Carbon Relations of Flowering in a Semelparous Clonal Desert Perennial

Abstract

Agave deserti is a long—lived, semelparous perennial of the northwestern Sonoran Desert that flowers after °50—55 yr. Measurements of CO2 exchange over 24—h periods indicated that leaves of flowering rosettes had 24% more net CO2 uptake than leaves of adjacent nonflowering rosettes during the first month of inflorescence production. Net CO2 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 CO2 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.