Agricultural and Food Science in Finland, Vol. 12 (2003): 155–164 155 © Agricultural and Food Science in Finland Manuscript received July 2003 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 12 (2003): 155–164. Allocation of added selenium in lettuce and its impact on roots Asko Simojoki Department of Applied Chemistry and Microbiology, University of Helsinki, Finland. Present address: Institute for Plant Nutrition and Soil Science, Christian-Albrechts University Kiel, Hermann-Rodewald-Str. 2, D–24118 Kiel, Germany, e-mail: a.simojoki@soils.uni-kiel.de Tailin Xue Institute of Geography, Chinese Academy of Sciences, Beijing 100101, China Kaarina Lukkari Finnish Institute of Marine Research, PO Box 33, FIN–00931 Helsinki, Finland Arja Pennanen and Helinä Hartikainen Department of Applied Chemistry and Microbiology, PO Box 27, FIN–00014 University of Helsinki, Finland Allocation of selenium (Se) in lettuce and its impact on root morphology were studied to better understand the growth responses of plants to added Se. Lettuce was grown in vermiculite under con- trolled growing conditions for seven weeks, and the allocation in the shoots and roots of selenate added in increasing dosages (0, 1, 10, 100, 500 and 1000 µg Se per 3.5-litre pot) as well as morpho- logical variables of the roots were determined. The intermediate additions of 100 and 500 µg Se per pot seemed to produce the highest biomasses, although this was nearly masked by large scatter in the data. The Se contents both in roots and shoots increased roughly proportionally to the amount of Se added. However, at small additions Se was preferentially allocated to roots, whereas at larger addi- tions the contents in roots and shoots (mg kg-1 dry matter) were roughly equal. Se treatments did not change the morphology of hypocotyls. On the contrary, the specific length and area of basal and lateral roots were smallest at intermediate Se additions, whereas the specific volume was largest at the largest Se addition. These effects of Se on root morphology were, however, not unambiguously related to plant growth. As the Se contents in roots increased, the roots grew thicker and the specific volume of lateral roots increased in agreement with a hypothesis of increased endogenous ethylene production. Key words: root morphology, selenium, lettuce, vegetables mailto: a.simojoki@soils.uni-kiel.de 156 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Simojoki, A. et al. Allocation of selenium in lettuce Introduction Selenium (Se) is an essential microelement for animals and human, but toxic at high concentra- tions. Although Se is generally not considered essential for higher plants, it may benefit their physiology and growth at small concentrations (Hartikainen et al. 1997, Xue et al. 2001, Pen- nanen et al. 2002). Due to low bioavailability of Se in Finnish soils (Yläranta 1985), the multinu- trient fertilizers for field crops have been sup- plemented with Na-selenate in Finland since 1984 to ensure adequate Se intake in domestic agricultural products by humans (Ekholm et al. 1995). The fertilization has to be repeated annu- ally, as selenate is reduced to selenite, which is much more strongly bound to Al- and Fe-oxides than selenate. In greenhouse production, Se fer- tilization is not yet allowed. This is partly due to the toxicity risk attributable to the fertiliza- tion technique where the plants are subjected to a continuous flow of nutrient solution. The positive growth responses of plants to Se added at small concentrations have been at- tributed to the antioxidative effect of Se coun- teracting the oxidative stress (Hartikainen et al. 1997, Hartikainen and Xue 1999, Xue et al. 2001, Seppänen et al. 2003). Plants take up Se as se- lenate (SeO4 -2) and selenite (HSeO3 -2, SeO3 -2) ions. In the plant, selenite is generally less mo- bile, more easily assimilated and more toxic than selenate (Hopper and Parker 1999). Plants uti- lize Se effectively in their amino acid and pro- tein synthesis (e.g. Fishbein 1991, Hartikainen et al. 1997) and replace it for S in amino acids (Läuchli 1993). The phytotoxicity of Se has been explained by the disruption of the protein me- tabolism due to the increased formation of se- lenoamino acids (Läuchli 1993, Terry and Zayed 1994) as well as the pro-oxidative effects of Se (Hartikainen and Xue 1999) at high concentra- tions. Selenoamino acids, in turn, enhance the pro- duction of ethylene (Konze et al. 1978), a plant hormone mediating plant responses to several stresses and affecting e.g. root morphology (Jackson 1991, Morgan and Drew 1997). How- ever, there is little information about the effects that Se may exert on plant growth by changing root morphology, although these changes obvi- ously affect e.g. the nutrient and water acquisi- tion by plants. In our earlier studies, added Se induced root morphological changes that could be partly explained by assuming that Se in- creased endogenous ethylene production (Har- tikainen et al. 2001). The aim of this study was to determine the allocation of Se in the shoots and roots of let- tuce at increasing additions. A special emphasis was paid to the responses in the roots, especial- ly the specific length and specific area of roots that affect nutrient and water acquisition by roots. Furthermore, the accumulation of Se in edible parts of plants was of interest: desirable Se contents in food and feed crops have been suggested to lie in the range 0.1–1 mg kg-1, whereas at above 5 mg Se kg-1 in the diet, there is a danger of toxicity (see Hakkarainen 1993, Mengel and Kirkby 2001). European legislation allow a maximum Se content of 0.5 mg kg-1 in all commercial feeds for domestic animals (Council directive 70/524/EEC, Commission Information 2002/C 329/EC), but no similar reg- ulations exist for mineral elements, including Se, in food for human consumption. Lettuce was chosen for the study, because it is an important vegetable in human diet. It was grown in ver- miculite, an inert growing medium, to promote effective utilization of added Se with minimal sorption and microbial uptake, and to allow easy study of roots. Material and methods Experimental setup Lettuce was grown in a growth chamber for 7 weeks under controlled conditions (day: 16 h, photosynthetically active radiation 180 µmol 157 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 12 (2003): 155–164. m2 s-1, 20˚C, relative humidity (RH) 80%; night: 8 h, total darkness 6 h, 10–20˚C, RH 80%) at 6 Se addition levels: 0, 1, 10, 100, 500 and 1000 µg per 3.5-litre pot added as H2SeO4. The exper- iment had a completely randomised design with 4 replications. Fifteen lettuce seeds were sown in each plastic pot filled with 350 g of vermicu- lite pre-moistened with deionised water (1500 ml pot-1). After the emergence, the seedlings were thinned to four and watered with deionised wa- ter (500 ml pot-1). During a period of 10 days from the emergence, the pots were fertilized with Se and slightly modified Hoagland and Arnon solutions (1000 ml pot-1) containing N 224 mg, K 235 mg, Ca 160 mg, Mg 49 mg, P 62 mg, S 32 mg, and trace amounts of Fe, B, Mn, Zn, Cu, Mo and Ni per pot. The solutions were added carefully avoiding leaching from the pots. The amount of S in the solution was smaller than or- dinary to reduce the antagonism of sulfate on the selenate uptake by plants. After this, the pots were watered from below with deionised water applied to the plates. Plant sampling and analysis At the harvest, one seedling in each pot was ran- domly selected for root morphological analysis. All roots were washed in cold water to remove the vermiculite particles and weighed fresh. Roots for the morphological analysis were stored in 18% ethanol at +5˚C before the analysis, whereas the rest, as well as the shoots, were weighed fresh, frozen in liquid nitrogen and stored at –70˚C for the later analysis of dry mat- ter and Se content. For the analysis, the samples were dried overnight at +70˚C. Se content was determined by graphic furnace atomic absorp- tion spectroscopy (Kumpulainen et al. 1983, Ekholm 1997). Reference samples (two in-house reference samples, one commercial certified sample NIST 1567) were included at an average frequency of 20% of all samples for data quality management. For morphological analysis, the roots were separated and classified manually according to their type into hypocotyls, basal roots or lateral roots (Fig. 1) and analysed by digital image analysis (Simojoki 2000). Statistical analysis The relationships between the response variables were explored by examining their linear corre- lations and principal components. The aim of the principal component analysis (Webster 2001) was to obtain a few variates that would capture most of the information in the numerous origi- nal response variables. Treatment means were examined by Tukey’s test. Fig. 1. Hypocotyl, basal roots and lateral roots of lettuce were separated manually and analysed by digital image analysis (Photo: Asko Simojoki). 