Constant distance between leaf initiation sites permits nondestructive analysis of apical meristem activity during cactus shoot growth.

Background and scope: Trunks of saguaro cacti (Carnegiea gigantea) grow for many years, and during this time, the shoot apical meristem (SAM) of each trunk not only grows in diameter, it also initiates new orthostichies (ribs). Several questions were examined: Is a saguaro SAM's diameter correlated with the number of orthostichies/ribs it is producing? Is SAM diameter tightly controlled, or does it vary among individuals of the same age? When saguaro trunks are about 3 m tall, their SAMs stop adding new orthostichies/ribs: do SAMs stop growing only after reaching a critical diameter, or do the SAMs vary in diameter when each stops growing?

Methods: Ribs were counted at various heights (corresponding to various ages) on saguaro plants in habitat. Shoot apical meristem diameter was measured by light microscopy in sectioned material. Shoot apical meristems of Echinocactus grusonii were also studied.

Key results: Shoot apical meristem diameter is strongly correlated with the number of ribs being maintained: the circumferential distance between newly initiated leaf primordia remains constant (145 ± 10.6 µm in C. gigantea; 193 ± 10.7 µm in E. grusonii) even as an SAM grows in diameter. An SAM's diameter and circumference can be estimated by counting the number of ribs it is maintaining. The diameter of each SAM of C. gigantea increases for many years but it eventually stabilizes; the final, stable diameter of each C. gigantea SAM varies from shoot to shoot.

Conclusions: Shoot apical meristem diameter in both species can be estimated nondestructively by simply counting the number of orthostichies/ribs the SAM is producing (or produced in the past). The growth rate of C. gigantea SAMs varies from plant to plant and can change with age. All C. gigantea SAMs stop increasing in diameter at some point, but that diameter varies from plant to plant.