The Timeless Contribution of Rootstocks towards Successful Horticultural Farming: From Ancient Times to the Climate Change Era
Ioannis E. Papadakis
DOI : 10.3844/ajabssp.2016.137.141
American Journal of Agricultural and Biological Sciences
Volume 11, Issue 4
Grafting is ahorticultural practice by which living vegetative plant organs (root, stem and/orbuds) of either the same genotype or, usually, different genotypes belonging tothe same species, genus, subfamily and/or family, are tightly connected to eachother to form a double-compound plant. In fact, it is a method of asexual(clonal-vegetative) propagation of plants. The basal part of a grafted plant iscalled "rootstock" ("stock" or "understock") that usually comprises the rootsystem and a portion of the lower above ground plant stem (trunk). The restabove-ground part of a grafted plant is termed "scion". Following successfulgrafting, the resulting composite plant continues growth as one individualorganism according to its life-span genetic potential (annual, biennial orperennial). Early humans observed non-anthropogenic (natural) grafting inforest natural ecosystems and tried to mimic nature and, thus, applyinggrafting (artificial grafting) not only in domesticated forest ecosystems, butalso to cultivated plants including fruit trees, grape vines, floricultural cropsand vegetables. In the writings of the Greek physician, Hippocrates of Kos(460-377 BC), and Greek philosophers, Aristotle (384-322 BC) and Theophrastus(371-287 BC), grafting was clearly reported as a well-established agronomicalpractice at that time period.
Froma practical perspective, the existence of words referring to the present usageof the terms "rootstock", "stock" and/or "understock" is inextricably linkedwith the existence of the grafting per se. In fact, without artificialor natural grafting, there is no "rootstock" and vice versa.
Historically, critical roles of rootstocks have evolvedthrough time periods expanding from years to millennia. This is due to dramaticchanges in farming management requirements and/or environmental, climatic andphytopathological conditions existing in the field of horticulturalcrops -including fruits, vegetables and ornamentals- from the ancient times up tothe era of climate change.
For many centuries,grafting was mainly used as a means of non-sexual (asexual) reproduction ofplant genotypes which were difficult to root by using other methods of asexualpropagation, such as cuttings, suckering, stooling and layering. The aim wasthen the successful cultivation of genotypeswith individual desirable agronomical and horticultural traits by grafting themonto pre-existed wild plants that produced commodities of inferior quality.Although the latter plants were used as rootstocks, their contribution was toospecific. They were just used as a background plant, since they had simplyprovided their root system and a part of their vascular system to thenew-composite plants formed after grafting. Through the following years andcenturies, macro observations of agronomists and farmers have given motivationto intensive scientific research projects, carried out all over the world,aiming to breed and/or select new superior genotypes for using them asrootstocks.
The second half of the 19th century, the French wine industry was reallyin panic when the insect phylloxera (Daktulosphaira vitifoliae Fitch)nearly destroyed the grape and wine production. Until nowadays, this insect hasnever been eradicated or controlled by other chemical or cultural means andstill remains a serious problem in susceptible wine/grape-producing regionsglobally. The ultimate solution remains the use of phylloxera resistantrootstocks, originating from native American Vitis species like V.riparia, V. berlandieri and V. rupestris (Mudge et al., 2009). Additionally, from thebeginning of the 20th century, the aphid-transmitted Citrus Tristeza Virus (CTV)has destroyed thousand hectares of citrus trees (more than 100 millions) allover the world, grafted on sour orange (Citrus aurantium L.) rootstock,which has been proved to be very susceptible to CTV. In this case, the bestsolution has also remained the prophylactic grafting of various citrusvarieties on CTV resistant rootstocks, such as trifoliate orange (Poncirus trifoliatasp.) and its grapefruit (Citrumelos) or sweet orange (Citranges) hybrids.Before the appearance of the CTV, sour orange had been widely used as arootstock to protect citrus against harmful effects of Phytophthora spp (Roistacher et al., 2010). Moreover, as a result ofthe limited availability of arable land, the high demand for off-seasonvegetables and the intensive production practices with limited crop rotations,vegetables are often cultivated under adverse conditions (Savvas et al., 2010). In fact, the intensivecontinuous monoculture of vegetables has led to the establishment andexponential growth of many soil-borne diseases, like Verticillium spp.,Fusarium spp. and bacterial wilts. This resulted in serious damages inplants, lower yields and production ofinferior quality commodities. To overcome this problem, in the middle of the20th century, Japanese and Korean researchers and farmers were the first whosucceeded in the large scale production of various commercial cultivars,belonging to Solanaceae and Cucurbitaceae families, grafted on appropriatedisease-resistant rootstocks. Today, grafting of vegetables is a widely spreadhorticultural practice that is commercially exploited in many countries all over the world (Mudge et al., 2009; Lee et al., 2010; Schwarzet al., 2010).
