Rodrigo Marcelli Boaretto

Absorption and Mobility of Boron in Young Citrus Plants

Boron (B) deficiency is widespread in Brazilian citrus orchards and has been considered an important soil constraint to citrus yield. The aim of this work was to study B uptake and its mobility in young citrus trees, under different B statuses, in two rootstocks. The experiment was carried out in a greenhouse, with ‘Valencia’ sweet orange trees budded on Rangpur lime or Swingle citrumelo. The plants were grown in pots containing nutrient solutions under either adequate or deficient B supply. Plants with different B levels were transplanted into solution with adequate level of B, enriched in 10B in different stages of development (vegetative growth and fruiting). About 20 to 35% of B content in the new parts of orange tree came from plant reserves. Boron mobility within the plant was influenced by its nutritional status; that is, the longer the period was that the plants were grown under deficient supply, the smaller was the mobility. Boron concentration in the sweet orange trees on Swingle was higher than that on Rangpur, suggesting higher demand of swingle Citrumelo for B.

Contribution of phosphorus ( 32 P) absorption and remobilization for citrus growth

Background and aimsPhosphorus (P) is a mobile nutrient in the plant so growth depends on its internal remobilization and a plant’s ability to respond to its availability in the growing media. This study was conducted to evaluate the influence of P status and rootstocks on the patterns of P uptake and remobilization in orange trees.MethodsSweet orange trees on Cleopatra mandarin (CM) or Rangpur lime (RL) rootstocks were grown for nine months in nutrient solution (NS) that was either P-deficient (DNS) or was P-sufficient (SNS). After this period, half of the trees were reciprocally transferred between DNS and SNS (from D to S and S to D), while the others remained in their initial P availability.ResultsTrees on RL had more shoot and root growth, accumulated more P and had greater efficiency of P absorption and transport to the shoot (PAE) than those on CM. The major source of P for growth was previously stored P even with an adequate current P supply to the roots. This suggested the dominance of P remobilization over P uptake and the requirement that trees had sufficient stored P to meet P demand of new growth. Trees on CM had greater concentrations of remobilized P in new shoots than trees on RL.ConclusionTrees grafted on rootstocks less able to take up P (CM) were more dependent on the internal reserves of P for new growth than rootstocks with higher PAE (RL).

BORON UPTAKE AND DISTRIBUTION IN FIELD GROWN CITRUS TREES

In low fertility tropical soils, boron (B) deficiency impairs fruit production. However, little information is available on the efficiency of nutrient application and use by trees. Therefore, this work verified the effects of soil and foliar applications of boron in a commercial citrus orchard. An experiment was conducted with fertigated 4-year-old ‘Valencia’ sweet orange trees on ‘Swingle’ citrumelo rootstock. Boron (isotopically-enriched 10B) was supplied to trees once or twice in the growing season, either dripped in the soil or sprayed on the leaves. Trees were sampled at different periods and separated into different parts for total B contents and 10B/11B isotope ratios analyses. Soil B applied via fertigation was more efficient than foliar application for the organs grown after the B fertilization. Recovery of labeled B by fruits was 21% for fertigation and 7% for foliar application. Residual effects of nutrient application in the grove were observed in the year after labeled fertilizer application, which greater proportions derived from the soil supply.

Anatomical and Physiological Responses of Citrus Trees to Varying Boron Availability Are Dependent on Rootstock.

In Citrus, water, nutrient transport and thereby fruit production, are influenced among other factors, by the interaction between rootstock and boron (B) nutrition. This study aimed to investigate how B affects the anatomical structure of roots and leaves as well as leaf gas exchange in sweet orange trees grafted on two contrasting rootstocks in response to B supply. Plants grafted on Swingle citrumelo or Sunki mandarin were grown in a nutrient solution of varying B concentration (deficient, adequate, and excessive). Those grafted on Swingle were more tolerant to both B deficiency and toxicity than those on Sunki, as revealed by higher shoot and root growth. In addition, plants grafted on Sunki exhibited more severe anatomical and physiological damages under B deficiency, showing thickening of xylem cell walls and impairments in whole-plant leaf-specific hydraulic conductance and leaf CO2 assimilation. Our data revealed that trees grafted on Swingle sustain better growth under low B availablitlity in the root medium and still respond positively to increased B levels by combining higher B absorption and root growth as well as better organization of xylem vessels. Taken together, those traits improved water and B transport to the plant canopy. Under B toxicity, Swingle rootstock would also favor plant growth by reducing anatomical and ultrastructural damage to leaf tissue and improving water transport compared with plants grafted on Sunki. From a practical point of view, our results highlight that B management in citrus orchards shall take into account rootstock varieties, of which the Swingle rootstock was characterized by its performance on regulating anatomical and ultrastructural damages, improving water transport and limiting negative impacts of B stress conditions on plant growth.

