Rootstock Shoot Research Articles

The Effect of Different Seed Cutting Treatments and Concentrations of BAP for the Successful In Vitro Micrografting of Mangosteen (Garcinia mangostana L.)

The efforts of the rooting of regenerants resulting from gamma-ray irradiation require plant tissue culture, which known as micrografting. This technique can help irradiated regenerants to develop a well root system, by combining non-rooting shoots with rooted in vitro cultured shoots of plant rootstock. The purpose of this study was to examine the effect of seed explants cutting and the application of different BAP concentrations for the successful micro-grafting of mangosteen in vitro. This experiment employed Complete Randomized Design (CRD) Factorial with two factors and ten replications. The first factor was the cutting treatments of mangosteen seeds explants for rootstock shoots, consisting of 2 types of seeds: uncut and cut seeds. The second factor was BAP concentrations: BAP 0 mg/l and BAP 2 mg/l. The results showed that the division of the seeds had an influence on the results of micro-grafting mangosteen in vitro. Micrografted mangosteen, which rootstock applied from undivided seeds, possessed faster growth, longer shoots, and produce more leaves compared to rootstock shoots from the divided seeds. BAP concentrations also contributed to the results. The application of BAP 2 mg/ demonstrated better effect on all variables observed. There were no interactions between seed divisions and BAP concentrations in all observed variables.

Using New Micro Grafting Techniques in Grape

Micro-grafted Egyptian grapes were evaluated under in vitro conditions. In vitro Flame seedless cv. scions were grafted on Freedom rootstock shoots. Two methods were used for micro-grafting; alginate matrix and rhizogenic calli. Fluorescein stain was used to sap transport detection through vessels of both scion and rootstock. Biochemical and molecular analyses were used to evaluate successful micro-grafts. Obtained results showed that, In vitro derived shootlets had satisfactory growth. The culture of derived explants formed a successful graft union by alginate matrix (75%) compared to rhizogenic calli (50%) grown shootlets maintained by some roots and leaves formation. There was a significant difference between the two methods in forming a successful graft union and the subsequent growth-related traits. Grafted explants were tested to salinity stress of NaCl: CaCl2 1000, 2000 and 3000 ppm. The shoot and root related growth characteristics of the alginate matrix micro grafts were more tolerant to salinity than rhizogenic calli. However, raising salinity level led to growth decrease but alginate plantlets continued forming leaves. RAPD- PCR analysis cleared genetic changes in graft union area reflects a complete scion- rootstock combination, also a hereditary relationship with both was detected and proved micro-grafting success with the two methods. To sum up, micro-grafting is a suitable alternative propagation tool leading to higher growth potential of grafted plants and improving their ability to tolerate different stresses.

Alpha monofluoro substitution at C5 in homotyphasterol enhances shoot production and multiplication rate of in vitro-grown marubakaido apple rootstock shoots

KEY MESSAGE: 5 fluoro-typhasterol is more effective than the most potent natural brassinosteroid, brassinolide, towards stimulation of shoot production in the marubakaido apple rootstock. Brassinosteroids (BRs) comprise a class of low-abundance plant steroids required for normal plant growth. Here we demonstrate that treatment of in vitro-grown shoots of the marubakaido apple rootstock with 2.5 µg per shoot of 5α-monofluoro homotyphasterol (5F-HTY), a synthetic derivative of the natural BR homotyphasterol (HTY), resulted in significant enhancement in the formation of primary lateral shoots. This treatment also resulted in increased length of primary lateral shoots and main shoots. This growth-stimulatory effect led to a significant increase in the multiplication rate for the rootstock. In contrast to what was found for 5F-HTY, neither HTY nor the synthetic derivative 3α-monofluoro homotyphasterol (3F-HTY) were able to significantly stimulate shoot formation. HTY and 3F-HTY were not able to stimulate primary lateral shoot elongation as well. However, both HTY and 3F-HTY were able to significantly stimulate main shoot elongation. The 5F-HTY-driven enhancement of the multiplication rate described here demonstrates the potential of this compound to improve micropropagation techniques, not only for the marubakaido apple rootstock.

