The effect of seed scarification and soil-media on germination, growth, storage, and survival of seedlings of five species of Prosopis L. (Mimosaceae)

Journal of Arid Environments (2001) 48: 171–184 doi:10.1006/jare.2000.0735, available online at http://www.idealibrary.com on

The effect of seed scarification and soil-media on germination, growth, storage, and survival of seedlings of five species of Prosopis L. (Mimosaceae)

A. E. Vilela & D. A. Ravetta IFEVA y CaH tedra de Cultivos Industriales, Fac. AgronomnH a, Universidad de Buenos Aires, Av. San MartnHn 4453 (1417) Buenos Aires, Argentina (Received 2 May 2000, accepted 22 September 2000)

Prosopis L. (Mimosaceae) species are important vegetation elements in arid and semi-arid environments where they offer shade, firewood, timber and food for man, wildlife and livestock. Many species, specially South-American ones, have been included in afforestation programmes and agroforestrysilvopastoral systems. Within this scope, there is a need of information on eco-physiological responses related to growth and development. The objective of this study was to assess the influence of seed scarification method and growing media on germination, seedling growth, survival and some physiological variables of three Prosopis species native to South America: P. alba, P. chilensis and P. flexuosa, and two native to North America: P. velutina and P. pubescens. All scarification methods can be used to promote germination in the species evaluated in this study, except for the chemical treatment for P. chilensis and the thermal treatment for the North American species. Nursery soil mix should be preferred to the soil under the canopy of Prosopis trees, since germination, plant size, and plant survival were higher in this growth media. An increase in biomass partition to above ground structures was an important mechanism that allowed for the superior response of plants in nursery mix. The addition of nutrients to Prosopis soil did not improve the rate of leaf appearance or biomass, or changed the shoot:root ratio. Both reserve carbohydrates and C:N ratio were lower in fertilized plants. Prosopis flexuosa showed a significant increase in plant biomass with the addition of nutrients to the growth media.  2001 Academic Press Keywords: mesquite; total non-structural carbohydrates; nitrogen content; net CO2 uptake

Introduction Several authors have reported the dramatic increase of woody plants on grasslands of the American Southwest ( Johnson & Mayeux, 1990; Bush & Van Auken, 1991), where honey mesquite (Prosopis glandulosa var. glandulosa Torrey) is one of the major woody ‘pest’ plants found. A similar phenomena, although in a minor extent, occurs in the temperate semi-arid rangelands of central Argentina, where P. caldenia Burkart is a dominant species (Distel et al., 1996). Although this increase in density of woody plants in areas considered grasslands suggests that germination and establishment of seedlings are succesful, the seedlings of woody species do not compete well with grasses 0140-1963/01/060171#14 $35.00/0