158 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Simojoki, A. et al. Allocation of selenium in lettuce Results Owing to the high variation between the repli- cates, the dry masses of shoots and roots and the shoot-to-root ratios were not significantly dif- ferent at the various Se addition levels (Table 1). However, the intermediate additions of 100 and 500 µg Se per pot seemed to produce the high- est biomasses. Dry mass allocated to the various root types was also fairly similar in all treatments (data not shown): the dry masses of lateral and basal roots were, on average, 239 ± 14 mg and 33 ± 3 mg, respectively (mean ± standard error, n = 24) with no significant differences between the treatments. However, slightly more dry mass was allocated to hypocotyls at the Se addition of 100 mg per pot (71 ± 5 mg, n = 4) than in other treatments (31 ± 3 mg, n = 20). In contrast, in the unfertilized treatment the Se content in roots was nearly 2.5-fold compared with that in shoots [mg kg-1 dry matter (DM), Table 1]. The increase in Se contents both in roots and shoots from these levels was roughly proportional to the amount of Se added. Howev- er, at small additions, Se was preferentially al- located to roots, whereas at larger additions the contents in roots and shoots did not differ from each other. The various root types differed from each oth- er by orders of magnitude with respect to the measured morphological variables (Table 2). The average widths were 312, 940 and 3200 µm, and the specific lengths 3, 28 and 355 m g-1 in the lateral roots, basal roots and hypocotyls, respec- tively. Specific volume was, on average, larger in lateral roots (27 cm3 g-1) than in the basal roots and hypocotyls (18 and 20 cm3 g-1, respectively). Addition of Se had diverse effects on root morphology (Table 2). No statistically signifi- cant changes were observed in the morphology of hypocotyls. In the basal and lateral roots, on the contrary, specific length and specific area were smallest at intermediate Se additions (100 or 500 µg pot 1), whereas specific volume was largest at the largest application. In addition, the tips of lateral roots were frequently dark brown, however, roughly to the same extent in all treat- ments (Fig. 1). Numerous linear correlations between the response variables were relatively strong (see Table 3), which allowed capturing most of the information into a few orthogonal linear combi- nations of original variates by principal compo- nent analysis (Table 4). The first three principal components explained 69% of the total variation in the data. The first component correlated pos- itively with plant growth and the width of hy- Table 1. Dry mass (g) and Se content (mg kg-1) in the shoots and roots of lettuce at different Se fertilization levels. Se added Dry mass Se contenta ANUEb µg per pot Shoot Root Root/Shoot Shoot Root Shoot/Root % 0 1.16 0.28 0.24 0.031 0.077 0.41 – 1 1.33 0.32 0.24 0.074 0.157 0.47 8.3 10 1.02 0.25 0.25 0.41 0.62 0.67 5.2 100 1.50 0.39 0.26 3.5 3.9 0.90 6.7 500 1.57 0.37 0.23 17 18 0.94 6.7 1000 0.99 0.26 0.25 39 42 0.94 4.9 HSD 0.73 0.20 0.06 HSR 1.3 1.5 1.7 HSD honestly significant difference, HSR honestly significant ratio, P < 0.05 a Statistical tests with logarithm-transformed data b Apparent nutrient use efficiency for fertilizer Se = (Dry mass of shoots × (Se content in shoots minus that in the unferti- lized treatment) + Dry mass of roots × (Se content in roots minus that in the unfertilized treatment)) / Amount of Se applied × 100 159 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 12 (2003): 155–164. Table 2. Morphology of lettuce roots at different Se fertilization levels. Se added Width Specific root length Specific