Growth responses of three legume species exposed to simulated acid rain

Environmental Pollution 62 (1989) 21-29

Growth Responses of Three Legume Species Exposed to Simulated Acid Rain

T. W. Ashenden & S. A. Bell Institute of Terrestrial Ecology, Bangor Research Station, Penrhos Road, Bangor, Gwynedd LL57 2LQ, UK (Received 19 January 1989; revised version received 14 April 1989; accepted 28 June 1989)

ABSTRACT Seedlings of Vicia faba L, Phaseolus multiflorus L. and Pisum sativum L. were raised during exposure to simulated acid rainfall treatments of pHs 5"6, 4"5, 3"5 and 2"5 at a rate of 3Omm per week. All three species were found to be adversely affected by the more acid p H 3"5 and p H 2"5 treatments after 7-8 weeks of exposure. There were totalplant dry weight reductions of 40% J'or V. faba, 31% for P. sativum and 28% for P. multiflorus exposed to the p H 2"5 treatment, as compared to those grown in the control ( p H 5.6 treatment). In addition, V. faba was found to be sensitive to the p H 4"5 treatment with an 18% reduction in total plant weights (compared to plants grown in the p H 5"6 treatment). In P. multiflorus, reduction in the dry weights of shoots in response to increasing acidity of rain was not accompanied by reduction in root weights, indicating an interference in the partitioning of assimilates. It is concluded that these three species, and V. faba in particular, may be growing below their potential in much of the UK.

INTRODUCTION Without additions of pollutants, rainwater would be slightly acidic, since water in equilibrium with carbon dioxide has a p H of 5.6. Further slight increases in acidity m a y occur by the removal of naturally occurring acids from the air, but, even then, the lower limit of the natural mean p H of rainwater is unlikely to be below p H 5-0. In Britain, the acidity of rainfall is substantially increased by the presence of pollutants and there have been 21 Environ. Pollut. 0269-7491/89/$03.50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

22

7". w. Ashenden, S. A. Bell

reports of sites in rural areas with monthly mean acidities as low as pH 3.4 and annual means down to pH 4.1 (see Barrett et al., 1983; RGAR, 1987). In urban areas, rainfall may be even more acidic because of the local 'washout' of high levels of gaseous pollutants (Fowler et al., 1982). The main pollutant gases responsible for increasing the acidity of rainfall are sulphur dioxide (SO2) and, to a lesser extent, nitrogen oxides. It is well documented that these pollutants may reduce the yields of plants when present in the atmosphere as gases at ambient concentrations (see reviews by Law & Mansfield, 1982; Bell, 1982) and that they may be more potent in combination, by acting synergistically (Ashenden & Mansfield, 1978). However, the effects of wet acid deposition on vegetation are much less well defined. Most studies which have demonstrated plant injury have needed exposures to rain more acid than pH 3-0 to obtain an effect (see Amthor, 1984) and this is clearly unrealistic in terms of ambient acidities of rainfall. At slightly higher pHs of 3.1 and 3.4, there are several reports of simulated rain treatments inducing leaf lesions in both woody and herbaceous species (Jacobsen & van Leuken, 1977; Evans et al., 1977, 1978), but many other investigators have failed to find effects on crop plants with treatments less acid than pH 3-0 (see review by Irving, 1983). However, Evans et al. (1982, 1983) found additions of simulated acid rain of pH4-0-4.1 to reduce the yields of field-grown soybean crops, when compared with plants exposed to rain of pH 5'6. More recently, our own studies have suggested 9-17% yield reductions in winter barley in response to the critical pH range of rainfall of 3.5-4.5 (Ashenden & Bell, 1987a). Subsequently, we have reported adverse effects of acidified rain treatments on other herbaceous and woody species, and have stressed the need for using large numbers of replicates in studies using native soils (Ashenden & Bell, 1987b, 1988). Here we describe an experiment to determine the effects of different simulated acid rainfall treatments on the growth of three leguminous crop species.

MATERIALS AND METHODS The exposure system consisted of a 7"3 x 3"3 m polythene tunnel which was divided internally by polythene sheeting into eight treatment bays of 1-8 x 1"3 m, and contained no supplementary lighting or heating. This allowed duplicate blocks of four different simulated rain treatments. The end doors on the polythene tunnel were kept open from 9.00 am until 5.00 pm to allow good ventilation with ambient air. On average, illumination was 10-15% below natural and temperatures were 2-3°C above ambient, but there was uniformity between the different treatment bays. Each bay was fitted with three Eintal spray nozzles at a height of 0.6 m above the ground.

