White Asparagus Economic Valuable Highly Demanded Fresh Vegetable Biology Essay

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White asparagus (Asparagus officinalis L.) is an economic valuable and highly demanded fresh vegetable, especially in Germany. As a highly seasonal vegetable, its availability is limited to just two months per year. The harvested spears of this monocotyledonous herbaceous perennial are succulent fleshy subterraneous shoots that grow from rhizomes. From the botanical view, they are "developmentally immature" and retain their high physiological activity in postharvest. Probably the most crucial postharvest changes that impair texture of fresh and processed white asparagus is that spears become increasingly tough and fibrousness within a few days of postharvest shelf life (Herppich and Huyskens 2008, Herppich et al. 2005a, Huyskens et al. 2005, Lipton 1990). Because texture is a major quality attribute that determines acceptance in several horticultural crops (Waldron et al. 1997) these changes negatively affect spears quality and shelf life.

B Toughening

Besides its overall morphology and histological structure, and its water relations, the biophysical properties and the biochemical composition of cell walls determine the toughness of the asparagus shoot. Basically toughening may result from a further cell wall thickening (Herppich and Huyskens 2008), an increased lignification of cell wall (Lipton 1990) and/or from a rapid increase in ferulic acid cross-linking of cell wall polymers of sclerenchyma sheath cells and of the vascular bundle elements (Rodriguez-Arcos et al. 2002). These reactions are assumed to be at least partially controlled by a wound-induced ethylene formation. They may also simply reflect the unaltered continuation of shoot differentiation (Herppich et al. 2005a). Furthermore, it is still not clear whether secondary cell wall thickening and lignification is fed by a turnover of asparagus cell wall polysaccharides (O'Donoghue et al. 1998) or by the consumption of stored soluble sugars or proteins (Herppich and Huyskens-Keil 2008).

II PREHARVEST AND HARVEST EFFECTS

A The cell wall and its properties

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Water relations, overall tissue structure, and the biochemistry and biophysics of plant cell walls are the major determinants of the biomechanical properties and, hence, texture of plant tissues. Plant cell walls are highly diverse and dynamic in composition and structure among species, plant tissues, cell types and even among specific regions of a cell wall. Cell walls are composed of many different polysaccharides, glycoproteins and, in specialized cell-types, complex aromatic polymers such as lignin.

In young, still developing tissues the cell are surrounded by primary walls mainly consisting of a framework of cellulose microfibrils and crosslinking glycans often referred to as hemicellulose. These two main elements together with structural and enzymatically active proteins are embedded in a matrix of pectic polysaccharides. This structure makes the primary cell walls elastic and allows further growth and, hence, incorporation of additional wall materials. With proceeding development, when elongation has been completed, the cell wall shape must be arrested. This is probably achieved by crosslinking the structural cell wall proteins or, at least in the so called commelinoid monocotyledoneous plants by esterification and etherification of phenolic compounds. Interestingly the latter phenolic cross bridges have also been reported for asparagus (Rodriguez et al., 2005), although the asperagales do not belong to the aforementioned group of monocots (Carpita and McCann, 2000).

Depending on their specific function many cell types such as sclerenchyma bundle, and xylem and xylem cap cells in asparagus undergo secondary cell wall thickening. This includes the assembly of many additional layers of exactly oriented cellulose microfibrils and, after commencement of secondary wall formation, the incorporation of the aromatic polyphenolic substance lignin. Some or all of these changes may largely increase strength and toughness, as well as the stiffness and rigidity of the cell walls and, hence of the entire tissue.

Among many other functions lignin is important for the structural integrity of cells and the stiffness and the strength of stems (Boerjan et al., 2003). Also in asparagus it is assumed that spear toughening is mainly due to an increase in lignification but not a result of fibre accumulation (Hsiao et al., 1981), contradictive findings are reported by Rodriquez et al. (1999c) indicating that toughening of asparagus spears was not directly related to an increase in lignin content nor depending on the absolute lignin content.

