Abstract
Argentine mesquite (Prosopis alba) seedlings were inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungi, Glomus intraradices, container-grown for five months, and transplanted into a simulated landscape in Tempe, Arizona. After transplanting, mesquite trees were either drip irrigated at regular intervals or nonirrigated for one year. Six months after transplanting, VAM fungal colonization was observed only in the roots of inoculated mesquite but by 12 months, roots of both inoculated and noninoculated mesquite were colonized by VAM fungi. Higher levels of VAM fungal colonization occurred in roots of irrigated mesquite. While irrigation promoted mesquite shoot growth, VAM inoculation inhibited shoot growth of nonirrigated trees. Trunk caliper was greater for irrigated trees than for nonirrigated trees and was not affected by VAM infection. VAM fungi promoted the growth of thinner roots for irrigated trees and thicker roots for nonirrigated trees after six months. At the conclusion of the study, root growth was enhanced by drip irrigation and was not affected by previous VAM fungal inoculation.
An estimated two-thirds of all terrestrial plants form vesicular-arbuscular mycorrhizal (VAM) associations (15), and this type of mycorrhizal association is especially predominate in arid climates (1). VAM fungi encourage root uptake of mineral nutrients such as phosphorus in exchange for photosynthetically-reduced carbon, and have been shown to alter plant water relations (3,6,10,12). Several studies have reported an inverse correlation between mycorrhizal colonization and soil phosphorus concentration (14,16). Thus, enhanced root colonization by VAM fungi may be associated with soils of poor fertility such as those found in many disturbed urban planting sites.
Transplant establishment of container-grown trees into urban landscapes is most successful when using landscape systems that promote root growth beyond the container rootball (7,8). Roots of trees grown in nurseries in soilless rooting media, unless specifically inoculated, generally will be nonmycorrhizal. Mycorrhizal colonization has been associated with changes in root morphology of herbaceous plants by affecting root thickness and branching patterns (4,11). Thus, landscape transplant systems that incorporate inoculation of tree roots with mycorrhizal fungi during the container phase might facilitate transplant establishment by altering root growth and development patterns.
Materials and Methods
We potted 150 seedling liners of Argentine mesquite (Prosopis alba) (25 - 35 cm height) into 27-liter containers filled with a 3 pine bark: 1 peat moss: 1 sand medium, pH 6.5. Before potting, 75 of the 150 trees were band inoculated at a depth of 8 to 12 cm below the growth medium surface with approximately 1,000 spores/g of Glomus intraradices affixed to attapulgite clay (Nutrilink, NPI, Salt Lake City, UT). All trees were topdressed with 15 g Osmocote 18 N - 2.6 P - 9.9 K (Grace-Sierra, Milpitas, CA) and 3 g Micromax (Grace-Sierra, Milpitas, CA) fertilizers and grown for five months in a fiberglass greenhouse under 50% light exclusion. During the container production phase, all trees were watered to container capacity every other day. Prior to transplanting into the landscape, shoot and roots were evaluated for growth. In addition, 20 root segments per tree were sampled, cleared, stained (13) and observed using a light microscope to check for mycorrhizal infection of inoculated trees and noninoculated trees. Percent colonization was calculated as the number of colonized root segments divided by the total number of root segments observed.
After five months, mesquite trees were transplanted into unamended planting holes in a simulated landscape. The soil was a Gilman sandy loam, pH 7.3, 11.62 μg P/g soil, 13.2 g organic matter/Kg soil. Each tree was loosely secured to a single 2.6-m tall lodgepole tree stake and pruned to a canopy base height of 1.3 m. Mesquite trees were either drip irrigated every third day with 30 liters per irrigation or nonirrigated. Irrigated trees were watered with two emitters at 7.6 liter/hr per emitter, while nonirrigated trees were flood irrigated once at transplanting. Monthly rainfall and mean daily temperature maxima and minima data are shown in Figure 1 (1).
onthly rainfall and mean daily temperature maxima and minima for Tempe, AZ, from December 1991 to November 1992 (2).
Tree shoot and root growth, and soil moisture content were measured 6 and 12 months after transplanting. Root growth was analyzed via the extraction of four soil core profiles in the rhizosphere oriented 60 cm from the tree trunk at the north, east, south, and west compass coordinates using a hand-held auger (4). Each soil core profile consisted of samples taken at 20 cm intervals from 0 to 140 cm below grade. Roots from the samples were then analyzed for fresh weight and for total length using a Model DIAS Digital Image Analysis System (Decagon Devices, Pullman, Wash.). Approximately one hundred 1-cm root segments per tree were cleared, stained and observed under a dissecting microscope (40X magnification). Percent and length of mycorrhizal colonization was calculated using the grid-line intercept method (9). Mycorrhizal root length density (cm root length colonized/cm3 soil) was calculated by dividing the estimated length of root colonized in a soil core by soil core volume. Soil cores were extracted over a similar depth range immediately before an irrigation event. Soil moisture content was determined as a percentage of moisture content by weight. The experiment was a 2 × 2 factorial arranged in a randomized complete block design with 12 blocks.
Results and Discussion
Container Production.
