Root anatomy of Pinus taeda L.:: seasonal and environmental effects on development in seedlings

The environmental and seasonal effects on anatomical traits of Pinus taeda L. seedling roots were studied in the laboratory in three contrasting root growth media and also in typical outdoor nursery culture. Growth media with lower water regimen and high penetration resistance caused a reduction in lengths of the white and condensed tannin (CT) zones and acceleration of development of suberin lamellae in the endodermis. As a possible counter to this reduction in zone lengths, second-order laterals were produced closer to the tips of first-order laterals. This suggested there may be an advantage to producing more shorter roots under stressful conditions. Under outdoor nursery conditions (June to mid-December) the white zone was always a rather small part of the root system surface area (4.5% in December), but it dominated as a provider of cortical plasmalemma surface area (CPSA) in contact with modified soil solution (65% in December) because of its live cortex and capacity to increase nearly three fold the amount of CPSA per unit root length. The CT zone always provided most of the total root surface area (80% in December). Although it had no live cortex, a few cells of the CT zone endodermis remained non-suberized passage cells, perhaps giving this major part of the root system some capacity for ion and water absorption. A late summer increase in CPSA was due largely to the rapid production of mycorrhizae. Root systems were capable of very rapid replacement of roots lost due to undercutting and lateral root pruning. The great variation in CPSA per unit root length contained in the white, mycorrhizal and CT zones suggested a capacity to adapt rapidly to changing conditions.