Nitrogen Uptake before Bud Breaks Influence on Nitrogen and Non Structural Carbon Reserve

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Photosynthesis and Carbon Storage

Plants are photosynthetic organism which use light energy to produce nutrients (i.e. carbohydrate). Such nutrient can be used at a later time to supply the energy needs of the cell. Photosynthetic carbon assimilation is the primary transducer of energy for growth and reproduction. The carbon metabolism of plant is regulated in order to maintain these different function and to sustain plant growth, development and metabolism in tissues. Moreover, environments factor can severely impact of photosynthetic capacity of leaf (Lawlor, 2009) such as temperature (Ow et al., 2010), light (Gansert and Sprick, 1998), drought (Regier et al., 2009), atmospheric CO2 (Curtis et al., 2000), and available nitrogen (Ripullone et al., 2003). For example, soil nitrogen availability limits Populus tremuloides photosynthesis under elevated CO2 environment, bud not ambient one (CO2 373 2 ml l-1) (Kubiske et al., 1997).
Products of photosynthetic carbon assimilation are allocated to storage tissues or growth and development tissues from “source” sites to “sink” sites by phloem and xylem. The assimilate fluxes from sources to sinks are mainly dependent on the source-sink distances and on the respective abilities of the different sinks to take up and the assimilation that are available to them (Lacointe, 2000). For example, Mayrhofer et al. (2004) calculated daily carbon balance of mature poplar leaves (Populus canescens). They found that young poplar uses carbon assimilation of mature leaver photosynthesis by isoprene emission and dark respiration amount of 1% (percent of total carbon delivered to a leaf per day) and 20%, respectively. All carbon mobilization is loaded into the phloem, accounting for 28% of carbon exported from poplar leaves. Moreover, the transport of carbon within northern red oak seedling during a second flushing show that more than 90% of labeled carbon (14C) assimilation translocated from first flush leaves was directed upward to developing second flush leaves and stem, while about 5% was found in lower stem and roots. When second leaves were fully expend, only about 5% of the 14C exported from first flush leaves was translocated upward, while 95% was translocated downward to lower stem and roots (Dickson, 1989).
In addition to such results, Dyckman et al. (2000) found that the main carbon sink in the first six weeks of beech (Fagus sylvatica L.) after bud break were the leaves, representing 56% of the new assimilation. Carbon assimilation was translocated to belowground about 12.4% of the new assimilation. The carbon assimilation reached its maximum from 7 to 12 weeks after bud break when leaves apparatus was established. Leaves accumulate 12% of new carbon assimilation, whereas increase translocation to belowground carbon was about 55%. From 13 to 18 weeks after bud break, the belowground dominance of assimilation transport, may indicate that the carbon accumulated towards the end of growth period is prepared for the next year’s leafing (Dyckmans et al., 2000). Therefore, the net photosynthesis assimilation transalocated to tissues would depend on tissue demand for growth and development or life cycle of plant.
Carbon is stored in many forms, but starch is the main source of carbon storage in plant. Carbon is also accumulated in the wood and bark in both root and stem via the photosynthesis between summer and autumn. Basically, carbon is allocated to storage reserves plant survival, when the current level of carbohydrates produced by photosynthesis is not enough to meet the carbohydrates demand for maintenance, growth or metabolism. Especially, the storage carbohydrate is maintenance respiration in the winter and builds leaves in the spring (Barbaroux et al., 2003; Landhäusser and Lieffers, 2003; Landhäusser and Lieffers, 2002; Rowe et al., 2002; Wong et al., 2003). Carbon stored in the root system is considered to be very important for regeneration and growth of Pinus taeda,and Populus tremuloides (Landhäusser and Lieffers, 2003; Landhäusser and Lieffers, 2002; Ludovici et al., 2002). The mobilization and utilization of stored carbon implies the hydrolysis of starch and the synthesis of sugar such as sucrose, glucose, fructose and raffinose (Sauter and Vancleve, 1994; Witt and Sauter, 1994; Wong et al., 2003).
Cycles of carbon remobilization and utilization change according to seasons. The seasonal patterns of production, accumulation, and utilization of non structural carbohydrates (NSC) of deciduous trees are closely correlated to phenological events and (or) physiological processes. Wong et al. (2003) studies on maple tree show that starch is the major storage reserve carbohydrate in sugar maple, (Acer saccharum Marsh.). Starch is accumulated in the xylem ray tissues in late summer and early fall. During the cold season, there is a close relationship between starch hydrolysis-accumulation and temperature. Starch concentration was decreased during the cold months with soluble sugars (sucrose, glucose, fructose, xylose, raffinose, and stachyose) was increased. Thus, these sugars were synthesis may play a role in cold tolerance. At the end of dormancy and the dehardening process, the levels of soluble sugars decline as starch level increases, prior to carbon mobilization for primary growth activities (Wong et al., 2003).
To confirm the role of season on the concentration of starch and sugar, Landhäusser and Lieffers (2003) undertook research on poplar trees. They determined that starch and sugar concentration in roots, crown and stem of Populus tremuloides also depended on season. Starch concentration in roots, crown and stem tissues remained high during late summer and autumn, dropped in the winter months. Starch content of roots and crown remained low during bud flush. Soluble sugar concentration in roots increased at late fall and remained relatively constant to the week before bud flush. Concentration of soluble sugars in the crown remained relatively constant during late summer and winter until before bud flush. Upon bud flush, soluble sugar concentration in the large branches decreased, while the soluble sugar concentration of the current shoots increased. Soluble sugars in the stem tissue increased during leaf abscission while sugar concentration increased in the phloem/bark and dropped in xylem during bud flush. Therefore, carbon storage can remobilize to allow respiration, growth and development. In addition to the seasonal changes affecting carbon storage, it also depends on cropping management in growth period. Some studies in apple and peach found that limiting available nitrogen during summer or autumn before leaves falling increased NSC in the next spring (Cheng et al., 2004; Cheng and Fuchigami, 2002; Jordan et al., 2011).