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(Van Auken & Bush, 1990) and the causes of these changes are most often attributed to the disturbance associated with selective grazing of grasses by domestic livestock and spread of seed in livestock dung (Johnson & Mayeux, 1990). Prosopis alba Grisebach, P. chilensis (Molina) Stuntz emend. Burkart and P. flexuosa DC are trees that occur in Argentinean subtropical dry forests and none of them is considered an invasive plant. All three provide timber of high value and their pods are harvested, stored and used as feed and fodder during the winter (Burkart, 1976). Many Prosopis species, especially these South-American ones, have been included in afforestation programmes and agroforestry-silvopastoral systems. Within this scope, little information is available on their requirements for growth and development and the influence of abiotic factors on seed germination and seedling initial growth. Hard-seeded species like those of Prosopis, require external stimuli to promote seed-coat rupture. In nature, there are various biotic and abiotic factors that produce seed scarification such as temperature (fire or chilling; Rees, 1997), changes in the chemical environment (i.e. seed ingestion by frugivores and its passage through the digestive tract; Mooney et al., 1977) or mechanical abrasion of the stony endocarp by sand and rocks in water courses. In natural stands of leguminous trees with indehiscent pods like those of Prosopis species, wild animals and domestic cattle that feed on pods, scarify the seeds triggering the germination process. Since Prosopis seeds require some kind of scarification for germination, all published experimental work mentions some way of promoting seed coat rupture (i.e. Villagra, 1995; Manga & Sen, 1995), although no information is available at this time of the benefits and problems involved with each method of scarification for each of the individual species. Even after scarification is completed, environmental conditions appropriate for seed germination and seedling establishment in arid and semi-arid zones, occur only in few occasions in which a set of requirements is fulfilled (Grubb, 1977), and usually the latter does not occur along a gradient but in spurts (Sosebee & Wan, 1987). After germination is initiated, the fate of seedlings depends on several environmental factors as well as physiological responses. Some of the mechanisms that can enhance plant performance under stress involve changes in biomass allocation (i.e. shoot:root ratio) triggered by shifts in the C:N relationship (Landsberg & Gower, 1997). The speed of adjustment of these mechanisms to the prevailing environmental conditions varies among plants and is a critical aspect of their survival strategies (Bazzaz, 1997). According to the optimization theory, the availability of each individual resource is capable of influencing the growth of the plant, promoting development of those organs that are involved in the acquisition of whichever resource limits growth most severely (Heilmeier et al., 1997). For example, fertilization is known to increase the allocation of carbon to foliage and decrease allocation of carbon to fine roots (Landsberg & Gower, 1997). In the genus Prosopis it was found that a reduction in light availability increased shoot:root ratio and decreased total biomass accumulation and stored carbohydrates ( Vilela & Ravetta, 2000). This experiment showed that if prevailing environmental conditions after germination combine both shade and water stress (i.e. under a tree canopy) seedling survival can be greatly reduced. The objective of this study was to assess the influence of seed scarification method and growing media on germination, seedling growth, survival and some physiological variables of three Prosopis species native to South America: P. alba, P. chilensis and P. flexuosa, and two native to North America: P. velutina Wooton and P. pubescens Bentham. Germination, rate of leaf appearance, CO2 assimilation, survival, biomass partition, protein content and non-structural carbohydrates content of leaves, stems and roots were evaluated. Material and methods A three-factor experiment was conducted in Tucson, AZ, U.S.A., in a greenhouse set at 143C/383C (night–day). Seeds of five Prosopis species (P. alba, P. chilensis, P. flexuosa,