Evaluation of grafting for processing tomato production in California’s Central Valley

The benefit of grafting for California processing tomato production in the lower Sacramento Valley was evaluated in commercial tomato fields, one each year from 2016 to 2017. The investigation focused on fruit yield and quality from rootstocks grafted onto popular cultivars. Treatments included 3 rootstocks (Maxifort, Multifort and DRO 138TX) and 3 scions (‘H 8504’, ‘DRI 319’ and ‘HM 3887’) plus each of the non-grafted scions as controls. In year 2, ‘N 6428’ replaced ‘H 8504’. Grafts were done by hand, at Growers Transplanting, Inc., coastal facility in Salinas, California. Rootstocks and scions were grown and later healed in 230-cell trays (~28 cc cell volume) within their commercial greenhouses in Salinas. The grafted plants were finished in their Central Valley greenhouse ahead of field planting. Within the field, seedlings were transplanted with the grower’s ‘finger-style’ Mechanical Transplanter® as per norm. The trial design was a randomized complete block with 4 replications. Plots were a single plant line on a 1.5-m centered bed for 30 m. Planting depth was 10 cm or more below the bed top without concern for the depth of the graft union which was buried. Emergence of wild rootstock shoots (which were removed) was minimal in year 1 and none in year 2. Irrigation was only with buried drip at a depth of 25 cm. Harvest was performed mechanically using the grower’s harvester and crew. Yields increased, on average, 10% in 2016 and 19% in 2017. Scion choice made a difference in yield outcome, but rootstock choice did not. Brix performance was related to scion, but not to rootstock. Factorial interaction did not occur between scion and rootstock for yield or Brix in either year, although grafting reduced Brix in 2017. Grafting for processing tomato production was not economically attractive without substantially higher yield gain, lower cost of grafting and/or reduced planting rate. The test will be conducted in the upper San Joaquin Valley from 2018 through 2020.

Regeneration of some grape rootstock shoot tips after cryopreservation by dropletvitrification

Regeneration of some grape rootstock shoot tips after cryopreservation by dropletvitrification

Absorption, Translocation, and Metabolism of14C-Halosulfuron in Grafted Eggplant and Tomato

Grafted plants are a combination of two different interspecific or intraspecific scion and rootstock. Determination of herbicidal selectivity of the grafted plant is critical given their increased use in vegetable production. Differential absorption, translocation, and metabolism play an important role in herbicide selectivity of plant species because these processes affect the herbicide amount delivered to the site of action. Therefore, experiments were conducted to determine absorption, translocation, and metabolism of halosulfuron in grafted and non-grafted tomato and eggplant. Transplant type included non-grafted tomato cultivar Amelia, non-grafted eggplant cultivar Santana, Amelia scion grafted onto Maxifort tomato rootstock (A-Maxifort) and Santana scion grafted onto Maxifort rootstock (S-Maxifort). Plants were treated POST with commercially formulated halosulfuron at 39 g ai ha-1followed by14C-halosulfuron under controlled laboratory conditions. Amount of14C-halosufuron was quantified in leaf wash, treated leaf, scion shoot, rootstock shoot, and root at 6, 12, 24, 48, and 96 h after treatment (HAT) using liquid scintillation spectrometry. No differences were observed between transplant types with regard to absorption and translocation of14C-halosulfuron. Absorption of14C-halosulfuron increased with time, reaching 10 and 74% of applied at 6 and 96 HAT, respectively. Translocation of14C-halosulfuron was limited to the treated leaf, which reached maximum (66% of applied) at 96 HAT, whereas minimal (<4% of applied) translocation occurred in scion shoot, rootstock shoot, and root. Tomato plants metabolized halosulfuron faster compared to eggplant regardless of grafting. Of the total amount of14C-halosulfuron absorbed into the plant, 9 to 14% remained in the form of the parent compound in tomato compared with 25 to 26% in eggplant at 48 HAT. These results indicate that grafting did not affect absorption, translocation, and metabolism of POST halosulfuron in tomato and eggplant.

Occurrence fungi causing black foot on young grapevines and nursery rootstock plants in Italy

