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La gestione del vivaio influenza laqualità delle plantule

Riassunto. Produrre trapianti vegetali di "buonaqualita" richiede una serie di pratiche e interventi orti-coli che, nel complesso, consentono la crescita dellepiante dall'inizio alla fine in maniera costante, ininter-rotta e con uno stress minimo, garantendo un tasso dicrescita adeguato, un rapporto ottimale tra radice egermoglio, una buona capacita di insediamento e unpotenziale alto rendimento dell’impianto che nederiva. Poiche la crescita del trapianto vegetale e unasintesi di molti fattori che si moderano a vicenda, piut-tosto che seguire raccomandazioni standardizzate,piantine di ortaggi di buona qualita possono essereprodotte solo attraverso un'attenta selezione del sub-strato e delle miscele di substrato, una gestioneappropriata del regime termico e della fertilizzazione,della dimensione del modulo e dell'eta del trapianto e, infine della corretta combinazione di promotori edelle tecniche di controllo della crescita delle piante.

Parole chiave: substrato, sostituti della torba,dimensioni del modulo, età del trapianto, fertiliz-zazione, biostimolanti vegetali, indurimento.

Introduction

Vegetable transplants have been used for decadesand the methods used to produce them, gradually,have tremendously changed (Balliu et al. 2017). Fiftyyears ago, the standard method for producing veg-etable seedlings was to sow the seed in a nursery bedand then dig up the seedlings when they were largeenough to be successfully transplanted, starting fromthe 1970s the use of cell trays for growing vegetableseedlings was largely used (Nichols, 2013). That is asystem, where each transplant grows in an individual

cell and so there is less competition among plants andgreater uniformity, the water and nutrients are deliv-ered through overhead booms and roots are air pruned(Nichols, 2013).

Since the quality of transplant affect plant estab-lishment, initial growth and consequently yield(Prunty et al., 2015), the production method of veg-etable seedlings is extremely important. Although theterm “good quality” leave room for some subjectiveinterpretation, and there are not well distinct stan-dards, high quality vegetable transplants are generallydefined as those with 1) no infections of diseases orpests, 2) ability to survive in unfavorable environ-ments after transplanting, 3) well developed root sys-tem, and 4) well developed leaf area without visualdefects of leaves such as chlorosis or necrosis (Balliuet al. 2017).

The quality of vegetable transplants is affected bymany factors and therefore the management of trans-plant production is complicated and requires broadknowledge’s and experience (Mcavoy and Ozores-hampton, 2015). Practically, it is a chain of severalhorticultural practices and interventions which overalldetermines the growth rate (Oagile et al., 2016), theroot to shoot ratio (Sallaku et al., 2009b), the standestablishment capabilities (Babaj et al., 2012) andyield potentials (Ali, 2016) of new plantlets. Goodquality vegetable seedlings in very small module sizescan only be produced through proper use and appro-priate manipulations of water (Liptay et al., 1998) andfertilization regimes (Liu et al., 2012) which overallaiming to raise the plant from start to finish by slow,steady, uninterrupted growth and with minimal stress(Mcavoy and Ozores-hampton, 2015). Furthermore,the quality of vegetable seedlings is affected by thephysical and chemical qualities of substrate (Ceglie et

al., 2015), module size (Oagile et al., 2016) and trans-plants age (Ibrahim et al., 2013; Jellani et al., 2016),plant growth promoters (Baum, 2015; De Pascale et

Review n. 33 – Italus Hortus 24 (3), 2017: 39-52 doi: 10.26353/j.itahort/2017.3.3952

Nursery management practices influence the quality of vegetableseedlings

Astrit Balliu*, Glenda Sallaku, Thoma Nasto

Agricultural University of Tirana, Albania

Ricezione: 26 gennaio 2018; Accettazione: 23 marzo 2018

* aballiu@ubt.edu.al

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al., 2018) and growth control and hardening tech-niques (Garner and Björkman, 1999; Agehara andLeskovar, 2015) .

Since the number of factors affecting the quality ofvegetable transplants is very large, and therefore hardto have all included in a single review, this one dealswith the recent findings and current trends of onlysome of nursery management practices regarding veg-etable transplant production; substrate and peat substi-tutes, cell size, transplant age, fertilization, plant bios-timulants use and growth control and hardening tech-niques, acknowledging that despite voluminousresearch done, often contradictory results are pub-lished, and yet, no clear answers are provided regard-ing many specific issues with practical importance forvegetable growers.

Substrate; peat and its substitution alternatives

Various growing media may be used in commer-cial seedling production, but still, the most commonmaterial used for vegetable seedling production ispeat (Gruda and Schnitzler, 2004; Díaz-pérez et al.,2011). Its long-time success is certainly due to thephysical properties (slow degradation rate, low bulkdensity, high porosity, high water holding capacity)and the chemical characteristics (relatively highcation exchange capacity) that makes peat particularlysuitable as growing media for a large number of veg-etables and ornamentals (Fascella 2015). However,the frequent irrigation deteriorates the physical char-acteristics (porosity and water and air volume) ofpeat. Therefore, commercial nurseries often mix peatwith perlite or vermiculite to increase the water-hold-ing capacity of growing substrate and avoid watercontent volume fluctuations of solely peat substrate(Balliu et al. 2017).

