Nursery performance of potentially promising rootstocks for citriculture in the south of Brazil

ABSTRACT: Diversification of rootstock varieties, with consequent reduction in phytosanitary risks, has great importance to the sustainability of citrus cultivation in the south of Brazil. This study evaluated the performance of 42 rootstocks in the nursery phase to generate ‘Valencia Late’ sweet orange seedlings. Therefore, nucellar seedlings from the rootstocks under study were analyzed in relation to plant height, stem diameter, mortality rate and percentage of bud set after grafting. The experimental design was completely randomized with four replications of 20 sample units. Not adapted genotypes had high mortality rates, and the ones of both hybrids LCR x CTSW - 009 and LVK x LCR - 038 were 100% and 90%, respectively. Other 13 genotypes had mortality rates above 30%, and ‘Sunki’ mandarin was the female genitor of ten of them. ‘Swingle’ citrumelo was the rootstock with the highest development (plant height and stem diameter), associated with low mortality rate. In relation to the other rootstocks, the best results were the fast grafting diameter, associated with low mortality rate obtained by hybrids HTR - 053, LRF x (LCR x TR) - 005, CLEO x TRBN - 245, CLEO x TRSW - 287, and citrandarins ‘Indio’, ‘Riverside’ and ‘San Diego’, as well as Trifoliate orange and ‘Rangpur’ lime. In addition to Trifoliate orange, which is widely used in Rio Grande do Sul, these rootstocks have great potential in citriculture in the south of Brazil.

Five Rootstocks for “Emperor” Mandarin Under Subtropical Climate in Southern Brazil

Rootstocks modulate several characteristics of citrus trees, including vegetative growth, fruit yield and quality, and resistance or tolerance to pests, diseases, soil drought, and salinity, among other factors. There is a shortage of scion and rootstock cultivars among the combinations planted in Brazil. “Ponkan” mandarin and “Murcott” tangor grafted on “Rangpur” lime comprise the majority of the commercial mandarin orchards in Brazil. This low genetic diversity of citrus orchards can favor pest and disease outbreaks. This study aimed to evaluate the agronomic performance, Huanglongbing (HLB) tolerance, and fruit quality of “Emperor” mandarin on five different rootstocks for nine cropping seasons under the subtropical soil-climate conditions of the North region of the state of Paraná, Brazil. The experimental design was a randomized block, with six replications, two trees per block, and five rootstocks, including “Rangpur” lime, “Cleopatra,” and “Sunki” mandarins, “Swingle” citrumelo, and “Fepagro C-13” citrange. The evaluations included tree growth, yield performance, fruit quality, and HLB disease incidence. “Emperor” mandarin trees grafted on “Rangpur” lime and “Swingle” citrumelo had early fruiting and high yield efficiency. “Rangpur” lime also induced the lowest tree growth, but low fruit quality. Trees on “Swingle” citrumelo and “Fepagro C-13” citrange showed low scion and rootstock affinity and produced fruits with high total soluble solids (TSS), with a lower number of seeds for those from trees on “Fepagro C-13” citrange. “Cleopatra” and “Sunki” mandarins induced higher juice content, while fruits from trees on “Cleopatra” also had higher TSS/titratable acidity (TA) ratio. “Emperor” mandarin trees were susceptible to HLB regardless of the rootstocks. Overall, “Cleopatra” and “Sunki” mandarins, “Swingle” citrumelo, and “Fepagro C-13” are more suitable rootstocks for “Emperor” mandarin under Brazilian subtropical conditions than “Rangpur” lime.