Carbon Remobilizes to Bud Break

Carbon is very important for bud break, during winter and spring. After come out of endodormancy to bud break period which bud is sink strength, plants need to use the large amount of carbon to have a high capacity to synthesize ATP, which is involved in many metabolic pathways. Bud break requires NSC supply for metabolic reactivation and leaf primordial growth. Local reserves could closely relate to local within the bud itself or in the neighboring tissues of the stem (Bonhomme et al., 2009; Maurel et al., 2004). If the NSC used for bud break is stored far away in the stem such as root, they have to be transported from the source tissue to the bud by xylem. Thus, xylem sap has been proposed as the principal route for soluble carbohydrate to be transported from exporting tissue to buds. The transport of NSC in xylem sap was studied in various plants. Decourteix et al. (2008) found that at the beginning of bud break in walnut (Juglans regia L. cv. ‘Franquette’), a higher xylem sap sucrose concentration and a higher active sucrose uptake by xylem parenchyma cells were found in the apical portion (bearing buds able to burst) than in the basal portion (bearing buds unable to burst) (Decourteix et al., 2008). Bonhomme et al. (2009) propose that close to bud break in walnut, buds were able to import high sugar quantities from the xylem vessels. The flow rates between xylem vessels and bud increased 1 month before bud break and reached 2000 μg sucrose h-1gDW-1. Maurel et al. (2004) accepted that hexoses are of greater importance than sorbitol or sucrose in the early events of bud break in peach. Therefore, the carbohydrate supplies of bud will depend on in xylem transport.
For poplar, Populus tremuloides, the transport of reserve carbohydrate was found in the study of Landhäusser and Lieffers (2003). They suggest that the major source of carbohydrate storage allowing bud break comes from the stem and tree crown. As stated earlier, before leaf flush the decreased of sugar concentrations in the phloem/bark tissues of the stem coincided with a build up of the starch reserves in the large branches of the crown. Thus, the actual total NSC reserves in the stem dropped at that time. During bud flush, the total NSC suddenly decline in the crown tissues. This decrease in starch and sugar content in the large branches, in combination with the significant increases of sugar concentrations in the current shoots and breaking buds at the flush, suggests a mobilization and transport of NSC reserves towards the sprouting buds and unfolding leaves. This large decline in total NSC concentrations in the branches was accompanied by a smaller and not significant decrease in the roots, in combination with a slight but significant increase in the sugar concentrations in the xylem at the time of leaf flush (Landhäusser and Lieffers, 2003). Accordingly, the decrease of carbon storage of the pervious year relates to the decrease of leaf area and re-growth biomass (Landhäusser and Lieffers, 2002). However, Cheng and Fuchigami (2002)’s study on apple revealed different result. It is there postulated that young plants with low carbohydrate reserves and high nitrogen reserves produced a larger total leaf area at the end of the re-growth period than plants with high carbohydrate reserves but low nitrogen reserves. In their study, rather than carbohydrate reserves, nitrogen is another nutrient playing a key role on plant re-growth. Therefore, similar to the carbon storage, nitrogen storage during bud break period could be another factor impacting plant re-growth.