Summary. Young grapevine plants with decline and wood necrosis symptoms were collected from vineyards and nurseries in the Apulia and Molise regions, Italy, from 2013 to 2015. Isolations of fungi were prepared from 45 diseased grapevine plants, and the cultures were identified. Several species commonly associated with Petri disease, Botryosphaeria dieback, and black foot disease were isolated. A detailed study was carried out, and 182 isolates resembling Cylindrocarpon -like asexual forms were identified through morphological characterisation and DNA analysis of internal transcribed spacer regions 1 and 2 of the rRNA gene and the partial β-tubulin gene. Dactylonectria torrensensis and Ilyonectria liriodendri were identified based on morphological features and the partial histone 3 gene, so these fungi can be defined as the causal agents of black foot on grapevine for the first time in Italy. Thelonectria blackeriella is also described as a new species, through morphological characterisation and multigenic analysis using sequence data for five loci (large subunit RNA, internal transcribed spacers, β-tubulin, actin, RNA polymerase II subunit 1). This new species was associated with black foot symptoms according to preliminary pathogenicity tests, with representative isolates of each of the three species. Pathogenicity tests showed that these species can cause black streaking in the wood of 1-year-old grapevine rootstock shoots. The identification of D. torresensis , I. liriodendri and T. blackeriella from young grapevine plants and rooted rootstock highlights the importance of black foot disease in Italy, which has previously been overlooked.

EFFICIENT IN VITRO PROPAGATION AND ROOTING OF ADULT EXPLANTS OF CITRUS ROOTSTOCKS

The influence of the basal medium and different plant growth regulators on the efficient micropropagation of nodal explants from mature trees of sour orange and ‘Cleopatra’ mandarin citrus rootstocks was studied. The preference of the two citrus rootstock shoot cultures for high-salt media, like MS or DKW, was observed. Several combinations of BA and AD, KIN or GA were made in an attempt to optimize the proliferation phase. The addition of BA and AD to the culture medium improved the proliferation in sour orange or ‘Cleopatra’ mandarin in the same way as BA/GA. The use of different combinations of BA/KIN did not improve any of the studied variables. The transfer of in vitro shoots to rooting media, containing different concentrations of IBA and IAA, produced complete plantlets of ‘Cleopatra’ mandarin, but very low rooting percentages in sour orange were obtained. To improve the rooting results, different combinations of NAA and IBA were tested and higher rooting percentages than in the IBA/IAA combinations were produced. In sour orange nearly 100% of rooted explants were obtained.

Efficient propagation and rooting of three citrus rootstocks using different plant growth regulators

The influence of various basal medium and plant growth regulators on the efficient micropropagation of nodal explants from mature trees of alemow, sour orange, and ‘Cleopatra’ mandarin citrus rootstocks was studied. All three citrus rootstock shoot cultures showed a preference for high-salt media, like Murashige and Skoog or Driver and Kuniyuki Walnut medium. Several combinations of N 6-benzyladenine (BA) and adenine (AD), kinetin (KIN) or gibberellic acid (GA) were tested to optimize the shoot proliferation phase. BA/GA combinations improved the proliferation of all the rootstocks studied, especially alemow. The addition of BA and AD to the culture medium improved shoot proliferation in sour orange and ‘Cleopatra’ mandarin in the same way as BA and GA. The addition of different combinations of BA/KIN did not result in further improvement of any of the studied variables. The transfer of in vitro shoots to rooting media, containing different concentrations of indolebutyric acid (IBA) and indoleacetic acid (IAA), resulted in regeneration of complete plantlets. Alemow and ‘Cleopatra’ mandarin shoots rooted well using these plant growth regulators; however, all combinations of IBA and IAA tested resulted in very low rooting percentages in sour orange. To improve rooting in sour orange and ‘Cleopatra’ mandarin, different combinations of naphthaleneacetic acid (NAA) and IBA were tested. All NAA/IBA combinations produced higher rooting percentages than did the IBA/IAA combinations, and in sour orange nearly 100 % of explants developed roots. An efficient and simple protocol for the micropropagation of three citrus rootstocks, alemow, ‘Cleopatra’ mandarin, and sour orange, by culturing nodes from mature plants, has been established.

Anatomy and osmotic potential of the Vitis rootstock shoot tips recalcitrant to cryopreservation

This study was carried out on Kober 5BB (Vitis Berlandieri × V. riparia) grape rootstock shoot tips during the preparatory steps preceding the direct immersion in liquid nitrogen, in order to overcome until now unsuccessful cryopreservation with this species. The exposure of shoot tips to 0.3-0.4 M sucrose leads to a high cell solute concentration. The treatment with plant vitrification solution (PVS2) alone, i.e., not followed by storage in liquid nitrogen, markedly affected shoot tip survival. After a 30 min exposure, regrowth percentage of shoot tips decreased from 94 % (control) to 57 %, and dropped to 15 % when the treatment was prolonged up to 60 min. After a 90 min exposure, no regrowth occurred. In addition, plantlets regenerated from shoot tips which underwent 60 min or more exposure to PVS2 showed signs of malformation. Microscope observations of shoot tips treated with 0.3 or 0.4 M sucrose and 30 min PVS2 showed the presence of cells starting to plasmolyze, localized in the area surrounding the apical meristem. A limited presence of starch grains in meristem and bract cells was also noted. However, the most conspicuous consequence of prolonged PVS2 treatment was convex plasmolysis. The phenomenon was dependent on the time of PVS2 exposure. Indeed, after a 30 min treatment, plasmolysis was minimal or absent, but it increased with longer exposure to PVS2 at 4 °C.