Tree Growth and Production of Rainfed Valencia Sweet Orange Grafted onto Trifoliate Orange Hybrid Rootstocks under Aw Climate

Brazil is the largest producer of sweet orange and its juice in the world. Extensive cultivated area is located under an Aw climate in the North–Northwest of the state of São Paulo and the Triângulo of Minas Gerais state, being subjected to severe drought events. Although 56% of the orchards are irrigated in these regions, there is a need for drought tolerant rootstocks as an alternative to traditional genotypes such as Rangpur lime and Volkamer lemon, which are susceptible to the endemic citrus sudden death disease (CSD). In this sense, the tree size and production of Valencia sweet orange grafted onto 23 rootstock genotypes were evaluated over a ten-year period in rainfed cultivation at 7.0 m × 3.0 m spacing. Most evaluated types resulted from the cross of Poncirus trifoliata with Citrus, but two interspecific hybrids of Citrus (Sunki mandarin × Rangpur lime hybrids), the Barnes trifoliate orange and a tetraploid selection of Swingle citrumelo were also tested. Tropical Sunki mandarin was used as the reference control. Those hybrids coming from the cross of Sunki × Flying Dragon induced large tree sizes to Valencia sweet orange as well as the other citrandarins, Tropical Sunki mandarin and the Sunki mandarin × Rangpur lime hybrids, whereas only the tetraploid Swingle citrumelo behaved as a dwarfing rootstock, decreasing the canopy volume by 77% compared to that induced by the most vigorous citrandarin 535. The citrandarins 543 and 602 and the citrange C38 induced the highest mean fruit production, 67.2 kg·tree−1, but they also caused pronounced alternate bearing and only the hybrid 543 led to a high production efficiency consistently. Graft incompatibility symptoms were not observed over the evaluation period, and the canopy shape of Valencia sweet orange was also influenced by the rootstocks tested. Two citrandarins and one citrange were selected as the most promising alternative rootstocks for Valencia sweet orange grown under an Aw climate, even though productivity would likely benefit from supplementary irrigation.

Effect of seventeen rootstocks on young ‘Valência’ sweet orange performance in western Santa Catarina, Brazil

Recommendations of rootstocks for ‘Valência’ sweet orange in southern Brazil have been partially based on empirical observations or information. Therefore, this study compared young ‘Valência’ trees budded on different rootstocks in western Santa Catarina, Brazil. Seventeen rootstocks were tested for growth (tree height and canopy projection area and volume) and fruit production (per tree, hectare, m² and m³ of canopy). The experiment comprised four replicates of three trees and was carried during six years under standard management, in a Cfa (humid subtropical) climate. In general, larger trees produced more fruit. However, ‘San Diego’ citrandarin, ‘Swingle’ citrumelo, ‘Cravo’ rangpur lime (among vigorous rootstocks), ‘Cravo’ x ‘Sunki’ EEI hybrid, ‘Fepagro C37 Dorneles’ citrange, ‘Rubidoux’ trifoliate orange (among medium vigorous rootstocks), ‘HFD25 EEI’ hybrid and ‘Flying Dragon’ trifoliate orange (among less vigorous rootstocks) stood out in cumulated production by area or volume of the canopy. ‘San Diego’ also stood out in production per tree. Cultivars ‘Cravo’ x ‘Sunki’ EEI, ‘San Diego’, and ‘HFD25 EEI’ should be further evaluated regarding disease susceptibility and fruit quality for possible release.

Rootstocks Genotypes Impact on Tree Development and Industrial Properties of ‘Valencia’ Sweet Orange Juice