Responses of legumes exposed to acid rain

23

The nozzles were positioned so as to give an even distribution of droplets of simulated acid rainfall over the growing area. Treatment solutions were made up in 120-1itre tanks and pumped to the exposure bays from an adjacent building. For this experiment, simulated acid rain at pHs of 2.5, 3.5, 4.5 and 5-6 (made up by additions to tap water ofsulphuric and nitric acids in a ratio of 7 : 3 by volume) was applied to each treatment bay in each block in 4 x 5 min episodes Monday to Thursday and 1 x 7 min episode on Friday, so as to provide a total input of 30 mm rainfall per week. The acidities of the rainfall treatments reaching the plant surfaces were confirmed by the collection of samples for pH determination. Seeds of Pisum sativum L. cv. Onward (pea), Vicia faba L. cy. Bonny Lad (broad bean) and Phaseolus multiflorus L. cv. Prizewinner (runner bean) were sown individually in 7"5 cm diameter pots of John Innes No. 2 potting compost on 12 June 1985 and placed in the polythene tunnel exposure system. The soil surface in the pots was not protected from the treatment solutions and there was no supplementary irrigation. Simulated rainfall treatments began after 24 h. Two weeks after sowing, pots which did not contain germinated seedlings were discarded and the minimum number of seedlings left in any bay for each species was determined. From an initial sowing of 25 peas, 15 broad beans and 15 runner beans, a minimum in any one bay of 22 pea, 14 broad bean and 12 runner bean seedlings remained. Excess plants were removed by random selection from all treatment bays containing more than the miminum, so that even numbers of seedlings of each species remained in each of the eight treatment bays. It should be noted that the numbers of seeds which did not germinate for all species was not related to the different rainfall treatments. The plants of P. multiflorus and V.faba were harvested after 7 weeks and the plants of P. sativum after 8 weeks of exposure to the rainfall treatments. It was noted that none of the plants had become pot bound by the end of the experiment. At harvest, plants were washed free of soil and divided into roots, leaves and the remaining stems for dry weight determinations. The yields obtained for the different treatments were compared using two way analysis of variance with blocks and treatments as main effects.

RESULTS The results of the analysis of variance tests on the dry weight yields of the different plant fractions of all three species after exposure to the four simulated rainfall treatments are summarised in Table 1. It is immediately apparent that there were highly significant effects of the treatments on all dry weight fractions measured, except for the roots portion of plants of

24

T. W. Ashenden, S. A. Bell

TABLE 1 F Ratios O b t a i n e d in Two Way Analysis o f Variance Tests on the Dry Weights o f Plant Fractions Following Exposure to Different Simulated Acid Rainfall T r e a t m e n t s

Vicia faba Roots Stems Leaves Total plant Phaseolus mult([torus Roots Stems Leaves Total plant Shoot/Root Pisum sativum Roots Stems Leaves Total plant

Treatment

Blocks

Blocks × Treatment

5.64** 12.52 *** 6-35** 16.07"**

3.71 ns 0-22 ns 1.21 ns 3.20ns

0.72ns 2-09 ns 2.34ns 2.34ns

2.41 ns 4.68"* 7.73*** 5.05** 7.46"**

1.43ns 0.65 ns 0.0! ns 0.84ns 0.07ns

0'19ns 0"81 ns 0-15ns 0.33 ns 0.29ns

11.77"** 4.61"* 6.99 *** 7.16"**

1.57ns 3.67* 0.12 ns 1.79ns

1.53ns 2.97* 2.22 ns 2.12ns

The levels o f significance are: * P_< 0-05; ** P _< 0'01; *** P _< 0"001; ns = not significant.

