HortTechnology October – December 1998 8(4) Understanding Root Systems to Improve, Seedling Quality Silvana Nicola 1 SUMMARY. Root architecture can be very important in plant productivity. The importance of studies on root morphology and development is discussed to improve seedling growth. Root systems of dicotyledonous species are reviewed, with emphasis on differences between growth of basal and lateral roots. The presence of different types of roots in plant species suggests possible differences in function as well. The architecture of a root system related to its functions is considered. Classical methods for studying root systems comprise excavation of root system, direct observation, and indirect analyses. While the first method is destructive and the third is effective in understanding root architecture only on a relatively gross scale, observation methods allow the scientist a complete a nondestructive architectural study of a root system. The three groups are reviewed related to their potential to give valuable information related to the root architecture and development of the seedling, with emphasis on the availability of a medium-transparent plant-growing system, enabling nondestructive daily observations and plant measurements under controlled environmental conditions. Effects of CO2 enrichment on seedling growth is reviewed, emphasizing the effects of CO2 on root growth. Vigorous root systems are as essential as vigorous shoots for growth and development of healthy plants. Early seedling root growth and development determine the optimum root system throughout the entire life of a plant, consequently affecting growth during this period and potentially leading to optimization of yields (Leskovar and Stoffella, 1995; Lynch, 1995). The spatial distribution in the soil of the root system can determine the potential of a plant to exploit the soil's resources, which are unevenly distributed on Earth's surface or subjected to localized depletion by the roots (Lynch, 1995). The production of a primary root system, i.e., the primary branching from the radicle, may have a major impact on growth and survival of a plant (MacIsaac et al., 1989). A primary root system increases the surface area available for the uptake of water and mineral elements. In addition, with its architecture, a primary root system provides physical support to the developing shoot. Morphology and development of young roots in dicotyledonous species Seedling root morphology differs between monocotyledonous and dicotyledonous species (Fahn, 1982; Leskovar and Stoffella, 1995; Sutton and Tinus, 1983; Zobel, 1975, 1986, 1995, 1996). Dicotyledonous species were described as presenting three types of roots: radicle, adventitious, and lateral (Esau, 1965, 1977). The radicle forms the taproot (pri- hypocotyl and upper portion of the al., 1979), and lettuce (Lactuca sativa mary root), adventitious roots are taproot. Assuming only three types of L.) (Nicola, 1997). initiated from the hypocotyl, and root, the double homozygote should Evidence that basal and lateral roots lateral roots are from the taproot. have had only a taproot. Because these differ in terms of morphology, point of Beginning with Zobel (1975), several roots were genetically not lateral, nor origin from the taproot and researchers have identified a fourth were they adventitious, Zobel (1975) development was reported by Nicola type of roots-basal roots. Zobel (1986) classified these roots as basal roots. (1997) in lettuce seedlings grown in indicated that initiation of basal roots is Basal roots have been reported to be transparent medium (Fig. 1). Basal and under different genetic control than produced by mungbean (Vigna radiata lateral roots in lettuce seedlings initiation of lateral and adventitious L.) (Leskovar and Stoffella, 1995), bell originated from two distinct re gions of roots. In fact, a double homozygote pepper (Capsicum annuum L.) the taproot and developed differently. from a lateralless tomato mutant (Leskovar et al., 1989; Stoffella et al., A thicker, short upper radicle was (recessive mu tant called diageotropica, 1988), tomato (Lycopersicon esculen- visibly distinguished from a smaller, dgt) and an adventitiousless tomato tum Mill.) (Zobel, 1975), beans long, lower radicle 2 d after seeding mutant (recessive mutant called (Phaseolus vulgaris L.) (Stoffella et (DAS). This distinction was even more rosette, ro ) originated roots in the evident 3 and 4 DAS. A restriction area 1 Dipartimento di Agronomia, Selvicoltura e Gestione del Territorio,Universita di Torino, Via Leonardo da Vinci, 44, 10095 Grugliasco (TO), Italy; e-mail: [email protected]. HortTechnology October – December 1998 8(4) separated the two portions of the defined lateral roots as "roots derived