Anatomy and osmotic potential of the Vitis rootstock shoot tips recalcitrant to cryopreservation

This study was carried out on Kober 5BB (Vitis Berlandieri × V. riparia) grape rootstock shoot tips during the preparatory steps preceding the direct immersion in liquid nitrogen, in order to overcome until now unsuccessful cryopreservation with this species. The exposure of shoot tips to 0.3-0.4 M sucrose leads to a high cell solute concentration. The treatment with plant vitrification solution (PVS2) alone, i.e., not followed by storage in liquid nitrogen, markedly affected shoot tip survival. After a 30 min exposure, regrowth percentage of shoot tips decreased from 94 % (control) to 57 %, and dropped to 15 % when the treatment was prolonged up to 60 min. After a 90 min exposure, no regrowth occurred. In addition, plantlets regenerated from shoot tips which underwent 60 min or more exposure to PVS2 showed signs of malformation. Microscope observations of shoot tips treated with 0.3 or 0.4 M sucrose and 30 min PVS2 showed the presence of cells starting to plasmolyze, localized in the area surrounding the apical meristem. A limited presence of starch grains in meristem and bract cells was also noted. However, the most conspicuous consequence of prolonged PVS2 treatment was convex plasmolysis. The phenomenon was dependent on the time of PVS2 exposure. Indeed, after a 30 min treatment, plasmolysis was minimal or absent, but it increased with longer exposure to PVS2 at 4 °C.

Stigmasterol-driven enhancement of the in vitro multiplication rate for the marubakaido apple rootstock

The treatment of in vitro-grown shoots of the marubakaido apple rootstock with 0.5 μg stigmasterol, an end-pathway sterol of the bifurcated sterol biosynthetic pathway, in 5 μL acetone per shoot led to a significant (p ≤ 0.05) enhancement of the multiplication rate (MR) from 5.1 (shoots treated with 5 μL acetone only) to 10.3. This increase in the MR was due to a significant enhancement of the number of newly formed main shoots suitable for micropropagation purposes (measuring at least 15 mm in length) from 2.6 to 3.3 per explant, and of the number of newly formed primary lateral shoots from 2.2 to 5.0 per explant as well. Shoots treated with stigmasterol at 0.5 and 2.5 μg per shoot presented primary and secondary lateral shoots with significantly (p ≤ 0.05) longer length compared to shoots treated with acetone only. These results provide an insight into the morphological responses of the marubakaido rootstock shoots to the treatment with an end-pathway sterol. To the best of my knowledge, this is the first report on the successful use of stigmasterol for the improvement of a micropropagation system. These results also demonstrate that stigmasterol-induced shoot proliferation is a low-cost and effective way to enhance the in vitro MR for the apple rootstock.

Grafting Technique to Eliminate Rootstock Suckering of Grafted Tomatoes

Vegetative grafting has been proposed as a technique for managing diseases in tomatoes under open-field conditions. Over 2 successive years, we investigated the use of grafting under open-field fresh tomato production and found a serious limitation with current grafting techniques, which resulted in recurring rootstock shoot regrowth (“suckering”) from the rootstock cotyledons when left intact. Left unchecked, the regrowth of tomato rootstocks can envelop the experimental scions, which can impact the growth of field-grown tomatoes. In the Fall of 2007, the cultivars Multifort, Aloha, and TX-301 with ‘FL-47’ scions were grafted by a commercial propagator. These grafted plants were planted in a field experiment and after a time period, the number of rootstock suckers was counted and removed. This process was repeated over five time periods. In 2008, we grafted all of the plants below the rootstock cotyledons. During five different time periods spanning 57 d, bud regrowth from the rootstock occurred in all studied rootstocks in 2007. The difference in the 2 years was dramatic. In the first year, the number of plants with rootstock regrowth was as high as 84.6% in some of the plots for ‘Multifort’, 30.7% for ‘Aloha’, and 15.4% for ‘TX-301’. In the second year, with a different grafting technique, no regrowth from the rootstocks was observed. Even when the rootstock regrowth was removed in 2007, the rootstock sprout growth would reappear from the rootstock. The statistical interaction of the percentage of plants with rootstock regrowth for all recorded dates in 2007 was significant for ‘Multifort’ (Tukey-Fischer P ≤ 0.05) but not ‘Aloha’ or ‘TX-301’. Although the study also contained fumigation treatments, no interaction of soil treatments on root sprouting was observed. Since adopting this method, rootstock regrowth has not been observed in subsequent field experiments for the past 3 years.