Phaseolus multiflorus. There were no effects of blocks or the interaction of blocks x treatments on dry weights except for the stems portion of Pisum sativum. Specific treatment effects on the dry weights of the three species studied are detailed below. The dry weights of the different plant fractions of V. faba exposed to the four rainwater treatments are shown in Table 2. It is immediately apparent that at pH 5"6 the yields of roots, leaves and total plants were significantly greater (P < 0"05) than for all other rainfall treatments. In addition, the dry weight of the stems fraction for the pH 5.6 treatment was greater than that obtained in the two most acid (pH 3.5 and pH 2-5) rainfall treatments (P _< 0.05). There were no significant differences between yields of V.faba at the pH 3"5 and pH 4-5 treatments but the yields obtained for the pH 2"5 treatment were significantly lower (P < 0"05) than any other treatment for total plant weights and all plant portions except roots. For P. multiflorus, there were no significant differences in the dry weights of roots obtained for the four rainfall treatments (Table 2). However, there were significantly higher dry weights (P < 0.05) of both the stems and leaves for plants exposed to the pH 5.6 and pH 4"5, as compared to the pH 3.5 and pH 2"5 rainfall treatments. For total plant dry weights of P. multiflorus, the

Responses of legumes exposed to acid rain

25

TABLE 2 Mean Yields (g) of Different Dry Weight Fractions of Viciafaba, Phaseolus multiflorus and Pisum sativum exposed to Simulated Acid Rainfall Treatments at pHs of 5"6, 4-5, 3"5 and 2"5 LSD

pH of rainwater treatment

(P < 0"05)

2"5

3"5

4"5

5"6

0'301 0"388 0"239 0'929

0'347 0.502 0"340 1.189

0-363 0.573 0-339 1.274

0"462 0"638 0.452 1.552

0"080 0.084 0'097 0.179

Phaseolus mult(florus Roots Stems Leaves Total plant Shoot/Root ratio

0-263 0"653 0.357 1"273 4.25

0"195 0-603 0.408 1"206 5.36

0'205 0.830 0.528 1'563 6"93

0"257 0.892 0.613 1.761 6-30

0'069 0" 177 0.116 0'319 1.26

Pisum sativum Roots Stems Leaves Total plant

0.100 0.399 0'258 0-758

0'110 0.495 0"331 0-937

0.203 0.561 0"421 1.184

0.143 0.534 0.420 1"097

0.042 0-103 0-082 0.195

Vicia faba Roots Stems Leaves

Total plant

yields obtained at the pH 5.6 treatment were greater than those of both the pH 2"5 and pH 3.5 treatments. However, the total plant dry weight at pH 4.5 was higher than that at pH 3.5, but not significantly higher (P < 0.05) than that for the pH 2.5 treatment. A lack of significant differences in the yields of roots to correspond with differences in shoot weights suggests an effect on the partitioning of assimilates for this species. Further analyses of the data confirmed highly significant differences (P < 0.001) in the proportion of dry matter in shoots compared to roots (see Table 1). There was a trend for more proportioning of dry matter into the shoots with the less acid rainfall treatments. The effects of the different rainwater acidities on the dry weight yields of seedlings of P. sativum are shown in Table 2. Again, it is readily apparent that all dry weight fractions decrease with increasing acidity of the rainfall treatments, and plants exposed to the pH 2.5 treatment had significantly lower total plant weights (P___ 0"05) than all other treatments. However, differences between the pH 3.5 and control (pH 5.6) treatment were less pronounced for peas, as compared to the other species studied: while yields were significantly lower at pH 3.5 (P _< 0.05) for the leaves dry weight fractions, there were no significant differences in the stems, roots and total

26

T. W. Ashenden, S. A. Bell

plant dry weights. For this species, there were overall differences in the yields of stems in the two treatment blocks (P _< 0.05). It is apparent that the yields of stems were higher in the pH 3-5 and pH 4"5 treatments, but not the pH 5"6 and pH 2.5 treatments for block 2.