B The lignin

Lignin is unusual as a plant polymer in that there are no plant enzymes for its degradation and exhibits high degree of heterogeneity (SEDEROFF et al, 1999). Lignin is derived from the dehydrogenative polymerization of 3 different hydroxycinnamyl alcohols (or monolignols): p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. They give rise to the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units of the lignin polymer, respectively. But the monomer composition of lignin is known to differ between species, between cell types within a species, and between developmental stages of a single cell, although the extent and physiological significance of this variation are not clear. Lignins differed in the ratio of products resulting from syringyl units (S) and guaiacyl (G) units, and G/S ratios can range from 39 to 0.2 (BUNZEL et al. 2005). The major monolignols in dicotyledonous angiosperm lignin are monomethylated guaiacyl (G) units and syringyl (S) units. In contrast in monocotyledonous gymnosperms lignin is primary derived from coniferyl alcohol (G) (Higuchi 1990). Studies on green asparagus showed that asparagus lignin consisted mainly of coniferyl (60%) and sinapyl (40%) units, and the majority of these were present as monoacetylated and diacetylated monolignols (JARAMILLO et al. 2008). Furthermore the insoluble fibre lignins of white asparagus were classified by BUNZEL et al. (2005) as G-rich lignins with a G/S ratio of > 3.

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Lignins occur in great quantity in the secondary cell walls of fibres, xylem vessels and tracheids. Several functions are recognized for lignin in the extracellular matrix of plants; it is crucial for structural integrity of the cell wall and stiffness and strength of the stem (BOERJAN et al., 2003) and therefore it supplies the cell wall with mechanical support (MONTIES, 1989) as well as acts as a water-impermeable seal for the xylem vessels (NORTHCOTE, 1989). Thereby the cell wall lignin of asparagus was detected for the guaiacyl-syringyl type in sclerenchyma, bundle sheath and pith parenchyma, and for the guaiacyl type in all other tissues (ARAKI et al. 1985).

C Physiological parameters affecting and controlling lignification

Plant development and the formation of lignin

During plant development, lignin is deposited mainly in the vascular tissues and provides additional strength and imperviousness to the cell wall (Akai 1959). Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components, especially in tracheids, sclereids and xylem. It is covalently linked to hemicellulose and thereby cross links different plant polysaccharides, conferring mechanical strength to the cell wall and by extension the plant as a whole (Bidlack et al, 1992). Depending on the developmental stage of plants its formation differs within the tissue as well the cell wall. But most of research works on lignin and its formation in developmental growing of plants was done on woody plants. So in normal lignifying wood cells lignin first appears in the cell "corners" (cellular junctions). It spreads between adjacent cells (in the middle lamella regions), and from there through the primary and secondary wall layers (Wardrop 1971). The lignin first laid down in angiospermous wood fibre cells (in the cell comer-middle lamella regions) has lower syringyl content than that laid down subsequently (cell wall lignin). As a result, younger tissues contain higher concentration of guaiacyl lignin and corresponding lower methoxyl content (Joseleau et al. 1977). Vessels apparently contain only guaiacyl lignin (Fergus et al. 1970). Lignin in leaves of Angiosperms and Gymnosperms are formed from both guaiacyl and syringyl precursors (Mischke and Yasuda 1977). In some tissues such as compression and tension woods and in bark (Anderssen et al. 1973) lignin differs from normal wood or leaf lignin in the proportion of p-hydroxyphenyl, guaiacyl, and syringyl units. Maturation of asparagus, either in the field or during storage, results in the development of an undesirable increase in toughness (Sharma et al. 1975a). Thereby lignin content in growing asparagus increased from the top to the bottom of the spear, which was acertained in green spears by Jaramillo et al. (2008).

Wound signalling and the formation of lignin

Moreover lignins also play an important role in plant defence, and indeed, one of the characteristic responses of plants to injury and disease is the reinforcement of cell walls in the vicinity of the wound/infection site with lignin, as well as with suberin (Vance et al. 1980; Nicholson and Hammerschmidt 1992). The lignin deposited in this case is also known as lignin-like material (Stange et al. 2001), or defence lignin. The reinforcement of cell walls with such material results in the formation of an impermeable barrier that protects the healthy tissue from water loss and opportunistic infection, as well as limiting the spread of the original pathogen (Vance et al. 1980; Trockenbrodt 1994). Defence lignin formation has also been implicated in the responses of plants to abiotic stresses such as ozone (Sandermann et al. 1998) and water deprivation (Costa et al. 1998). It is also possible that monolignols have antimicrobial activity, as has been reported by Keen and Littlefield (1979).