Nowadays, the search for alternative high-qualityand low-cost materials as growing media in horticul-ture is a necessity due to the increasing demand andrising costs for peat, and it’s uncertain availability inthe near future owed to environmental constraints(Fascella 2015). Therefore, extensive research hasbeen carried out regarding the use of different farm,industrial and consumer waste by-products as compo-nents of nursery substrates. Different residual bio-masses, such as coir (coconut (Cocos nucifera L.)husk fibre), rice (Oryza sativa L.) hulls, switchgrass(Panicum virgatum L.), spent mushroom compost(Agaricus bisporus (J.E. Lange) Imbach, andPleurotus ostreatus (Jacq.) P. Kumm), beachedPosidonia residues (Posidonia oceanica L.), extractedsweet corn tassel (Zea mays L.), and giant reed

(Arundo donax L.) wastes have been studied as partialor total substrate components (Ceglie et al., 2015).Numerous studies have meantime reported the use oforganic residues, after proper composting, as peatsubstitutes in potting media, such as municipal solidwaste compost, animal manure compost, green wastecompost, and agro-industrial compost (Ceglie et al.,2015). Recently, dewatered aquaculture effluent arealso investigated as potential partial substitutes invegetable seedling production (Danaher et al., 2016).However, although some by-products obtained fromwaste recycling of human activities, agricultural andfood industry, and/or energy production processesrepresent valid alternative to peat, their use as poten-tial substitutes of conventional materials in vegetabletransplants production needs further clarifications(Russo 2005).

Currently, industrial growing mixes are widelyused in the vegetable transplant industry, composedby growing media constituents which include combi-nations of peat and other organic or inorganic materi-als, and growing media additives include fertilizers,liming materials, and biocontrol or wetting agents(Fascella 2015). While their cost is considerablylower and they also may suppress many soilborne dis-eases, various composts act as well as peat moss doesin container media (Raviv, 2005). Assessing the via-bility of using compost from vine pomace, agricultur-al solid wastes and urban solid wastes Díaz-pérez et

al., (2011) have found that that the partial substitutionof peat in the mixture of substrates in watermelonseedling production is possible and the dose shalldepend on compost salinity and the type of salts that ithas. Evaluating bovine manure compost (BMC) and agreen compost (GC) as components of substrates inpartial substitution of peat for organic melonseedlings production, Tittareli et al., (2009) have alsofound that seedling growth in treatments containing30% and 50% of composts was significantly higherthan control.

Anyway, plant response to different substrates isstrictly related to the tested species and also dependson the materials used and on the proportions in themixtures and it is very complex to establish the mostsuitable materials and especially, the best proportionsto obtain good results concerning plant growth andproductivity (Ceglie et al., 2015). The use of compostinto substrates in different ratios may cause someproblems as a consequence of its high salt content,unsuitable physical properties (water holding capaci-ty, aeration etc.), heavy metal toxicity and variablequality and composition (Chrysargyris et al. 2013;Fascella 2015). Thus, examining the impact of munic-

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ipal solid waste compost (MSWC) mixed with con-ventional peat substrates at various ratios, as a growthmedium in the nursery production of melon(Chrysargyris et al. 2013) found that seed emergenceenhanced when low MSWC content (< 30%) wasused while increased MSWC content (>60%) reduced(up to 80%) emergence and delayed up to 6 days theemergence time. Under nursery conditions, additionof MSWC (especially in content greater than 30%)reduced leaf numbers produced, seedling fresh weight(and dry matter content), chlorophyll and totalcarotenoids content. In similar experiments,(Chrysargyris, 2017) found that addition of > 30%MSWC reduced pepper seedling height, leaf numberand fresh weight, while leaf chlorophyll and totalcarotenoids content decreased in > 60% MSWC intothe peat, and (Papamichalaki et al., 2014) found thatlow content (15% to 30%) of MSWC may act as analternative substitute for peat in watermelon seedlingproduction. Analyzing the influence of MSWC incucumber seedlings production (Mami and Peyvast,2010) recommends that MSWC in mixtures with peatshould not exceed the level of 5%. Gruda andSchnitzler (1997) did also report that wood fiberscould be used as organic substrates, but they warn thatN-immobilization can be a cause of nutritional imbal-ance on young vegetable seedlings. There are reportsthat olive-mills waste composts could be used asreplacement for peat in growing media. According toUrlic et al. (2015), they should be used in a rate nothigher than 50% for Brassicas and not higher than25% for lettuce and endive transplant production.Meantime, Ceglie et al., (2011) reports that treat-ments with 20% olive pomace compost showed thebest performances of tomato seedlings, while 45%could be considered as the upper limits for producingsatisfactory tomato seedlings. Similarly, grape-vine-compost can also be used in tomato transplant produc-tion in a rate of up to 40% (Kritsotakis andKabourakis, 2011).

One promising method of processing organicwastes into a value-added product is vermicompost-ing. In this process, organic material is stabilized intoan odorless peat-like substance in the digestive tractsof various species (e.g., Eisenia fetida and E. andrei)of earthworms (Paul and Metzger, 2015). The processof vermicomposting tends to result in higher levels ofplant availability of most nutrients than does the con-ventional composting process. Therefore, its presencein compost mixtures stimulates plant growth. Thus, ahigher dry matter per plant and a higher relativegrowth rate was found by Babaj et al. (2009) andArouiee et al., (2009) in tomato seedlings growing in

a mixture of vermicompost with commercial peat (upto 50%, vol:vol). Improved transplant quality in pep-pers and eggplants (up to 20%, vol:vol), but not intomato transplant was also reported by (Paul andMetzger, 2015). Parallel to increased shoot length,fresh weight and leaf area of tomato seedlings due toaddition of vermicompost from 10 to 50 % of totalpot volume, Dintcheva and Tringovska (2012) reportssignificant contribution of vermicompost to N and Ksupply of tomato transplants. In addition, Sallaku et

al. (2009a) and Kaciu et al. (2011) reports enhancedsalinity stress alleviation capabilities of respectivelycucumber and pepper seedlings grew in a mixture ofvermicompost with commercial peat (50%:50%,vol:vol) after transplanting in a saline environment.However, due to variability of organic wastes, vermi-composts might have varying nutrient content levels(Dintcheva and Tringovska, 2012), which subse-quently might yield different results under varyingpotting mixtures.

