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Biological Sciences 作業代寫

    Growth responses to nitrogen in three species

    Introduction

    Biological Sciences 作業代寫
    Growth responses to nutrients vary greatly in different plant species, which may lead to species distribution by fertility gradients (Taub, 2002). For example, relative growth rates (RGR) of different plant species increased in the high-nutrient treatment by varying coefficient of variance; species with higher RGR in fertile conditions had their RGR most depressed in the low-nutrient treatment (Shipley & Keddy, 1988; Meziane & Shipley, 1999). Growth responses to nitrogen are also different among different plant species. When low nitrogen is available, C4 species, including maize, had higher radiation use efficiency than C3 species (Sinclair & Vadez, 2002). Despite the significance of interspecific variations in ecology, there are relatively few studies exploring the variation between species in growth responses to nutrients.
    Nitrogen is an important nutrient content, which is the composition of protein and structural element of nucleic acids, and plays a vital role in enzymatic activity (Marschner, 2011). Therefore, plants require more nitrogen than other nutrients, but nitrogen needs more energy to obtain and utilize (Gutschick, 1981). Legumes can be regarded as biological nitrogen fixers owing to its symbiotic bacteria. Legumes generate high N-content soil, so it seems that legumes are able to increase crop production in low N-content environments (Sinclair & Vadez, 2002). However, despite biological N fixation, plant growth in most non-tropical ecosystems is limited to N content (Vitousek & Howarth, 1991). Therefore, advantages of legumes on N-deficit soils may be not as huge as past thought.
    Methods, like Relative Growth Rates (RGR), Net Assimilation Rate (NAR), Leaf Area Ratio (LAR) and Specific Leaf Area (SLA), are used to analyze the growth condition of plants. RGR means the increase in plant mass per weight per time, using to analyze the inherent variation in growth rate. NAR variation depends most on variation in the photosynthesis rate (Konings, 1989). SLA is leaf area per unit leaf weight, and LAR means leaf area per unit plant weight (Poorter & Van der Werf, 1998).
    Lack of studies on interspecific variation in growth responses to nitrogen, this research aims to study the variation among corn (Zea mays), barley (Hordeum vulgare) and field peas (Pisum sativum) under different N-content environments. Shoot growth, leaf area, root growth, RGR, NAR, LAR and SLA are calculated to measure the plant growth at two levels of nitrogen availability.

    Materials and Methods

    Growth of the plants.
    Biological Sciences 作業代寫
    This study used corn (Zea mays cv. SR73), barley (Hordeum vulgare cv. Schooner) and field peas (Pisum sativum) as subjects. Seeds were sown on the 2nd March 2012 into trays with seed raising mix (Debco). Seedlings were selected to keep similar size and developmental stage. They then were sown into pots of soil-sand (1: 4) mix, and grew in the Glasshouse Complex. After 12th March, plants were watered three times a week with nutrition solution. A modified Hoagland solution was used containing 16 mM NO3- and 8 mM NH4+ (High N) or no NO3- or NH4+ (Low N).

    On 21st March, first leaf of several corn plants became yellow, which demonstrated nitrogen deficiency. Nutrient solution was applied double in amount. On 23rd March, most corn plants showed the symptoms of nitrogen deficiency. Then the nutrient solution was changed to Aquasol® (Hortico), which contained 0.3 mM MAP, 1.5 mM KNO3 and 5.3 mM Urea. Plants were watered with double-strength Aquasol® for the following experiment.

    Growth analysis

    For each nutrient treatment and species, there were three replicate trays containing 5 pots each (15 in total). On 12th and 13th March, initial measurement was carried out. After 21 days, final measurement was carried out. Leaf area, leaf dry weight, leaf number, root dry weight, shoot dry weight, shoot height, stem dry weight and tiller number were recorded. Total plant dry weight was calculated by the sum of leaf, stem and root dry weight. Dry matter partitioning to roots was the quotient of root dry weight and total plant dry weight. SLA was equal to the division of leaf area and leaf dry weight, while LAR was equal to the division of leaf area and total plant dry weight. Total plant dry weight was calculated at both initial measurement (time: t1; weight: W1) and final measurement (time: t1; weight: W1), where t2-t1 was 21 days. RGR is calculated by the following formula:

    NAR is calculated by the formula:

    Results

    Biological Sciences 作業代寫

    Figure 1. Shoot dry weight of corn, barley and field pea under different N treatments. Error bars equal standard error (n = 15).
    Significant differences existed in the result of dry weight of corn and barley shoot between different N treatments (P < 0.01). With N treatments, weight of dry matter increased 89.5% and 75.0% relative to non-N treatments in corn and barley, respectively. Though without N treatments, the shoot dry matter of pea was lower than that with N treatments, there was no statistically difference (P > 0.05). In addition, tiller numbers for barley were low.

