‘local’ (K), 57°N; Kent Wild White (KWW), 51°N; Regal Ladino (RL), 45°N; a Portuguese mat-type (P), 40°N; a Spanish ecotype, C1067 (S), 39°N; Grasslands Huia (H), 400S; Tamar (T), 32°N; and Louisiana S.1 (LS1), 31°N. In all lines inflorescene initiation occurred earlier under cooler conditions. Those from lower latitudes (T and LS1) initiated earliest in the field (March-April) whereas those from the highest latitudes (J, K, KWW, and RL) and higher altitudes (P) initiated latest (JulyAugust). Little (in T only) or no initiation occurred in August-September in plants grown throughout autumn and winter in the warm. Cool pretreatment in the field led to weak initiation in a warm glasshouse in August-September in J, K, KWW, RL, and H, and to strong initiation in S, T, and LSI. Most lines behaved as long-day plants in warm glasshouse conditions, but initiation in T and LSI was weaker in long days than short days, and S remained vegetative in warm long days. In lines which flowered strongly in the long days of early summer, all except RL and P stopped initiation almost completely after December, both inside and out. Thus, 2 broad categories of plants were recognisable: higher latitude lines tended to initiate late in response to cool short days and were typical long-day plants; lower latitude lines initiated earlier in cool short days and flowered weakly or not at all in long days. The time at which initiation started in autumn-winter in hybrids between Hand KWW, T, or S was intermediate between that in the parents. The differences in flowering behaviour between lines are mostly explicable in terms of differences in the amount (duration and level) of low temperature required for initiation as a direct response, and variation in the inhibitory effect of warm long days on initiation.