The growth of terrestrial plants is primarily limited by edaphic factors such as drought, nutrient deficiencies, and mineral toxicities. Roots express an array of adaptive traits that help plants cope with these stresses, but such traits often incur significant costs, including direct metabolic costs, tradeoffs for contrasting resources, opportunity costs, and increased risks of biotic and abiotic stress. As an example, root cortical aerenchyma appears to improve crop growth under drought and low soil fertility by reducing the metabolic costs of soil exploration. Production of root cortical aerenchyma, however, may involve tradeoffs by reducing radial transport, mycorrhizal colonization, and by increasing disease susceptibility. A better understanding is needed of the full costs associated with specific root traits if we are to develop crops with better growth and yield in the stressful soil environments that increasingly dominate the earth's surface.
Jonathan P. Lynch
Robert C. Ebel and Jonathan P. Lynch
Kinematic analysis allows accurate description of physiological changes along root axes by additionally taking into account changes due to dilution as cells expand. In previous studies using kinematic analysis, roots have been marked with ink by fine-tipped pens or single hair brushes. These methods have occasionally reduced root growth and limited resolution to the width of the marks, usually 1 mm. We describe a new method of marking roots with the fluorescent dye calcofluor which does not reduce root growth. The terminal 7 mm of bean root tips were grown vertically in a glass chamber into which a constant flow of aerated nutrient solution was passed. A 0.001% calcofluor solution was pulsed through the chamber for 1 min. Excess calcofluor was removed rapidly by a high rate of nutrient flow (200 ml·h–1) for 3 min. after which flow was reduced to 20 ml/hr. Roots were magnified 11.5× under a microscope mounted horizontally and five digitized images captured every 5 min. Imaging software allowed determination of fluorescence of individual pixels along the length of the root. Fluorescence decreased in the zone of cell elongation due to dilution as cells expanded. This method may improve resolution of kinematic analysis to the length of individual pixels, which was 18 microns at 11.5× magnification.
Jonathan P. Lynch and Stephen E. Beebe
Andrew Riseman, Richard Craig and Jonathan P. Lynch
Interspecific hybrids of exacum (Exacum L.) display variable responses to zinc nutrition. Our research compared two genotypes with contrasting zinc efficiency phenotypes in terms of root cation exchange capacity (CEC), whole plant 65Zn uptake, and the effects of Cu+2 and Mg+2 on 65Zn uptake and partitioning to shoot tissues. Results show that the zinc efficient and inefficient genotypes had significantly different root CEC [27.2 and 16.9 cmol(+)·kg-1 root dry weight (DW), respectively] and whole plant 65Zn uptake rates (0.048 and 0.026 μmol·h-1·g-1 DW, respectively). In equimolar concentrations to Zn+2, Cu+2 reduced Zn+2 uptake by approximately 50% in both genotypes while supplemental Mg+2 enhanced Zn+2 uptake. In addition, Mg+2 facilitated a larger proportion of absorbed 65Zn to the upper shoot of the efficient genotype. We conclude zinc is absorbed through a specific Zn+2/Cu+2 transporter and that zinc efficiency in exacum is based on a combination of apoplastic and symplastic traits. In addition, a secondary Mg+2 × Zn+2 interaction may contribute to the zinc efficiency phenotype.
Yuan-ling P. Lin, E. Jay Holcomb and Jonathan P. Lynch
Alumina granules charged with P were used as an amendment to improve the ability of a soilless medium to retain P and provide it to plants. Commercially available alumina was acidified, saturated with P, and evenly distributed in a medium of peat, vermiculite, and sand to grow potted marigolds (Tagetes spp.) to a commercially salable stage. Marigolds grown in medium amended with P-charged alumina had adequate nutrition and similar or superior shoot growth (as measured by height, number of branches, and flower production) and fresh and dry weights compared to marigolds grown using commercial fertilizer. Phosphorus-charged alumina at 1% or 2% of total medium volume was sufficient to grow marigolds for at least 8 weeks and substantially reduced P leaching compared to conventionally fertilized controls. Alumina amendments in this range did not cause Al toxicity, as evidenced in root growth and leaf Al content.
Yuan-ling P. Lin, E. Jay Holocomb and Jonathan P. Lynch
Soilless growing media are used extensively in the greenhouse, especially for the potted plant production. Unlike soil having a phosphorus (P)-fixing ability, soilless media allows greater P leaching from the media. Leaching of excess P results in inefficient fertilizer utilization and effluent pollution. In hydroponic and sand-culture systems, alumina adsorbed with P (P-alumina) has been developed as a P source to maintain buffered P concentrations in nutrient solutions. This P-alumina has not been used with soilless media; however, it may have a potential of serving as a P source for plant growth and a P buffer to alleviate P leaching in soilless media. Marigolds were grown in soilless media (peat moss: vermiculite: sand=2:2:1, v/v/v) with P-alumina at various concentrations being substituted for sand. These marigolds were fertilized with a nutrient solution containing no additional P, while the control was fertilized with complete nutrient solution. In four cultivars of marigolds, me P-alumina treatments produced comparable or superior growth and floral production compared to plants provided with complete nutrient solutions or conventional fertilizer. 70% of applied P was leached in conventional treatments compared to only 2% in the P-alumina treatments.