Module size and shape

According to (Nesmith and Duval, 1998), the trendamong many commercial transplant producers istoward more cells per tray (smaller containers), whichreduces propagation costs per plant. Yet, it is unclearhow plants grown in smaller root volumes will per-form under post transplant field conditions. It is large-ly admitted that reducing transplant container sizegenerally increases the probability of root restriction,but the length of time a plant remains in the containeris also a major factor to be considered (Nesmith andDuval, 1998). However, analyzing the influence ofpot size, (Poorter et al., 2012) have found that reducedgrowth in smaller pots is caused mainly by a reduc-tion in photosynthesis per unit leaf area, rather than bychanges in leaf morphology or biomass allocation.

In general, larger cells will enable production of alarger transplant, the plant has more room to grow, soit is possible to produce an older, more mature trans-plant without it becoming spindly or root bound(Balliu et al. 2017). That is an well known fact andaccording to Poorter et al., (2012) a doubling of thepot size increased biomass production by 43%. Thus,tomato transplants grown commercially in a containercell size of 4.4 cm had greater dry matter accumula-tion at planting and 30 days after planting than plantsgrown in 2.5 cm cells (Vavrina and Arenas, 1997).Similarly, higher plant height, leaf number and leafarea, and higher shoot fresh and dry weight of tomatoseedlings due to a larger module size were alsoreported by Shopova and Cholakov (2014) and Oagile

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et al., (2016). An early report from Maynard et al.

(1996) did also confirms that seedling leaf area, andshoot and root weights of muskmelon seedlingsbefore transplanting increased linearly with increasingcell volume.

The cell size influences the field performance ofthe transplant (Balliu et al. 2017). Obviously trans-plants with relatively large root systems generally suf-fer less post-plant shock and thus come into produc-tion earlier than plants with small root systems(Weston and Zandstra, 1986). Maynard et al. (1996)have also reported that shoot dry weight 20 days aftertransplanting and vine length of muskmelon seedlings30 days after transplanting increased linearly withincreasing cell volume.

Plant development can also be influenced by mod-ule size. Evidences brought by (Nesmith and Duval,1998) confirms that as rooting volume increased, thetime from sowing to anthesis was shortened, butmeanwhile a delay in fruit maturation was shown forroot restricted tomatoes. In contrast, root restrictionresulting from small containers did not have anyinfluence on duration of flowering or time to anthesisin summer squash.

Varying transplant container size has shown mixedand even contradictory results on harvested yield anddiffering observations between yields of species andcultivars in response to transplant container size havenot been thoroughly explained (Nesmith and Duval,1998). According to Vavrina and Arenas, (1997),tomato transplants grown commercially in a containercell size of 4.4 cm produced earlier and yieldedgreater than transplants grown in 2.5 cm cells. Highertransplant biomass and higher early and total yield oftomato seedlings due to increased module size wasalso reported by Bouzo and Favaro, (2015). However,Nicola and Cantliffe (1996) indicated that yield andearliness of lettuce (Lactuca sativa L.) was morerelated to growing season and soil type than to trans-plant quality resulting from various container sizes.The same fact is admitted from Vavrina and Arenas,(1997) whose found that in case of tomato seedlingsthe cell size impact was more dramatic in the springthan in the fall, perhaps due to environmental compli-cations experienced in the fall. In an earlier report(Maynard et al., 1996) have also found that earlyyields increased linearly as transplant cell volumeincreased, but cell volume did not consistently affecttotal yield of muskmelon.

Attention was also paid to the effect of cell shapeon the plug transplant growth in different vegetablecrops. In study published by (Chen et al. 2002), wasnoticed that in lettuce and cucumber plug transplants,

round cells promoted root circling while triangularcells retarded root growth compared to pentagonal andsquare cells. Surprisingly, before root ball formation,round cells promoted plant growth, but on the con-trary, they inhibited plant growth after root ball forma-tion compared with square and pentagonal cells. Intomato plug transplants, the rate of root circling wasincreased in order of round, pentagonal, square, andtriangular cells. However, cell shape did not affect theplant growth before root ball formation, while thegrowth of plants in triangular cell was inhibited com-pared with all other cells after root ball formation. Inconclusion, square or pentagonal cells are more suit-able for lettuce and cucumber plug transplants thanround and triangular cells. Except of triangular cells,the cells with other shapes are suitable for the produc-tion of tomato plug transplant. A deeper-celled trayhas a larger cell volume so more water and fertilizerare available to the plant. Deeper cells tend to promotefaster growth and also they will not need watering asfrequently as shallow cells (Kubota et al. 2013).

Transplant age

The definition of ‘ideal’ transplant age is a longtime questions, which has not yet received a decisiveanswer. Lack of standardized methods, multiple culti-vars, processing versus fresh market types, bare-rootsvs. containerized, various container cell sizes, super-imposed studies, postharvest storage, and otheraspects all led to varying results (Vavrina, 1998).Young plants that were tender and capable of quickgrowth resumption were often associated with largeyields, whereas it was believed that hardeningreduced early yields in proportion to the degree ofhardening (Casseres 1947 after Vavrina and Orzolek,1993). Over mature transplants generally have spindlystems and excessive leaf growth, whereas their rootgrowth is limited because of the small rooting volumeof high-density plug trays (Agehara and Leskovar,2015). In addition, following transplanting olderplants have only a limited time for the readjustment oftheir vegetative development before the initiation ofreproductive growth or the maturation of the vegeta-tive phase (Dullforce, 1954 after Vavrina 1998).However, young plants had to be carried longer in thefield to reach optimal yields, and that could reducemarket window opportunities (Vavrina and Orzolek,1993).

In fact, numerous studies have already confirmedthat the transplant age window for certain crops mightbe wider than previously thought and current moderncultivars, improved production systems, and technical

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expertise enable us to produce high yields regardlessof transplant age (Vavrina, 1998). As an example, in areview of 1993, (Vavrina and Orzolek, 1993) admitsthat after more than 60 years of transplant ageresearch, it appears that comparable production can beobtained from tomato transplants of various ages.

