Estimating the cost of flowering in a grapefruit tree

The objective of the present study is to evaluate a Citrus tree's investment in the flowering process in relation to its photoassimilate resources, as a part of its annual reproductive effort. The overall requirement for carbohydrate of a single flower of grapefruit (Citrus paradisi Macf. cv. 'Marsh seedless') is evaluated as 8.33 x 10^-3 mol C over 3 weeks. The direct cost of production of a single flower is estimated to be 5.75 x 10^-3 mol C, most of which is allocated to the petals, anthers and style - organs designated to abscise. About 2-58 x 10^-3 mol C is consumed by respiration not associated with growth processes. Growth respiration (R(g)) occurs mostly during early stages of flower growth and development. However, the total respiration rate increases sharply during anthesis, when growth processes have almost ceased. Ethylene evolution also reaches remarkably high rates during anthesis. High temperatures increase the rate of flower respiration (Q¹⁰ = 2.12) but shorten the duration of flowering. A grapefruit tree may bear each year 20 000-50 000 flowers, only 0.5-2.5% of which develop into mature fruit. The amount of carbohydrate invested each year in bloom at the whole-tree level is 166-400 mol C per tree (depending on the number of flowers), amounting to 10-20% of the carbohydrate consumed for fruit growth. The overall daily demand for carbohydrate by the flowers of a grapefruit tree during anthesis may exceed the daily carbohydrate production by the leaves. High temperatures lead to a further increase in the dally demand for carbohydrate. In such cases, the management of flowering must rely on carbohydrate reserves recruited from other tree organs. The ecophysiological and evolutionary aspects of Citrus flowering are discussed.