Influência da adubação orgânica no crescimento de tangerineira cv Clemenules e nos atributos químicos e microbiológicos do solo

O estudo objetivou avaliar o efeito de adubos orgânicos de diferentes origens, empregados como fontes de nitrogênio, sobre a atividade vegetativa das plantas de tangerineira (cv Clemenules/Poncirus trifoliata) durante a fase inicial de desenvolvimento e atributos químicos e microbiológicos do solo, nas condições pedológicas e climáticas da região de Pelotas-RS. O experimento foi conduzido durante dois anos (2002-2003), utilizando-se de plantas de 18 meses de idade, em vasos de 35 kg de capacidade, irrigadas por gotejamento, com seis repetições, sendo cada unidade experimental constituída por uma planta. Os tratamentos confrontados foram os seguintes: Controle (sem adubação); Adubação mineral (AM); Vermicomposto (VC); Vermicomposto + sangue bovino seco (VC + SB) e Resíduo da indústria de transformação de sucos cítricos (RITC). Ao final de cada estação vegetativa, foram avaliados os seguintes parâmetros biométricos: área de seção do porta-enxerto, altura da copa, número de brotações, folhas totais produzidas por planta e massa fresca e seca das folhas. Ao fim do experimento, três plantas de cada tratamento foram desplantadas, separados os principais órgãos, determinando-se a área total das folhas produzidas, a massa fresca e seca dos ramos principal e secundários e das raízes, separadas em grossas e finas. Na mesma época, amostras de solo de cada vaso foram coletadas e utilizadas para as análises químicas e microbiológicas. Em ambos os anos, foram coletadas folhas avaliando-se a concentração dos macronutrientes. Os efeitos mais marcantes sobre o desenvolvimento das plantas foram observados naquelas em que, além do vermicomposto (VC), foi fornecido periodicamente sangue seco (VC + SB). No solo desses tratamentos, também foi observado aumento do teor de nitrogênio total e da atividade microbiana do solo, avaliada pela respiração basal. O aporte de sangue seco (VC + SB) supriu satisfatoriamente as necessidades de N ocorrido nas plantas fertilizadas com adubo orgânico mesmo que estas últimas tenham recebido, no biênio de experimentação, cerca de 40% a menos de nitrogênio em relação às plantas adubadas com a fonte mineral (AM). Por isso, os tratamentos VC e RITC induziram um efeito limitado no desenvolvimento das plantas, enquanto, provavelmente, estas duas últimas fontes, e nas quantidades utilizadas, não forneceram suficientemente nitrogênio disponível para suprir a demanda das plantas. De maneira geral, a concentração dos macronutrientes foliares foi adequada em relação ao P, K, Ca e Mg e abaixo do normal para o N, demonstrando a elevada exigência desse nutriente na fase inicial de crescimento e desenvolvimento de tangerineiras cv. Clemenules.

Differential performance of marubakaido apple rootstock shoots grown in culture media containing different agar brands: dynamic rheological analysis

ABSTRACT Shoots of the marubakaido apple rootstock grown in culture medium containing BBL agar presented significantly lower multiplication rate (MR) compared to MRs found for shoots grown in medium containing A-7002, A-7921, Select, and Phytagar as gelling agents. In addition, significant hyperhydricity was found for shoots grown in Phytagar and A-7921 agar-containing media. Analysis of elastic (G′) and viscous (G ) modulus showed that for all of the five agar brands used in this study, G′ was always much higher, i.e., typically one order of magnitude higher than G, which characterizes a strong gel. G′ changed randomly with time for all of the agar brands studied, except for BBL, which presented progressive decline in G′ throughout the culture cycle. Examination of G′, within the same week, showed that Select agar always had the smallest G′, while Phytagar always had the highest G′. Analysis of the loss tangent (tan δ =G/G′), a better indicator for gel behavior compared to G′ isolated, showed that tan δ ...

