Mycorrhizae in relation to crop rotation and tillage Terence McGonigle, Dept. of Biology, Brandon University, Brandon, MB R7A 6A9 E- mail: mcgoniglet@brandonu.ca Abstract: Many crops form mycorrhizae, which are the combined growth of a fungus with the root to form an organ capable of enhanced plant uptake of phosphate from the soil. The fungi involved are a particular group of species not related to plant pathogens. These fungi form arbuscules in the roots. The arbuscules are tree-like structures formed by branching of fungal filaments inside root cells, which provides a high surface area of fungus-plant contact for the plant to deliver sucrose to the fungus in exchange for phosphate. The other half of the fungus is outside the root in the soil and gathers phosphate. Grasses, flax, and legumes form mycorrhizae, whereas canola does not. Where mycorrhizae are reduced, crops may have impaired phosphate nutrition unless adequate fertilizer is provided. Two common causes of reduced mycorrhizae are tillage and rotation with a previous crop not forming mycorrhizae. Fungal filaments left in soil after harvest are able to colonize the roots of the following crop the next spring. Tillage acts to break apart the network of fungal filaments in the soil, thereby slowing the growth of mycorrhizae in the crop to follow. Data are reviewed below to illustrate reductions in mycorrhizae and phosphate nutrition of corn in response to tillage. Rotation with a crop not forming mycorrhizae will reduce the density of fungal inoculum in soil, in turn leading to a reduction in the growth of mycorrhizae in the crop to follow. Recent data from Manitoba show that mycorrhizae and phosphate uptake are both impaired for flax following canola compared to flax after wheat, illustrating this effect. Phosphate fertilization interacts with these processes in two ways. First, the fertilizer will stimulate plant phosphate uptake. Second, addition of fertilizer to a crop will reduce the growth of mycorrhizae on that crop. Introduction Mycorrhizae are the combination of particular fungi with roots. Various crops form arbuscular mycorrhizae, in which arbuscules are formed by repeated tree-like branching of hyphal filaments inside root cells. These structures provide high surface area of contact for the plant to deliver sucrose to the fungus in exchange for phosphate. The rest of the fungus is outside the root in the soil, where it gathers phosphate to be moved through the hyphae into the roots. The fungi forming this association are a group of about 150 species and are classified within the Glomeromycota. There are no plant pathogens known within this group (Smith and Read, 2008). Arbuscular mycorrhizae are formed by grasses including wheat, barley, and corn. Flax and soybeans also form arbuscules, but canola is non-mycorrhizal. Where less mycorrhizae are formed, crops have reduced ability to capture phosphate, and greater demand is placed on provision by fertilizer (Grant et al., 2005). Both tillage and rotation with a previous crop not forming mycorrhizae are common causes of reduced mycorrhizal colonization. Hyphae in soil following harvest are able to colonize the roots of the following crop. Tillage breaks apart this network of fungal filaments and causes a delay in the colonization of roots the next year, because more time is needed for the inoculum around the roots to accumulate. Reductions in mycorrhizae and phosphate nutrition are well known for corn in response to tillage (McGonigle and Miller, 1993b), and the effect can be replicated in laboratory systems (McGonigle and Miller, 1993a). The timing of the reduction is colonization is important, because the crop demands adequate phosphate early in the season for yield potential to be reached (Grant et al., 2001).
A crop not forming mycorrhizae will reduce the quantity of fungal inoculum in soil by temporal loss without replacement, in turn leading to a reduction in the mycorrhizal colonization of the crop to follow. Canola will reduce development of arbuscular mycorrhiza in a subsequent crop of maize compared to continuous corn (Gavito and Miller, 1998a), leading to a reduction in early season growth, harvest index, and yield (Gavito and Miller, 1998b). An extract of recent data from Manitoba (McGonigle et al., not published) is given below to show that mycorrhizae and phosphate uptake are both impaired for flax following canola compared to flax after wheat. Flax is reported in Manitoba to be a crop highly dependent on mycorrhiza (Entz et al., 2004). Phosphate fertilization will stimulate plant phosphate uptake but reduce the growth of mycorrhizae on maize in the field (Guttay and Dandurand 1989).The phosphate-fertilizer-induced reduction of mycorrhizal colonization is also known for flax in both the field (Kahiluoto et al., 2001) and growth chamber (Kahiluoto et al., 2000). Materials and Methods The experimental area covered approximately acres and was planted to flax in 2008. Plots in 2007 comprised a number of variety trials for wheat, canola, and juncea. Border sections in 2007 between plots were planted to wheat. Identical fertilization was applied across the site in 2007 and 2008. Replication varied from 2 to 4 among varieties in 2007. The variety trials presented limited opportunities for statistically valid comparisons for wheat history and canola history for particular varieties of previous crop. Blocked replication was used to account for soil variability across the site. The study was configured as 16 blocked comparisons of wheat history and canola history across the site, with canola variety as a background variable, that is, with all 16 canola plots for a different variety. The 16 pairs corresponded to newly designated replicates in 2008, and they were scattered across the site at random. Among replicates, we took samples from 8 pairs with canola to the west of the wheat, and from 8 pairs with canola to the east of the wheat, as dictated by a coin toss in each replicate. The border row for wheat was used for the wheat plot adjacent to each canola plot in each replicate. Samples were taken from the 32 plots on 14 and 15 July, 2008. Each soil sample was cylindrical and 7.3 cm diameter by 15 cm deep. One soil sample was taken for each plot and was centered on flax. A shoot sample of 30 cm of flax row was removed using scissors for estimation of shoot dry mass and shoot nutrients. At the laboratory, roots were rinsed free of soil and preserved in 70% ethanol. After rinsing following storage, root length was determined using the method of Tennant (1975). A root subsample was then taken buy cutting roots in fragments 1-2 cm in length and dispersing them in excess water. Root subsamples were picked free of debris with forceps and fixed overnight in formyl acetic alcohol (FAA), which was composed 18:1:1 by volume of 95% ethanol, glacial acetic acid, and 37% w/v formaldehyde. For the following steps, roots were rinsed in deionized water during transfer among solutions. Roots were cleared for 10 minutes at 121 o C in 10%KOH and then bleached for 20 minutes in dilute alkaline peroxide, which was composed 8:1:1 by volume of deionized water, ammonium hydroxide, and 30% hydrogen peroxide. Following the method of Brundrett et al. (1984), roots were then stained for 90 minutes at 90 o C in 0.05% chlorazol black E made up in equal volumes of lactic acid, glycerin, and deionized water. The stain was first dissolved in the lactic acid during preparation. Roots were mounted on microscope slides in glycerin and scored for mycorrhizal colonization (McGonigle et al. 1990).