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P. velutina and P. pubescens) were scarified by three methods (mechanical, thermal and chemical) and planted in three different growing media (Prosopis soil, Prosopis soil#nutrients, and Nursery Mix). Seeds of P. alba, P. chilensis and P. flexuosa were obtained from ripe pods collected in the phytogeographic region of Monte, Argentina. Prosopis velutina and P. pubescens seeds, collected in Tucson, AZ, U.S.A., were provided by the Desert Legume Program of the University of Arizona. Voucher specimens of South-American species are deposited in BAFC herbarium. Scarification methods were applied according to Fflolliott & Thames (1983): (1) mechanical: seeds were nicked with a razor blade; (2) chemical: seeds were scarified with sulphuric acid 1 N for 15 min, washed in running tap water three times for 2 min and soaked in water for 15–30 min; (3) thermal: seeds were dipped in boiling water until water reached room temperature. Prosopis soil (PS) was obtained from a natural mesquite (P. velutina) stand in Tucson, AZ, U.S.A. Soil analysis was as follows: pH 8)2; total organic C: 0)5%; total N: 0)055%; soluble Ca: 16 ppm; soluble Na 42 ppm. For the preparation of Prosopis soil#nutrients (PSN) 3g of NPK fertilizer/plant (Osmocote, Sierra; 14:14:14 Total N: P2O5:K2O) were added to PS. Commercial nursery mix (NM) was made of ground sphagnum, peat moss and vermiculite (1:1:1 by volume). Seedlings were treated with systemic fungicide (dimethyl 4,4-0 phenylenebis -3-thioallophanate). Plants in nursery mix were fertilized once (45 days after seeded) with 0)112 g/plant of NPK fertilizer (Peters Company; 9:45:15 NH4:P2O5:K2O). One scarified seed was randomly planted in each tree tube (40;150 mm) filled with one of the three growing media (50 reps/treatment). All plants were irrigated between three and five times a week. The rate of leaf appearance was recorded every other day for 60 days after germination had occurred. Net CO2 uptake was measured using an LCA-3ADC portable infrared analyser open gas exchange system (Analytic Development Co, Hoddesdon, U.K.) on 2 days during the experiment. On each of three individual plants per species the latest fully expanded leaf was selected, its leaf area determined, and CO2 uptake recorded at 9 a.m., 12 noon and 3 p.m. At the end of the experiment, the height of every plant was recorded; 10 plants/treatment were harvested, and leaves, shoots and roots were separated and weighed. Total non-structural carbohydrates were determined by autoclaving (0)1 Mpa, 15 min) 50 mg of biomass in 100 ml of distilled water. Samples were homogenized in 100 ml of water and in every case the solubilised sugars were determined by the anthrone method (Yemm & Willis, l954). Nitrogen determination was based on Kjedahl method. A protein Digestion System G 1007 (Tekator, Hoganas, Sweden) with a Kjeltec System 1002 nitrogen determinator was used. The C:N ratio was determined by computing the amount of total non-structural carbohydrates (C) and nitrogen (N) content, respectively. Results and discussion All species germinated 8–10 days after seeded; no significant differences in germination timing were found among species, while final germination differed significantly among species, scarification and soil treatments (p(0)01). An interaction between scarification treatment and species was found in final germination (Fig. 1(a); p(0)01). No differences in seed germination were found among scarification methods for South-American species, except for P. chilensis in which chemical scarification reduced seed germination (Fig. 1(a); p(0)01). Arce & Balboa (1988) found no negative effect of sulfuric acid on germination of P. chilensis, but in this experiment radicle protrusion through the seed coat was used as germination criteria while in our experiment the seed was not considered germinated until cotyledons were erect. The thermal method decreased germination of the North-American species (97% in

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Figure 1. Influence of scarification method and soil-media on seed germination for five Prosopis species. Scarification methods were mechanical (MM, ), thermal (TM, ), and chemical (CM, ); soil-media were Prosopis soil (PS, ), fertilized Prosopis soil (PSN, ), and ). Germination is given as percentage. Horizontal bars indicate $1 nursery mix (NM, S.E.M.; n"50 seeds.

P. velutina and 92% in P. pubescens) compared to that found for the chemical method (Fig. 1(a)). Germination of seeds (all species) in NM was 57% higher than in PS. No significant differences were found in germination between PS and PSN for P. alba and P. chilensis but differences were appreciable in the three other species (Fig. 1(b); p(0)01), although no explanation was found for this pattern. Within the North-American species, the germination rate in PSN was lower than in PS (Fig. 1(b); p(0)01). Among the five species tested, P. alba had the highest rate of germination in every soil (Fig. 1(b)). In general, the seeds of most Prosopis species have high

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Figure 2. Influence of soil-media on the rate of leaf appearance (RLA) for five Prosopis species (a). Soil-media were Prosopis soil (PS, ), fertilized Prosopis soil (PSN, ), and nursery mix (NM, ) (b). The RLA for each species was calculated pooling individual RLAs of 50 plants per soil-medium for a total of 150 plants. The RLA for each soil-medium was calculated pooling individual RLAs of 50 plants per species for a total of 250 plants per type of soil-medium. Horizontal bars indicate $1 S.E.M. Species: P. alba ( ), P. chilensis ( ), P. flexuosa ( ), P. velutina ( ), P. pubescens ( ).

germination rates, after appropriate scarification and placed under controlled conditions (optimum temperature and high water potential; i.e. Villagra, 1995; Warrag, 1994; Manga & Sen, 1995). Similarly, in our experiment when commercial nursery mix was used, high germination rates were achieved. On the other side, our data suggests low germination rates for seeds in soil under natural stands, although a comparison with data on germination in natural environments is not possible, since that kind of data is rarely available. In PS most seed did imbibe, but since we considered germinated a seed with erect cotyledons, the reason for the low ‘germination’ rate should be explored in the phase between seed imbibition and cotyledon emergence, and is probably related to the physical properties of this soils (i.e. the formation of a superficial crust).