root surface area Specific root volume µg per pot µm m g-1 cm g-1 cm3 g-1 Lateral roots 0 304a 371ab 3533b 26.9ab 1 306a 363ab 3492ab 26.9ab 10 297a 384b 3569b 26.5ab 100 313a 323a 3170a 24.9a 500 325a 346ab 3511ab 28.5ab 1000 330a 343ab 3564b 29.5b Basal roots 0 917a 26.4ab 754ab 17.2a 1 884a 29.4ab 805ab 17.7ab 10 852a 37.1b 937b 19.4ab 100 978a 23.7ab 724ab 17.6ab 500 1089a 20.0a 669a 18.0ab 1000 939a 29.2ab 857ab 20.1b Hypocotyl 0 2993a 2.8a 247a 18.3a 1 3771a 2.2a 246a 22.5a 10 2400a 3.9a 303a 17.3a 100 3840a 1.9a 219a 20.6a 500 3358a 2.6a 232a 18.3a 1000 2778a 3.8a 313a 21.4a Means denoted by a different letter on the same column and root class differ significantly (P < 0.05) Table 3. Linear correlations between the growth, Se content and root morphology of lettuce. Dry mass of shoots Se content in rootsa Lateral roots Basal roots Hypocotyl Lateral roots Basal roots Hypocotyl Dry mass 0.90*** 0.73*** 0.66*** 0.04 –0.22 –0.40 Width –0.08 0.36 0.70*** 0.55** 0.21 –0.11 Specific length –0.27 –0.42* –0.74*** –0.27 –0.10 0.15 Specific surface area –0.49* –0.54** –0.79*** 0.18 0.04 0.30 Specific volume –0.41* –0.52** 0.29 0.53** 0.47* 0.18 n = 24; * P < 0.05, ** P < 0.01, *** P < 0.001 a Average including all root types (lateral, basal, hypocotyl) pocotyl, and negatively with the specific length of hypocotyl and the specific surface areas of all root classes (Table 4). Thus, it accounted for observations where the increases in plant growth were accompanied by thickening and shorten- ing of hypocotyls and by reduction of specific root surface area. The second component accounted for the in- crease in plant Se content accompanied by thick- ening and shortening of lateral and basal roots, 160 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Simojoki, A. et al. Allocation of selenium in lettuce Table 4. First three eigenvalues and eigenvectorsa from a principal component analysis of the data on growth, Se content and root morphology of lettuce in all experimental treatments. Principal components 1 2 3 Eigenvalue 7.481 4.125 2.291 Explained variance, % 37.41 20.62 11.45 Cumulative variance, % 37.41 58.03 69.48 Eigenvectors Variates 1 2 3 1 Se content in shoots –0.283 0.738 –0.444 2 Se content in roots –0.267 0.726 –0.470 3 Dry matter content in shoots –0.470 –0.190 0.537 4 Dry matter content in roots –0.161 0.213 0.768 5 Dry mass of shoots 0.880 0.032 –0.015 6 Dry mass of lateral roots 0.831 0.223 –0.151 7 Dry mass of basal roots 0.600 –0.228 –0.112 8 Dry mass of hypocotyls 0.707 –0.294 0.004 9 Width of lateral roots –0.150 0.872 0.109 10 Width of basal roots 0.432 0.715 0.277 11 Width of hypocotyls 0.845 –0.004 –0.319 12 Specific length of lateral roots –0.338 –0.656 –0.227 13 Specific length of basal roots –0.568 –0.595 –0.329 14 Specific length of hypocotyls –0.873 –0.033 0.179 15 Specific surface area of lateral roots –0.657 –0.038 –0.233 16 Specific surface area of basal roots –0.696 –0.426 –0.375 17 Specific surface area of hypocotyls –0.879 0.022 –0.001 18 Specific volume of lateral roots –0.567 0.620 –0.080 19 Specific volume of basal roots –0.647 0.395 –0.275 20 Specific volume of hypocotyls 0.456 0.048 –0.548 a Product-moment correlations between the principal components and the original data; largest contributions are shown in bold as well as by the increase in the specific volume of lateral roots (Table 4). The third component accounted for the inverse relationship between the plant dry matter content and the specific vol- ume of hypocotyls. As the principal components are due to their orthogonality independent of each other, the ex- amination of their mean scores could be used to sum up the essential responses of lettuce to the experimental treatments (Table 5). Only small part of the total variation was related to the Se treatments. The first principal component (PC1) that explained 37% of the total variation was not linearly related to the Se content in plants but showed maximum scores at the Se addition of 100 µg per pot and smallest ones at the largest addition. The second order polynomial fitted to the data (PC1 = a + bx + cx 2) had a maximum near 400 µg per pot (a = –0.0488, b = 9.69 × 10-3, c = –0.012 × 10-3; x = added Se, µg per pot). However, the regression explained only 23% of the total variation in PC1 (R 2 = 0.23). The scores of the second component increased roughly pro- portionally to the amount of Se added. Additions of Se did not change the third component sig- nificantly. 