Genetic differentiation in white clover (Trifolium repens) populations during 8 years of contrasting phosphorus supply in New Zealand hill country

Abstract

Genetic differentiation under the selection force of variable soil P availability was measured in populations of two white clover cultivars introduced to a grazed, low fertility grassland site. The resulting mixed grass‐clover pastures subsequently received contrasting levels of phosphorus (P) fertiliser for 8 years. Afield‐based reciprocal transplant experiment and two controlled‐environment experiments were conducted using plants removed from the 8‐year field experiment and appropriate controls to test the hypotheses that: (1) genetic differentiation occurred within the white clover populations in response to variation in P supply; and (2) surviving plants which demonstrated genetic differentiation under high‐P conditions would continue to do this when grown in mixtures with plant material removed from unsown pasture. This study demonstrated that genetic differentiation did occur. In the reciprocal transplant experiment, when returned to a low‐P environment the mean of the populations removed from a low‐P site yielded more than the mean population removed from a high‐P site. Genetic differentiation was demonstrated most strongly with ‘Grasslands Huia’. ‘Huia’ survivors growing in the high‐P source environment retained the potential of the original seed line to respond to P additions, but this potential was lost in ‘Huia’ populations grown under low‐P fertility. In contrast, ‘Grasslands Tahora’ did not retain the potential of the original seed line to respond to additional P and behaved similarly to the resident material. High‐P inputs led to the retention of ‘Huia’ genotypes with the capacity for high growth rates and superior competitive ability. This competitive ability was demonstrated when resident plants grown in a mixture with ‘Huia’ plants removed from the high‐P source treatment suffered reductions in yield compared with their yield in pure pots. The drift by both ‘Tahora’ and ‘Huia’ away from the potential of the original seed attributes towards the resident type demonstrates the adaptive nature of the traits found in the resident population that favour a conservative growth strategy. P availability does not appear to be a dominant selection factor where new germplasm has been introduced to pastures with the objective of stimulating production; maintenance of fertility levels may assist in retaining a full complement of P‐responsive genotypes.