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Figure 3. Influence of soil-media on total plant biomass (dry weight) for five Prosopis species. Soil-media were Prosopis soil (PS, ), fertilized Prosopis soil (PSN, ), and nursery mix (NM ). Species were Prosopis alba (P. alba), P. chilensis (P.chil), P. flexuosa (P. flex), P. velutina (P. vel), and P. pubescens (P. pub). Horizontal bars indicate $1 S.E.M., n"10 per species and soil-medium.

Species significantly differed (p(0)01) in the rate of leaf appearance from leaf number 1 to 5. Prosopis pubescens showed the lowest rate (7)3 days/leaf ). From leaf 6 onwards, no significant differences in the rate of leaf appearance were found among species (Fig. 2(a)). This pattern resulted in a similar number of leaves appeared at the end of the experiment among species (Fig. 2(a)). Differences in leaf area duration were observed among species, P. pubescens retaining almost all emerged leaves and the rest of the species dropping several of the basal (older) leaves. It has been documented for Prosopis that moderate water stress results in leaf drop (Mooney et al., 1977), that leaf drop can occurs for variable periods of time and that species differ in their leaf-fall period (Balboa et al., 1988), although no explanation for these patterns has been given. Soil type affected the rate of leaf appearance up to leaf 7 (p(0)01; Fig. 2(b)). Plants in NM showed the highest rate (5)9 days/leaf), while those in PS and PSN did not differ and showed the lowest rate (6)2 and 6)9 days, respectively; Fig. 2(b)). Plant height, 60 days after seeded, differed among species and soils (p(0)01; data not shown). A significant interaction between soil and species was found (p(0)01). Plant height did not differ among soil types for P. flexuosa and P. velutina, while for other species, plants growing in NM were taller than those in PS or PSN. The addition of nutrients to PS increased plant height only in P. chilensis and P. alba. Under similar growth conditions Arce & Balboa (1988) reported that the addition of nitrogen to seedlings growing in N-free Hoagland solution produced a significant increase in plant height in P. chilensis but not in P. alba. Total plant biomass decreased between 36)0% and 58)5% on average, for plants growing in PSN and PS, with respect to those in NM (Fig. 3; p(0)01). Only in P. flexuosa a significant increase in total biomass was found with the addition of nutrients to PS (p(0)01). Similarly, Arce & Balboa (1988) reported that the addition of nitrogen

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Figure 4. Influence of soil-media on biomass allocation (dry weight basis) to leaves ( ), stems ( ) and roots ( ) for five Prosopis species. Soil-media were Prosopis soil (PS), fertilized Prosopis soil (PSN), and nursery mix (NM). Species were Prosopis alba (P. alba), P. chilensis (P.chil), P. flexuosa (P. flex), P. velutina (P. vel), and P. pubescence (P. pub). Horizontal bars indicate $1 S.E.M. n"10 per species and soil-medium.

to P. chilensis seedlings growing in N-free Hoagland solution did not cause changes in whole plant’s dry weight. All soil types averaged, P. pubescens accumulated significantly less biomass than the other species, while the highest values were found in P. alba and P. chilensis (p(0)01). No differences in the shoot:root ratio (data not shown) were found among species, although this ratio was significantly affected by soil type (p(0)01; Fig. 4). Plants in PS and PSN showed a lower shoot:root ratio than those in NM (1)25; 1)3 and 1)9 respectively). The lack of difference in shoot:root ratio between plants growing in Prosopis soil with or without the addition of nitrogen, and the highest ratio of those plants growing in nursetry mix would suggest that the different response in both growth media is related to water availablity or other factors but not to nitrogen availability. Prosopis pubescens invested more dry matter in leaves while P. velutina partitioned less of its biomass to leaves than the three South American species (42%, 22%, and 29–32%, respectively; p(0)01; Fig. 4). When all species were taken together an inverse relationship between the proportion of root biomass and plant total dry weight was found (r 2"0)54; Fig. 5). In general, soil fertility strongly affects foliage growth, and nitrogen fertilization and irrigation tend to increase the allocation of carbon to foliage. Greater allocation of carbon to foliage and stem growth makes a species a better competitor for light, whereas greater allocation of carbon to fine roots enhances the capacity to compete for nutrients and water (Landsberg & Gower, l997). These two strategies are incompatible and according to the optimal partitioning models (Bloom et al., 1985) shoot:root ratio would be near optimum in each particular set of