Nitrogen

Nitrogen (N) is the main mineral element in plant tissues and almost the entire amount is acquired from soil by roots. The N availability commonly limits plant productivity (Finzi et al., 2007) by supporting growth process and increasing quality and quantity of new shoot. In this regard, N uptake during deciduous trees growth has two sources: (i) uptake of external sources such as NO3- and NH4+; and (ii) remobilization of internal reserves (Millard et al., 2006; Miller and Cramer, 2004; Tagliavini et al., 1997).

Nitrate Uptake

Generally, nitrate is actively transported across the plasma membranes of epidermal and cortical root cells, but net uptake is the balance between active influx and passive efflux. This transportation requires energy input from the cell over almost the whole concentrations range encountered in the soil (Glass et al., 1992 ; Glass et al., 2002; Miller and Smith, 1996; Zhen et al., 1991). It is accepted that NO3- uptake is coupled with the movement of two protons down an electrochemical potential gradient, and is therefore dependent on ATP supply to the H+-ATPase that maintains the H+ gradient across the plasma membrane (Crawford and Glass, 1998; Forde, 2000; Miller and Cramer, 2005; Miller and Smith, 1996). It is commonly accepted that roots operates nitrate uptake by three types of transport systems, to manage with the different external NO3- concentrations: two high affinity transport system (HATS) are about to take up NO3- at low external concentration. The constitutive system (cHATS) is available even when plants have not been previously supplied with NO3-. The inducible system (iHATS) is stimulated by exogenous NO3-. A low affinity transport system (LATS) is most important at high external NO3- concentration (Chen et al., 2008; Miller and Cramer, 2005; Miller et al., 2007; Rennenberg et al., 2010).
After nitrate is uptake into the cell, nitrate has four fates: (1) reduction to NO2- by the cytoplasmic enzyme nitrate reductase (which enters the plastid and is reduced to ammonia and then incorporated into amino acid); (2) efflux back across the plasma membrane to the apoplasm; (3) influx and storage into the vacuole; and (4) long-distance translocation to the leaves by entering into xylem vessels (Fig. 2.5). Following long-distance translocation, NO3-must leave the xylem and enter the leaf apoplasm to reach leaf mesophyll cells, where NO3- is again absorbed and either reduced to NO2- or stored into the vacuole (Crawford, 1995; Crawford and Glass, 1998).

