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Dariusz Swietlik

The effects of soil and foliar Zn applications on growth, yield, and fruit quality of `Rio Red' grapefruit were studied in the field for 4 years. Two annual foliar sprays applied in winter (W), spring (S), or W+S were compared to a single application of 10 or 30 g of Zn/tree applied to the soil around the tree as ZnDTPA or ZnEDTA chelate. In the first 2 years, when control trees displayed severe Zn deficiency symptoms affecting 60% to 70% of the tree foliage, the W and W+S sprays resulted in significant yield increases. Similar yield increases were obtained after a single soil application of 30 g Zn as ZnEDTA. The effects of other soil treatments were statistically insignificant. Foliar Zn deficiency symptoms were much more severe in winter than summer months irrespective of treatment. As the trees aged, however, the severity of symptoms decreased in all treatments. Corrective foliar or soil Zn applications were found to increase grapefruit yield when 15% or more of the canopy foliage showed Zn deficiency symptoms in January, ≈2 months before anthesis.

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Dariusz Swietlik

Several soil moisture measuring sensors are used to schedule irrigation of horticultural crops. However, without precise knowledge on root distribution, these devices may not provide accurate data on soil moisture conditions around the majority of roots. Also, some devices are expensive and/or require a tedious calibration and careful maintenance, limiting their use by growers. ET models have been successfully used for many crops grown in arid climates; however some models are not very precise and do not account for seasonal differences in plant water needs. Moreover, in humid regions it is difficult to assess the magnitude of rainfall contribution to the plant water needs particularly when only a part of the root system is irrigated. Research is needed to characterize plant variables which are useful for scheduling irrigation.

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Dariusz Swietlik

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Dariusz Swietlik

Sour orange seedlings were grown in water culture to which one of seven aromatic compounds, associated with allelopathic effects, was added to produce concentrations ranging from 0.5 to 2.0 mM. Leaf water potential (ψ1), leaf stomatal conductance (gs), and whole plant transpiration (T) were measured during a 7-day treatment period. At the end of that period, the total and average leaf surface area, shoot elongation, and fresh weight gain of seedlings were determined. Solutions of vanillic, coumaric, and ferulic acids of 2mM concentration reduced ψ1, gs, and T. Reductions of gs, and T but not (ψ1) occurred when vanillic acid of 1mM concentration was applied. Solutions of vanillic (0.5; 1.0; 2.0mM), coumaric (1; 2mM), cinnamic (1mM), or chlorogenic (1; 2mM) acids reduced fresh weight gain of seedlings. Only the coumaric and chlorogenic acids treatments of 2mM concentration reduced shoot elongation. No treatment affected total or individual leaf area. Gallic and caffeic acids had no effect on sour orange water relations and growth.

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Dariusz Swietlik

Root distribution of trickle–and flood-irrigated 4-year-old `Ray Red' grapefruit (Citrus paradisi Macf.) trees on sour orange (C. aurantium L.) rootstock was studied utilizing a trench method. Irrigation treatments were: flooding at 50% soil water depletion, trickle irrigation (2 drippers per tree) at 0.5 Class A Pan evaporation or at 0.02 MPa soil tension. Two trees from each treatment were studied. Five 2.5 m deep trenches positioned perpendicular or parallel to the tree row at 0.6, 2.1, or 4.3 m from the tree trunks were dug per tree. After washing off a 0.5 cm thick layer of soil from the trench wall, 0.5 cm long root sections were marked on a transparent plastic film attached to the wall. Many roots of trickle-irrigated trees grew past the trickle wetted zone and extended beyond 2.1 but not 4.3 m of the trunk. However, the roots of flood-irrigated trees were present at all distances from the trunk. From 26 to 51% of the roots of trickle–irrigated trees were found 90-230 cm deep, despite the clayey texture of the top 1 m of soil which was underlaid by a sandy clay loam. The root systems-of flood-irrigated trees were shallower and in most cases confined to the top 90 cm soil layer.

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Dariusz Swietlik

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Dariusz Swietlik

Zinc (Zn) deficiency is widespread throughout the world causing economic losses on a number of crops. Despite the fact that much information was generated during the last 20 years on Zn soil chemistry and its inorganic phase equilibrium, the mechanism controlling the amount of free Zn+2 present in the soil solution is not yet completely understood. This information is critical for the development of effective techniques of supplying Zn through the soil. As Zn moves very slowly through the soil, however, and a large portion of fruit tree root system occupies deep soil layers, foliar sprays with Zn are generally more effective than soil treatments in alleviating Zn deficiency symptoms. That is why many extension specialists recommend this approach. In view of the poor mobility of foliar-absorbed Zn in plants, however, we may need to reexamine this approach. Zinc foliar sprays may be effective in controlling Zn deficiency in leaves, but not in alleviating Zn deficiency in roots or subsequent flushes of growth. Also, the conditions under which fruit trees are most likely to respond to corrective Zn treatments are not well understood and the critical periods for Zn supply to assure optimal fruit set, fruit growth, and high fruit external and internal quality are not well defined. Field studies on fruit trees suggest that Zn deficiency must be quite severe to make the application of this element economically justifiable. In well-controlled greenhouse studies, however, growth responses were realized on plants only mildly affected by Zn deficiency. If considerable field variability may explain this discrepancy in the data, then future field research must use improved methodologies to properly quantify the impact of various levels of Zn deficiency on tree growth, fruit yield, and fruit quality.

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Dariusz Swietlik

The public is increasingly concerned with the danger of ground water pollution with fertilizer nitrogen and other chemicals. This is because slow water movement in underground aquifers assures the long lasting existence of contaminants. Citrus orchards commonly are heavily fertilized with nitrogen and other mineral nutrients. Fertigation through a low volume irrigation system is a promising new method of efficient use of fertilizer materials because it places mineral nutrients only in the wetted zones where roots are most active. Preliminary studies in Texas indicate that applying nitrogen fertilizers through a low volume irrigation system is a potentially powerful tool in minimizing N fertilizer leaching. When coupled with partial sodding in close tree proximity further reductions in NO3 leaching may be achieved presumably through uptake into the cover plants and/or indirectly by enhancing biological fixation in the soil. Other potential benefits of frequent N fertigations in citrus orchards will also be discussed based on the experimental data collected in various parts of the world.

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Dariusz Swietlik

The purpose of this study was to determine the effect of Ca: NH4 ratio in the rhizosphere of hydroponically grown sour orange seedlings (SO) (Citrus aurantium L.) on the plants' vegetative growth and N uptake. The experiment was prompted by our observation that application of N in the form of NH4 in conjunction with CaCl2 was more efficient in eliminating N deficiency in field-grown grapefruit trees than the same rates of N applied in the form of NH4NO3 without CaCl2. About 40-cm-tall SO were pruned back to the 4th leaf and grown for 6 weeks in nutrient solutions containing 5 mm NH4 + at CaCl2: NH4 + molar ratios of 1.0, 1.3, 1.6, 1.9, 2.2, or 2.5. In an additional treatment, NO3 was used as the sole source of N at CaCl2: NO3 ratio of 1:1. The level of Ca:NH4 ratio had no effect on new leaves number, shoot growth, total and average leaf area, specific leaf weight, as well as leaf, stem, and tap root dry weight. However, lateral root dry weigh decreased at Ca: NH4 ratio of 2.5. No growth differences were found when the plants were supplied with NH4 + vs. NO3 at Ca:N molar ratio of 1:1.