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Figure 5. Relationship between root biomass (expressed as percentage of the plant’s total biomass) and total biomass for five Prosopis species growing in three soil-media. Each point represents 10 replicates of each species grown in each soil media. Prosopis alba ( ), P. chilensis ( ), P. flexuosa ( ), P. velutina ( ), P. pubescens ( ).

environmental conditions. Our data for these Prosopis species show that the addition of nutrients to PS did not increase the proportion of biomass allocated to leaves or aboveground biomass, although total biomass significantly increased with the addition of nutrients (Fig. 3). CO2 uptake (A) under PPFD saturating conditions can be limited by nitrogen availability (Salisbury & Ross, 1985). Although in our experiment radiation availability was probably close to saturation (Vilela & Ravetta, 2000), CO2 uptake, when measured at noon, was significantly higher in plants growing in PSN than in PS or NM (Fig. 6) indicating photosynthesis limitation by nitrogen. No differences among species were found in A (data not shown) and the rates of A found were similar to those reported for these species (Ortiz et al., 1995; Vilela & Ravetta, 2000), but lower than those reported for P. glandulosa (Wan & Sosebee, 1990; Reynolds et al., 1999). Protein content in stems was between 40 and 50% of that found in leaves, for all species except for P. pubescens, in which the proportion was higher (Table 1; 81% and 64% of that of leaves for PS and PSN, respectively). Root protein content was about 35–50% of that found in leaves for all species except for P. alba, in which the proportion was lower (14–27%). Maximum protein contents were found in plants growing in PSN (Table 1). In contrast to our data, Arce and Balboa (1988) did not find significant changes in whole-plant N content in plants growing with the addition of nitrogen or with Rhizobium nitrogen fixation. In general, plant nitrogen content is highly associated with protein content, photosynthetic enzymes (mainly Rubisco) accounting for most of it (Salisbury & Ross, 1985). Thus, as found in our experiment, leaf green tissue has higher nitrogen contents than support and conduction tissues (Poorter & Villar, 1997). An exception to this pattern was found in P. pubescens, with green (photosynthetically active?) stems, containing

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Figure 6. Influence of soil media on net CO2 uptake of Prosopis. Each point represents three replicates for each of the five species and two dates (for a total of 30 measurements). Prospis soil ), fertilized Prosopis soil (PSN, ), nursery mix (NM, ). (PS,

nearly as much nitrogen as found in leaves. At the whole plant level, nitrogen contents in our study were in the higher limit of those reported in a review made by Poorter & Villar (1997), probably indicating higher nitrogen requirements for Prosopis species than for Table 1. Protein content of leaves, stems and roots for five Prosopis species (% on DW basis)

Species

Prosopis alba P. chilensis P.flexuosa P.velutina P.pubescens

Plant part

Leaves Stem Roots Leaves Stem Roots Leaves Stem Roots Leaves Stem Roots Leaves Stem Roots

Soil-type Prosopis soil

Prosopis soil#N

Nursery mix

18)7 8)4 2)6 19)2 9)2 6)9 19)3 8)8 7)7 20)2 8)9 7)2 17)7 14)3 8)3

24)5 11)8 6)3 22)2 11)5 8)9 23)2 10)6 8)7 24)4 10)9 8)2 21)6 13)8 11)3

20)4 7)7 5)5 21)3 7)7 8)5 19)0 9)9 8)0 24)0 10)3 8)8 22)7 11)5 11)3

Four plants harvested 61 days after seeded were pooled in each sample.