Dry mass was recorded at Brandon University after shoot drying to constant mass at 70 o C. Analysis of nutrients for shoots was undertaken by AGVISE Laboratories (P.O. Box 510, Northwood, ND 58267, USA). Results and Discussion Early season shoot growth was significantly (P=0.002) greater, and by a factor of 1.6, in flax following wheat compared to flax following canola, but root growth was similar for both treatments (Table 1). Similar growth responses were seen previously for flax after canola and wheat (Grant et al., 2009). Compared to flax after a previous crop of canola, development of arbuscular mycorrhiza was stimulated for flax roots following wheat in terms of both arbuscules (P=0.017) and hyphae (P<0.001) in the root cortex (Table 1). Concentrations were reduced in flax shoots following wheat, relative to those following canola, for all essential nutrients except P, Cu and Zn (Table 2). These reductions in concentrations form part of well-known lowering of shoot nutrient concentrations associated with plant growth and advancement of plant development during the early season (Westfall et al., 1990). For P, Cu, and Zn, sustainment of concentrations in the larger plants following wheat indicates enhanced nutrient inflow per unit root length that is associated with the more extensive mycorrhizal development following wheat compared to that for flax after canola. The stimulation by mycorrhizae of nutrition of Cu and Zn, in addition to P, has been noted previously for maize (McGonigle and Miller 1996) and is attributable to the immobility of these elements in soil. The quantity of mycorrhiza in the flax following canola, although less than that following wheat, was still substantial at the time samples were taken (Table 1). Thus, it is likely that differences in colonization earlier in the season would have been more pronounced than those found here between treatments at the sample date in mid-july. Conclusion Tillage and phosphorus fertilizer addition reduce development of mycorrhizae. Previous cropping of canola restricts mycorrhizal development of flax relative to that seen following wheat. Further, such reduction in the mycorrhizal symbiosis is to the detriment of the flax growth and early season nutrition for P, Cu, and Zn. Crop rotation should avoid placement of flax after canola, and where this does occur, additional use of P fertilizer should be considered. Literature review indicates that the rotation with canola will also reduce mycorrhizal colonization and growth of maize.
Table 1. Early season growth and mycorrhizal colonization of flax in 2008. Colonization values are percentages of root length colonized, respectively, by arbuscules, hyphae, and vesicles. Probabilities (P) are given for a significant difference between cropping history as determined by randomized block analysis of variance. Percentage data were given the arcsine transformation prior to statistical analysis. Data are means ± s.d.; n=16. Data of McGonigle, Hutton, Greenley, and Karamanos (not published). Property Flax after canola Flax after wheat P Shoot dry mass (t ha -1 ) 1.12 ± 0.52 1.80 ± 0.52 0.002 ** Root-length density (cm ml -1 ) 2.12 ± 0.34 2.09 ± 0.34 0.74 n.s. Arbuscular colonization (%) 36 ± 10 45 ± 10 0.017 * Hyphal colonization (%) 49 ± 11 65 ± 11 <0.001 *** Vesicular colonization (%) 0.4 ± 0.4 1.0 ± 0.4 0.11 n.s. Table 2. Early season shoot nutrient concentrations of flax in 2008. Probabilities (P) are given for a significant difference between cropping history as determined by randomized block analysis of variance. Data are means ± s.d.; n=16. Data of McGonigle, Hutton, Greenley, and Karamanos (not published). Element Flax after canola Flax after wheat P N (%) 3.43 ± 0.49 2.88 ± 0.49 0.006** P (%) 0.268 ± 0.049 0.251 ± 0.049 0.33 n.s. K (%) 1.92 ± 0.20 1.60 ± 0.20 <0.001 *** Zn (mg kg -1 ) 11.8 ± 2.2 12.4 ± 2.2 0.48 n.s. Fe (mg kg -1 ) 311 ± 139 156 ± 139 0.006** Mn (mg kg -1 ) 193 ± 50 133 ± 50 0.004** Cu (mg kg -1 ) 5.19 ± 0.82 5.18 ± 0.82 0.98 n.s.
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