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  • Author or Editor: Richard Zobel x
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Different root types have different temporal and morphological patterns of development, functionality and adaptation. Root based adaptation to stresses can not be assessed on a root system basis, but must be employed on an individual root type basis. Three different types of root are developed by most seedlings: tap root, basal roots, and lateral roots. These have been demonstrated to have temporally and spatially different developmental and functional patterns. If a stress occurs prior to the onset of an especially sensitive type of root or after that type has shut down functionally, the seedling will demonstrate resistance, when it is not correctly resistant. Timing of screening treatments and scoring of the results is, therefore, extremely critical. Different genotypes within a species demonstrate strong differences in numbers and timing of root initiation and functional maturity for each of the root typos. In addition, different types of root demonstrate sensitivity/tolerance to different chemicals, suggesting that functionality and, therefore, resistance/tolerance mechanisms may differ. All root types present on a seedling must be scored for resistance/sensitivity to the stress, even if morphological/physiological symptoms are not readily apparent. The technology (knowledge, software and equipment) necessary to detect tolerance/resistance and to establish genetic selection schemes is available. Root type differences and the potential for genetic selection will be discussed.

Free access

Abstract

Plants exist as integrations of their many parts and processes. Each part is conditioned by a distinct collection of genes that interact and integrate with the genes that condition other plant parts or processes (44). Root characteristics are conditioned by about 30% of the plant genome, and one-third of these (10% of the total) condition only root characteristics (39). This level of control for a single plant organ is in agreement with that of other plant organs (25) and implies that root characteristics are as amenable to genetic manipulation as the characteristics of any other plant organ or tissue. Root characteristics are not normally emphasized in plant breeding programs because of the difficulty in observing them in situ, rather than because of a reduced potential for improvement. The level of genetic control described (39) should allow the development of isogenic root mutants that have modified physiological and developmental controls to be used for precise experimentation. Data derived from experimentation with these isogenic root mutants would provide a sound basis for developing and testing hypotheses leading, ultimately, to genetic improvement of vegetable root systems.

Open Access

Published data on the spatial patterns and periodicity of root growth in asparagus are limited. During the 1989 growing season growth and distribution of both fleshy and fibrous roots were monitored in a 7 year old asparagus planting. Soil cores were removed at 15 cm intervals to a depth of 90 cm at 40 and 80 cm from the plants in asparagus beds which had been maintained under conventional (CT) and no-till (NT) production systems. Fleshy and fine roots were separated from the soil and root length densities calculated. Harvests began in late March and continued at three week intervals until early November. Fine root growth was greater in the NT than CT in all depths and at both locations in March. Greatest lengths of fine roots were at the 15-60 cm depths for both CT and NT. This pattern was consistent throughout the season. Fine root lengths decreased by one half by the middle of the year (July) and remained at those levels until the last harvest (Nov). Fleshy root lengths were more variable, however NT generally had greater lengths than CT. Greatest length of fleshy roots were located in the 15-60 cm depths for both CT and NT treatments. Few fleshy roots were found below the 60 cm depth.

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Greenhouse experiments were conducted in 1987 and 1988 to evaluate the effect of timing of a 4-day flooding stress on growth and yield of snap bean (Phaseolus vulgaris L. cv. Bush Blue Lake 274, BBL). Plant survival was reduced when flooding was imposed at postflowering growth stages, but most plants survived when flooded before flowering or when reproductive development was prevented by deflowering. Early yields of surviving plants were very low in all flooded treatments, regardless of timing, in both years. Total yield response to timing of flooding was linear in 1987, with lowest yields when flooding was imposed at later growth stages. The trend was not linear in 1988, but in both years the latest flooding treatment (36 days after planting) had few surviving plants and no measurable pod yield. Additional greenhouse experiments revealed that leaf conductance of BBL and another bean cultivar, Luna (LN), declined within the first day of flooding. This decline was concomitant with one in leaf water potential and photosynthesis (Pn) in BBL, but decline of these responses occurred 1 to 2 days later for LN. After 4 days of flooding, Pn fell to near 0 for BBL, and to 15% of the prestress value for LN. Pn of both cultivars had recovered to 18.5 μmol·m-2·s-1 10 days after termination of flooding. LN had a larger adventitious root biomass, higher percentage of adventitious roots, and a consistently lower leaf: root ratio than BBL during recovery.

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Root and shoot growth periodicity were determined for Fraxinus pennsylvanica Marsh. (green ash), Quercus coccinea Muenchh.,Corylus colurna L. (Turkish hazehut) and Syriaga reticulara (Blume) Hara `Ivory Silk' (tree lilac) trees. Two methods for determining root growth periodicity using a rhizotron were evaluated. One method measured the extension rate of individual roots, and the second method measured change in root length density. A third method, using periodic counts of new roots present on minirhizotrons, was also evaluated. The root extension method showed the least variability among individual trees. Shoot growth began before or simultaneously with the beginning of root growth for all species with all root growth measurement methods. Species with similar shoot phenologies had similar root phenologies when root growth was measured by the root extension method, but not when root growth was measured by the other methods. All species had concurrent shoot and root growth, and no distinct alternating growth patterns were evident when root growth was measured with the root extension method. Alternating root and shoot growth was evident, however, when root growth was measured by the other methods.

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Abstract

Three black bean cultivars (Phaseolus vulgaris L.), ‘Midnight’, ‘Black Turtle Soup’, and ‘T-39’, were grown in a sand culture in a series of greenhouse experiments. Seedlings were subjected to various flooding treatments beginning at the time the first true leaves were fully expanded and continuing for up to 14 days. Root systems were partitioned into adventitious, basal, and tap components at harvest. Basal roots accounted for the greatest proportion of the total root biomass in control plants. Adventitious roots became the dominant root component in plants which had been flooded for up to 7 days and then allowed to recover for 7 days. The production of adventitious roots by flooded plants reduced their shoot:root ratios to values closer to those of unflooded control plants. ‘Midnight’ plants usually had larger basal roots and higher total root and shoot weights than plants of the other 2 cultivars. All 3 cultivars, however, were proportionally affected to the same extent by flooding.

Open Access

The objectives of this study were to determine root and shoot growth periodicity for established Fraxinus pennsylvanica Marsh. (green ash), Quercus coccinea Muenchh. (scarlet oak), Corylus colurna L. (Turkish hazelnut), and Syringa reticulata (Blume) Hara `Ivory Silk' (tree lilac) trees and to evaluate three methods of root growth periodicity measurement. Two methods were evaluated using a rhizotron. One method measured the extension rate (RE) ofindividual roots, and the second method measured change in root length (RL) against an observation grid. A third method, using periodic counts of new roots present on minirhizotrons (MR), was also evaluated. RE showed the least variability among individual trees. Shoot growth began before or simultaneously with the beginning of root growth for all species with all root growth measurement methods. All species had concurrent shoot and root growth, and no distinct alternating growth patterns were evident when root growth was measured by RE. Alternating root and shoot growth was evident, however, when root growth was measured by RL and MR. RE measured extension rate of larger diameter lateral roots, RL measured increase in root length of all diameter lateral roots and MR measured new root count of all sizes of lateral and vertical roots. Root growth periodicity patterns differed with the measurement method and the types of roots measured.

Free access