Microscopic analyses indicated that the barrier zone was formed by the reinforcement of cell walls with lignin-like material in both ground tissues and vascular tissue, and that, in addition, the lumen of certain xylem cells (vessels and fibres) were blocked by the deposition of polymeric phenolic material. Furthermore histochemical characterization of the polymer revealed that the lignin-like material (defence lignin) deposited in ground tissue cell walls and xylem cell blockages was poor in syringyl (S-type) lignin units and therefore differed from the usual mixed guaiacyl-syringyl (G-S) lignin unit composition of developmental lignin (Hawkins and Boudet 2003). A significant amount of data (Hammerschmidt et al. 1985; Messner and Boll 1993; Lange et al. 1995) demonstrates that the chemical composition of defence lignin is different to that of developmental lignin, but changing in lignin composition depends on plant species. For example, in cucurbits, it appeared that defence lignin is extremely rich in p-coumaraldehyde units (H-units) as opposed to cucurbit developmental lignin, which is a typical angiosperm guaiacyl-syringyl (G-S) lignin (Stange et al. 2001). Similarly, in wounded poplar and almond trees, defence lignin was apparently associated with G-lignin (Biggs et al. 1984; Bostock and Middleton 1987) as opposed to the normal G-S developmental lignin found in these species. Similar studies on asparagus have not been performed till now. Further research in order to get useful informations about lignin composition in wounded tissue of asparagus could be also tackle following questions: What is the benefit of a different lignin composition for the wounded plant? Is there a lesser energetic effort for producing wound-lignin in order to react very fast with wound healing? Does the plant use the same pool of phenolic precursors for synthesis of wound lignin versus "developmental" lignin? And, what kind of influence does the different lignin-composition has on rigidity and firmness of plant tissue?

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Wounding induces expression of genes encoding defence-related proteins involved in wound healing. An important signal molecule that intermediates in the wound signal to cellular response is the plant hormone jasmonic acid (JA) and its methyl ester methyl jasmonate (MJ). JA and MJ are widespread natural regulators involved in many processes during plant development (Creelman and Mullet, 1995). Jasmonates have also been shown to stimulate the production of secondary metabolites (Gundlach et al., 1992), including phenols (Keinänen et al. 2001). Phenolic compounds are the major secondary metabolite products of plant metabolism, and their biosynthesis involves the induction of phenylalanine ammonialyase (PAL), the first and rate-limiting enzyme in the phenylpropanoid metabolism. These compounds have a wide range of structural classes and biological functions; however, within a single tissue, not all of their pathways may be expressed (Rhodes 1985). The accumulation of phenolic compounds in plant tissues may be induced by different abiotic and biotic stresses (Huyskens). As they are lignin-precursors an increased production of phenolic compounds due to stress also enhances lignification (Ali et al. 20062; Fan et al. 2006 3).

The role of Ethylen and its related enzymes

It is well known, that ethylene plays a highly role in plant growth and development and is involved in a number of processes, including germination, senescence, abscission, and fruit ripening. Ethylene also participates in a variety of defence responses as well as abiotic stress responses (Ecker, 1995), and, ethylene enhances the lignification process (Haard et al., 1974). The most important enzymes involved in ethylene biosynthesis are ACC-synthase (ACS, EC 1.4.3), which produces the precursor of ethylene- 1-aminocyclopropane-1- carboxylic acid (ACC) (Adams and Yang, 1979 5), and ACC-oxidase (ACO, EC 4.4.1.14), that converts ACC to ethylene. ACS genes have been identified as early wound-response genes (Ecker, 1995; Reymond and Farmer, 1998), and also ACO is also stimulated by wounding, i.e. in avocado (Owino 2002), bamboo shoots (Matsui 2001) or melon (Bouqin 1992). But it has also been demonstrated that both ACS and ACO are encoded by multigene families, members of which are differentially expressed in response to various environmental stresses and during specific developmental stages known to induce ethylene (Fluhr et Mattoo, 1996). A well-studied case of transcriptional regulation of ACS and ACO is in tomato fruit (Barry et al. 2000; Barry 1996; Lincoln et al. 1993; Yip et al. 1992; Olson et al. 1991; Rottmann et al. 1991). Furthermore the expression of the JA-inducible and pathogen- and wound responsive gene (PDF1.2) is also regulated by ethylene (Penninckx et al. 1998). Intriguingly, JA has been found to be conjugated to ACC in Arabidopsis plants, suggesting that JA-ACC conjugates could be involved in the co-regulation and crosstalk between JA- and ethylene-dependent pathways in plants (Staswick and Tiryaki 2004).