The low diversification of rootstock genotypes in orchards limits the expansion of the citrus industry, restricting increases in productivity and cost-saving via phytosanitary treatments and other horticultural practices. Therefore, the aim of this study was to assess the impact of rootstock genotypes on tree development and industrial properties of ‘Valencia’ sweet orange juice (Citrus sinensis). Twenty rootstock genotypes were evaluated by measuring tree growth and industrial properties of orange juices, including ‘Trifoliata’ hybrids with tangerine (citrandarins) and grapefruit (citrumelos), as well ‘Rangpur’ lime and other potential rootstocks. The experimental orchard was planted in Rancho Alegre, PR, Brazil, under clay soil and subtropical rainfed conditions. A randomized block design with four replicates was used. Trees grown on IPEACS–239 and IPEACS–256 citrandarins, and on ‘US–802’ pummelo hybrid had low vigor, high production efficiency and high industrial properties of orange juice, and are therefore potential alternatives for high-density plantings. The F.80–3 and F.80–5 citrumelos also had good dwarfing potential and high production efficiency, but lower industrial properties of juice compared to the other ‘Trifoliata’ hybrid rootstocks. Trees grown on ‘US–812’ citrandarin rootstock had low vigor, good productive performance, accumulated production and production efficiency similar to ‘Rangpur’ lime, and high industrial properties of juices. Although the ‘Rangpur’ lime and the ‘Florida’ rough lemon allowed high yields, the trees are very vigorous, with low-quality fruits. A Quick Reference Chart was created to provide practical and objective identification of the best rootstock alternatives for ‘Valencia’ orange trees in terms of tree development and industrial properties of juices.

Response of Nagpur mandarin (Citrus reticulata Blanco) to high density planting systems

High density planting system i.e. accommodating a higher number of plants than routine in a given area is an innovative agro-technology to increase yield and thereby early net returns. Due to conventional wide spacing plantation in Nagpur mandarin (Citrus reticulata Blanco), the land remains unutilized as the plant canopy gradually increases over the years. In the present study, Nagpur mandarin (Citrus reticulata Blanco) budded on Rangpur lime rootstock was evaluated under six different planting spacings. It was observed that the organic carbon (1.10–1.82%) and major nutrients viz. N (309–430 kg ha−1), P (20–54 kg ha−1) and K (291–810 kg ha−1) increased vis-à-vis plant density and was highest under 2 × 2 m spacing. Plants were tallest at 2 × 2 m spacing with the higher PAR interception (88.2) and the lowest leaf area index (1.09). Fruit yield on area basis, under 2 × 2 m spacing was 26, 7.1, 4.6 times more as compared to conventional plantation during the first, second and third year, respectively. At fifth year of crop harvest, the highest B:C ratio (6.36) was recorded in 6 × 3 m followed by 4 × 2 m and 2 × 2 m.

Less Is More: A Hard Way to Get Potential Dwarfing Hybrid Rootstocks for Valencia Sweet Orange

As in several fruit crops, citrus trees with decreased size allow for a higher planting density, which may lead to higher productivity and facilitate operations such as harvesting and spraying. The use of dwarfing rootstocks is one of the most feasible methods for tree size control, but few commercial varieties are available to date. In this work, the long-term performance of Valencia sweet orange grafted onto 51 hybrid citrus rootstocks was evaluated in rainfed cultivation at 6.0 m × 2.5 m tree spacing in Northern São Paulo State, Brazil. About a third of the evaluated hybrids were classified as dwarfing and semi-dwarfing rootstocks, that is, respectively inducing a relative canopy volume of <40% and 40–60% compared withthe standard rootstock, the Rangpur limeSanta Cruz selection. The production efficiency and soluble solids concentration were conversely related to the canopy volume. Three citrandarins of Sunki mandarin (TSKC) × Flying Dragon trifoliate orange (TRFD) were grouped within the most productive dwarfing rootstocks. Other hybrids that expressively decreased tree size were mainly sensitive to drought; therefore, the mean fruit yield was low, indicating the need for irrigation, albeit fruit quality was high. Estimated productivity on the selected TSKC × TRFD rootstocks would double to an average of 40 t·ha−1·year−1 if tree spacing wasadjusted to the smaller tree size. Although the HTR-208 citrandarin and the LCR × CTSW-009 citrumelimonia were as vigorous as the Santa Cruz Rangpur lime, they induced an outstanding fruit yield due to their highertolerance to drought and, hence, can be considered for rainfed cultivation at wider tree spacing.

