Separate effects of high temperature on root growth of Vigna radiata l. and colonization by the vesicular-arbuscular mycorrhizal fungus Glomus versiforme

Separate effects of high temperature on root growth of Vigna radiata l. and colonization by the vesicular-arbuscular mycorrhizal fungus Glomus versiforme

Soil Biol. Biochem. Vol. 25, No. 0, pp. 1127-l 129, 1993 Printed in Great Britain. All rights reserved 0038-0717/93 $6.00 + 0.00 Copyright 0 1993 Per...

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Soil Biol. Biochem. Vol. 25, No. 0, pp. 1127-l 129, 1993 Printed in Great Britain. All rights reserved

0038-0717/93 $6.00 + 0.00 Copyright 0 1993 Pergmon Press Ltd

SHORT COMMUNICATION SEPARATE EFFECTS OF HIGH TEMPERATURE ON ROOT GROWTH OF VIGNA RADIATA L. AND COLONIZATION BY THE VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGUS GLOMUS VERSIFORME

BONNIE VOGELZANG,

HELEN PARSONSand SALLY SMITH*

Department of Soil Science, Waite Institute, The University of Adelaide, Glen Osmond, South Australia 5064, Australia (Accepted 20 February 1993)

The rate of mycorrhizal infection of roots is influenced by many environmental factors including soil temperature. Most experiments have been carried out between 10 and 30°C and an increase in soil-root temperature in that range has been shown to increase colonization of roots (e.g. Furlan and Fortin, 1973; Hayman, 1974; Schenck and Schroeder, 1974; Smith and Bowen, 1979). Only a few investigations have used temperatures above 3O”C, yet selection of suitable fungi for use in tropical agriculture and horticulture requires information on the effects of higher temperatures on the development of infection and on plant growth.

Changes in the extent to which roots become infected may be the result of direct effects on fungal growth or on effects on the rate of root growth, or a combination of both. The differential responses of plants and fungi to changes in temperature need to be taken into account in interpretation of data from experiments designed to investigate mycorrhixal interactions. Examples of differential responses include the observation (Smith and Roncadori, 1986) that maximum growth response to infection and maximum root growth and mycorrhixal infection by Glomus introradices occurred at different temperatures. In this case cotton roots remained infectible at the highest temperature used (36°C). In our experiments (Haugen and Smith, 1992) cashew (Anacardium occidentale L.) did not become infected at 38°C despite the fact that the mycorrhixal fungus used (Glomus intrurudices) germinates rapidly and inoculum retains infectivity (with respect to mung bean) for up to 6 weeks at this temperature. Lack of infection of cashew was therefore probably the result of poor root growth rather than reduction in the infectivity of the fungus. In preliminary experiments Gfomus uersifortne (Karsten) Berth was shown to infect mung bean (Pignu radiata L.) rapidly and produce a significant growth response at 30°C. Here we report effects of temperatures between 30 and 38°C on growth of mung bean and the early phases of infection by this fungus. Seedlings were planted into 1 litre non-draining pots containing 9: 1 washed, steamed sand:autoclaved soil, very low in available phosphate (1.6 pg g-’ o.d. soil). The water content was maintained at 12.5% (w/w) by watering to weight three times a week. Plants were inoculated by placing 2 g of soil and infected root pieces from pot cultures -of G.- v&ifo*me on Triforiwn shbterranewn- L. immediately below the seedling just before planting. Control pots received root pieces and soil from uninfected T. *Author for correspondence.

subrerraneum. Plants were grown in temperature tanks set at 30,34 or 38°C to maintain constant root temperatures. The pots were arranged in a randomized complete block design with blocking on temperature. As in our previous work (Haugen and Smith, 1992) the assumption was made that the effect of temperature was the main cause of variation induced by block. The experiments were conducted under summer conditions in a glasshouse with air temperature between 15 and 37S”C and a mean daily light intensity of co 760mol me2 s-’ Three plants of each treatment were destructively sampled 3, 5, 7, 10, 14, and 21 days after planting. Shoot fresh and dry weights (24 h at 8o”C), and root f&h weights were recorded at-all harvests. Roots were cleared in 10% KOH and stained with 0.05% trvoan blue in lactophenol (Phillips and Hayman, 1971). b;xial and lateral root lengths (up to 14 days) and levels of infection were measured separately, either directly in very young plants, or by the grid intersect method (Pennant, 1975) under a dissecting microscope. Effects of temperature on shoot dry weight and root fresh weight only became apparent at 21 days. Data for this harvest time are shown in Fig. 1. The trends are the same for both variables. There was a general decrease in growth with increasing temperature, with mycorrhixal plants being more severely affected than non-mycorrhixal plants, particularly with respect to root fresh weight. At 30°C mycorrhixal plants were larger than non-mycorrhizal plants, but this growth stimulation was reversed (though not significantly) at higher temperatures. The greater sensitivity of mycorrhizal plants to changes in temperature was also observed by Smith and Roncadori (1986). Roots extended throughout the pots at 30 and 34T, but were confined to the top one-third of the pot at 38°C. The roots became longer and thinner at higher temperatures as the fresh weight to length ratio decreased (results not shown, but compare Fig. 1B with Figs. 2A and B), confirming the observations of Haugen and Smith (1992) for both mung bean and cashew. The total length of roots per plant (main plus lateral) varied significantly in the first 7 days, with plants grown at 34 and 30°C having longer root systems than those grown at 38°C (results not shown). After 10 days, while the lengths of main roots grown at 38°C were lower than at the other temperatures, the total root length was not significantly different due to the increase in length of lateral roots. The effects of temperature on lengths of lateral roots were small (Fig. 2). There were no significant differences between temperatures for non-mycorrhizal plants. In mycorrhixal plants, roots grown at the higher temperatures were slightly shorter.