The influence of ethylene on lignification is undisputable, also on white or green asparagus as well as on other monocotyledones like bamboo (Haard et al., 1974; Liu et Jiang, 2006; Luo et al., 2008). But as the asparagus spears are with 2.1- 4.9 µl/ kg/ hr not a large ethylene producing commodity (Haardt, 1974), very few research have been conducted on the rates of own ethylene production and its deteriorative effects during storage. Harvesting and handling impose a series stresses on the tissue, including wounding, dehydration, and separation from nutrient supply. While its influence on postharvest of climatic plants is well-known, the hormone is also generally active at very low concentrations in non-climatic plants (Will et al. 1999 !!!). It could be demonstrated, that the rate of ethylene production increases after harvest by white (Beever et al. 1985, Hennion et al. 1991) and green spears (Bhowmik et al. 20021). These authors and others (Goldstein et al., 1972) suggested that severing of asparagus during harvest enhances ethylene production via a wounding reaction and thus enhances lignification.

Research work on the enzyme kinetically and molecular genetically base was primarily done on green asparagus. Here, Bhowmik et al. (2002) isolated a cDNA clone (pAS-ACO) prepared from the tip section of green asparagus spears and referred from an increase of pAS-ACO mRNA until 3 d at 20°C after harvest, which coincided with ethylene production and the ACO activity. Moreover studies two years later on ACS in green asparagus by Bhowmik et al. (2004) encoded a cDNA clone (pAS-ACS) and determined an increase of pAS-ACS mRNA until 8 h at 20°C in postharvest. Here and for the studies of 2002 it was suggested that the increase might be a response to the wounding associated with harvest. Thus, Bhowmik et al. (2004) reported also, that in contrast to the top of spears, the activity of ACS in the bottom portion was too low to induce ethylene production. However, wounding occurs on the basal part of asparagus and might influence so-called wounding-depend enzymes and therefore also the ethylene production. On the other hand Hennion et al. (1990), who analysed harvested white asparagus spears during aging at 20°C, revealed, that ethylene production did not vary greatly during the first days of aging, but increased after 10 days. ACC steadily accumulated in the tip of the spear after 1 day and exhibited a maximum at Day 10. Though, Yang et Hoffmann (1984), who has been competently reviewed the biosynthesis of ethylene, described that the stimulation of ethylene production by stress typically occurs with a lag of 10-30 minutes and subsides later reaching a peak within several hours. Furthermore, ACC-synthase and -oxidase in white asparagus could respond differently from green spears, because white spears are not able to accomplish photosynthesis and therefore they differ in metabolism.