Interaction Between Salinity Stress and Verticillium Wilt Disease in Three Pistachio Rootstocks in a Calcareous Soil

ABSTRACT The interaction between soil salinity and infection caused by Verticillium dahliae was studied in pistachio (Pistacia vera) in a greenhouse experiment. Treatments consisted of 0, 1400, 2800, and 4200 mg sodium chloride (NaCl) kg− 1 soil and three rootstocks (Sarakhs, Badami, and Qazvini cultivars). They were gradually exposed to salinity stress before and/or after root inoculation with a water suspension of 107 conidia/mL of a pistachio isolate of V. dahliae. Salt stress significantly increased rootstock shoot and root colonization by V. dahliae. All rootstocks were susceptible to V. dahliae, but symptoms of the disease appeared earlier in Sarakhs, a salt sensitive cultivar. Moreover, salinity and V. dahliae interaction increased the concentrations of sodium (Na), potassium (K) and chloride (Cl), but decreased the K/Na ratio in all rootstocks. Shoot and root tissues of inoculated Sarakhs and Qazvini (a salt tolerant) contained the highest and the lowest concentrations of Na, K,and Cl, respectively. In salinity treatments, shoot and root dry weight of all rootstocks decreased as compared with controls. Sarakhs showed smaller shoot and root dry weight than Qazvini and Badami. Also, increasing the NaCl level increased accumulation of Na, K, and Cl in shoot and root of the rootstocks. Sarakhs showed higher concentrations of ions in the shoot and root. Based on shoot and root dry weights and ion accumulation, Sarakhs and Qazvini were susceptible and tolerant to salinity, respectively.

Cryopreservation of in vitro almond shoot tips by vitrification

SummaryShoot tips of two almond scion cultivars, ‘Ne Plus Ultra’ and ‘Nonpareil 15-1’, and one almond/peach hybrid rootstock were successfully cryopreserved using a one-step vitrification technique. Three week old in vitro cultures were cold-hardened at 4°C on the multiplication medium (Murashige and Skoog for ‘Ne Plus Ultra’ and the hybrid rootstock; Almehdi and Parfitt for ‘Nonpareil 15-1’) for three weeks. Shoot tips, 2–2.5 mm long, were excised and precultured for 1 d at 4°C on the same basal medium, without plant growth regulators, supplemented with 0.7 M sucrose. After the preculture, the shoot tips were incubated in vitrification solution at 25°C for 45 min for the almond scion cultivars and 60 min for the hybrid rootstock, and then stored under liquid nitrogen (LN) for at least 3 d. After rapid thawing at 30°C, the shoot tips were washed with the appropriate liquid basal medium containing 1.0 M sucrose and then cultured on the same basal medium, solidified with agar, but excluding NH4NO3 or (NH4)2SO4. Shoot regeneration was usually observed within 2–3 weeks. Survival after LN, recorded as the percentage of shoot tips that produced at least one new shoot four weeks after thawing, was 87.5, 60.0 and 72.5% for ‘Ne Plus Ultra’, ‘Nonpareil 15–1’ and the hybrid rootstock respectively. The one-step vitrification method is a promising simple technique for cryopreserving almond scion and rootstock shoot tips from in vitro cultures.

Forcing Method and Time of Rootstock Shoot Removal Affect Growth of Containerized Citrus Nursery Trees

I conducted an experiment to determine the effects of time of rootstock shoot removal on growth of citrus nursery plants forced by bending and lopping. `Hamlin' orange [Citrus sinensis (L.) Osb.] budded on Carrizo citrange [C. sinensis (L.) × Poncirus trifoliata (L.) Raf.] and Cleopatra mandarin (C. reticulata Blanco) seedlings were grown in 2.8-L plastic pots and forced by bending or lopping the rootstock shoots. For Carrizo plants, rootstock shoots were removed 0, 21, 36, 170, or 235 days after forcing. For Cleopatra plants, rootstock shoots were removed 0, 11, 21, 36, 170, and 235 days after forcing. Bending and lopping with rootstock shoots remaining attached for 36 days after forcing resulted in greater nursery tree growth (scion, root, and wholeplant dry masses; scion length; and leaf area) than when rootstock shoots were removed the day of forcing. Growth of scion leaves (both rootstocks) and stems (Cleopatra) was greatest at a rootstock shoot removal time (RSRT) of 36 days after forcing. However, root and whole plant dry masses increased as RSRT increased up to 235 days after forcing. These results suggest that roots continue to be a major sink for rootstock photosynthates of bent or lopped plants during periods after the first scion growth flush. Proportionally greater root growth occurred on plants forced by bending or lopping when rootstock shoots were left attached for more than the usual 4 to 5 weeks after forcing.