Since 1993, (NeSmith, 1993) has reported that 28to 35 day-old greenhouse-grown summer squashtransplants grew more slowly after transplanting thanplants that were 10,14, or 21 days old. Older trans-plants flowered earlier; however, earlier flowering didnot result in higher early yield. In support of theseconclusions, a number of recently published reportsunderline the benefits of using younger transplants. Ahigher leaf gas exchange and photosynthetic produc-tivity were found for 20-25 day old compared to 30-35 and 40-45 day old tomato seedlings (Shopova andCholakov, 2014). Similarly, Ibrahim et al. (2013),reports that pepper seedlings transplanted at 8 and 10week after sowing (WAS) produced larger leaves,greater number of leaves, more branches per plant,and also grew taller than the seedlings transplanted at12 or 14 WAS. The variation in the seedling-trans-planting age had no statistical significant differenceon the fresh fruit yield, but however, pepper seedlingstransplanted at 8 and 10 weeks produced more fruitsand higher fresh fruit yield per hectare than thoseseedlings transplanted at 12 and 14 WAS (Ibrahim et

al., 2013). Analyzing the effects of 21, 28 and 35 oldday cucumber seedlings Tanweer et al. (2005) foundthat 28 day old seedlings provided the maximum sur-vival rate after transplanting, the maximum fruit num-ber and the highest yield per plant, while the least val-ues of each of above parameters were obtained in 35day old seedlings. Similarly, Grabowska et al., (2007)have found that cauliflower plants obtained from 4-week old transplants gained a significantly highermarketable yield than plants obtained from 10-weekold transplants, and in an experiment aimed to deter-mine the effects of 40, 50 and 60 days old seedlings togrowth and yield of bitter gourd Jellani et al., (2016)have found that the minimum days to 1st picking, andthe maximum availability period, yield and net profitwere observed in 40 days old seedlings.

Contrary to the abovementioned evidences, in anexperiment conducted by Jankauskien et al., (2013),older tomato transplant (9–10 leaves) started to flowerbefore 5–6 leaves transplant, which also did also havethe least early yield, while the higher total yield wasproduced by 7–8 leaves transplants. In addition,Essilfie et al., (2017) showed that in chili pepper 44day old transplants had the highest yield, tallest plant,and highest number of branches and canopy width

compared to 30 and 37 old seedlings, and similarly,comparing 30, 45 and 60 days seedlings, Ali, (2016)have found that transplantation of 45 and 60 days oldcucumber seedlings produced the higher early andtotal yield with higher gross return and gross margin.

Obviously, the definition of best transplanting ageremains an open question. It seems that the optimumage for vegetable transplants depends on crop, the cellsize used and the growing conditions. Generally, rela-tively young vegetable transplants produce the beststand and fastest crop development. On the contrary,overgrown plants face difficulties regarding thereestablishment of their root system and commonlyexperience strong growth retardation after transplanti-ng. Older transplants might result in earlier yields;however younger transplants will produce compara-ble yields, but it take longer to do so. As a conclu-sion, there is no unique definition of the bestseedlings age or the most appropriate phenologicalstage of transplant age. As an example, the best trans-planting age for tomato in Canada is considered whenfirst flowers are showing, while seedlings over 5weeks old are less desirable in Mediterranean coun-tries (Zeidan, 2005).

Fertilization

Since the nutrient content of growing medium isgenerally quite low, vegetable transplants nutritiondepends on the frequency of fertigation and the con-centration of the feeding solution (Balliu et al., 2009).Overhead irrigation is the most used method growersuse to apply water and dissolved fertilizer nutrients tovegetable transplants during greenhouse production(Leskovar, 1998; Liu et al., 2012). Under this irriga-tion method, it is advised that the watering should beto cell capacity and with minimum drainage, in orderto prevent nutrients leaching from the growing medi-um. However, in specific cases the grower may applywater in excess of container requirements, so thatsome water and dissolved nutrients can leach out thebottom of the container and reduce the buildup of sol-uble salts in the container (Liu et al., 2012). Sub irri-gation, commonly referred to as ebb-and-flow orflotation systems is also using in some cases(Leskovar, 1998). However, sub irrigation tends toresult in elevated EC values in the upper regions ofcontainer medium compared to overhead irrigation(EC values are high toward the top of containers),which is a consequence of water evaporation from themedium surface while leaving behind soluble salts.Therefore, nurseries having irrigation water with highdissolved salts should be cautious with sub irrigation

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and consider the potential risk of elevated EC(Schmal et al., 2011).

The differential provision of water to root systemsinfluences the morphology, dry-matter partitioning,and physiology of transplants (Leskovar, 1998).Analyzing the effects of different irrigation methodsAhmed et al. (2000) found that sub irrigated lettuceand tomato transplants were taller, had longer leaves,a larger leaf area and more leaves than overhead irri-gated transplants. Significant effects of irrigationmethods (overhead versus sub irrigation) on fresh anddry weight, height and stem diameter of collards, kale,kale, pepper and tomato transplants were also report-ed by (Liu et al., 2012). When averaged across all fer-tilizer treatments, the fresh weight, dry weight, as wellas height of sub irrigated plants were generally greaterthan overhead irrigated plants. In addition theyemphasized that in most cases the optimum growth ofsub irrigated vegetable transplants was obtained insignificantly lower nutrient concentration than over-head irrigated transplants.

The necessity of scheduling the delivery of trans-plants to commercial growers requires that the growthrate of transplants to be strictly controlled. Practically,the most effective way of controlling transplantgrowth is to moderate their nutritional regime(Dufault, 1998). Unfortunately, despite numerouspublications regarding vegetable transplant nutrition itis not possible to summarize the one best way to growany vegetable transplant, simply because of manyinteracting and confounding factors that moderate theeffects of nutritional treatments (Dufault, 1998; Liuet al., 2012). In a review, (Zandstra and Liptay, 1999)does also underlines that in different climates, and fordifferent crops, there is a great disparity in feedingrequirements for plants. As an example they bringsthe case of very diverse recommendations regarding‘ideal’ N concentration for tomato feeding; 400 mg L-

1 in northern climate versus only 20 mg L-1 in a warmsoutherly climate.