E Lignin biosynthesis

Relevant enzymes of lignin pathway

It is known that, lignification occurs trough a series of enzymatic steps, starting with a phenylalanine ammonialyase catalyzed reaction to produce lignin precursors, and terminating with a process that requires H2O2 and a cell-wall-bound peroxidase to bring about polymerization of the C6-C3 units into lignin. Phenylalanine ammonialyase (PAL, EC 4.3.1.5.) is one of the most extensively studied enzymes in plant metabolism, because it plays an important role in activating plant defence system. It generally serves as the entry point to phenylpropanoid metabolism. With producing of phenylpropanoid intermediates the next step of lignification is the lignin branch pathway, which consists of two enzymes that sequentially reduce the hydroxycinnamyl-CoA thioesters to their corresponding hydroxycinnamyl alcohols, the monolignols. Here, Cinnamoyl CoA-Reductase (CCR, EC 1.2.1.44) is the first enzyme and it has been hypothesized to play a key regulatory role in lignin biosynthesis. Cinnamyl-alcohol dehydrogenase (CAD, EC 1.1.1.195), the second and last enzyme in the lignin branch pathway is directly responsible for the formation of monolignols. The final step in lignin biosynthesis involves an enzymatic oxidation of p-phenylpropanoid monolignols followed by a free radical coupling reaction. Peroxidases (EC 1.11.1.7) have been postulated to carry out the peroxidative coupling of monolignols (coniferyl-, sinapyl-, and p-coumaryl alcohol) (BOUDET et al. 2000).

Down-regulating of enzyme genes appears to be one of the most efficient ways to demonstrate their involvement in lignin biosynthesis. In most ways it led to a reduction of lignin, e.g. for PAL in tobacco (Sewalt et al. 1997a, b), for CCR in tomato (VAN DER REST et al. 2009) and tobacco (PIQUEMAL et al. 1998), for CAD in loblolly pines (MACKAY et al. 1997) and sorghum (PILLONEL et al. 1991), and for peroxidase in tobacco (Blee et al. 2003). Otherwise CAD regulation could also have no influence on the lignin content, which was presented on tobacco plants (YAHAIOI et. al. 1997, BERNARD-VAILHE 1998). Moreover down-regulation of enzymes also led to an altered lignin composition as well as to a change of lignin precursors. For PAL, SEWALT et al. (1997a, b) reported from a decrease in G lignin composition as well as from increased methoxyl content in transgenetic tobacco. Furthermore CCR mutant tomatoes showed a dramatic change in soluble phenols mainly in stem and leaves (VAN DER REST et al. 2009), and CCR mutant tobacco plantlets revealed an increased S/G ratio due to a decrease in extractable G units (PIQUEMAL et al. 1998). On the other hand DIXON et al. (2001) suggested the involvement of CCR in the synthesis of both, G and S lignin. Hence, the importance of CCR in S and G lignification is still not understood. Furthermore, CAD mutant plants showed decreased S lignin content in tobacco (YAHAIOI et. al. 1997, BERNARD- VAILHE 1998) and an increase of conferaldehyde units in loblolly pines (MACKAY et al. 1997).

Regulation of enzymes of lignin biosynthesis

Gen encoding studies detected a different regulation of respective genes, which are involved in activating lignin precursor enzymes. While the enzymatic activities participated in the biosynthesis of defence lignins appear to be similar to those contributed in developmental lignin biosynthesis, the signal transduction pathways involved in the production of these two lignin types appear to be different. For example, different PAL genes are regulated differentially during development or in response to elicitation (Logemann et al. 1995). Such regulation presumably enables the plant to direct its pool of phenolic metabolites to where they are needed (defence phenolic molecules or developmental lignin precursors).

For CCR gene encoding studies reported that the number of isoforms identified per species has never exceeded two, such as in Arabidopsis (LAUVERGEAT et al. 2001), maize (PICHON et al. 1998) and poplar (LEPLE et al. 1998). Furthermore expression analyses suggested different functions of both gene types. The recent isolation of two CCR genes (AtCCR1 and AtCCR2) from Arabidopsis thaliana (Lauvergeat et al. 2001, Raes et. al 2002, Cano-Delgado et al. 2003) demonstrated different expression patterns during development, or in response to pathogen attack, would seem to suggest that the production of developmental versus defence lignin is also controlled at this level of the biosynthetic pathway. Similarly, the isolation of two lucerne CAD cDNAs (MsaCAD1 and MsaCAD2) (Brill et al. 1999), and of two different types of CAD activities in eucalyptus (Hawkins et al. (2003), which are differentially regulated during development and plant defence would seem to support the hypothesis of separate signal transduction pathways for the formation of developmental and defence lignins. Furthermore recently transgenic cell cultures expressing the constitutively active OsRac1 in rice accumulated lignin through enhanced CCR activity and increased reactive oxygen species production (Kawasaki et al. 2006). Authors argued that OsRac1 controls lignin synthesis through regulation of both NADPH oxidase and OsCCR1 activities during defence responses.