Although biotic factorsare usually the main reasons of using rootstocks in fruit, ornamental andvegetable crops, there are many other reasons for grafting commercial varietieson different rootstocks. For example, grafting represents an important means toavoid or reduce yield losses due to salinity stress. The benefits of graftinghigh-productive genotypes onto salinity-resistant rootstocks include bettergrowth and higher yield, higher photosynthesis, water content, elevatedconcentrations of osmoprotectants, antioxidants, abscisic acid and polyaminesin leaves and lower contents of sodium and/or chloride in various plant parts,compared to un-grafted plants (Colla et al., 2010; Penella et al., 2016). Rootstock can also affect various aspects of plantnutrition under conditions of mineral deficiency or toxicity. Indicatively, therole of the rootstock is critical when citrus trees are cultivated in orchardsirrigated with water containing high boron. More specifically, 'Clementine'mandarin and 'Navelina' orange plants grafted on 'sour orange' rootstock provedto be more sensitive to boron excess than those grafted on 'Swingle citrumelo'.This is mainly due to lower absorption of boron from the roots of 'Swingle citrumelo'and to higher boron retention in the stem of this rootstock, resulting inlesser boron accumulation in leaves of the two aforementioned cultivars whenthey are grafted on 'Swingle citrumelo' (Papadakis et al., 2004a; 2004b).
Besides from the premium contribution of rootstocks in the adaptation ofgrafted varieties on adverse abiotic and biotic conditions, their effects ondifferent stages of farm management and cultivation practices per sehave been well documented. For instance, the use of shading nets is a common,although too expensive, procedure to decrease the impact of high solar irradiance stress during thespring-summer harvesting period, especially in Mediterranean areas. The use ofsome rootstocks has been reported to be an economically-efficient alternativemeans to maintain commercial fruit yield and quality under non-shadedgreenhouse conditions (Lopez-Marin et al., 2013). Therefore, the overall benefit is at least double,decreasing production costs (there is no need to buy shading nets) andprotecting the environment (less manufacture of shading nets). Similarly,rootstock can affect the overall performanceof scion, when plants are exposed to suboptimal air and soil temperatures,resulting in either prompt-earlier vegetative and reproductive growth (fruitset, growth, maturation and harvest), or extension of the growing season. Therespective earlier and/or delayed harvest, combining with higher yieldssucceeded over the time together with the production of better qualitycommodities, result in a significant increase of farmers' net income (Schwarzet al., 2010). Finally,the establishment of high density fruit treeplantings is exclusively based on dwarfing rootstocks which not only inducescion precocity and higher yields per unit of orchard area, but alsosubstantially decrease labor and production costs per each unit of producedfruits (Robinson, 2011).