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Table 2. Non-structural carbohydrates content of shoots and roots for five Prosopis species (% on DW basis)

Species

Prosopis alba P.chilensis P.flexuosa P.velutina P.pubescens

Plant part

Soil-type Prosopis soil

Prosopis soil#N

Nursery mix

14)70 6)09 13)74 9)21 — — 16)34 8)10 12)55 7)00

8)11 8)58 9)69 9)74 7)06 6)37 7)85 5)04 — —

14)21 13)00 15)92 16)08 19)63 12)80 17)21 12)96 6)70 16)38

Shoot Roots Shoot Roots Shoot Roots Shoot Roots Shoot Roots

Four plants harvested 61 days after seeded were pooled in each sample.

a large range of woody species. The fact that Prosopis is capable of symbiotic nitrogen fixation supports this idea (Diagne, 1996; Johnson & Mayeux, 1990). Perennials have a large capacity for storage of carbohydrates which reduces their growth potential in the early vegetative stage but enables these plants to re-start growth early each season and to survive unfavorable conditions for CO2 assimilation or nutrient absorption (Lambers et al., 1998). In our experiment, whole-plant non-structural carbohydrate content decreased with the addition of nutrients to PS (Table 2). The greater reserve carbohydrate content in PS plants compared to that of PSN plants was probably due to an excess of available carbon caused by the reduction in biomass accumulation (greater than 30% in all species except for P. pubescens) while CO2 assimilation was maintained (only slightly lower at noon) in plants growing in PS (Figs 3 and 6). Also, the high shoot:root ratio (i.e. high investment on aboveground biomass instead of root biomass; Fig. 4) found in plants growing in NM with similar CO2 assimilation rates, probably contributed to the high reserve carbohydrates contents (Table 2) and biomass accumulation (Fig. 3) found in these plants. In general, our values for total non-structural carbohydrates in the roots were lower than those reported

Table 3. C:N ratio (whole plant) for five species of Prosopis

Species

Prosopis alba P. chilensis P. flexuosa P. velutina P. pubescens

Soil-type Prosopis soil

Prosopis soil#N

Nursery mix

1)4 1)5 — 1)6 1)1

0)8 0)9 0)7 0)7 —

1)6 1)6 1)9 1)6 0)9

C:N ratio estimated as TNC/Total N content.

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Figure 7. Influence of soil-media on plant survival (measured 60 days after germination) for five Prosopis species. Soil-media were Prosopis soil (PS, ), fertilized Prosopis soil (PSN, ), and nursery mix (NM, ). Species were Prosopis alba (P. alba), P. chilensis (P.chil), P. flexuosa (P. flex), P. velutina (P. vel), and P. pubescens (P. pub). Horizontal bars indicate $1 S.E.M., n"10 per species and soil-media.

for P. glandulosa even during the time of active growth, after bud-break (Wan & Sosebee, 1990). As seen with the reduction in total non-structural carbohydrates contents, the C:N ratio (non-structural carbohydrates:total nitrogen) decreased with the addition of nutrients to PS (Table 3). The lower C:N ratio found in fertilized plants could affect the chances of survival and regrowth after herbivory damage and the defense capability of plants. According to the carbon-nutrient balance hypothesis (Bryant et al., 1983) plants allocate carbon preferentially to growth when nutrients are available, and invest less carbon in defenses. In Prosopis, the increase in non-structural carbohydrates content should theoretically enhance the production of arabinogalactans exudates, common in this genus (Anderson & Farquhar, 1982), which are similar to those of another legume tree, Anacardium occidentale. In this species the exudates have been shown to inhibit growth of fungi and bacteria as part of a biochemical defense mechanism (Marques et al., 1992). Plant survival at 60 days, was affected by soil type and differed among species ( p(0)01), although a significant interaction between soil and species was found. NM was the soil type that resulted in the highest plant survival (95% or more) independently of plant species (Fig. 7). The addition of nutrients to PS increased plant survival in P. chilensis, P. flexuosa and P.velutina but not in P. alba and P. pubescens (p(0)01; Fig. 7). Survival of P. pubescens was higher than 80% in all soil-types (Fig. 7). Plant survival was positively related to total biomass (r 2"0)65) and root total nonstructural carbohydrates (r 2"0)39) and inversely related to shoot:root ratio (r 2"0)56) excluding P. pubescens from the analysis (data not shown). Prosopis pubescens allocated fewer resources to growth and proportionally greater allocation to storage, a strategy that could improve survivorship in a harsh environment (PS), like what has been found in plants growing in low-resources environments (Chapin, 1991). No significant relation-