hypothesized that basal roots would taproot. Basal roots originated at two from lateral endogenous primordia provide a means for plants to uptake opposite sides of the short upper formed in preexisting roots" (p. 149). surface-applied nutrients and water portion of the taproot and located close The author said that lateral roots during crop production, and they may to the hypocotyl, and did not produce appeared at a relatively constant also play a role in supporting the plant secondary branches during the first 18 distance behind the tip of a growing (Bole, 1977; Eshel and Waisel, 1996; DAS. Lateral roots originated at ˜120o root, that lateral roots initiated in rows Jackson, 1995). Findings in lettuce that apart along the longer portion of the or ranks, and that within each rank they lateral roots originated in three taproot and produced secondary appeared to initiate and emerge in directions with respect to the taproot, branches. Basal roots presented a acropetal sequence under normal at 120o apart and extending deep in the horizontal extension in the medium, conditions. Charlton (1996) reported soil, is an indication that lateral roots while lateral roots presented a growth that between the basipetally emerged can explore more soil volume for extension downward into the medium. lateral roots, particularly in dicots, resources than basal roots, thus lateral The number of lateral roots 18 DAS additional latera ls may arise for a long roots may be able to reach and exploit was 2- to 3-fold the number of basal period in roots with secondary growth. localized patches of nutrients in the roots. Esau (1965) referred to these soil (Lynch, 1995). Conversely, basal Adventitious roots cannot be con- additional lateral roots as adventitious. root bidirectional and superficial founded with lateral roots for two main formation gives the root system a reasons. First, adventitious roots The architecture of a root horizontal extension in the soil surface, originate from the stem, while lateral system related to its function assuring the basal roots the capacity to roots originate from the taproot. exploit the most fertile portions of Second, the former type originates Fitter and Stickland (1991) and agricultural soils (Bole, 1977; Eshel from tissues other than the pericycle, Lynch and van Beem (1993) suggested and Waisel, 1996). while the latter type originates from the that the architecture of a root system Zobel (1996) reported that species pericycle. Conversely, basal roots are may have ecological implications for that demonstrated the most drought not clearly classified with respect to uptake of water and nutrients from soil. tolerance had the most deeply penetrat- their point of initiation. Zobel (1986) Fitter (1986, 1987, 1996) suggested ing root system, implying that plants demonstrated that basal roots initiate that, in general, plants with a more with an extensive lateral root system from the pericycle of the lower herringbone-like distribution of roots, would be favored in these instances. hypocotyl and upper taproot. Con- that is, with branches mainly along the Eshel and Waisel (1996) suggested that sequently, basal roots were not central root axis, may occur under low the major function of basal roots was adventitious in anatomical origin, nor soil resource availability, whereas in to exploit the most fertile portions of lateral or adventitious in genetic plants with a dichotomous-like distri- agricultural soils more efficiently than control of their initiation (see above). bution, secondary branches increase lateral roots. In addition, Bole (1977) Detailed information of the when resources are present in abundant found that basal roots of rape (Brassica sequence of events occurring in root supply, thereby increasing acquisition campestris L.) were capable of branch forma tion of young seedlings of water and nutrients. Variation in absorbing phosphorus more efficiently could be valuable to further determine root architecture among species was the root development of field-grown indicated by Seiler (1994) to be an plants. In bell pepper seedlings, basal important factor determining differ- roots emerged prior to lateral roots ences in drought tolerance among (Stoffella et al., 1988) and only after species. Early, rapid root growth and full cotyledon expansion. A similar branching were suggested to confer an development was found in lettuce adaptive advantage in more efficient seedlings (Nicola, 1997). In tomato use of soil water. Root elongation can seedlings, lateral roots emerged before be advantageous to plants in drying basal roots (Aung, 1982). Weinhold soil, and may be particularly important (1967) described basal roots as arising for seedling establishment. Growth of acropetally (toward the shoot apex) new plants