volkamer lemon
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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.

Water shortage is among the major abiotic stresses that restrict growth and productivity of citrus. The existing literature indicates that tetraploid rootstocks had better water-deficit tolerance than corresponding diploids. However, the associated tolerance mechanisms such as antioxidant defence and nutrient uptake are less explored. Therefore, we evaluated physiological and biochemical responses (antioxidant defence, osmotic adjustments and nutrient uptake) of diploid (2x) and tetraploid (4x) volkamer lemon (VM) rootstocks grafted with kinnow mandarin (KM) under two water-deficit regimes. The KM/4xVM (VM4) and KM/2xVM (VM2) observed decrease in photosynthetic variables, i.e., photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (E), leaf greenness (SPAD), dark adopted chlorophyll fluorescence (Fv/Fm), dark adopted chlorophyll fluorescence (Fv´/Fm´), relative water contents (RWC) and leaf surface area (LSA), and increase in non-photochemical quenching (NPQ) under both water-deficit regimes. Moreover, oxidative stress indicators, i.e., malondialdehyde (MDA) and hydrogen peroxide, and activities of antioxidant enzymes, i.e., superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APx), glutathione reductase (GR) were increased under both water-deficit regimes. Nonetheless, increase was noted in osmoprotectants such as proline (PRO) and glycine betaine (GB) and other biochemical compounds, including antioxidant capacity (AC), total phenolic content (TPC) and total soluble protein (TSP) in VM2 and VM4 under both water-deficit regimes. Dry biomass (DB) of both rootstocks was decreased under each water-deficit condition. Interestingly, VM4 showed higher and significant increase in antioxidant enzymes, osmoprotectants and other biochemical compounds, while VM2 exhibited higher values for oxidative stress indicators. Overall, results indicated that VM4 better tolerated water-deficit stress by maintaining photosynthetic variables associated with strong antioxidant defence machinery as compared to VM2. However, nutrient uptake was not differed among tested water-deficit conditions and rootstocks. The results conclude that VM4 can better tolerate water-deficit than VM2. Therefore, VM4 can be used as rootstock in areas of high-water deficiency for better citrus productivity.

Our study aimed to evaluate the physiological responses following salinity treatment of three putatively salt-tolerant Citrus rootstocks recently developed by the University of Florida’s Citrus breeding program. Four-month-old seedlings from each of the three rootstocks (HS1, HS17, and HC15) were irrigated with 0, 60, 80, and 100 mm NaCl solution. The seedlings were evaluated together with the salt-tolerant Cleopatra mandarin as a positive control, Volkamer lemon as a moderately salt-tolerant rootstock, and the salt-sensitive Carrizo rootstock as a negative control. Our results demonstrated that chlorophyll content, net CO2 assimilation rate (A), transpiration rate (E), and stomatal conductance (gsw) significantly decreased in response to salinity. Na+ and Cl− levels were higher in leaf tissues than in the roots. Relatively little damage to the cellular membrane was recorded in HC15 and Cleopatra rootstocks under the 100 mm NaCl treatment, along with high accumulation of total phenolic content (TPC), while HS17 had the highest proline levels. Our results indicate that HC15 and HS17 rootstocks exhibited salt tolerance capacity via different strategies under salt stress and could be suitable replacements to the commercially available, salt-tolerant Cleopatra rootstock.

Arid and semiarid regions are vulnerable to water deficits and salinity. Citrus plants are sensitive to saline stress and require the use of tolerant scion-rootstock combinations. Thus, this study aimed to evaluate and classify citrus scion-rootstock combinations with respect to their tolerance to salinity during seedling formation in a protected environment. An experiment was conducted in a randomized block design with a 5 x 12 x 2 factorial scheme corresponding to five levels of water salinity (0.8, 1.6, 2.4, 3.2, and 4.0 dS m-1) applied in 12 citrus rootstocks grafted with two scion varieties: ‘Tahiti’ acid lime and ‘Star Ruby’ grapefruit. The scion-rootstock combinations were evaluated for accumulated dry matter and survival index at 330 days after sowing the rootstocks. Salinity exerted different effects on the dry matter formation of scion-rootstock combinations. ‘Star Ruby’ was less sensitive to salinity, particularly when the rootstocks were the hybrids from ‘Sunki of Florida’ mandarin (TSKFL) with ‘Troyer’ citrange (CTTR) – 013 (TSKFL x CTTR – 013), common ‘Sunki’ mandarin (TSKC) with ‘Argentina’ citrange (CTARG) – 019 (TSKC x CTARG – 019), TSKC with ‘Swingle’ citrumelo (CTSW) – 031 (TSKC x CTSW – 031), and the trifoliate hybrid (HTR) - 069, as well as the varieties Volkamer lemon and Santa Cruz Rangpur lime. When grafted with ‘Tahiti’ acid lime, the rootstocks displaying the less sensitive to salinity were TSKFL x CTTR - 013 and TSKC x CTARG - 019.

Just because of geographical spread, citrus species generally grow in places sensitive to salinity. Testing methods have a significant role in breeding and cultivar development programs. This study was conducted to investigate in vitro salt response of Cleopatra mandarin (Citrus reshni Tan.), sour orange (Citrus aurantium L.), rough lemon (Citrus jambhiri Lush.), Volkamer lemon (Citrus volkameriana Tan & Pasq.), Carrizo citrange (Poncirus trifoliata L. Raf. X Citrus sinensis L. Osbeck) and trifoliate orange (Poncirus trifoliata Raf.) rootstocks at different NaCl concentrations. Seeds were germinated in MS medium with 0, 45, 90 and 135 mM NaCl concentrations. In general, the greatest germination rates in all salt concentrations in Volkamer lemon and sour orange rootstocks and the lowest values were observed in rough lemon and trifoliate orange rootstocks. Present findings revealed that in vitro conditions could reliably be used in salt tolerance tests of citrus rootstocks.

Metarhizium brunneum is a generalist entomopathogenic fungus known to be virulent against Acari. We investigated Metarhizium brunneum-7 (Mb7) interactions in three systems of phytophagous mites and their respective plant hosts: Volkamer lemon (Citrus volkameriana) and the citrus rust mite Phyllocoptruta oleivora; common bean (Phaseolus vulgaris) and the two-spotted spider mite Tetranychus urticae; and spring onion (Allium cepa) and the bulb mite Rhizoglyphus robini. All three mite species were susceptible to directly applied Mb7 conidia. Results obtained using the standard method for studying endophytic colonization vs. live confocal imaging of plant tissues using the green fluorescent protein (GFP)-transformed fungus differed markedly, demonstrating that microscopy validation was more definite than the standard process of recovery from plant tissue. Endophytic colonization was observed in conidium-infiltrated citrus leaves and in roots of onion plants treated with soil-drenched conidia, but not in common bean treated by either spray or drench of conidia. Endophytic colonization of citrus leaves did not affect the citrus mite population. Drench application in common bean reduced two-spotted mite population. Similarly, drench application in onion reduced bulb mite population. This study emphasizes the importance of the host plant effects on Mb7 control efficacy of mite pests, and the merits of live-imaging techniques in studying endophytic interaction.