The beneficial impactsof the rootstocks may also be extended beyond harvest. Some indicative examplesare addressed below, although more research has to be carried out in the futurefor all related issues. Firstly, rootstock can improve the main quality indicesof various harvested products, including the overall nutritive value andaromatic substances of horticultural products (Rouphael et al., 2010; Orazem et al., 2011; Krumbein, 2013; Legua etal., 2014). They may alsoenhance the levels of various particular health-promoting substances (Rouphael etal., 2010; Turhan et al., 2011; Chavez-Mendoza et al.,2013. Krumbein, 2013; Legua et al.,2014; Cardenosa et al.,2015) or decrease those of health hazardous ones (e.g., organic pollutants) (Schwarzet al., 2010). Secondly,postharvest storage and shelf life of fruits and vegetables are alsorootstock-dependent (D'Hallewin et al., 1993; Ritenour et al., 2004). Thirdly, the effect of rootstock may also be criticalin the processing of fruits and vegetables. For instance, since pH adjustmentis a significant part of the total cost of vinification, the identification ofrootstocks that result in lower potassium concentrations and pH values in grapemust and wine is of high interest for thewine industry (Walker and Blackmore, 2012; Bouza et al., 2013), particularly in world'sregions producing wine with relatively high values of pH. In the latter case,suitable rootstocks could help in reducing winemaking production cost andimproving some key quality attributes of wine.
It is well-known that global warming and the impact to the environment areissues of increasing importance and a subject of international debate, mainlyover the last years, attracting the attention of several scientific studies (Landiand Benelli, 2016), including research on new rootstocks. In particular, at thecurrent era of the climate change, breeding and selecting rootstocks withspecific traits (e.g., heat, drought, salinity, waterlogging and/or floodingtolerance), due to continuous changing environmental issues, will help insuccessful growing of fruits and vegetables even in the most affected areasworldwide. As a result of global warming, the accumulated winter chill iscontinuously decreasing in many regions of the world (Baldocchi and Wong, 2008;Atkinson et al., 2013), a fact thatmay cause serious limitations in the cultivation of temperate-zone deciduousfruit and nut tree species due to incomplete dormancy release. Based on theliterature (Ghrab et al.,2014), the role of rootstocks could be vital thanks to their beneficial effectson the scions' buds requirements for chilling units to overcome dormancy.However, more research is needed to evaluate more appropriate rootstock-scioncombinations for areas having relatively low chilling hours. Moreover, as theclimate continues to be warmer, the frequency, the intensity and the durationof serious precipitation events is expected to increase, enhancing theprobability for soil waterlogging/flooding in many areas of the world (Kundzewiczet al., 2014). Selectingand/or creating new rootstocks to resist anaerobic soil conditions are reallyimportant challenges for horticultural breeders.
The research onrootstocks is really multidisciplinary, requiring the contribution ofdifferent, usually well-separated, horticultural and agricultural branches.Since 1900s, huge funds have been invested worldwide in breeding, selecting andevaluating genotypes having the proper traits to be used as superiorrootstocks. According to an independent assessment commissioned by the AustralianGrape and Wine Authority (AGWA), every dollar invested in vinerootstock-associated research generates $11 in return to users of rootstocksonly across the Australian wine industry (AGWA, 2015).
A superior rootstock enables the genetically composite plant, viz.each one specific rootstock-scion combination, to adapt to a number ofdifferent factors including root- and/or shoot-associated both abiotic [e.g.,drought, waterlogging, flooding, salinity, mineral toxicity, mineraldeficiency, heavy metal toxicity, heat, cold, low chilling units, low soiltemperature, low soil oxygen, wet or poorly drained soils, soils with highcalcium carbonate content, high or low soil pH (Papadakis et al., 2004a; 2004b; Colla et al., 2010; Hartmann et al., 2013; Savvas et al., 2010; Ghrab et al., 2014; Castle et al., 2016)] and biotic [e.g., fungaland bacterial pathogens, virus and virioid diseases, insect and nematodes (Mudgeet al., 2009; Shokrollahet al., 2009; Roistacheret al., 2010; Louws etal., 2010; Castle et al., 2016)]. Furthermore, rootstock cantheoretically affect every characteristic of the scion [e.g., overall plantgrowth, plant shape, fruit shape, fruit weight, fruit color, fruit firmness,content of phytochemicals in fruits and juices and postharvest storability andshelf life of fresh fruits (Ritenour et al., 2004; Rouphael et al., 2010; Orazem et al., 2011; Turhan et al., 2011; Castle et al., 2016). In applied horticulture, all these factors play adeterminant role for the final choice of the best rootstock/s for each onegiven grove having its own particular characteristics.