    Figure 2. Leaf area in corn, barley and field pea under different N treatments. Error bars equal standard error (n = 15).
    The leaf area of corn and barley with N treatments was 72.9% and 75.1% larger than that without N treatments, respectively. Significant differences existed (P < 0.01). There is no difference between leaf area of pea under different N-conditions (P > 0.05).

    Figure 3. Dry matter partitioning to roots of corn, barley and field pea under different N treatments. Error bars equal standard error (n = 15).
    Significant differences existed in all of the three plants about dry matter partitioning to roots (corn and barley, P < 0.01; pea, 0.01 < P < 0.05). Corn, barley and field pea reduced the dry matter partitioning to roots if N is abundant by 37.3%, 19.6% and 11.9% respectively. In addition, very few nodules were observed on the roots of the field peas.

    Figure 4. Relative growth rates of corn, barley and field pea under different N treatments. Error bars equal standard error (n = 15).
    RGR of corn and barley were significantly increased with N treatment by 29.1% and 30.6% relative to non-nitrogen treatments, respectively (P < 0.01). RGR of field pea was independent of nitrogen statistically (P > 0.05).

    Figure 5. Leaf area ratio of corn, barley and field under different N treatments. Error bars equal standard error (n=15).
    LAR of corn with N treatments was significantly different from that without N treatments (P < 0.01), which reduced by 9.2%. There were no significant differences in barley and field pea between different N treatments (P > 0.05).

    Figure 6. Net assimilation rate of corn, barley and field pea under different N treatments. Error bars equal standard error (n=15).
    NAR of corn and barley with N treatments significantly increased by 42.7% and 27.7% relative to non-nitrogen treatments (P < 0.01). No difference existed in the NAR of field pea (P > 0.05).

    Figure 7. Specific leaf area of corn, barley and field pea under different N treatments. Error bars equal standard error (n=15).
    There was significant difference in SLA of corn between two treatments of corn with an increase ratio of 9.8% under N treatments (P < 0.01). SLA of barley and pea did not vary with different N treatments (P > 0.05).

    Discussion

    Effects of nitrogen on shoot and root growth

    Our results showed that nitrogen did not influence the shoot and leaf growth of field pea and the influence on root growth was less than that of corn or barley. Without N treatments, shoot and leaf growth of corn and barley were depressed remarkably, while root growth was encouraged.
    Field pea is able to drive N required from gaseous nitrogen, which is fixed by nodules. Thus, it requires less nitrogen than other species, like crop or barley. Low concentration of NO3- or NH4+ in the soil will influence the pea less than other plants. This is why there were differences between growth responses of pea and other plants. However, the root growth of pea was encouraged by the low N concentration as well. This demonstrated that pea was also influenced by the concentration of nitrogen. Two reasons may explain the results. First, some field experiments showed that nitrogenous fertilizers increased the growth of pea plants (Mulder, 1948). Actually, fertilizer N has a negative impact on nodulation and nitrogen fixation (Chen & Phillips, 1977), while small doses of nitrogenous fertilizers are beneficial to growth of field pea (Mahon & Child, 1979). Therefore, without N treatments and with high N treatments, the growth of pea was both depressed to some extent. Second, we only observed very few nodules on the roots of the field peas in this research, which led to the need for nitrogen absorption from the soil. Nitrogen fixation of pea depends on the nodules. In the future study, pea seeds can be applied with cultures of Rhizobium (Clayton et al., 2004), which may prevent the shortage of nodules.
    The shoot and root growth of corn and barley were both related to the N concentration. Under N deficiency, construction of nucleic acids and enzymes is limited, leading to an especially huge influence on shoot and leaf cell development. The leaf area was reduced without N treatments in this study in accordance with previous studies. For example, corn and sorghum dry matter of leaves decreases under a deficit of nitrogen (Muchow, 1988). Trying to absorb sufficient nitrogen, plants grow their root when nitrogen is lacking, causing the more dry matter partitioning to roots. Specifically to barley, the number of tillers, which were depended on by barley growth, was relatively small in quantity under both low and high N treatments. Total tiller number can be increased by nitrogen (Birch & Long, 1990). The few tillers showed the lack of nitrogen to some extent. In our studies, it happened that most of corn plants under high N treatments showed yellowing of first leaf, which was the symptom of nitrogen deficiency. It demonstrated that plants under high N treatments might still lack nitrogen. The shortage of nitrogen may lead to the low number of tillers in barley, and underestimation of growth of corn and barley in high N treatments.
    The above results showed the ability of plants to change their phenotype including the dry matter in shoot and root, and leaf area. To dictate plant growth fitness, weight of shoot dry matter to weight of root dry matter ratio (S: R) is regarded as a major factor (Iwasa & Roughgarden, 1984). Optimal S: R is related to the nitrogen availability. In this study, shoot dry matter of pea independent and root dry matter dependent on N concentration illustrated that S: R would increase with N treatments, which is the same as previous report that there was a positive correlation between S: R and the availability of nitrogen (Andrews, Sprent, Raven & Eady, 1999). Furthermore, S: R of corn and barley increased more with N treatments consistent with the theory.

