AbstractThe effects of copper deficiency on the growth and flowering of 21 chrysanthemum cultivars were studied in a peat‐sand substrate (3:1 v/v); nine cultivars were grown as ‘sprays’, ten as ‘standards’ and six as pot plants. Copper deficiency either prevented or severely delayed floral development in many of the ‘spray’ and ‘standard’ cultivars, and resulted in continued vegetative growth. In cultivars which were less seriously affected, flowering was delayed and the number of blooms per plant was reduced. Cultivars grown in pots were sprayed with the growth retardant B‐Nine (daminozide); these plants showed the least response to omission of copper from the substrate, and flowering was delayed by only a few days. In the absence of added copper the substrate and the leaves of the plants contained 0.3‐0.5 μg Cu ml−1and 1‐4 μg Cu g−1, respectively. When copper was supplied the corresponding values were 49‐55 fig μg Cu ml−1and 7‐19 /μg Cu g−1. Copper deficiency reduced phenolase activity (Δ OD min−1g−1of fresh tissue) in the leaves of ‘spray’ and ‘standard’ cultivars from 25‐113 units to 4 units or less, and in the pot‐grown plants from 56‐169 units to 4‐47 units. No phenolase activity was detected in four of the ‘spray’ and t
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