Control of Nitrogen Uptake

Nitrogen is available in many different forms in the soil; the three most abundant forms are nitrate, ammonium, and amino acids. The relative importance of these different soil N pools is difficult to measure and depends on many different environmental factors (Miller and Cramer, 2004). Basically, N acquisition from soil depends on both external and internal factors. External factors are salinity (Dluzniewska et al., 2007; Ehlting et al., 2007), heat, drought, flooding (Rennenberg et al., 2009) and the atmospheric CO2. In addition, elevated CO2 concentration in the atmosphere is reported to increase N uptake in loblolly pine (Pinus tadea), sweetgum (Liguidambar styraciflua) and poplar (Populus tremuloides, P. nigra) (Curtis et al., 2000; Finzi et al., 2007; Luo et al., 2006; Luo et al., 2008). Dong et al. (2001) reported that apple tree has no N uptake before bud break when growing in soil at 8°C, whereas N uptake enhances with increasing soil temperature, between 12°C and 20°C. European beech (Fagus sylvatica) and Norway spruce (Picea abies) uptake nitrate at the soil temperatures of 10°C – 15°C, amounted 16% and 11%, respectively, of maximum uptake at 25°C. By contrast, net uptake of ammonium at 10°C reached 73% and 31% of the maximum uptake for spruce and beech trees, respectively (Gessler et al., 1998). Poplar uses NO3- and NH4+ according to soil pH : NH4+ uptake favoured at high soil pH and NO3- uptake favoured at low soil pH (DesRochers et al., 2003; DesRochers et al., 2007). Other reports showed that differential N availability modulated N pools in plant tissues (Cooke et al., 2005; Nicodemus et al., 2008; Otto et al., 2007; Rowe et al., 2002). However, external factors have to be sent and converted to shoot-to-root signals that are part of a signalling cascade. Such signalling cascade ultimately controls N acquisition by roots under changing developmental and environmental conditions.
Concerning internal factors, plant roots are regulated to adapt to current N demand for growth and development, leaf and root senescence, as well as the N storage requirement of the whole plant (Kunkle et al., 2009; Millard, 1996; Ozbucak et al., 2008; Silla and Escudero, 2003; Tian et al., 2005). Net NO3- uptake is regulated by whole plant demand and concentration of N metabolites, including NO3- in the tissue (Vidmar et al., 2000). Kirkby and Armstrong’s (1980) study provided direct evidence that root NO3- uptake is regulated by NO3- reduction in the leaf. Touraine et al. (1992) showed that N uptake increased when NO3- supply of shoots increased, and decreased when the nitrate reductase activity in shoots was inhibited by tungstaten (WO42-). In addition, internal signals communicating the N status of the plant are of importance as they coordinate root N uptake with the actual N demand of the whole plant (Imsande and Touraine, 1994). Reduced N compounds, which are able to cycle between shoot and root via xylem and phloem transport, can signal the N demand of the shoot to the roots and/or exert direct feedback regulation on N uptake in the roots (Grassi et al., 2003). An exogenous supply of particular amino acids results in a significant decrease in NO3- uptake in different species and glutamine seems to play a dominant role in this process (Miller et al., 2007). Exogenous supply of glutamine feeding also increases various amino acids (Gln,Glu, Ala and GABA) and NH4+ content in poplar roots, which are negatively correlated to NO3-uptake. Including trans-zeatin riboside (tZR), an active form of cytokin, in the nutrient solution reduces NO3- uptake in poplar saplings (Dluzniewska et al., 2006).

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Nitrogen Storage

Nitrogen storage in plant is crucial to support plant growth in the early growth after bud break, when roots N uptake conditions are sub-optimal (Millard, 1996). The N storage correlates to available N supply during growth season. The N availability increase in summer, as well as foliar urea application in autumn, can be used to build up N storage in young apple and poplar. In addition, they also are often used to increase the development of new growth in spring (Cheng et al., 2004; Cheng and Fuchigami, 2002; Dong et al., 2002; Dong et al., 2004).
Generally, the sites of N storage by trees are restricted to specific organs and depend on leaf habit (Table 1). Deciduous species tend to store N in the wood and bark of root or trunk (Millard and Grelet, 2010). Nitrogen is stored in plant as proteins and amino acids (Rennenberg et al., 2010). For example, poplar trees initiated vegetative storage proteins (VSPs) accumulation in new shoots during new shoots development in spring, under high temperature and long days conditions (Tian et al., 2005). VSPs accumulate in wood, bark and roots (Langheinrich, 1993) and the major VSPs in Populus are bark storage proteins (BSPs) (reviewed by Cooke and Weih, 2005). N allocation to storage is programmed seasonally and is, therefore, intimately linked to tree phenology (Millard and Grelet, 2010). Later in the season, the growth rate declines. In the fall, the growth of deciduous trees stops as day length and temperatures decrease, and the trees drop their leaves. Leaf proteins’ N is transferred to bark storage proteins, stored over the winter, and then remobilized and used for growth in the next spring (Black et al., 2001; Cooke and Weih, 2005). The absorption of N from senescing leaves varied from 40% in Quercus suber L.(Orgeas et al., 2003) to 80% in Populus tremula (Keskitalo et al., 2005). In addition, leaf N remobilization depends on plant age. Yuan and Chen (2010) showed that Populus tremuloides Michx. in boreal forest, standing of different ages about 7, 25, 85, and 139 years, leaf N remobilization is about 68.5%, 65.6%, 63.1%, and 58.4% of dry weight of leaf, respectively.

