Table

Growth characteristics of species with determinate root growth (Stenocereus gummosus and Ferocactus peninsulae) and indeterminate root growth (Pachycereus pringlei) in the first week of the seedling stage

Quantity

S. gummosus F. peninsulae P. pringlei

Primary root growth rate (pm hour-1) Length of mature epidermal cells (pm) Cell number in a file of epidermal cells in the meristem Duration of the cell division cycle (hours)

12.0 ± 0.2 20.9 ± 0.4 20.8 ± 0.4 9.7 ± 1.1 11.7 ± 0.9 14.5 ± 0.6

The duration of the cell division cycle is estimated by the rate of cell production method (Ivanov and Dubrovsky 1997). Data are means ± SE (n = 11) and are from Dubrovsky et al. (1998a).

time (days)

Figure 3.3. Primary root length for Ferocactus peninsulae, Pachycereus pringlei, and Stenocereus gummosus for 7 days after the start of radicle protrusion. Data are means ± SE (n = 12) and are from Dubrovsky et al. (1998a).

time (days)

Figure 3.3. Primary root length for Ferocactus peninsulae, Pachycereus pringlei, and Stenocereus gummosus for 7 days after the start of radicle protrusion. Data are means ± SE (n = 12) and are from Dubrovsky et al. (1998a).

during the first week after germination (Fig. 3.3) and then stops growing. The roots have much shorter meristems, averaging 21 epidermal cells in a cell file (Table 3.1). Smaller root meristems are more typical for young seedlings, because meristem size is correlated with root diameter, and primary and lateral roots are typically thinner than adventitious roots.

The rate of root growth depends on various factors. A primary external factor is temperature, which affects the duration of the cell cycle (cycle time). At an optimal temperature, the average cycle time in the root apical meristem for O. ficus-indica is 28 hours (Dubrovsky et al. 1998b) and for P pringlei is 15 hours (Dubrovsky et al. 1998a). A primary internal factor is the final size of elongated cells. The cell length for O. ficus-indica averages 378 pm (Dubrovsky et al. 1998b) and for P pringlei, 176 pm (Dubrovsky et al. 1998a). The average cell cycle time within the root apical meristem can vary within and among species, but, in general and assuming elongated cells are the same size, the shorter the cycle, the greater the rate of root growth (Dubrovsky et al. 1998a).

Determinate Root Growth

Determinate root growth is the result of a developmental program in the root apical meristem, in which a finite number of cell division cycles takes place. For cacti, determinate root growth was first described for lateral roots of Opuntia arenaria and O. tunicata var. davisii (Boke 1979).

Two types of determinate roots occur for these species. First, relatively long lateral roots (a few cm long) are developed as a part of the absorptive or feeder root system. The apical meristem of these roots has a normal organization, but functions for only a limited time. Soon the root tip dies, and a new meristem is formed behind the dead root tip, giving rise to a sympodially branched system of roots of various lengths. On relatively long roots, short determinate roots of a second type, called "root spurs," develop. The root spur is a cluster of sympodially branched roots less than 1 mm long. The root tip of such spur roots is atypical, lacking a root cap. All cells of the small apical meristem become differentiated, and the tip of the spur root becomes completely covered by root hairs. Spur roots may allow a rapid increase in root surface area, increasing water uptake during infrequent periods of rainfall (Boke 1979).

A determinate pattern of root growth in primary roots is highly unusual for flowering plants. Indeed, continuous elongation of primary roots seems generally more advantageous, particularly for seedling establishment. Surprisingly, determinate root growth is exhibited by the primary root of Sonoran Desert cacti belonging to two tribes, Pachycereeae (P pringlei, Stenocereus thurberi, and S. gummosus; Dubrovsky i997a,b) and Cacteae (Ferocactus peninsulae; Dubrovsky 1997b). Determinate growth of primary roots is also shown by S. pruinosus and S. standleyi growing in the Tehuacan Valley of Mexico (Dubrovsky 1999). A similar pattern occurs for epiphytic cacti, including Epi-phyllum phyllanthus, in which the radicle dies after elongating by only 200 to 400 pm (G. B. North, unpublished observations).

A typical characteristic of cactus roots with determinate growth is the relatively short duration of primary root growth and early meristem exhaustion. For example, P. peninsulae and S. gummosus grow for only 2 days after the start of radicle protrusion (Fig. 3.3). When seedlings are grown on filter paper, the radicle does not exceed i0 mm, similar to its length in soil (Dubrovsky i997a,b). When seedlings are grown on sterile medium supplemented with 2% sucrose, the pattern of growth is the same, even though the final size of the roots is greater (Dubrovsky 1997b). During the period of steady-state growth (24-36 hours), meristematic activity is maintained and the size of the root apical meristem remains unchanged. After this period, meristematic cells cease dividing, leave the meristem, and undergo rapid elongation. Because no new cells are produced, the meristem rapidly decreases in size and becomes exhausted. As epidermal cells elongate and differentiate producing hairs, the root hairs grow to the tip of the

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