Nitrogen is the most powerful nutrient elements tocontrol the growth of young plants. It has been the tar-get of most nutritional research on transplants and haslong been recognized to have the greatest effect ontransplant growth (Zandstra and Liptay, 1999).Significant growth effects of increased fresh and drymatter biomass, and increased growth rate due toincreased N concentration in nutrient solutions werereported in pepper and eggplants (0-300 mg L-1)(Balliu et al., 2007b; De Grazia et al. 2008 a), pepper(0-1200 mg L-1) (De Grazia et al. 2008b) and tomato(25-100 mg L-1) ( Balliu et al., 2007a). In fact, theoptimum N concentration in a nutrient solution is

highly crop depended. In a recent publication whichanalyses the effect of N concentration (50 mg L-1 to500 mg L-1) on the transplant quality parameters offive different vegetable crops (Liu et al., 2012), reportsthat the maximum shoot dry weight were received bythe application of nutrient solutions contained 100 mgL-1 N for pepper, 200 mg L-1 N for tomato, and 350 mgL-1N for collards, kale, and lettuce.

Internal plant tissue N status is important forimproving post-transplanting seedling growth(Widers 1989 after Dufault 1998), because a higher Nin seedling tissues at transplanting may be usedimmediately for growth than that available in the soil(Liptay and Nicholls, 1993). In addition there arereports that earliness in tomato is improved in trans-plants grown with 200 ppm versus 50 to 100 ppm(Melton and Dufault, 1991a). Masson et al., (1991)has also reported that tomato transplants grown with300 to 400 ppm N yielded earlier, but howeverincreased N rate did not affect the total yield. Quitesimilar results were also reported from (Edelstein andNerson, 2001) in muskmelon.

The way the nutrient composition influences thephotosynthetic products partitioning between the dif-ferent plant parts, namely leaves, stems and roots, isof an utmost importance for the quality parameters ofvegetable seedlings. Basically this is a question of thetrade-off between biomass investment in leaves,which would increase the supply of photosynthates,and in roots, which would stimulate the uptake ofnutrients and water (Poorter and Nagel, 2000).Indeed, while N content in transplants dry matter wasincreased with increasing N concentration in thenutrient solution, a steady decrease of root dry weightversus total plant dry weight and smaller NRE (Nrecovery efficiency) values was found in tomato(Balliu et al., 2007a), eggplant (Balliu et al., 2008)and pepper seedlings (Balliu et al., 2009). Commonly,the increase of N concentration enhances the relativegrowth rate (RGR) of vegetable seedlings (De Graziaet al. 2008 a). However, while at low concentration(25-100 mg L-1) any increase of N concentration, hasincreased the seedling growth rate by both; fasterexpansion of plant leaf area (LAR) and higher physio-logical plant productivity (NAR), at higher N concen-tration (100-400 mg L-1), increased RGR values wereachieved only due to higher values of LAR (Balliu et

al., 2007a; Balliu et al., 2007b) which might cost mis-balances between root and shoot growth in fast grow-ing transplants.

Although N is commonly applied to the container-ized vegetable transplants through the irrigation (eitheroverhead or floating), other methods are available. A

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simpler method is the addition of nutrients to growingmedia prior to seeding. According to (Grazia et al.,2008b), transplants growing on substrate premixedwith 600 mg L-1 N produced greater individual bio-mass than those receiving N by fertirrigation at both600 and 1200 mg L-1 rates. Even though there arereports that such a practice could improve transplantquality (Dufault, 1998), the risk of high salinity in caseof high doses of inorganic fertilizers and respectivesalt toxicity effects on young transplants should beconsidered. Thus, Zhang et al. (2017) warns that theaddition of high levels of N (5.0 g·kg-¹) with high (7.0g·kg-¹) or low (0.7 g·kg-¹) levels of K not only signifi-cantly decreased the root dry weight but alsodecreased leaf area of cabbage (B. oleracea) seedlings.

Despite of delivery method, too high nitrogen maycause overgrowth or even toxicity to the plants.Seedlings grown on high rates of nitrogen fertilizationare succulent, less resistant to dry weather and solarradiation, and this leads to a low rate of plant survivalafter transplanting in the open field (Balliu et al.

2017). In an experiment that evaluated the effect of Nconcentration in pepper seedlings in the range of 0 to1200 mg L-1 N was found that the application of 1200mg N L-1 led to transplants that had thinner leaves anda very low shoot:root biomass ratio (De Grazia et al.,2008b). Deleterious effects (reduced shoot DW) werereported in kale, lettuce, and pepper at concentrationgreater than 350–440 mg L–1N under sub irrigation,but not with overhead irrigation (Liu et al., 2012).

Phosphorus (P) and potassium (K) are also impor-tant to guarantee steady and balanced growth of veg-etable seedlings. Therefore, combined nutrient solu-tions with the presence of the three most used ele-ments; N (50-200 ppm), P (10-40 ppm) and K (100-300 ppm) are common in nursery application (Balliuet al. 2017). The effect of phosphorus on the vegeta-tive growth of vegetable transplants is not as dramaticas N, and often no effects on growth are noted(Zandstra and Liptay, 1999). In a two year experiment(Melton and Dufault, 1991a), found that stem diame-ter, leaf number, leaf area, fresh shoot weight, and dryshoot and root weights of tomato seedlings were high-er with 40 than with 10 mg L-1 P, with no furthereffect above 40 mg· L-1. Rare evidences are found toreport an effect of P concentration on yield of subse-quently field transplanted seedlings. In the abovementioned experiment of Melton and Dufault (1991a),P concentration did not affected tomato yields afterfield transplanting in both years of experiment. Thesame authors reports that production of quality trans-plants requires at least 225 mg· L-1 N, 45 mg· L-1 Pand 25 mg· L-1 K (Melton and Dufault, 1991b).

