Carbon relations of flowering in a semelparous clonal desert perennial

.4bstmct. Agave deserti is a long-lived, semelparous perennial of the northwestern Sonoran Desert that flowers after -50-55 yr. Measurements of CO, exchange over 24-h periods indicated that leaves of flowering rosettes had 24% more net CO, uptake than leaves of adjacent nonflowering rosettes during the first month of inflorescence production. Net CO, uptake for leaves of flowering rosettes was 32% less thereafter than for leaves of nonflowering rosettes, as senescing leaves of flowering rosettes exhibited dramatic reductions in nitrogen and chlorophyll contents. During the course of flowering, levels of total nonstructural carbohydrate (TNC) in the leaves of flowering rosettes dropped from 38 to 6% of the leaf dry mass, indicating substantial translocation of stored carbon to the inflorescence. TNC reserves of the rosette provided 70% of the carbon required to produce the 1.53 kg inflorescence, and CO, uptake by the leaves and the inflorescence provided the remaining 30%. Rosettes must attain a minimum size (>1000 g dry mass) to initiate flowering, unless they are connected to a large flowering rosette. Small rosettes did not produce inflorescences when their rhizome connection to a large rosette was severed -4 wk before inflorescences emerged, suggesting that a chemical signal is transmitted through the rhizome that induces the small rosette to flower precociously. Small flowering rosettes could not complete formation of the inflorescence unless partially supported by carbon supplied by the connected large rosette. The contribution of the large rosette declined from 74% for a 0-30 g dry mass connected rosette to 35% for a 200-600 g rosette. For both small and large flowering rosettes, the translocation of substantial carbohydrate reserves from the leaves is essential for production of the inflorescence. -50-55 yr (Nobel 1987). Within a population the Most perennial plants flower more than once, but number of rosettes that flower each year varies up to semelparous species, or Big Bang reproducers (Gadgil 50-fold, depending on soil water conditions 2 yr prior and Bossert 1970), experience a single, massive repro- to flowering and the number of rosettes that flowered ductive episode that is followed by fairly rapid plant in the previous year (Nobel 1987). Most rosettes that death. Semelparous species exhibit postreproductive flower are large, but - 10% of the flowering rosettes are mortality in part because they allocate resources to small and are connected to large flowering rosettes. their single reproductive event that otherwise would The period from the initial emergence of the inflobe used for maintenance and growth of vegetative rescence of A. deserti until seed dispersal lasts -5 mo structures. The evolution and maintenance of semel- and involves the production of a single, 3-4 m tall, parity in perennials is promoted by increased fecundity bracteate inflorescence from the shoot apex of the with age that offsets the delay in reproductive maturity basal rosette of leaves, with lateral branches arising (Schaffer 1974, Bell 1980). Delayed reproduction is from the upper sixth of the main axis of the infloresoften accompanied by increased stored resources that cence (Nobel 1977). Evolution of such large inflorescan be used for greater flower and fruit production cences in Agavaceae has been attributed to pollinator (Aker 1982a, b) and greater seed production (Janzen preference for inflorescences with the greatest pollen 1976).