Spread of endomycorrhizal colonization and effects on growth of apple ...

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determine longer-term effects of colonization by tbese fungi on tbe growtb of apple seedlings. Sbort-term cbaracteristics of colonization were determined by ...
New Phytot. (1989), U l , -S1-S9

Spread of endomycorrhizal colonization and effects on growth of apple seedlings

BY DIANE DOUD MILLERS, MAJA BODMER^ AND HANNES SCHUEPP^ ^Department of Horticulture, The Ohio State University, Columbus, Ohio 43210, USA ^ Swiss Federal Research Station, CH-8820 Wddenswil, Switzerland (Received 19 August 1988; accepted 30 August 1988) SUMMARY

An experiment to determine tbe sbort-term cbaracteristics of root colonization by two vesicular-arbuscular (VA) mycorrhizal fungi witb apple (Mains x dotnestica Borkh.) was initiated concurrently witb an experiment to determine longer-term effects of colonization by tbese fungi on tbe growtb of apple seedlings. Sbort-term cbaracteristics of colonization were determined by sequential barvesting of a cuvette system wbicb also allowed monitoring of bypbal spread from inoculated 'spreader' plants, tbrougb a root-free soil region, to non-inoculated 'receiver' plants. Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe exceeded Glomus macrocarpttm Tul. & Tul. in rate of initial colonization, amount of maximum colonization, and persistence of arbuscules and external hypbae. Colonization by eitber fungus reduced sboot fresb weigbt 11 days after inoculation but increased sboot and total fresb weigbt by day 38. Hypbae of G. mosseae bad traversed tbrougb 2 and 3 cm of root-free soil to colonize non-inoculated 'receiver' roots by 29 days, and bypbae of G. macrocarpum by 38 days. G. mosseae colonized ' receiver' roots more intensely tban did G. macrocarpum at 2 and 3 cm, and 3 and 4 cm distance, at subsequent harvests. Tbe fungi were effective, singly and in combination, in increasing apple plant biomass, as compared to non-mycorrbizal controls. Relative beigbt growth rate, and colonization at barvest, of G. macrocarpum-'n\Qcw\&t 0) apple seedlings grown in cuvette sections at six 9 d intervat harvests Mycobiont inoculated at distance (cm) = 0 Glotnus mosseae Root colonization Harvest 1 (day 11) Class**t Intensity**§ Oomments: Harvest 2 (day 20) Class** Intensity** Oomments: Harvest 3 (day 29) Class**t Intensity**! Comments:

0

1

Glotnus macrocarpum 2

3

4

0-0 0-0 1-0 0-0 0-0 1-0 15—40 independent colonization site.s/per plant; thick, coarse hyphae (H) ||

00

0-0

0-0 1-2 0-0 Thick, external hyphae most common; some arbuscules, vesicles (H,A,V)

4-4 — 0-8 0-5 — 2-6 — 0-7 0-5 — Heavy colonization; external vesicles; much external hyphae; colonization spreading to receivers (A, H, V)

Harvest 4 (day 38) Class** Intensity** Comments:

4-2 2-3

Harvest 5 (day 47) Class**! Intensity**! Comments:

4-0 2-4

Harvest 6 (day 56) Class**! Intensity! Comments;

40 1-5

0-8 0-8

0-9 0-8

Predominately arbuscules ; some vesicles; predominately hyphae in 'receivers' (A,H,V)

2-5 1-6

1-3 10

— —

3-6 12

0

1

0-9 0-5

0-0 0-0

2

3

4

0-0 ()•()

3-20 independent colonization sites/per plant; fine hyphae (H) 2-2 13

0-0 0-0

0-0 0-0

— —

— —

Arbuscules common in inner cortex (A, H, V) 3-6 2-4

— —

0-0 0-0

0-0 0-0

— ^

Colonization sites coalesced (A,H,V)

4-0 0-5 10 2-1 1-0 0-5 Increased vesicles, decreased arbuscules in 'spreaders'; hyphae wrapped around outside of ' receivers ' (A, H, V)

3-7 0-7 0-1 1-8 0-7 0-1 Strong colonization remains in 'spreaders'; Colonization has faded in 'spreaders'; coloprogression from root tip = hyphae to ar- nization strongest right behind root tips; buscules to vesicles to no colonization in old slight infection in 'receivers', sloughing roots (A,H, V) cortex (H, V,A) — —

1-6 09

0-6 1-4 0-5 0-8 Colonization faded in 'spreaders' but vig- Best infections in new roots (V, H,A) orous in 'receivers' (A, H, V) 3-6 M