Weston and Zandstra (1989) cited by Dufault, (1998)reports that tomato transplants fertilized with 400 mg·L-1 N and 30 mg· L-1 P produced the greatest earlyand total yields.

No significant effects were found due to N:P ratioin the nutrient solutions used during nursery stage ofseedling growth. In an experiments conducted withmuskmelon seedlings, (Edelstein & Nerson 2001) didnot found any effect of N:P ratio (in the range of10:1; 5:1, 1:1) on root and shoot dry matter ofseedlings, nor in their yield after field transplanting.The ratio of N with P and K did not, also, have anysignificant effect on the relative growth rate (RGR) ofpepper and eggplant seedlings (Balliu et al., 2007b).Very low P concentration in nutrient solutions couldhowever lead to P deficiencies in vegetable seedlings.Deficiencies of P resulted in chloroplasts filled withstarch, and dark green and rich in anthocyanin leaves.Overall this may in fact reflect a type of increasedhardening not necessary desirable because of reducedrate of transplant root initial growth (Zandstra andLiptay 1999). The symptoms of P deficiency could bedeteriorated under low temperature growing condi-tions.

It is accepted that K level in the nutrient solutionscan be varied to a great extend with little effect ontransplant performance (Zandstra and Liptay 1999).The common response of vegetable seedlings toincreased K concentration in the nutrient solutionfrom 50 to 250 mg· L-1 is the reduction of rootgrowth, while no effects was noticed regarding theshoot growth (Melton and Dufault, 1991b).

There are nurseries imposing to plants a constantmild salt stress by treating transplants with concen-trated nutrient solution, compounded by high concen-tration of P and K. The salt stress reduces theseedlings growth rate and also stimulates a generativeprocess which favors the earliness (Balliu et al.

2017). However, high levels of nutrient concentrationdecreases stem diameter, plant height, leaf number,root dry weight, stem dry weight, leaf dry weight,photosynthesis and stomata conductance (Hossain et

al., 2010).

Plant biostimulants

Plant biostimulants contain substance(s) (humicacids, seaweed extracts and protein hydrolysate; Collaet al., 2017) and/or micro- organisms (mycorrhizaand PGPR) whose function when applied to plants orthe rhizosphere is to stimulate natural processes toenhance nutrient uptake, nutrient efficiency, toleranceto abiotic stress, yield and crop quality (Abdel Latef

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and Chaoxing, 2011; Calvo et al., 2014; Rouphael et

al. 2015; Kapoor and Singh, 2017). Among them, themost known categories are:

microbial inoculants,•humic acids,•fulvic acids,•protein hydrolysates and amino acids•seaweed extracts (Calvo et al., 2014).•

Although their effects are crop depended, the mostinteresting biostimulants for vegetable nursery indus-try are microbial inoculants which include free-livingbacteria, fungi, and arbuscular mycorrhizal fungi(AMF) (Calvo et al., 2014). These are also named asbiofertilizers and when applied to seed, plant surfaces,or soil, promote growth by several mechanisms suchas increasing the supply of nutrients, increasing rootbiomass or root area, and increasing nutrient uptakecapacity of the plant (Cavagnaro et al., 2006;Rouphael et al. 2015). In addition, improved plant tol-erance to salinity (Dodd and Perez Alfocea, 2012;Porcel et al., 2012; Meça et al., 2016), the biologicalcontrol of plant pathogens (Heydari and Pessarakli,2010) and improvement of the chemical quality ofvegetable species (Baum, 2015) are also largelyacknowledged.

Due to their beneficial effects on terrestrial ecosys-tems, AM fungi are widely used in plant nurseries toimprove the growth of economically importantspecies (Corradi and Bonfante, 2012), by enablingstronger growth of seedlings in nursery and improvedperformance after planting in the field (Raviv et al.

1998; Al-Karaki 2017). Thus, it is reported that AMfungi have significantly increased shoot and root drymatter and plant height of AMF inoculated pepperplants (Al-Karaki 2017), improved the stand estab-lishment rate of tomato seedlings and enhanced theirresistance to soil salinity immediately after transplant-ing (Vuksani et al., 2015; Balliu et al., 2015) andimprove the yield of AMF pre inoculated tomatoplants (Al-Karaki, 2006; Abdel Latef and Chaoxing,2011; Vuksani et al., 2015). Increased yield due toAM inoculation of transplants were also reported incabbage (Raviv et al. 1998), onion (Makus, 2004),and green pepper (Al-Karaki 2017). Despite manyencouraging reports on profits of using AMF, oneshould mind that the efficiency of inoculation withAMF on their host plants is controlled by the geno-type combinations (host plant × AMF), soil propertiesand by the inoculation method (Baum, 2015). As anexample, different from above authors Makus, (2004)reported only improved earliness due to mycorrhizalinoculation of tomato transplants, but not of the finalyield, fruit number and average fruit weight.

The use of free living bacteria, collectively knownas plant growth-promoting bacteria (PGPB) is alsoincreasing in vegetable seedling production. Theytypically promote plant growth in two ways: directstimulation and biocontrol (Gamalero and Glick,2014) and includes activities such as; nitrogen fixa-tion, phosphate solubilization, iron sequestration, syn-thesis of phytohormones, modulation of plant ethyl-ene levels, and control of phytopathogenic microor-ganisms (Gamalero and Glick, 2014, Picta andPastucha 2008). Enhanced growth effects due to inoc-ulation of a certain strain of Burkholderia sp. werereported from (Nowak et al. 2004) in tomato, cucum-ber and sweet pepper. Similarly, (Ekinci et al., 2014)have showed that the use of strains of Bacillus mega-

terium, Pantoea agglomerans and Bacillus subtilis

was associated with increased fresh shoot weight, dryshoot weight, root diameter, root length, fresh rootweight, dry root weight, plant height, stem diameter,leaf area and chlorophyll contents of cauliflowertransplants. However, it is noticed that the degree ofgrowth stimulation varied with species and method ofinoculation, and that the early growth of transplantsand developmental stimulation caused by the bacteri-um did not always translate into significant effects onyield (Nowak et al. 2004).