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ship was found between N content, total non-structural carbohydrates or C:N with survival. To fully understand the biological significance of root reserve carbohydrates and C:N ratio on plants performance, longer term experiments should be undertaken.

Conclusions All scarification methods can be used to promote germination in the species evaluated in this study, except for the chemical treatment for P. chilensis and the thermal treatment for the North-American species. It is clear that nursery soil mix used should be preferred to the soil from underneath the canopy of Prosopis trees, a substrate widely used in propagating Prosopis species, since germination, plant size, and plant survival were higher in the former. Also, other important variables related to plant survivorship as nitrogen content and carbohydrate reserves which were significantly higher for all species when grown in nursery mix. An increase in biomass partition to above ground structures was an important mechanism that allowed for the superior response of plants in nursery mix. The addition of nutrients to this low-nitrogen substrate did not improve RLA or biomass, or change the shoot:root ratio, while nursery mix was clearly superior to Prosopis understory soil. The lack of response to nitrogen fertilization is common in arid-adapted shrubs and trees (Chapin 1991) and has been reported for P. chilensis (Arce & Balboa, 1988). This response limits the capacity of plants to acquire more photosynthates but improves survivorship in harsh environments (Chapin, 1991), although in our experiment survival was extremely low for plants growing in Prosopis soil ((5% for P. chilensis and P. flexuosa). On the other side, both reserve carbohydrates and C:N ratio were lower in fertilized plants, which should result in a reduction of carbon based defenses (like gum exudates) and in the chances for regrowth after herbivory damage. An exception to this general response pattern was found in P. flexuosa which accumulated less biomass than P. alba and P. chilensis when grown in Prosopis understory soil but showed a significant increase in plant biomass with the addition of nutrients to the growth media. The authors would like to thank Matthew Johnson, William Feldman and the Desert Legume Program of the University of Arizona for providing research facilities; Ken Coppola for greenhouse assistance and Steven P. McLaughlin for providing the ADC equipment. This work was funded in part by the International Foundation for Science, the Fulbright Program and Agencia Nacional de PromocioH n CientmH fica y TecnoloH gica (PID 009).

References Anderson, D.M.W. & Farquhar, J.G. (1982). Gum exudates from the genus Prosopis. The International Tree Crops Journal, 2: 15–24. Arce, P. & Balboa, O. (1988). Some aspects of the biology of Prosopis growing in Chile. In: Habit, M. & Saavedra, J. (Eds), The Current State of Knowledge of Prosopis juliflora, pp. 313–322. FAO. Rome. 460 pp. Balboa, O., Parraguez, J. & Arce, P. (1988). Phenology studies of Prosopis species growing in Chile. In: Habit, M. & Saavedra, J. (Eds), The Current State of Knowledge of Prosopis juliflora, pp. 259–267. FAO. Rome. 460 pp. Bazzaz, F.A. (1997). Allocation of resources in plants: state of the science and critical questions. In: Bazzaz, F.A. & Grace, J. (Eds), Plant Resource Allocation, pp. 1–37. San Diego: Academic Press. 303 pp. Bloom, A.J., Chapin III, F.S. & Mooney, H.A. (1985). Resource limitation in plants—an economic analogy. Annual Review of Ecology and Systematics, 16: 363–392.

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