161 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 12 (2003): 155–164. Discussion In contrast to our earlier experiments (Hartikai- nen et al. 1997, Hartikainen and Xue 1999, Har- tikainen et al. 2001, Xue et al. 2001, Pennanen et al. 2002), practically no significant effects of Se addition on the dry matter production of let- tuce could be proved in this experiment owing to large scatter in the data (Table 1). However, the relative shoot yields and Se contents in this experiment conformed rather well to the gener- al pattern from the earlier experiments that plant growth is increased by small Se additions and decreased by large additions (Fig. 2). At low contents of 1–5 mg kg-1 DM, Se tended to en- hance plant growth. When the Se content exceed- ed that range, the shoot yields started to dimin- ish, the drop being drastic above 20 mg kg-1 DM. Moreover, by adopting a broader view on the growth process that takes into account the cor- relation of plant growth with certain root mor- phological features (first principal component), an optimum for plant growth could be established at intermediate Se additions also in this study (Tables 4 and 5). The results of this experiment were obvious- ly partly confounded by some extraneous varia- bles that increased the error variance so that even some large effects were statistically insignificant. Uneven distribution of moisture within the pots is the most likely reason for large error varia- tion in this study. Watering the pots from below was probably not successful due to the poor wa- ter conductivity of unsaturated vermiculite, al- though no attempt was made to confirm this sug- gestion by measurement. Low illumination in the phytotron partly explains, why the yields were much smaller than in some earlier studies (Har- tikainen et al. 1997, Pennanen et al. 2002). These factors are also probable major reasons for the very low apparent nutrient use efficiencies (5– Table 5. Mean scores from the first three principal compo- nents of the data on growth, Se content and root morpholo- gy of lettuce in all experimental treatments. Se added Component order µg per pot 1 2 3 0 –0.11ab –1.01ab 1.09a 1 –0.72ab –1.15a –0.11a 10 –2.23a –2.01a –0.21a 100 2.89b –0.19ac 0.33a 500 1.09ab 1.90bc 0.61a 1000 –2.37a 2.46c –1.71a Means denoted by a different letter on the same column differ significantly (P < 0.05) 0.01 0.1 1.0 5 20 100 1000 Se content (mg kg-1 DM) 0 50 100 150 200 Relative shoot yield (100 = no added Se) 0.01 0.1 1.0 5 20 100 1000 Se content (mg kg-1 DM) 0 50 100 150 200 Relative shoot yield (100 = no added Se) 0.01 0.1 1.0 5 20 100 1000 Se content (mg kg-1 DM) 0 50 100 150 200 Relative shoot yield (100 = no added Se) 1 1 1 1 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 0.01 0.1 1.0 5 20 100 1000 Se content (mg kg-1 DM) 0 50 100 150 200 Relative shoot yield (100 = no added Se) 0.01 0.1 1.0 5 20 100 1000 Se content (mg kg-1 DM) 0 50 100 150 200 Relative shoot yield (100 = no added Se) Fig. 2. Relative yield as related to the Se content in lettuce shoots. The numbers below the points stand for the experimental data: 1 = Hartikainen et al. 1997, 2 = Har- tikainen and Xue 1999, 3 = Xue et al. 2001, 4 = Pennanen et al. 2002, 5 = Simojoki et al. 2003 (this study). 162 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Simojoki, A. et al. Allocation of selenium in lettuce 9%) for fertilizer Se by plants compared with our earlier experiments on soils (Hartikainen et al. 1997, Hartikainen and Xue 1999, Xue et al. 2001, Pennanen et al. 2002). The frequently ob- served dark brown root tips may be attributable to unfavourable microsites in the growth medi- um, although the exact reason for their occur- rence remains unknown. Leaching of Se out of the pots can be exclud- ed as a cause for large variation in plant growth, because leaching was avoided during the fertili- zation, and as the pots were later on watered from below. Furthermore, the plant Se contents in all treatments differed significantly from each