One of the most used microorganisms in agricul-ture is Trichoderma sp., which is commonly used dur-ing the composting process of organic wastes.Composts inoculated with Trichoderma sp. have beeneffective for disease control of different fungalpathogens, such as Choanephora cucurbitarum,Rhozoctonia solani, Sclerotinia cepivorum, Botrytis

cinerea, Phytophthora sp. and Fusarium oxysporum

(López-Mondéjar et al 2011). According to them, thesynergistic effect of compost and Trichoderma hasfavored a higher melon seedling growth in compost-based growing media production. Recent papersclearly demonstrated that fungi belonging toTrichoderma genus can also alleviate plants from abi-otic stresses (salinity, drought, extreme temperature)(Balestrini et al., 2017).

Recently, plant-derived protein hydrolysates (PHs)have gained prominence as plant biostimulantsbecause of their potential to increase the germination,productivity and quality of a wide range of horticul-tural and agronomic crops. PHs can improve soil res-piration, microbial biomass and activity, sincemicroorganisms can easily use amino acids and pep-tides as C and N source. They also impacts the plantnutrition by forming complexes and chelates betweenpeptides/amino acids and soil micronutrients (i.e.,Cu,Fe, Mn and Zn), therefore contributing to nutrients

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availability and acquisition by the root system (DePascale et al., 2018). The application of PHs, eitherby foliar or root (substrate drench) application canalleviate the negative effects of abiotic plant stressdue to salinity, drought and heavy metals (Colla et al.,2017). Recently, there are interesting experimentsindicating that PHs could be applied as a componentof a seed coating blend, too. Amirkhani and coauthors (2016) have reported that a 10% suspensionof soy flour mixed with cellulose, and diatomaceousearth, applied with rotary pan seed coating equipmentat 25, 30, 40 and 50% of the seed weight in broccoliseeds, although have reduced the percentage germina-tion and the germination rate compared with the nontreated control, have shown significant improvementson seedling root and shoot growth compared withcontrols, at the tenth and the thirty day after applica-tion. Yet, despite several encouraging evidences, theapplication of PHs in the nursery industry is still rare.

Growth control and hardening techniques

Vegetable transplants can quickly outgrow theoptimal size for shipping and transplanting, and thuscausing a major concern for commercial nurseries,especially when transplanting is delayed because ofinclement weather at the time of field establishment(Agehara and Leskovar, 2015). Therefore, transplantconditioning alternatives were commonly applied inorder to obtain shorter stem, stronger and more elastictransplants to facilitate long distance transport andbetter resist potential adverse post transplant condi-tions. Effective growth control and transplant harden-ing can be achieved through proper combination ofindividual practices such as; day-night temperaturedifference, less frequent watering, increased ventila-tion, restricted fertilization, mechanical hardening anduse of plant growth retardants. In any case, transplanthardening needs to be done without totally stoppingthe growing process. Over-hardening will require toomuch time for growth to resume and earliness will bedelayed (Prunty et al., 2015).

Day-night temperature difference (DIF)

Plant stem growth rates of some vegetable speciesare positively correlated to the difference in day tem-perature (DT) and night temperature (NT), which istermed DIF (DIF=DT-NT). Using DIF helps to keepthe seedlings compact in size without using growthregulators (Erwin and Heins, 1995). Since high tem-peratures during the first three to four hours after sun-rise can cause considerable elongation in vegetableseedlings, it can be mitigated by keeping the green-

house temperature the few hours early in the morning,4-5 ºC cooler than night temperature (Wien, 1997).

Irrigation deficit / vapor pressure deficit

Water deficit stress can be applied in varyingdegrees to seedlings. Its main effect at moderate lev-els is the reduction of plant leaf area, before photo-synthesis or respiration is affected, with the result of ahigher rate of dry matter accumulation per unit leafarea (Liptay et al., 1998). A desirable level of waterrestriction results in stocky, stress resistant seedlingsable to withstand environmental stresses after trans-planting outdoors. However, as plant transpirationrate is largely affected by environmental conditions,determination of irrigation timing before imposingtoo much water stress requires a great deal of experi-ence (Balliu et al. 2017). Severe water deficits resultsin overstressed seedlings with stunted growth andpoor establishment when transplanted into the field(Liptay et al., 1998).

There are evidences that hardening with a highvapor-pressure-deficit (VPD) can decrease water con-ductance and thereby enhance drought tolerance ofcucumber seedlings by reducing transpiration(Shibuya et al., 2017). This is particularly useful dur-ing transplant establishment. The high-VPD harden-ing could enhance the tolerance to short-term droughtwithout stomatal or non-stomatal limitation of photo-synthesis (Shibuya et al., 2017). Someone shouldmind that after plants are hardened, they will require afull watering before outplanting and good soil mois-ture availability at the outplanting site will neededduring their early establishment in the field (Jacobs et

al., 2014).

Low temperature conditioning

Low temperature represents one of the mostharmful abiotic stresses affecting temperate plants(Janská et al., 2010). Chilling stress causes manychanges in the metabolism of plants which lead tobiochemical, physiological and morphological modi-fication and promote plant’s defense systems againststress factors (Kalisz et al., 2014). In fact, most tem-perate plant species have evolved a degree of coldtolerance, the extent of which is typically dependenton a combination of the minimum temperature expe-rienced and the length of exposure to cold stress(Janská et al., 2010). Temperate plant species canacclimatize to low nonfreezing temperature throughsynthesis of cryoprotectans, increasing activity ofantioxidative enzymes and launching the synthesis ofnon-enzymatic antioxidants (Kalisz et al., 2014).Therefore, acclimated plants should better withstand

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field conditions and they should be characterized bymore intensive growth.