Sink strength of citrus rootstocks under water deficit

Carbon allocation between source and sink organs determines plant growth and is influenced by environmental conditions. Under water deficit, plant growth is inhibited before photosynthesis and shoot growth tends to be more sensitive than root growth. However, the modulation of source-sink relationship by rootstocks remain unsolved in citrus trees under water deficit. Citrus plants grafted on Rangpur lime are drought tolerant, which may be related to a fine coordination of the source-sink relationship for maintaining root growth. Here, we followed 13C allocation and evaluated physiological responses and growth of Valencia orange trees grafted on three citrus rootstocks (Rangpur lime, Swingle citrumelo and Sunki mandarin) under water deficit. As compared to plants on Swingle and Sunki rootstocks, ones grafted on Rangpur lime showed higher stomatal sensitivity to the initial variation of water availability and less accumulation of non-structural carbohydrates in roots under water deficit. High 13C allocation found in Rangpur lime roots indicates this rootstock has high sink demand associated with high root growth under water deficit. Our data suggest that Rangpur lime rootstock used photoassimilates as sources of energy and carbon skeletons for growing under drought, which is likely related to increases in root respiration. Taken together, our data revealed that carbon supply by leaves and delivery to roots are critical for maintaining root growth and improving drought tolerance, with citrus rootstocks showing differential sink strength under water deficit.

How to drive phloem gene expression? A case study with preferentially expressed citrus gene promoters

New approaches for developing disease-resistant genetically modified organisms have included specific targets for gene expression to enhance the chances for pathogen control. Gene expression driven by phloem-derived Citrus sinensis gene promoters could be evaluated and compared with the expression induced by a strong constitutive promoter in the same tissue, leading to the production of transgenic sweet oranges potentially more resistant to diseases caused by phloem-limited bacteria. ‘Carrizo’ citrange [ (Poncirus trifoliataL.) Raf. x Citrus sinensis (L.) Osbeck] was transformed, via Agrobacterium tumefaciens, with the binary vector pCAMBIA2301 bearing the uidA gene (ß-glucuronidase) driven by the CaMV35S constitutive promoter (CaMV35S::uidA) or by the CsPP2.B1 (CsPP2.B1::uidA) or by the CsVTE2 (CsVTE2::uidA) citrus promoters. In vitro regenerated shoots were grafted onto ‘Rangpur’ lime (C. limonia Osbeck). The genetic transformation was confirmed by Southern blot analyses. uidA gene expression was evaluated by RT-qPCR, and gene histolocalization controlled by these three promoters was accessed by X-GLUC treated stem sections. uidA gene expression exhibited by tissue-specific promoters was overall lower than from the constitutive promoter CaMV35; however, constructs driven by tissue-specific promoters may lead to expression in restricted tissues. CsPP2.B1 and CsVTE2 promoters can be considered adequate for the utilization in gene constructs aiming disease resistance.

Performance of ‘Valência’ sweet orange grafted on trifoliate orange hybrid rootstocks

The objective of this work was to evaluate tree size, production, and fruit quality of ‘Valência’ sweet orange (Citrus sinensis) grafted on various trifoliate orange rootstocks, in order to select genotypes with a high performance. Twenty rootstock genotypes were evaluated, including trifoliate orange hybrids with mandarin (citrandarins) and with grapefruit (citrumelos), as well as ‘Rangpur’ lime. The experiment was implemented in the northwestern region of the state of Paraná, Brazil, in a 6.0×2.5 m spacing, in a sandy soil under subtropical and rainfed conditions. The statistical model used was the randomized complete block design with four replicates and four trees per plot. Tree size and fruit yield and quality were analyzed during three consecutive harvest seasons. Data were subjected to the analysis of variance, and means were grouped by the Scott-Knott test. Principal component analysis and agglomerative hierarchical clustering were also carried out. F.80-18, F.80-5, and F.80-3 citrumelos and IPEACS-239 citrandarin are adequate rootstock options for ‘Valência’ sweet orange, with dwarf trees and a high production efficiency. These rootstocks, except F.80-18, are also adequate options to obtain oranges with good industrial properties.