Robert H. Snyder, Jonathan P. Lynch, Donald Kaufman and Terry Schettini
Sustainable agricultural systems favor high organic amendments over chemical fertilizers for maintaining long-term soil fertility. To study root responses bell pepper was grown in soil treated with dairy compost, raw dairy manure, and a chemical fertilizer mix at Rodale Institute Research Center, Kutztown, Pa. Root crowns were excavated at 2-week intervals and total length determined from root subsamples by computer-based image analysis. Roots from compost amended plots displayed a simple branching pattern; a first order branch with short second order branches. Fertilizer stimulated a complex branching; short, thickened first and second order branches that supported long and thin third and fourth order roots. An intermediate form in the raw dairy plots yielded both simple and complex branching forms. All forms were dynamic within each treatment over time. Crown length averaged 250-300 m across treatments 6 weeks after transplanting. Raw dairy and fertilizer treatments decreased slightly in length by week 10, while compost remained constant. After heavy rainfall crown length increased to 400 m for compost and raw dairy, and to 750 m for the fertilizer treatment by week 13. Length for the fertilizer treatment dropped nearly 200 m by week 14. though an increase of 100-200 m occurred for compost and raw dairy treated roots respectively.
Kristian Borch, Carter Miller, Kathleen M. Brown and Jonathan P. Lynch
A series of experiments was conducted to investigate the response to drought stress of marigold (Tagetes patula L. `Janie Tangerine') plants grown with reduced phosphorus. Plants were grown with convention al phosphorus fertilization (1 mm, control) or one of two levels of alumina-buffered phosphorus (Al-P), 21 or 5 μm. Plants supplied with 21 μm Al-P produced plants with equal total dry weight, more flowers and reduced leaf area compared to control plants. Whole-plant photosynthetic CO2 assimilation expressed on a leaf area basis was nearly twice as high in 21 μm Al-P plants as in controls, probably as a result of reduced intraplant shading. In plants supplied with 21 μm Al-P, smaller leaf area resulted in reduced whole-plant transpiration. Moreover, the relative water content of the growing medium was significantly lower at wilting with 21 μm Al-P than for control or 5 μm Al-P regimes. The improved water acquisition with 21 μm Al-P could be explained by increased root proliferation via longer main roots and less densely distributed lateral roots. The results indicate that optimizing phosphorus nutrition with solid-phase buffered-phosphorus fertilizer improves drought tolerance by reducing transpiration and increasing water acquisition from the medium.
Eric M. Lyons, Robert H. Snyder and Jonathan P. Lynch
Root distribution in turfgrass systems influences drought tolerance and resource competition with undesirable species. We hypothesized that spatial localization of phosphorus (P) supply would permit manipulation of turfgrass root distribution. To test this hypothesis, creeping bentgrass (Agrostis stolonifera L.) plants were exposed to localized P supply in two experiments. The first experiment split the root zone horizontally into two different growth tubes and the second used alumina-buffered P (Al-P) to localize P availability deeper within a continuous root zone. In the horizontally split root zones, heterogeneous P availability led to no difference in shoot growth compared with uniform P availability. Root proliferation was greatest in the growth tube with available P compared with the growth tube without P. The use of Al-P, regardless of its spatial distribution, doubled root-to-shoot ratios compared with soluble P. Much of the increase in the ratio was accounted for by reduced shoot growth. Use of Al-P increased rooting deeper in the root zone, especially when the Al-P was mixed only in the lower portion of the root zone. Our results are consistent with the hypothesis that root distribution of creeping bentgrass can be manipulated by spatial localization of P supply in the root zone and indicate that relative biomass allocation to roots and shoots may be manipulated with buffered P sources.
Kristian Borch, Kathleen M. Brown and Jonathan P. Lynch
Bedding plants are frequently exposed to water stress during the postproduction period, resulting in reduced quality. We demonstrated that alumina-buffered P fertilizer (Al-P) provides adequate but much lower P concentrations than conventionally used in soilless mixes. When impatiens (Impatiens wallerana Hook. f. `Impulse Orange') and marigold (Tagetes patula L. `Janie Tangerine') plants were grown with reduced phosphorus using Al-P, P leaching was greatly reduced and plant quality was improved. Diameter of impatiens plants and leaf area of plants of both species were reduced by Al-P. Marigold plants grown with Al-P had more flowers and fewer wilted flowers. Flower wilting was also reduced for impatiens plants grown with Al-P. In marigold plants, roots were confined to a small volume beneath the drip tube in control plants, while roots of Al-P plants were well distributed through the medium. There was no obvious difference in impatiens root distribution. When plants at the marketing stage were exposed to drought, the Al-P plants of both species wilted more slowly than the conventionally fertilized controls. The reduced leaf area in both species and the improved root distribution of marigold may account for the improvement in drought tolerance of the Al-P plants.