oth- er and roughly proportionally to the Se addition (Table 1). This confirms that the Se treatments were established succesfully. As for the allocation of Se into various parts of lettuce, the results show that in a growth me- dium rather inactive in retaining selenate, the Se content in lettuce increases effectively with in- creasing Se dosages (Table 1). Nevertheless, the effects in roots and shoots can be different. At small Se additions, roots contained relatively more Se than shoots (as mg kg-1 DM), whereas at larger additions, the Se contents were similar. Selenate as such is easily translocated from roots to shoots (Hopper and Parker 1999), while se- lenite remains in roots and is accumulated in organic Se (Asher et al. 1977, Zayed et al. 1998). The results thus suggest that the roots are rela- tively more efficient in metabolising selenate when added in small amount. At higher addition levels more selenate is translocated from roots to shoots, which smoothens the difference in their Se contents. As the dry mass of shoots is usually much larger than that of roots, most of the Se taken up by the plant is, however, allocat- ed to shoots (63–80% in this study). The mean Se content of fresh lettuce in Fin- land is low, only 2 µg Se kg-1 (National Public Health Institute 2001), which is close to the Se content in the shoots of the unfertilized treat- ment in this study. Taking into account that about 95% of lettuce is water, our results suggest that Se fertilization up to at least 100 µg per pot (29 µg dm-3 soil) should not increase the Se con- tents beyond desirable range. The contents in plants growing in soil, peat and other non-inert media at a given fertilization are likely to be smaller than those in this study due to the sorp- tion and microbial uptake of Se as well as the sulfate antagonism on selenate uptake by plants. Moreover, in better growing conditions, the in- crease in apparent Se use efficiency is likely to be larger than the relative enhancement in plant growth (Hartikainen et al. 1997, Xue et al. 2001, Pennanen et al. 2002), so that a given Se fertili- zation will produce smaller contents in plants. The results of image analysis and statistical principal component analysis revealed that root morphology was related to both plant growth and Se content (Tables 3 and 4). Furthermore, the results confirmed that Se fertilization changes root morphology by exerting diverse effects on different parts of the root system (Tables 2 and 5). The morphology of hypocotyl seems not to be affected by Se addition. In contrast, moder- ate Se additions decrease the specific length and specific surface area in basal and lateral roots, and large Se additions increase the specific vol- ume of roots. This implies that moderate Se ad- ditions decrease the efficiency of photosynthate use for constructing root surface area, and larg- er Se additions increase the root porosity. These responses may have been caused by ethylene, as its production is known to be enhanced by se- lenoamino acids (Konze et al. 1978) and as plants commonly react this way to stresses mediated by ethylene (Jackson 1991, Morgan and Drew 1997). Positive correlation between root Se con- tent and the thickness and specific volume of lateral roots gives further indirect support to our hypothesis (Hartikainen et al. 2001) that added Se induces endogenic ethylene production in roots. The hypothesis thus deserves further study with direct measurements of ethylene produc- tion. On the other hand, better plant growth was found to be associated with more inefficient use of photosynthates for constructing root surface area (Tables 3 and 4). Thus, in this experiment the effects of Se fertilization on root morpholo- gy were not unambiguously related to plant 163 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 12 (2003): 155–164. growth. Moreover, Se use efficiency was appar- ently not related to root morphology. Overall, the results on root morphology thus suggest that the commonly observed plant growth enhance- ment by small Se additions are not likely due to more efficiently functioning roots, but more probably due to the positive effects of added Se on shoots such as enhanced production and ac- cumulation of carbohydrates (Arvy et al. 1995, Mazzafera 1998, Pennanen et al. 2002). Conclusions Small Se additions that increase Se contents in lettuce shoots up to 1–5 mg kg-1 DM tend to en- hance plant growth. The yields drop drastically at Se contents above 20 mg kg-1 DM. Selenium contents in the shoots and roots of lettuce increase roughly proportionally to the amount of Se added. At small Se fertilization given to lettuce, the roots contain relatively more Se compared with the shoots, whereas at larger amounts the contents are similar. Selenium fertilization changes root morphol- ogy, and the effects are diverse in different parts of the root system. The morphology of hypocotyl is not affected. Moderate Se additions decrease the specific length and specific surface area of basal and lateral roots, whereas large additions increase the specific volume of roots. Root morphology of lettuce depends both on plant growth and Se content. In this study, bet- ter plant growth was associated with thicker and shorter hypocotyls as well as more inefficient use of photosynthates for constructing root surface area. The effects of Se fertilization on root morphology were not unambiguously related to plant growth. The positive correlation between the thickness and specific root volume of lateral roots gives support to the hypothesis that added Se induces endogenic ethylene production. Acknowledgements. We gratefully acknowledge the tech- nical assistance by Ms. Maija Ylinen for the Se analyses, and by Mr. Teemu Halme for the morphological analysis of roots. This study was financed by the Research Funds of the University of Helsinki. References Arvy, M.P., Thiersault, M. & Doireau, P. 1995. Relation- ship between selenium, micronutrients, carbohy- drates, and alkaloid accumulation in Catharanthus roseus cells. Journal of Plant Nutrition 18: 1535– 1546. Asher, C.J., Butler, G.W. & Peterson, P.J. 1977. Seleni- um transport in root systems of tomato. Journal of Experimental Botany 28: 279–291. Commission Information 2002/C 329/EC. List of the au- thorised additives in feedingstuffs published in ap- plication of Article 9t (b) of Council Directive 70/ 524/EEC concerning additives in feedingstuffs. Of- ficial Journal of the European Communities C 329/ 1–142. Council directive 70/524/EEC. Council directive of 23 November 1970 concerning additives in feeding- stuffs. CONSLEG: 1970L0524-31/3/2003. Office for Official Publication of the European Communities. 38 p. Ekholm, P. 1997. Effects of selenium supplemented com- mercial fertilizers on food selenium contents and se- lenium intake in Finland. EKT-Series No. 1047. Hel- sinki. 82 p. (Academic dissertation, University of Hel- sinki). Ekholm, P., Ylinen, M., Koivistoinen, P. & Varo, P. 1995. Selenium concentration of Finnish foods: Effects of reducing the amount of selenate in fertilizers. Agri- cultural and Food Science in Finland 4: 377–384. Fishbein, L. 1991. Selenium. In: Merian, E. (ed.). Metals and their compounds in the environment, occurrence, analysis and biological relevance. Weinheim VCH. p. 1153–1190. Hakkarainen, J. 1993. Bioavailability of selenium. Nor- wegian Journal of Agricultural Sciences, Supplement 11: 21–35. Hartikainen, H., Ekholm, P., Piironen, V., Xue, T., Koivu, T. & Yli-Halla, M. 1997. Quality of the ryegrass and lettuce yields as affected by selenium fertilization. Agricultural and Food Science in Finland 6: 381–387. Hartikainen, H., Pietola, L. & Simojoki, A. 2001. Quanti- fication of fine root responses to selenium toxicity. Agricultural and Food Science in Finland 10: 53–58. Hartikainen, H. & Xue, T. 1999. The promotive effect of selenium on plant growth as triggered by ultraviolet irradiation. Journal of Environmental Quality 28: 1372–1375. Hopper, J.L. & Parker, D.R. 1999. Plant availability of se- lenite and selenate as influenced by the competing ions phosphate and sulfate. Plant and Soil 210: 1999–207. Jackson, M.B. 1991. Ethylene in root growth and devel- opment. In: Mattoo, A.K. & Suttle, J.C. (eds.). The plant 164 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Simojoki, A. et al. Allocation of selenium in lettuce hormone ethylene. Boca Raton, FL: CRC Press. p. 159–181. Konze, J.R., Schilling, N. & Kende, H. 1978. Enhance- ment