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Roots became infected by 3 days at 30 and 34”C, and by 5 days at 38°C. Arbuscules had developed at all temperatures by day 5. However, at 34 and 38°C fewer infection units had developed arbuscules and the infection did not spread within the root from the entry points as much as it did at 30°C. Vesicles were more abundant in infected roots grown at 30°C. Infected root length was significantly higher at 30°C than for the other temperatures at 10 and 14 days (Fig. 2C). As total root length was almost unaffected by high temperatures (compare Figs. 2B and C) the outcome was much lower fractional infection at 38 and 34°C than at 30°C. At 14 days the percentages of the root length infected at these temperatures were 2.2kO.4, 3.3 50.8 and 39.6+ 3.0, respectively. The reduction in fractional infection may explain the reduction in mycorrhizal growth response at high temperatures noted above. We have shown here that growth in length of roots and colonization by G. uersiforme do not respond in the same way to elevated temperatures. Fungal colonization and hence mycorrhizal growth responses were more sensitive than plant growth. Lower infection at higher temperature is apparently not related to the amount of root available (i.e. to reduced root length) but to factors directly affecting the ability of the fungus to colonize and become established within the plant root. There is evidence that Gfomus uersiforme has a specific temperature range within which it can infect and grow within the root tissues. Temperature may act directly on the fungus itself or the effects may be mediated via the plant through such mechanisms as struc-

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6 time (d) Fig. 2. Effects of temperatures of 30°C (e), 34°C (H) and 38°C (A) on the growth of lateral roots of V. rudiutu. Non-rny~o~hi~ plants (A), mycorrhizal plants (8) and infected root length of mycorrhizal plants (C). Means and standard errors of means of three replicate plants per treatment.

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Fig. 1. The effects of temperature on shoot dry weight (A) and root fresh weight (B) of mycorrhizal (black) and non-mycorrhizal (hatched) V. rudiuru at 21 days. Means and standard errors of means of three replicate plants.

tural changes in the root tissues or differences in the ava~ability of nutrients to the fungus. Direct effects are suggested by the fact that the response of G. versiforme was different from that of G. intrurudices. With the latter fungus percentages of root length infected at 38°C in the same host species were much higher, being 23 and 61% at 14 and 42 days, respectively. These preliminary investigations make it clear that effects of high temperature may be very complex and may differ between host-fungus combinations. Further

Short Communications investigations comparing the responses of different fungi would be useful in selecting isolates for use in tropical conditions. are grateful for financial support from the Rural Industries Research and Development Council, in conjunction with Voyager Enterprises Kununurra, Western Australia.

Acknowledgements-We

lar-arbuscular mycorrhiza IV. Effect of light and temperature. New Phytologist 73, 71-80. Phillips J. M. and Hayman D. S. (1970) Improved procedures for clearing roots and staining parasitic and vesicular-arbuscular mycorrhizal fungi for rapid assessment of infection. Transactions of the British Mycological Society 55, 158-160.

Schenck N. C. and Schroeder V. N. (1974) Temperature response of Endogone mycorrhiza on soybean roots. Mycologia

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Furlan V. and Fortin J. A. (1973) Formation of endomycorrhizae by Endogone calospora on Allium cepa under three temperature regimes. Natural&e Canadien 100, 467477.

Haugen L. M. and Smith S. E. (1992) The effect of high temperature and fallow period on infection of mung bean and cashew roots by the veiscular-arbuscular mycorrhizal fungus Glomus intraradices. Plant and Soil, 71-80. Hayman D. S. (1974) Plant growth responses to vesicu-

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Smith G. S. and Roncadori, R. W. (1986) Responses of three vesicular-arbuscular mycorrhizal fungi at four soil temperatures and their effects on cotton growth. New Phytologist 104, 89-95. Smith S. E. and Bowen G. D. (1979) Soil temperature, mycorrhizal infection and nodulation of Medicago truncatula and Trifolium subterraneum. Soil Biology & Biochemistry

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Tennant D. (1975) A test of a modified line intersect method of estimating root length. Journal of Ecology 63, 995-1001.