    Effects of nitrogen on growth components

    RGR, NAR, LAR and SLA of field pea with N treatments were the same as those without N treatments. RGR and LAR of corn and barley with N treatments increased, while NAR and SLA reduced, except for no differences in LAR and SLA of barley.
    As discussed above, nitrogen affects growth responses of field pea little, for it can fix nitrogen by nodules. Growth components of corn and barley, however, are changed by N concentration.
    RGR is product of NAR and LAR: RGR = NAR  LAR. Studies revealed that plants could change their phenotype to maximize RGR (Hilbert, 1990). Nevertheless, lack of nitrogen RGR still reduces. It was reported that plants with a higher RGR when N concentration was high would decrease greatly in RGR when N concentration was low (Shipley & Keddy, 1988; Meziane & Shipley, 1999), but our results refuted it. RGR of corn and barley both increased with N treatment by approximately 30% relative to non-nitrogen treatments, though RGR of corn was significantly higher than that of barley (P < 0.01). The possible reason may be that corn and barley have both lower RGR. In the study of Meziane & Shipley (1999), RGRs of 22 herbaceous species with high N treatments were between 0.18 and 0.33. However, with high nitrogen RGR of corn was 0.133 and RGR of barley was 0.111 in our study. They were significantly lower. On the one hand, shortage of nitrogen caused by the error during the experiment may reduce the RGR while growing under the high N treatments. On the other hand, corn and barley may be species with lower RGR comparing with other species, which could explain why RGR of corn and barley increased by the similar percentage with high nitrogen concentration.
    Increasing nitrogen availability can increase the LAR (Fichtner & Schulze, 1992). As discussed above, low nitrogen concentration depressed the shoot growth and leaf area but encouraged root growth. It meant that more biomass was allocated to leaves, and consequently LAR increased. The result of LAR of corn in our study corresponded with previous study and theory. However, LAR of barley did not change with the varying N availability. LAR is the quotient of leaf weight ratio (LWR) and specific leaf weight (SLW). LWR increased with N-supply (Fichtner & Schulze, 1992). But SLW is influenced by increased photosynthetically active compounds (Sage & Pearcy, 1987) and reduced starch levels (Stulen, 1981). SLW in some plants is not influenced by nitrogen (Fichtner & Schulze, 1992), which with increased LWR causes increased LAR. However, SLW of barley may increase under the high nitrogen. Photosynthetically active compounds increased in this study because NAR increased, which will be discussed in the following. Therefore, starch levels of barley probably reduce less or do not reduce under high N treatments. The variation between corn and barley in the LAR needs further research. And also, LAR = SLA  LMR (leaf mass ratio). SLA of barley was also independent of N concentration, which may contribute to the unchanged LAR.
    It was reported that the effect of nitrogen NAR was low (Fichtner & Schulze, 1992), which was different from our results in which NAR of corn and barley with N treatments significantly increased by 42.7% and 27.7%. Actually, NAR was the best predictor of variation in RGR (Shipley, 2006). Konings (1989) stated that NAR variation depended most on variation in the photosynthesis rate. In addition, increasing N concentration causes greater investment in photosynthetically active compounds (Sage & Pearcy, 1987). Therefore, high nitrogen treatments should increase the NAR of plants, just as our results.
    SLA has an important impact on plant’s allocation and growth rate. Species with high SLA are fast growing species, implying thin leaves or leaves with less biomass (Poorter, Remkes & Lambers, 1990). This indicates that pea and corn grow faster than barley, whose SLA is lower. Poorter and Remkes (1990) and Westoby (1998) presented that SLA was strongly correlated to RGR. Our result disagreed with this. RGR of corn and barley both increased with increased N concentration, but SLA of barley was independent of N concentration. It seemed that there were problems with the result of barley SLA in this study, which should decrease with increased nitrogen. Therefore, it is unclear that whether the different variance of SLA between corn and barley is due to interspecific variances or due to other unknown factors during the experiment. Further researches are needed to make it certain.
    309-336.

    Appendix      

    Biological Sciences 作業代寫

    Table 1. Shoot dry weight of corn, barley and field pea under different N treatments.

    Table 2. Leaf area in corn, barley and field pea under different N treatments.

    Table 3. Dry matter partitioning to roots of corn, barley and field pea under different N treatments.

    Table 4. Relative growth rates of corn, barley and field pea under different N treatments.

    Table 5. Leaf area ratio of corn, barley and field under different N treatments.

    Table 6. Net assimilation rate of corn, barley and field pea under different N treatments.

    Table 7. Specific leaf area of corn, barley and field pea under different N treatments.

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