of ethylene formation by selenoamino acids. Plant Physiology 62: 397–401. Kumpulainen, J., Raittila, A.M., Lehto, J. & Koivistoinen, P. 1983. Electrochemical atomic absorption spectro- metric determination of selenium in foods and diets. Journal of the Association of Official Analytical Chem- ists 66: 1129–1135. Läuchli, A. 1993. Selenium in plants: uptake, functions, and environmental toxicity. Botanica Acta 106: 455– 468. Mazzafera, P. 1998. Growth and biochemical alterations in coffee due to selenite toxicity. Plant and Soil 201: 189–196. Mengel, K. & Kirkby, E.A. 2001. Principles of plant nutri- tion. 5th ed. Dortrecht: Kluwer Academic Publishers. 849 p. Morgan, P.W. & Drew, M.C. 1997. Ethylene and plant re- sponses to stress. Physiologia Plantarum 100: 620– 630. National Public Health Institute 2001. Fineli. Food com- position database. Helsinki: National Public Health Institute, Nutrition unit. Available on the Internet: http:/ /www.ktl.fi/fineli/. Pennanen, A., Xue, T. & Hartikainen, H. 2002. Protective role of selenium in plant subjected to severe UV irra- dation stress. Journal of Applied Botany 76: 66–76. Seppänen, M., Turakainen, M. & Hartikainen, H. 2003. Selenium effects on oxidative stress in potato. Plant Science 165: 311–319. Simojoki, A. 2000. Calibration of a desktop scanner and digital image analysis procedure for quantification of root morphology. Agricultural and Food Science in Finland 9: 223–230. Terry, N. & Zayed, A. 1994. Phytoremediation of seleni- um. In: Frankenberger, W.T., Jr. & Benson, S. (eds.). Selenium in the environment. New York: Marcel Dekker. p. 343–367. Webster, R. 2001. Statistics to support soil research and their presentation. European Journal of Soil Science 52: 331–340. Xue, T., Hartikainen, H. & Piironen, V. 2001. Antioxida- tive and growth-promoting effect of selenium on se- nescing lettuce. Plant and Soil 237: 55–61. Yläranta, T. 1985. Increasing the selenium content of ce- real and grass crops in Finland. 72 p. (Academic dis- sertation, University of Helsinki). Zayed, A., Lytle, M.C. & Terry, N. 1998. Accumulation and volatilization of different chemical species of seleni- um by plants. Planta 206: 284–292. Seleeni on eläimille ja ihmisille välttämätön alkuaine, jota lisätään Suomessa moniravinteisiin lannoitteisiin. Lisäyksen tarkoituksena on parantaa peltokasvien se- leenin ottoa ja taata ihmisille riittävä seleenin saanti ruoasta. Kasvihuoneviljelyssä seleenilannoitus ei kui- tenkaan ole vielä sallittua, osittain koska kasvien se- leenin otto ja jakautuminen kasvissa sekä siihen liit- tyvä myrkytysvaara tunnetaan puutteellisesti. Selee- nin jakautumista versoihin ja juuriin sekä vaikutusta juurten morfologiaan tutkittiin kokeessa, jossa salaat- tia kasvatettiin vermikuliitissa kontrolloiduissa olois- sa seitsemän viikon ajan nousevilla selenaattilisäyk- sillä (0, 1, 10, 100, 500 ja 1000 µg Se 3,5 litran as- tiassa). Suurin biomassa saatiin keskisuurilla lisäyksillä (100 tai 500 µg seleeniä astiaa kohti), joskin aineis- tossa oli paljon satunnaisvaihtelua. Juurten ja verso- SELOSTUS Seleenin jakautuminen salaatin versoihin ja vaikutus juuriin Asko Simojoki, Tailin Xue, Kaarina Lukkari, Arja Pennanen ja Helinä Hartikainen Helsingin yliopisto, Kiinan tiedeakatemia ja Merentutkimuslaitos jen seleenipitoisuudet nousivat seleenimäärää lisätes- sä lähes verrannollisesti lisättyyn määrään. Pienillä lisäyksillä seleenipitoisuudet olivat suurempia juuris- sa kuin versoissa, kun taas suurilla määrillä juurten ja versojen seleenipitoisuuksissa ei ollut eroa. Selee- nikäsittelyt eivät muuttaneet juurenniskan morfolo- giaa. Sitä vastoin basaali- ja lateraalijuurten pienim- mät ominaispituudet ja ominaispinta-alat saatiin kes- kisuurilla seleenilisäyksillä (100 tai 500 µg Se per astia), ja suurin ominaistilavuus suurimmalla selee- nilisäyksellä. Seleenin vaikutuksia juurten morfolo- giaan ei kuitenkaan voitu kytkeä yksiselitteisesti kas- vin kasvuun. Lateraalijuurten paksuus ja ominaisti- lavuus kasvavat juuren seleenipitoisuuden noustessa, mikä tukee hypoteesia, että seleenilisä indusoi endo- geenista etyleenin tuotantoa. Allocation of added selenium in lettuce Introduction Material and methods Results Discussion Conclusions References SELOSTUS