After four months in containers, roots of inoculated mesquite trees were colonized (47% of roots sampled) while noninoculated trees remained nonmycorrhizal. The VAM fungal isolate used in this study reduced mesquite total root length and specific root length but did not affect shoot growth or root dry weight (Table 1). These data suggest that the effect of VAM infection on mesquite under near-optimal production conditions in a production container medium was to change mesquite root morphology to shorter, thicker, less-branched roots. This manner of root morphological change suggests that the role of VAM fungi was to substitute for small fine feeder roots and cause the mesquite to allocate more energy resources to enhance root caliper.
Growth of Argentine mesquite in 27-liter containers after 5 months following inoculation (+ or -) with Glomus intraradices.
Post Transplant.
Irrigated soils were generally wetter than nonirrigated soils prior to an irrigation event, particularly in the top 80 cm of soil (Fig. 2). This difference was more pronounced in November than June, most likely due to cooler weather and lower soil evaporation rates.
Mean percent soil moisture content of drip irrigated and nonirrigated Argentine mesquite soil cores (0 to 140 cm depth) at A) 6 and B) 12 months (June and December, 1992, respectively) after transplanting (n = 12).
Six months after transplanting, VAM colonization was observed only in the roots of inoculated mesquite. By 12 months roots of both inoculated and non-inoculated mesquite were colonized by VAM fungi; however, the level of colonization and mycorrhizal root length density were higher for irrigated than for non-irrigated trees (Table 2). Consequently, a strong relationship is suspected between mycorrhizal colonization of roots and maintenance of ample soil moisture.
The effect of irrigation on shoot extension (SE), trunk caliper (TC), percent VAM fungal infection (% VAM), and mycorrhizal root length density (MRLD) of Argentine mesquite, inoculated or noninoculated (+ or -) with Glomus intraradices 6 and 12 months after transplanting.
Drip irrigation promoted mesquite shoot extension, whereas VAM infection inhibited shoot extension of nonirrigated trees (Table 2). Trunk caliper was greater for irrigated trees than for nonirrigated trees (Table 2). Trunk caliper was not affected by VAM infection. Inhibition of shoot extension by VAM fungi under dry conditions suggests 1) a competition with shoots for photosynthetic assimilates or 2) a change in the root to shoot hormonal signaling patterns resulting in slowed shoot growth (2). The later is more likely because mycorrhizal colonization rates were low under dry conditions making it unlikely that VAM fungi were engaged in photoassimilate competition with shoots.
Six months after transplanting, mesquite rooting density (RD) decreased with increased depth and was not affected by drip irrigation or mycorrhizal treatments (Fig. 3a). Twelve months after transplanting, mesquite RD decreased with increased depth, but was much higher for irrigated trees 0 to 80 cm below the ground surface than for nonirrigated trees (Fig 3b). VAM inoculation did not affect RD. Drip irrigation and mycorrhizal treatments interacted at 6 months to affect specific root length density (SRLD) (Fig. 4a). Under drip irrigated conditions, roots of inoculated trees were generally thinnerthan roots of noninoculated trees; whereas, under nonirrigated conditions roots of inoculated trees were thicker than roots of noninoculated trees. At 12 months, SRLD was not affected by irrigation or mycorrhizal treatments (Fig. 4b) .SRLD varied with depth and was highest in the top 80 cm indicating a higher concentration of thinner roots at this depth. At lower depths, roots were fewer in number and thicker (Fig 3b and 4b).
The effect of irrigation on rooting density (mm root length/cm3 soil volume) of VAM inoculated (M) or noninoculated (N) Argentine mesquite, 60 cm from the trunk at depths of 0 to 140 cm below ground surface, A) 6 and B) 12 months (June and December, 1992, respectively) after transplanting into a simulated landscape (n = 12). Statistical probabilities shown above are for the interaction of irrigation and mycorrhizal treatments. W = drip irrigated regularly; D = dry.
The effect of irrigation on specific root length density (mm root length/g fresh root weight/cm3 soil volume) of VAM inoculated (M) or noninoculated (N) Argentine mesquite, 60 cm from the trunk at depths of 0 to 140 cm below ground surface, A) 6 and B) 12 months (June and December, 1992, respectively) after transplanting into a simulated landscape (n = 12). Statistical probabilities shown above are for the interaction of irrigation and mycorrhizal treatments. W = drip irrigated regularly; D = dry.
Conclusions
Pre-transplant inoculation of Argentine mesquite roots enhanced mycorrhizal root colonization, especially under irrigated conditions for up to one year after transplanting. Low soil moisture drastically reduced growth of Argentine mesquite and mycorrhizal colonization of tree roots. If mycorrhizal symbiotic efficiency is measured in terms of a stimulation of host plant growth, then the isolate of G. intraradices used to inoculate mesquite was not a very efficient symbiont under low soil moisture content because it suppressed growth after transplanting. However, this growth suppression might be caused by mycorrhizal-induced changes in the root to shoot hormonal signalling patterns and therefore may act as a survival mechanism resulting in the sequestration of photoassimilates particularly in the roots as evidenced by root thickening or decreased SRLD. Other geographic isolates of VAM fungi, especially those indigenous to more arid regions, might promote an alternate tree response.
Acknowledgments.
We thank Tina Divis for her technical assistance with this project. We also recognize the Southwest Center for Environmental Research and Policy (SCERP), Center for Environmental Studies, Arizona State University, Tempe, AZ 85287-3211, and the ISA Memorial Trust Fund for partial funding of this project. Use of trade names in this publication does not imply endorsement by the SCERP of products named or criticism of similar ones not mentioned.
- © 1994, International Society of Arboriculture. All rights reserved.
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