Nitrogen Remobilization

Bark storage proteins (BSP) degradation occurred considerably like temperature and day length of the situation spring condition before bud burst (Coleman et al., 1993; Langheinrich, 1993). BSP are hydrolysed and amino acids are then translocated into flushing bud and leaves and used for de novo protein synthesis (Cooke and Weih, 2005). Several studies have shown that a peak of nitrogenous compounds and coupling sap flow velocity in the xylem sap have been attributed to the remobilized N transport during bud break (Frak et al., 2002; Grassi et al., 2003; Grassi et al., 2002; Malaguti et al., 2001). The peak of N concentration in xylem sap during bud break and leaf growth was attributed to N remobilization. The majority of nitrogenous compounds remobilization in xylem depends on plant species. In particular, above 90% of N compounds of the xylem spring sap of apple (Malus domestica Borks.) was recovered from amino acid, asparagine (Asn), aspartic (Asp) (Geisler-Lee et al.) glutamine (Gln) and glutamic (Glu) (Malaguti et al., 2001). In walnut, arginin (Arg) , citrulline (Cit) , Glu and Asp always represented around 80% of total amino acid and amide N in xylem of walnut (Juglans nigra × J. regia) (Frak et al., 2002). Gln and Asn are the major N compounds translocated by xylem in cherry (Prunus avium L.) (Millard et al., 2006). In poplar, Millard et al. (2006) reported Gln as an important N compound translocated by xylem in poplar (Populus trichocharpa × P. balsamifera). Nitrogen mobilization from storage to new tissue depends on N uptake duration the previous year. The amount of N remobilized from reserve to recovered in new growth of nectarine (Prunus persica var. nectarine) accounted 42% and 49% of total N uptake in early (May to mid July) and late (mid August to the beginning of October) previous season, respectively (Tagliavini et al., 1999). In Ligustrum ovalifolium remobilizs about 55% of N assimilation in previous spring and raises to 68% of N assimilation during the previous autumn (Salaün et al., 2005). Therefore, the recent N uptake in previous season is more efficiently used for support re-growth than that absorbed N of the earlier previous season.
As stated for early re-growth, there are two main N sources to sustain growth: remobilization of stored N and N uptake. Nitrogen mobilized from storage tissues to sustain spring growth was reported to account for 15% of total N found in new shoots in beech (Fagus sylvatica L.) (Dyckmans and Flessa, 2001), 46% in pear (Pyrus communis L.) (Tagliavini et al., 1997) and up to 54% in walnut (Juglans nigra × J. regia) (Frak et al., 2002). Nitrogen from remobilization was recovered in the growing leaves before any root uptake of N occurs, 7-18 days in cherry and 36 days in poplar (Grassi et al. 2002; Millard et al. 2006), while Pinus sylvestris L. and Betula pendula Roth. were clipped, root uptake contributed N for leaf growth immediately after bud break, concurrently with N remobilization (Millard et al., 2001).
Nitrogen remobilization for new spring growth also depends on the current N supply and the amount of stored N remobilized. When no N was provided during re-growth, then all utilized N for support new growth was sustained by N remobilization. When adequate N was supplied during re-growth, N remobilization in Ligustrum ovalifolium was 15% lower than in unfertilized plants (Salaün et al., 2005). In apple, when N was supplied during re-growth, N remobilization provided about 43% of the total N in trees with low N status before bud break. This N remobilization increased to 85–90% in trees with medium to high N status before bud break (Cheng and Fuchigami, 2002). Similar pattern has been found in beech (Fagus sylvatica L.) (Dyckmans and Flessa, 2001) and cherry (Prunus avium L.) (Grassi et al., 2003). Therefore, N storage remobilization is the main N support to new growth and depends on quality of N storage. Additionally, the current N supply in spring affects the N status of the new growth.