Forcing Treatment and Rootstock Affect Budbreak and Growth of Containerized Citrus Nursery Trees

Two experiments were conducted to determine the effects of rootstock and bud-forcing treatment on scion budbreak and nursery tree growth of `Hamlin' orange. In Expt. 1, `Carrizo' citrange, `Swingle' citrumelo, and `Cleopatra' mandarin were budded with `Hamlin' orange and forced by one of the following methods: cutting off (purning away the rootstock top about 2 cm above the inserted scion bud); lopping (cutting half to two-thirds of the way through the rootstock stem 2 cm above the bud union, and breaking over the stem but leaving it attached); or bending (bending the rootstock shoot above the inserted scion bud and tying it to the base of the plant). For `Swingle' only, percent budbreak was less for bending or lopping compared to cutting off. For `Carrizo' and `Swingle', scion dry weights were less when plants were forced by cutting off compared to bending or lopping. For all rootstocks, whole-plant dry weights were greater for plants forced by bending and lopping than for plants forced by cutting off. In Expt. 2, scion buds on `Swingle' and `Cleopatra' plants were forced by the three methods in Expt. 1 plus combinations of bending with notching (making an inverted V incision through the bark and into the wood on the rootstock stem directly above the scion bud) and/or topping (removing the teminal 2 cm of rootstock shoot tips of plants forced by bending). Percent scion budbreak was high for `Cleopatra' plants regardless of forcing treatment. For `Swingle', scion budbreak was greater when bending was combined with notching than for bending alone. For `Cleopatra', plant dry weight was greatest for plants forced by lopping. When bending was combined with notching, or notching with topping, `Swingle' scion budbreak was comparable to cutting off, but plant dry weights were greater with these combination treatments than when cutting off was used.

Growth of Budded, Containerized, Citrus Nursery Plants when Photosynthesis of Rootstock Shoots Is Limited

Two experiments were conducted to determine the effects of rootstock shoot defoliation or shading on growth during the first two scion flushes of budded, containerized, citrus nursery trees. Both experiments were conducted in a greenhouse with Cleopatra mandarin (Citrus reticulata Blanco) and Swingle citrumelo [C. paradisi (L.) Osb. × Poncirus trifoliata (L.) Raf.] seedlings budded with `Hamlin' orange [C. sinensis (L.) Osb.]. Scion buds were forced by cutting off the rootstock shoots above the bud union or by bending the rootstock shoots and tying them to the base of the plants (bending). In one experiment, shoots from both rootstocks that were forced by bending received one of four defoliation treatments: treatments were 0%, 40%, 60%, or 85% (based on leaf area) defoliation for Cleopatra and 0%, 30%, 60%, or 100% defoliation for Swingle. In the second experiment, rootstock shoots of plants forced by bending were subjected to a maximum photosynthetic photon flux (PPF) of 820, 225, 90, or 30 μmol·m–2·s–1. Growth of Cleopatra plants declined linearly with increasing levels of rootstock shoot defoliation. When rootstock shoot defoliation was ≤40%, whole-plant (minus rootstock shoot) dry weights were higher for plants forced by bending than for those forced by cutting off rootstock shoots. For Swingle, most growth responses to rootstock shoot defoliation were curvilinear. Higher levels of rootstock shoot defoliation resulted in less growth than lower defoliation levels. Low PPF reduced whole-plant (minus rootstock shoot) and root dry weights for both rootstocks compared to high PPF. For Cleopatra, whole-plant and scion dry weights were highest at the highest PPF. For Swingle, whole-plant and scion dry weights were highest at 225 μmol·m–2·s–1. For both rootstocks, plants forced by bending had higher dry weights at 820 and 225 μmol·m–2·s–1 than plants forced by cutting off the rootstock shoots. For Swingle, most of the reduction in scion growth from rootstock shoot defoliation occurred during the first scion flush. Our results suggest that recently produced rootstock shoot photosynthates are the primary source of greater plant growth achieved by bending compared to cutting off rootstock shoots.