** P ^ 0-01, distance 0 > distance 1, 2 or 2, 3, or 3, 4; data analysed by harvest. j P < O'Ol G. mosseae > G. macrocarpum; data analysed by harvest. X Percentage root length colonized. 0, 0 % ; 1, 1-5%; 2, 6-25%; 3, 26-50",,; 4, 51-75",,; 5, 76-100"o. § Intensity of colonization. 0, none; 1, small, widely scattered; 2, larger colonization sites, not coalescing; 3, colonization in most cortical cells. II Colonization structures listed in order of prevalence (H, hyphae; A, arbuscule; V, vesicle). arbuscules in the inner cortex as hyphae spread rapidly internally. Arbuscules from independent colonization sites coalesced and vesicles began to form spottily in the outer cortex. By day 29 (harvest 3) hyphae of G. mosseae had extended through 2 and 3 cm of soil mix, and colonization of 'receiver' roots had begun (Table 2). With G. macrocarpum, no colonization of ' receivers ' was evident. 'Spreader' plants inoculated with G. mosseae were colonized intensely over roughly 60 % of t h e root system. Arbuscules were the predominant

structure, then hyphae and vesicles, but external vesicles were rare. 'Spreader' plants inoculated with G. macrocarpum were intensely colonized o\'er 40 % of the root length, with arbuscules more common, then hyphae and vesicles. With both fungal species, colonization of newly formed roots occurred by internal growth of hyphae and also by new independent sites of colonization on the root surface. Although with both fungi long lengths of roots were completely colonized, some roots were inexplicably uncolonized.

56

Diane D. Miller, Maja Bodmer and H. Sehiiepp

'Spreader' plants with G. mosseae as the mycobiont remained at roughly 60% of the total root, length colonized at day 38 (harvest 4), day 47 (harvest 5) and day 56 (harvest 6) (Table 2). However, since root fresh weight increased over this time (Table 1) so did the total amount of colonization. Arbuscules remained the predominant structure until harvest 6 when 'vigour' of colonization, as measured by presence of arbuscules and external hyphae, had declined and vesicles predominated. Colonization of 'spreaders' by G. macrocarpum never acbieved tbe overall quantity and intensity of tbat of G. mosseae (Table 2). The peak of colonization occurred at day 38 (harvest 4) with approximately 50% of the root length colonized, but the 'vigour' of the colonization had declined, with vesicles the predominant structure. At day 47 (harvest 5) and day 56 (harvest 6) vesicles were most common, with external hypbae and arbuscules occurring adjacent to root tips. ' Receiver' plants of G. mosseae were again slightly colonized at day 38 (harvest 4) at 2 and 3 cm distance (Table 2). By day 47 (barvest 5) colonization had become more intense and by tbe final barvest (day 56) 'receiver' roots at 3 and 4 cm distance contained more arbuscules and external byphae than did the spreader plants. 'Receiver' plants of G. macrocarpum were initially colonized on day 38 (harvest 4) at 2 and 3 cm distance (Table 2). Colonization had not increased in intensity at day 47 (harvest 5). At day 56 less then 5 % of the root lengths of 'receiver' plants were colonized and colonization was widely scattered, with vesicles the predominant structure.

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Growth study {a) Parameters of growth. Relative height growth rates among plants inoculated with G. mosseae, G. macrocarpum, G. mosseae-\-G. macrocarpum, and the non-mycorrhizal control did not differ between weeks 2 and 4 (Eig. 2) but from week 6 to tbe termination of tbe experiment at week 14, tt-eatments did differ. G. mosseae and G. mosseae-\-G. macrocarpum grew at a significantly faster relative rate tban G. macrocarpum. Relative beight growtb rate of tbe non-mycorrbizal control was the smallest of all treatments. Analysis of relative diameter growth revealed only two significantly difTerent rates from week 6 to week 14, as mycorrhizal plants gained diameter at a significantly greater relative rate than did nonmycorrhizal plants (Eig. 3). Plant leaf number, fresh and dry weights of leaves, stems, and roots, and shoot-to-root ratio among mycorrhizal plants were similar after 14 weeks (Table 3) while non-mycorrbizal plants were significantly smaller in all parameters.

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VA mycorrhizal colonization of apple seedlings

57 G. mosseae G. mosseae + G. macrocarpum G. macrocarpum

4-0

30

.e Non-mycorrhizal All 3 rates significantly different P< 0 05

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Week Fig 2. Effect of fungal treatments on relative beight growth rates of greenhouse-grown apple seedlings. G. mosseae ) G. mosseae + G. macrocarpum G. macrocarpum

1.5 1.4 • 13 1.2 I'l E 1-0 ^ 0-9 So.8 E 0.7

Non-mycorrhizal

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