Nitrogen Impact Plant Growth and Development

Nitrogen availability is necessary as it affects most physiological process of plant development. Increasing in the level of N nutrient increases shoot dry weight and number of bud set per plant. Furthermore, altered inorganic, amino acid and glucose in the xylem sap are also affected by N increasing (Grassi et al., 2002). Number of studies undertakes experiments on the effect of N availability on plants during growth period (late spring and summer). For examples; poplars (Populus spp.) include fast growth response to fertilization, especially N availability (van den Driessche et al., 2008; Yin et al., 2009); and the changes of N availability in poplars has an effect on several processes of plant growth such as light-saturated net photosynthesis, water-use efficiency and leaf area. These results change whole-plant architecture, secondary xylem formation, and carbon accumulation (Coleman et al., 2004; Cooke et al., 2005; Pitre et al., 2007; Ripullone et al., 2003; Ripullone et al., 2004). Therefore, nitrate may play a role in plant growth and development.
The stored N mobilization and supply during re-growth are used to increase the quality of new growth. Dyckmans and Flessa (2001) shown that N supplied to beech (Fagus sylvatics L.) the previous year increased carbon assimilation and whole plant dry weigh of re-growth. In addition, the N supplied the previous year induced an earlier leaves formation. These were complete 6 weeks after bud break with N supply, whereas no N supply show that leaves formation were not complete until 12 weeks. Total leaf area in apple, at the end of the re-growth period, increased curvilinear with the stored N remobilisation increases. Current N supply in the spring increased tree total leaf area only about 10% (Cheng and Fuchigami, 2002). The study of Dong et al. (2004) showed that when cuttings from poplar stock plant were grow in medium without N, the new biomass growth in the second year has a positive relationship with N content per cutting at the start of re-growth. When N was supplied to the cutting, total new biomass was significantly increased, but the strength of the relationship between the new growth and N content per cutting at the start of re-growth was significantly reduced. So biomass growth of new cutting was influenced by N supply in the second growth season.

Table of contents :