Uptake and distribution of soil applied zinc by citrus trees-addressing fertilizer use efficiency with 68Zn labeling.

The zinc (Zn) supply increases the fruit yield of Citrus trees that are grown, especially in the highly weathered soils of the tropics due to the inherently low nutrient availability in the soil solution. Leaf sprays containing micronutrients are commonly applied to orchards, even though the nutrient supply via soil could be of practical value. This study aimed to evaluate the effect of Zn fertilizers that are applied to the soil surface on absorption and partitioning of the nutrient by citrus trees. A greenhouse experiment was conducted with one-year-old sweet orange trees. The plants were grown in soils with different textures (18.1 or 64.4% clay) that received 1.8 g Zn per plant, in the form of either ZnO or ZnSO4 enriched with the stable isotope 68Zn. Zinc fertilization increased the availability of the nutrient in the soil and the content in the orange trees. Greater responses were obtained when ZnSO4 was applied to the sandy loam soil due to its lower specific metal adsorption compared to that of the clay soil. The trunk and branches accumulated the most fertilizer-derived Zn (Zndff) and thus represent the major reserve organ for this nutrient in the plant. The trees recovered up to 4% of the applied Zndff. Despite this relative low recovery, the Zn requirement of the trees was met with the selected treatment based on the total leaf nutrient content and increased Cu/Zn-SOD activity in the leaves. We conclude that the efficiency of Zn fertilizers depends on the fertilizer source and the soil texture, which must be taken into account by guidelines for fruit crop fertilization via soil, in substitution or complementation of traditional foliar sprays.

Acúmulo de nutrientes e destino do nitrogênio (15N) aplicado em pomar jovem de laranjeira

Information about nutrient absorption of citrus orchards is important to establish guidelines for best soil fertility management. However, studies on the fate of applied N fertilizers and validation of nitrogen (N) dose recommendations are scarce in the literature. The present work evaluated (i) the accumulation of nutrients and the distribution of N (15N) applied to citrus orchard and (ii) validated the N fertilization rate applied to young bearing orange trees. Three- to four-year-old Pera sweet orange trees Pera grafted on Rangpur lime were fertilized with 150, 300, 450, and 600 g of N per tree, as ammonium sulfate, split in three applications from spring to summer. A control treatment without N was included. In the same orchard, three plants were fertilized with 300 g per tree of N-[(15NH4)2SO 4)] labeled with 15N to study the fate of N in the soil-orange tree system. Fruit yield and recovery of 15N by tree biomass were evaluated. The efficiency of fertilization, estimate by tree N absorption, varied from 20 to 27% of the total applied N. Fruits exported 35% of the N taken up from fertilizer. Furthermore, the highest fruit yield was attained with N rate of 400 g/tree.

Oxidative stress induced by Cu nutritional disorders in Citrus depends on nitrogen and calcium availability

Nutritional stress caused by copper (Cu) deficiency or toxicity affects fruit production of citrus orchards worldwide, but this could be minimised by fine-tuned fertilisation in the orchards. Two experiments were performed aiming to evaluate the photosynthetic capacity and the antioxidant enzyme activities of Swingle citrumelo seedlings, grown in nutrient solution (NS) with two levels of nitrogen (N) in the first experiment (adequate-N and high-N) and two levels of calcium (Ca) in the second (low-Ca and adequate-Ca). Plants were then exposed to various Cu levels (low, medium and high) for 15 days. Plants under Cu-toxicity exhibited specific effects on reactive oxygen species formation and root-to-shoot plant signalling. Copper absorption was greater with increased Cu concentration in the NS, which reduced plant biomass accumulation, gas exchange measurements, the activity of nitrate reductase and affected Cu partitioning between roots and shoots. Despite these effects, oxidative stress induced by excess-Cu was reduced at the highest N dose when compared to control and, on the contrary, increased with low-Ca supply. Therefore, a rational supply of N or Ca minimises Cu-induced stress damages to roots and leaves of plants, by directly enhancing the antioxidant system and protecting the associated antioxidative enzyme activities, whilst maintaining photosynthesis.