For peroxidases HOLM et al. (2003) identified three typical class III plant peroxidases in asparagus (Aoprx1-3) in terms of conserved regions with most basic nature. It was reported, that the Aoprx2 and Aoprx3 sequences were lignin- or polyphenol-related peroxidases, whereas the down-regulation of Aoprx1 may suggest that this peroxidase is involved in the early steps of the wound response or not related to the formation of lignin. Further the deduced amino acid sequences of Aoprx1 and Aoprx2 showed a high identity with tobacco peroxidase, which was encoded as a pathogen-induced enzyme (EGEA et al., 2001). In asparagus somatic embryogenesis the determination of expression and function of cell wall bound cationic peroxidases showed, that the AoPOX1 was effective in the metabolism of feruloyl (o-methoxyphenol)-substituted substrates, including coniferyl alcohol (TAKEDA et al. 2003).

Research work on asparagus

The availability of results of research concerning lignin precursor enzymes in asparagus is rare. Most studies had concentrated on carbohydrate and amino acid metabolism of stored asparagus. Enzyme kinetically studies on other monocotyledons were primarily done on green asparagus spears or on bamboo. Here authors concluded that changes in enzyme activities might be due to harvest injury and the defensive responses of the tissues. However, enzyme activity differs with plant species, plant tissue and maturation stage. In asparagus spears PAL activity was very low in apex, but on a high level in the middle and base section (HENNION et al. 1992). In contrast Bamboo spears showed a low activity in the base and high levels in the apex, but only PAL activity of the base section correlated with a rise of lignin content after cold storage (XU et al.). Authors suggested that PAL activity in the basal section of bamboo has a corresponding impact on the changes of lignin, but not in the apical and middle section. Hence, bamboo is a gramineous plant with a basal stem development so that injury due to harvesting occurs in the mersitematic zone. In contrast in asparagus injury happens in the already differentiated zone. Therefore it could be hypothesised, that wounding leads to an increase of lignin content only in the affected and neighbouring zone of damaged cells. Lignification in middle and apical zone might be mainly influenced due to developmental growing of asparagus spears. This will be supported by studies of BHOWMIK et MATSUI (2004a), who determined increased PAL activity in bottom portions but not in excised tops of green asparagus after harvest.

In respect of CCR studies on white asparagus HENNION et al. (1992) demonstrated that neither the lignin content nor the activity of CCR changed in the spear apex during storage, regardless of treatments (water and water containing silver thiosulfate). In contrast the levels of CCR activities in spear base decreased during the first 2 to 4 h in every storage treatment (HENNION et al. 1992). The authors supposed that the common, initial decline of enzymes, also in CCR, represented the end of a transient surge in activity, induced by excision and handling at the time of harvest.

LIU et al. (2006) showed in green asparagus that PAL, CAD and POD activities in asparagus stalks increased rapidly during the first 3 days after harvest, and this increase was correlated with the accumulation of total phenols and subsequent lignin deposition. Only datas from HENNION et al. (1992) described CAD activity as well as POD activities in white asparagus, whereas the activities of CAD and syringaldazine oxidase (SPOX) gradually increased with storage time. Hence, these results indicated that postharvest lignification in etiolated asparagus spears is caused primarily by enhanced SPOX activity, not by enhanced CAD. However, in this study only SPOX from the POD family was analyzed, because it based on research work of GOLDBERG et al. (1985), who suggested that SPOX is specifically involved in lignin formation in situ. In respect of molecular genetically studies only datas from Bhowmik () on PAL of green asparagus and Holm (2003) on POD of white asparagus spears are available (see above chapter). Nevertheless, the work on peroxidases on lignification is still at a very primary stage, especially in white asparagus.