Chapter 1: Introduction 
Chapter 2: Literature Review 
2.1 Poplar Biology
2.1.1 Plant Area
2.1.2 Botany and Morphology
2.2 Bud Development
2.2.1 Bud Formation
2.2.2 Bud Break
2.3 Carbon
2.3.1 Photosynthesis and Carbon Storage
2.3.2 Carbon Remobilizes to Bud Break
2.4 Nitrogen
2.4.1 Nitrate Uptake
2.4.2 Control of Nitrogen Uptake
2.4.3 Nitrogen Storage
2.4.4 Nitrogen Remobilization
2.4.5 Nitrogen Impact Plant Growth and Development
2.5 Justification of Hypothesis and Objectives
Chapter 3: Experiments 
3.1 Experiment I: Nitrogen Supply before Bud Break Strongly Impacts Spring Development of One- Year Scion Poplar
3.1.1 Introduction
3.1.2 Material and Method
3.1.2.1 Plant Material
3.1.2.2 Mineral Uptake
3.1.2.3 Root Pressure
3.1.2.4 Sap Flow
3.1.2.5 Plant Harvest and Architecture
3.1.2.6 Nitrogen Analysis
3.1.2.7 Non Structural Carbon Analysis
3.1.2.8 Data Analysis
3.1.3 Results
3.1.3.1 Environments Control
3.1.3.2 Architecture before Start Experiment
3.1.3.3 Bud Break
3.1.3.4 Nitrogen Uptake
3.1.3.5 Root Pressure
3.1.3.6 Sap Flow
3.1.3.7 Plant Architecture
3.1.3.8 Nitrogen
3.1.3.9 Non Structural Carbon
3.1.3.10 Mineral Uptake
3.1.4 Discussion
3.1.4.1 Nitrogen Uptake
3.1.4.2 Root Pressures
3.1.4.3 Sap Flow
3.1.4.4 Plant Development
3.1.4.5 Nitrogen Content
3.1.4.6 Non Structural Carbon Content
3.1.4.7 Mineral Uptake
3.2 Experiment II: Nitrogen Supply before Bud Break Influence on Early Spring Development of Stump-Poplars
3.2.1 Introduction
3.2.2 Materials and Methods
3.2.2.1 Plant Materials
3.2.2.2 Mineral Uptake
3.2.2.3 Root Pressure
3.2.2.4 Sap Flow
3.2.2.5 Plant Characteristics and Harvest
3.2.2.6 Nitrogen Analysis
3.2.2.7 Non Structural Carbon Analysis
3.2.2.8 Data Analysis
3.2.3 Results
3.2.3.1 Environment Control
3.2.3.2 Architecture before Start Experiment
3.2.3.3 Nitrogen Uptake
3.2.3.4 Root Pressure
3.2.3.5 Plant Architecture
3.2.3.6 Water Content
3.2.3.7 Nitrogen Content
3.2.3.8 Non Structural Carbon Content
3.2.3.9 Mineral Uptake
3.2.4 Discussion
3.2.4.1 Nitrogen Uptake
3.2.4.2 Root Pressure
3.2.4.3 Plant Development
3.2.4.4 Nitrogen Content
3.2.4.5 Non Structural Carbon Content
3.2.4.6 Mineral Uptake
3.3 Experiment III: Nitrogen Supply before Bud Break Influence on Early Spring Development of Poplars Dependent on Low Temperature during Winter
3.3.1 Introduction
3.3.2 Materials and Methods
3.3.2.1 Tree Preparation
3.3.2.2 Mineral Uptake
3.3.2.3 Plant Characteristics and Harvest
3.3.2.4 Water Content
3.3.2.5 Nitrogen Analysis
3.3.2.6 Data Analysis
3.3.2 Results
3.3.3.1 Architecture at Start Experiment
3.3.3.2 Environment Control
3.3.3.3 Nitrogen Uptake
3.3.3.4 Water Content
3.3.3.5 Timing of Bud Break and New Shoot
3.3.3.6 Nitrogen of New Shoot
3.3.3.7 Mineral Uptake
3.3.4 Discussion
Chapter 4: General Discussion 
4.1 Nitrogen Uptake
4.2 Nitrogen Uptake before Bud Breaks Influence on Nitrogen and Non Structural Carbon Reserve
4.3 Nitrogen Uptake before Bud Breaks Influences the development of Re-growth early Bud Break
4.4 Nitrogen Uptake before Bud Breaks Influence on Mineral Uptake
Chapter 5: General Conclusions and Recommendations 
5.1 Conclusions
5.2 Recommendations
Chapter 6: References 
Chapter 7: Appendix 
7.1 Nutrient Uptake Measurement
7.1.1 Recirculating Nutrient System
7.1.2 Computation of Nutrient Uptake
7.2 Root Pressure Measurement
7.3Sap Flow Heat Balance Measurement
7.3.1 The Theory
7.3.2 Sensor Design
7.3.3 Sensor Installation
7.4 Nitrogen Content Measurement
7.5 Estimation Nitrogen Mobilization of Compartment Tissues
7.6 Non Structure Carbon Content Measurement
7.6.1 Extraction Non-structure Carbon
7.6.2 Analysis of Glucose, Fructose, and Sucrose
7.6.3 Analysis of Starch
7.6.4 Preparation Chemical for Analysis Non Structure Carbon Content .
7.7 Estimation Non Structure Carbon Mobilization of Compartment Tissues
7.8 Embedding in LR-White Resin and Bud Sections
7.8.1 Embedding in LR-White Resin
7.8.2 SectionBud Sample
7.9 Data Analysis.

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