Disponibilidade de boro em substrato para produção de porta-enxertos de citros em fase de sementeira

ABSTRACTBORON AVAILABILITY IN THE GROWTH MEDIA FOR PRODUCTION OF CITRUSROOTSTOCK SEEDLINGS The production of citrus nursery trees under protected environment using growth media requireshealthy management practices. Problems associated with deficiency and/or nutrient excess have frequentlycaused economic losses in the nursery. Citrus grow differently depending on boron (B) availability inthe growth media, even tough B toxicity commonly cause damage to plants. This research aimed toevaluate the effects of B supply in the growth media on nutrient uptake and seedling growth of tworootstock citrus varieties. A factorial design was set up with two factors: two rootstocks (Rangpur limeand Swingle citrumelo) and four B levels added to the substrate (0, 3, 6, e 12 g m -3 B) as boric acid, with3 replications. Boron availability in the substrate was evaluated at beginning the experiment, 55 and 110days after rootstock seeding, as well as seedling growth and B uptake by seedlings in the same timeintervals. Boron concentration in the treated substrate varied from 0.1 to 4.7 mg L

Photosynthesis is differently regulated during and after copper-induced nutritional stress in citrus trees

Antioxidant enzymatic responses in Citrus leaves under Cu-induced stress depends on rootstock genotypes. However, there is a lack of information about how woody plants recover growth capacity after exposure to elevated Cu and whether growth is affected by the redistribution of the metal to new vegetative parts and consequently whether photosynthesis is affected. Therefore, the biomass of plants and Cu concentrations in new leaf flushes were determined in young citrus trees grafted onto contrasting rootstocks [Swingle citrumelo (SW) and Rangpur lime (RL)]. Photosynthetic rate, chlorophyll fluorescence and antioxidant enzymatic systems were evaluated in plants previously grown in nutrient solution with Cu varying from low to high levels and with no added Cu. Both rootstocks exhibited reduced plant growth under Cu toxicity. However, trees grafted onto RL exhibited better growth recovery after Cu excess, which was dependent on the modulation of antioxidant enzyme activities in roots and leaves that maintained the integrity of the photosynthetic apparatus. In contrast, plants grafted onto SW exhibited a lower photosynthetic rate at the lowest available Cu concentration. Although the highest accumulation of Cu occurred in citrus roots, the redistribution of the nutrient to new vegetative parts was proportional to the Cu concentration in the roots.

Soil boron fertilization: The role of nutrient sources and rootstocks in citrus production

Abstract Boron (B) is a key element for citrus production, especially in tropical regions, where the nutrient availability is commonly low in the soil. In addition, information about doses, fertilizer sources, methods of application, and particularly, differential nutrient demand of scion/rootstock combinations are required for efficient fertilization of commercial groves. In a non-irrigated sweet orange orchard (cv. Natal), grafted onto Rangpur lime, Swingle citrumelo or Sunki mandarin, we studied the application of two sources of B: boric acid (17% B, soluble in water) and ulexite (12% B, partially soluble in water) at four levels of supply (control without B, and soil application of 2, 4 and 6 kg ha−1 yr−1 of B). The experiment was carried out for three years (2004–2006). Boron availability in the soil and concentration in the leaves, as well as the fruit yield and quality of trees were evaluated. Soil B extracted with hot water and total leaf B positively correlated with doses of the nutrient applied to the trees. Levels of B in the soil and in the leaves did not vary with fertilizer sources. Fruit yield of trees grafted onto Rangpur lime and Swingle citrumelo was more responsive to B doses than those grafted onto Sunki mandarin. The maximum fruit yield of trees grafted onto Swingle was obtained with 3.2 kg ha−1 yr−1 of B, and leaf B level of 280 mg kg−1 that point out to a highest demand for B when this combination was compared with other rootstocks. Furthermore, fertilization with B did not affect the quality of fruits, but correlated with B and potassium (K) concentrations in the leaves. These results also support that the current recommendations for levels of B in leaves should be revisited.