Although there arethousands of research papers describing the special effects of variousrootstocks on agronomical, phytosanitary, phenotypical, morphological,anatomical, physiological and biochemical aspects of scions, belonging to awide range of plant species, the implicated mechanisms are still quite obscureand need to be elucidated in the future. Recently, Aloni et al. (2010)and Goldschmidt (2014) reported that long-distance protein, mRNA and small RNA graft-transmissible signals currently emerge asnovel mechanisms which regulate nutritional and developmental root/toprelations and may play a crucial role in investigating basic processes inrootstock-scion communication. They further noted that available moleculartools (e.g., gene silencing) today are expected to advanceour understanding and eventually resolve the long standing grafting mysteries,relating mainly to the interactions of each rootstock-scion combination(Aloni et al., 2010; Goldschmidt,2014). Apart from that, anatomical, physiological and genetic basis forcompatibility between each rootstock-variety combination need to be examined ina wider biological context (Mudge et al., 2009). Undoubtedly, such studies are easier to be carried outwith herbaceous vegetables or some plant models, compared to fruit andornamental perennial trees. The very short biological cycles of vegetables andother species used widely as model plants offer the opportunity to researchersto investigate the effects of rootstocks on all critical vegetative andreproductive growth stages of plants, including studies not only at genetic,but also epigenetic level. In any case, breeding, selecting and testing of newrootstocks with desirable attributes will greatly benefit from theunderstanding of how exactly the rootstocks can affect the growth anddevelopment of scion. However, since the results from several horticulturalplants may not be applicable to other species, research on species-specific graftingresponses is required. Moreover, given that most of the published paperspresent the response of grafted plants to only a single stress, in the absenceof any other biotic or abiotic one, the question remains whether the concurrentimposition of two or more stresses alters dramatically the specific responsesof a rootstock to each stress separately. Towards acquiring a better knowledgeconcerning the rootstock-scion interactions, every scientific paper about basicand/or applied research on the rootstocks is well-welcome for publication,after rigorous peer review process, to the American Journal of Agricultural andBiological Sciences (AJABS).
Overall, each rootstock is a multi-dynamic horticultural tool functioningas a bridge, i.e., as anartificial Trojan horse, by which a cultivar with important agronomical traitscan be ideally cultivated to a farm with too specific unfavorableenvironmental, phytosanitary and/or cultural management conditions. Actually, theuse of appropriate rootstocks increases the tolerance of cultivated varietiesunder adverse biotic and abiotic conditions. Rootstocks can further affectsignificantly the overall behavior of scions, including specific growthcharacteristics, yield efficiency and the main quality indices of horticulturalproducts. As a result, the production of quantitatively more and qualitativelybetter agricultural products and by-products can be achieved by using suitablerootstocks. From a practical point of view, the choice of the most appropriaterootstock (s), for a given farm, variety and cultural management conditions, isconsidered as an environmentally friendly and sustainable farming practicelowering the use of agrochemicals (e.g., pesticides, fertilizers) and othercostly inputs (e.g., water). It also helps in protecting the environment, theconsumers and the farmers, per se,but also in decreasing the production costs and in increasing the overallfarm's profitability. Except for the conventional farms, the role of rootstockis undoubtedly vital under conditions of sustainable, integrated and organic,farming. Concluding, the unique value of rootstocks as key factors for thesuccessful cultivation of horticultural crops is unquestionable, rendering themtimeless allies for farmers growing vegetables, grape vines and/or fruit trees.
The author would liketo thank Prof. I. Therios, Dr. N. Kavroulakis, Dr. G. Doupis and Dr. A.Malandrakis for their valuable and constructive comments and suggestions thatcontributed to improve the quality of the current editorial.
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© 2016 Ioannis E. Papadakis. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.