DISCUSSION The data show that all three of the leguminous species studied may be adversely affected by exposure to acidified rainfall. At the extreme pH 2.5 treatment, there were substantial reductions in total plant dry weights, as compared to the yields obtained for plants exposed to the control (pH 5"6) treatment: 40% for V.faba, 31% for P. sativum and 28% for P. multiflorus. Such high yield reductions, in response to very high levels of rainfall acidity, are not unusual. In earlier studies, we have reported dry weight reductions of 37% for winter barley and 21% for white clover exposed to acidified rain at pH 2"5, as compared to pH 5.6 (Ashenden & Bell, 1987b), although those values were derived from observations of plant growth over a range of native soils. Here, the experiments were conducted using a rich, well-buffered potting compost, the pH of which was found not to have been altered by the rainfall treatments over the short exposure time. However, it is possible that the treatments may have differentially affected the nutrient status of the soils, which could have affected the nitrogen fixation processes in the three species. There were notable differences in the threshold acidity of rainfall which was needed to induce dry weight reductions in the three species studied. For P. sativum and P. multiflorus, there were no significant differences between yields at the pH 4.5 and pH 5"6 treatments. However, for V.faba, there were significant reductions (P < 0-05) of 21% in roots, 25% in leaves, and 18% in total plant dry weights at pH4-5, compared with the control (pH5"6) treatment. The level of rainfall acidity required to cause any significant (P < 0"05) dry weight yield reductions (as compared to the pH 5"6 treatment) in P. sativum and P. multiflorus was the pH 3"5 treatment. While the majority of researchers have not found adverse effects on plants in response to rainfall acidities above pH 3-0 (see Irving, 1983), yield reductions of 9-15% have been reported recently for soybeans exposed to pH4-4, as compared to plants exposed to rain at pH 5-6 (see Evans et al., 1986). In the same paper, the authors show that the magnitudes of yield reductions may vary considerably according to the cultivar used in the study. It is interesting to note the differential effects of the rainfall treatments on root growth. For P. sativum and V. faba, decreasing yields of shoots with increasing acidity of rainfall treatments was reflected in decreasing yields of

Responses of legumes exposed to acid rain

27

roots, indicating a general reduction in plant mass in response to the stress exerted by the pollutants. In contrast, for P. multiflorus, changes in shoot growth were not reflected by changes in the weights of roots. Indeed, there were no significant (P < 0.05) differences in the yields of roots between the four rainfall treatments for P. multiflorus. Further analysis of the data revealed a change in the partitioning of assimilates in P. multiflorus as a response to the treatments, whereby a greater proportion of assimilates were transferred to the shoots with decreasing levels of acidity. Similar increases in the proportion of assimilates in shoots with less acid rainfall treatments have been found for radish (Troiano et al., 1982). An alteration in the partitioning of dry matter between roots and shoots has been shown to be a common effect of gaseous pollutants on plant growth (see Kasana & Mansfield, 1986). However, gaseous pollutants are generally considered to reduce the growth of roots in proportion to shoots and a disturbance to translocation has been suggested as a cause (Mansfield, 1988). It would appear that wet acid deposition affects plant development in a different manner. In Britain, rainfall has typical ambient acidities of pH 3-5-4.5 (see Fowler et al., 1982; Barrett et al., 1983; RGAR, 1987). It is apparent from the data presented here that this level of acidity may be expected to have detrimental effects on the growth of the three legumes studies. V. faba is particularly sensitive to acidity showing dry weight yield reductions at the pH4.5 treatment, compared to plants grown at pH5-6. Hence this crop, in particular, may be expected to be growing below its potential in much of the UK. There are, of course, difficulties of interpretation when comparing the results of pot experiments to the field environment. Even in controlled experiments, it is well known that methodology and type of exposure system may significantly influence the sensitivity of plants to simulated acid rain. Temperature, water supply and nutrient conditions during exposure may affect plant sensitivity as well as differences in the duration and frequency of rain events (see Evans et al., 1985; Caporn & Hutchinson, 1987). In addition, there are difficulties in extrapolating effects on seedlings to plants reaching maturity. Reductions in plant dry weight need not necessarily be reflected by lower yields of seed pods--the cash part of the crop. Despite these limitations, it is apparent that there is an effect of rainfall acidity on dry matter production in the early stages of growth which could affect later crop yield. Furthermore, the large differences in the sensitivities of crops to wet acid deposition are likely to be reflected in the responses of species in natural plant communities, where small differences in the abilities of species to grow and compete against others may cause changes in community structure.

28

T. W. Ashenden, S. A. Bell

ACKNOWLEDGEM ENTS This research was partially funded by the Welsh Office. We are grateful to M r R. A. Page (Welsh Office), Professor C. Milner, Dr D. F. Perkins and Dr D. Moss for helpful discussion throughout the investigation.