III EFFECTS OF STORAGE CONDITIONS

Changes in biomechanical and biochemical properties and their controllability by various storage conditions and postharvest treatements have been intensively studied (Herppich and Huyskens-Keil 2007, 2008, Herppich et al. 2006a, 2005a, 2004, Huyskens-Keil and Herppich 2008a,b, 2007, Huyskens-Keil et al. 2005, 2004, Morrin et al. 2005, Prolygina et al. 2006). It could be shown that all functional cell wall components of asparagus spears increased closely temperature dependent. In parallel the content of soluble glucose declined with a similar temporal dynamics and to a comparable degree, indicating a major carbon flow of this storage sugar into cell walls (60 to 70 %). Lower temperatures reduce cell wall development but do not significantly affect the relative carbon flow from storage sugars into cell walls or maintenance respiration. Compared to cell walls maintenance respiration is by far the smaller carbon sink in stored asparagus spears. Temperature differentially affects the absolute amount and the relative contribution of the different cell wall components and the temporal dynamics of changes in structural carbohydrate and lignin content. At higher temperatures, secondary cell wall thickening resulted mainly from a large increase in cellulose content. The pronounced increase in the fractions of cellulose and especially lignin may stress the important role of lignin in cell wall strengthening. While the fraction of cell wall proteins decreased those of hemicellulose and the pectic components was not influenced.

The main effect of temperature may be due to it direct influence on the overall metabolic activity. Similarly, storage of asparagus spears under high CO2 concentration may also retard the physiological activity of shoots thus inhibiting the synthesis of cellulose and, to some extent, lignin accumulation (Herppich and Huyskens-Keil 2007, Huyskens-Keil and Herppich 2007). Additionally, storage at 20°C and elevated CO2 inhibited the degradation of soluble carbohydrates. However, tissue toughness increased significantly. Changes in toughness were associated primarily with the dynamics of lignin and cellulose. In contrast, slightly lower temperatures of 10°C in combination with high CO2 did not have a pronounced effect on changes in structural carbohydrate pattern (lignin, cellulose, hemicellulose and pectins).

In contrast, exposure of asparagus spears to short-term UV radiation and ozonated wash water, known for disinfection of pathogenic organisms in numerous food products, did not prevent changes in mechanical properties during subsequent storage (Herppich et al. 2009, Huyskens-Keil and Herppich 2008a, b). Tissue strength was again associated with changes in structural carbohydrates. However, O3 and O3+UV-C treatment significantly retarded the increase in cell wall components. A concomitant decline in cell wall protein might indicate an inhibition of the key enzyme of lignin synthesis (PAL) or a possible utilization of phenols for other metabolic processes.

IV FUTURE PROSPECTIVES

During recent years many attempts have been undertaken to improve postharvest handling for perishable white asparagus from the engineering point of view. However, any successful development of effective approaches to reach this goal inevitably depends on the comprehensive understanding of the physiological, biochemical and biomechanical basis of spear development in pre- and postharvest. Postharvest toughening and excessive fibre development is still the most important postharvest problem rapidly reducing spear quality, determining consumers' rejection and thus resulting in enormous economic losses.

In detail the main emphasis in future research work should be put on the following aspects:

To what extend are the dynamic changes in mechanical properties of shoots and their specific tissues directly related to cell wall metabolism in developing spears?

To what extend are cell wall thickening and lignification, respectively involved in toughening?

To what extend are changes in cell wall structure and -chemistry based on endogenous (development) and exogenous (wounding) factors?

Does the process of toughening of asparagus spears occur in planta in response to endogenous factors such as stage of plant development or to exogenous cues such as wounding?

Does the process of lignification occur in planta in response to endogenous factors such as stage of plant development or to exogenous cues such as wounding?

Can ethylene elicit cell wall thickening and/or lignification during spear development?

Hence, for any further effective optimisation of storage and handling the comprehensive knowledge of the physiological, biomechanical and molecular aspects of the developmental and/or stress-induced regulation of cell wall metabolism in white asparagus is essential. Although many attempts have been made to gain a better understanding of this topic (Herppich and Huyskens-Keil 2008, Herppich et al. 2005a, Kadau et al. 2005a,b, Zurera et al. 2000, Rodriguez et al. 1999a,b,c, O'Donoghue et al. 1998, Redondo-Cuenca et al. 1997), several important questions have been left open for further scientific analysis.