Chilling treatment, applied at juvenile stage, canalso induce transition to the generative phase. Positiveeffects of low temperature conditioning were reportedin broccoli Grabowska et al., (2013) and Kalisz et

al., (2014). According to Grabowska et al., (2013),dark-chilling (2°C/2 weeks), 4-week-old broccoliseedlings were more advanced in the generativedevelopment the non-chilled control plants. However,chilling of oldest, 8- and 10-week-old seedlings hasnegatively affected the inflorescence quality, throughplaned and spread shape of heads and non-uniformshape of flower buds. Similarly, Kalisz et al., (2014)have found that chilling of broccoli seedlings for 1week or 2 weeks at 6, 10 and 14°C have positivelyinfluenced the weight of broccoli heads, and the heav-iest heads were obtained as a result of transplant chill-ing at 6°C. However, the cold stress has negativelyaffected the relative growth rate (RGR) of cauliflowerseedlings during the cold treatment period. Due tothat, end of nursery period they have recorded asmaller dry weight (W) and leaf area (LA) comparedto common non treated seedlings (Kuçi et al., 2012).

The exposure of plants of a tropical or subtropicalnature, such as members of the Cucurbitaceae andSolanaceae, to chilling temperatures may stunt plantgrowth, induce wilting and leaf necrosis, and increasedisease susceptibility (Korkmaz and Dufault, 2001).Experiments conducted with watermelon and melonhave evidenced that exposure of watermelon andmelon seedlings to cold stress in the range from 9 to81 hours, was followed by decreased seedling’s shootand root fresh and dry weights, leaf area, chlorophylland carbohydrate contents. Plants cold stressed for upto 81 hours transpired more for first week after trans-planting than those exposed to shorter periods of coldstress, and vining (date first runner touched theground), flowering, and fruit set were delayed signifi-cantly as cold stress hours increased. Although earlyyields were unaffected, total yields decreased linearlywith increasing hours of cold stress (Korkmaz andDufault, 2001).

Mechanical conditioning

Stem elongation can be reduced by mechanicalstimulation such as brushing the upper canopy, shak-ing, and vibration by wind or forced aeration. Thesemechanical conditioning methods inhibit stem elonga-tion by stimulating ethylene production, which in turninhibits cell elongation and promotes stem thickening(Agehara & Leskovar 2014).

Brushing has proved successful in Solanaceous

crops, (tomato, pepper, and eggplant), but care shouldbe taken with cucurbits, which are more fragile andcan be damaged by brushing. Mechanically condi-tioned transplants of processing tomatoes resumedgrowth after transplant shock as quickly as diduntreated plants (Garner and Björkman, 1999), andsubsequent canopy development and yield frommechanically hardened tomato (Garner andBjörkman, 1999) and melon (Ayastuy et al., 2011)plants was similar to those from control plants.However, mechanical stress by vibration during trans-portation is known to negatively impact the trans-plants. It could physically damage the transplants orpromote ethylene production which in high concentra-tion induces adverse physiological impacts such asflower abortion or leaf yellowing especially duringlong distance transportation (Kubota et al., 2013).

Plant growth retardants and plant hormones

Plant growth retardants, such as daminozide,paclobutrazol, and uniconazole, are used in ornamen-tal plug production to improve plant compactness,marketable value, and shelf life. There are also reportsshowing that melon seedlings hardened with lowpaclobutrazol concentration (10 mg L-1) showed agood post-transplanting growth which was reflectedin a higher crop yield compared to the control(Ayastuy et al., 2011). However, regulations for theiruse are rather restrictive for vegetable crops, and atpresent, uniconazole is the only registered chemicalfor use in solanaceous crops (Agehara and Leskovar2014, 2015). In fact, the growth modulating effect ofuniconazole appears to be limited to height control,which is beneficial for producing compact transplants,rather than as a growth holding strategy (Agehara andLeskovar, 2015).

While growth retardants acts mostly as gibberellinbiosynthesis inhibitors (Ayastuy et al., 2011), abscisicacid (ABA) is a plant hormone, which triggers adap-tive growth responses to water stress (Agehara andLeskovar, 2015). Its immediate physiologicalresponse is stomatal closure, which in turn inhibitsphotosynthesis and transpiration-driven mass flow ofnutrients, whereas the morphological response is inhi-bition of leaf expansion. Thus, the overall effect ofABA is shoot growth suppression (Agehara andLeskovar, 2015). It is proved that treatment of tomatoseedlings with an ABA analog prior to transplantingcan help prevent the desiccation that is an importantpart of transplant shock without compromising long-term growth and yield potential of crops (Sharma et

al., 2005; Agehara and Leskovar, 2014). Earlier,Berkowitz and Rabin (1988) reports that ABA appli-

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cation can reduce transplant shock and increase yieldof bell pepper. Different from long lasting effect ofuniconazole, application of ABA at a specific rate andlength of exposure (5-day ABA-exposure at 1.0 mM)in pepper seedlings could eliminate the risk of stress-ing young seedlings to the point of physiologicalinjury as previously reported for some hardening tech-niques (Agehara and Leskovar, 2015; Ban et al.,2017). However, the efficiency of ABA application iscrop and time depended. Multiple dose applicationsinitiated at the cotyledon plant growth stage wereproved to be more effective to control transplantheight of bell pepper seedlings (Biai et al., 2011).

Abstract

Producing ‘good quality’ vegetable transplantsdemands a series of horticultural practices and inter-ventions which overall enable the raise of plant fromstart to finish by slow, steady, uninterrupted growthand with minimal stress and guarantee appropriategrowth rate, optimum root to shoot ratio, good standestablishment capabilities and high yield potentials ofnew plantlets. Because growth of vegetable transplantis a summary of many interacting and confoundingfactors which moderate each other, rather than follow-ing standardized recommendations, good quality veg-etable seedlings can only be produced through carefulselection of substrate, or substrate mixtures, appropri-ate manipulations of temperature regime, fertilizationregime, the module size and transplanting age, andproper combination of plant growth promoters andgrowth control and hardening techniques.

Key words; substrate, peat substitutes, module size,transplant age, fertilization, plant biostimulants, hard-ening.

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