REFERENCES Amthor, J. S. (1984). Does acid rain directly influence plant growth? Some comments and observations. Environ. Pollut., Ser. A, 36, 1-6. Ashenden, T. W. & Bell, S. A. (1987a). Yield reductions in winter barley grown on a range of soils exposed to simulated acid rain. Plant and Soil 98, 433-7. Ashenden, T. W. & Bell, S. A. (1987b). The effects of simulated acid rain on the growth of three herbaceous species grown on a range of British soils. Environ. Pollut., 48, 295-310. Ashenden, T. W. & Bell, S. A. (1988). Growth responses of birch and Sitka spruce exposed to acidified rain. Environ. Pollut., 51, 153-62. Ashenden, T. W. & Mansfield, T. A. (1978). Extreme pollution sensitivity of grasses when SO 2 and NOz are present in the atmosphere together. Nature, Lond., 273, 142 3. Barrett, C. F., Atkins, D. H. F., Cape, J. N., Fowler, D., Irwin, J. G., Kallend, A. S., Martin, A., Pitman, J. I., Scriven, R. A. & Tuck, A. F. (1983). Acid Deposition in the United Kingdom. Warren Spring Laboratory, Stevenage. Bell, J. N. B. (1982). Sulphur dioxide and the growth of grasses. In Effects of Gaseous Air Pollution in Agriculture and Horticulture, ed. by M. H. Unsworth & D. P. Ormrod, Butterworth Scientific, London and Boston, pp. 225--46. Caporn, S. J. M. & Hutchinson, T. C. (1987). The influence of temperature, water and nutrient conditions during growth on response of Brassica oleracea L. to a single, short treatment with simulated acid rain. New Phytol., 106, 251-9. Evans, L. S., Gmur, N. F. & Da Costa, F. (1977). Leaf surface and histological perturbations of leaves of Phaseolus vulgaris and Helianthus annuus after exposure to simulated acid rain. Am. J. Bot., 64, 903-13. Evans, L. S., Gmur, N. F. & Da Costa, F. (1978). Foliar responses of six clones of hybrid poplar to simulated acid rain. Phytopathology, 68, 847-56. Evans, L. S., Lewin, K. F., Cunningham, E. A. & Patti, M. J. (1982). Effects of simulated acid rain on yields of field grown crops. New Phytol., 91, 429~,1. Evans, L. S., Lewin, K. F., Patti, M. J. & Cunningham, E. A. (1983). Productivity of field grown soybeans exposed to simulated acid rain. New Phytol., 93, 377-88. Evans, L. S., Lewin, K. F., Santucci, K. A. & Patti, M. J. (1985). Effects of frequency and duration of simulated acidic rainfalls on soybean yields. New Phytol., 100, 199-208.

Evans, L. S., Lewin, K. F., Owen, E. M. & Santucci, K. A. (1986). Comparison of yields of several cultivars of field-grown soybeans exposed to simulated acidic rainfalls. New Phytol., 102, 409-17. Fowler, D., Cape, J. N., Leith, I. D., Paterson, I. S., Kinnaird, J. W. & Nicholson, I. A. (1982). Rainfall acidity in northern Britain. Nature, Lond., 297, 383-6.

Responses of legumes exposed to acid rain

29

Irving, P. M. (1983). Acidic precipitation on crops: A review and analysis of research. J. Environ. QuaL, 12, 442-53. Jacobsen, J. S. & Van Leuken, P. (1977). Effects of acid rain on vegetation. In Proceedings of the First International Symposium on Acid Precipitation and the Forest Ecosystem. ed. L. S. Dochinger & T. A. Seliga. USDA Forest Service, Northeastern Forest Experiment Station, Pennsylvania. Kasana, M. S. & Mansfield, T. A. (1986). Effects of air pollutants on the growth and functioning of roots. Proceedings of the Indian Academy of Science (Plant Science), 96, 429-41. Law, R. M. & Mansfield, T. A. (1982). Oxides of nitrogen and the greenhouse atmosphere. In Effects of Gaseous Air Pollution in Agriculture and Horticulture, ed. M. H. Unsworth & D. P. Ormrod. Butterworth Scientific, London and Boston, pp. 93 112. Mansfield, T. A. (1988). Factors determining root:shoot partitioning. In Forest Decline Symptomology CEC COST 612 Workshop. RGAR (1987). United Kingdom Review Group on Acid Rain. Acid Deposition in the United Kingdom 1981-1985. Warren Spring Laboratory, Stevenage. Troiano, J., Heller, J. & Jacobsen, J. S. (1982). Effect of added water and acidity of simulated rain on growth of field-grown radish. Environ. Pollut., 29, 1 11.