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Rashid Al-Yahyai, Bruce Schaffer and Frederick S. Davies

Two-year-old `Arkin' carambola (Averrhoa carambola L.) trees were grown in containers in a greenhouse and the field in a very gravelly loam soil. Trees in the field were subjected to four soil water depletion (SWD) levels which averaged, 10.5%, 26.5%, 41.0%, and 55.5% and trees in the greenhouse were maintained at field capacity or dried continuously to produce a range of SWD levels. The relationships between SWD and leaf (ΨL) and stem (Ψs) water potential, net CO2 assimilation (ACO2), stomatal conductance of water (gs) and transpiration (E) were determined. Coefficients of determination values between physiological variables were higher for trees in the greenhouse than in the field, which may have been due to greater fluctuations in vapor pressure deficit (VPD) in the field. Soil water depletion levels above 50% caused a reduction in Ψs that subsequently decreased gs. This reduction in Ψs was correlated with a linear reduction in E and a considerable decline in ACO2 when gs fell below about 50 mmol·m–2 ·s–1. Leaf gas exchange parameters were better correlated with Ψs than with SWD level. Therefore, Ψs may be a better predictor of carambola tree water status than SWD in a well-drained, very gravelly loam soil.

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Rashid Al-Yahyai*, Bruce Schaffer and Frederick S. Davies

The effect of soil water depletion on plant water potential and leaf gas exchange of carambola (Averrhoa carambola L. cv. Arkin) in Krome very gravelly loam soil was studied in an orchard and in containers in the field and in a greenhouse. The rate of soil water depletion was determined by continuously monitoring soil water content with multi-sensor capacitance probes. Stem water potential and leaf gas exchange of carambola in containers were reduced when the soil water depletion level fell below 50% (where field capacity = 100%). Although there was a decrease in the rate of soil water depletion in the orchard as the soil dried, soil water depletion did not go below an average of 70%. This was presumably due to sufficient rainfall and capillary movement of water in the soil. Therefore, soil water content did not decline sufficiently to affect leaf gas exchange and leaf and stem water potential of orchard trees. A decline in soil water depletion below 40% resulted in a concomitant decline in stem water potential of the container trees in the field and greenhouse to below -1.0 MPa. Stomatal conductance, net CO2 assimilation, and transpiration declined significantly when stem water potential was below -1.0 MPa. The reduction of net CO2 assimilation and transpiration was proportional to the decline in stomatal conductance of container trees in the field and greenhouse. Thus, soil water depletion in Krome very gravelly loam soil must be less than 50% before water potential or leaf gas exchange of carambola is affected. Based on these results, irrigation scheduling should be based on physiological variables such as stem water potential and stomatal conductance or the amount rather than the rate of soil water depletion.

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Rashid Al-Yahyai, Bruce Schaffer, Jonathan H. Crane and Frederick S. Davies

Phenological cycles were determined for carambola (Averrhoa carambola) trees in a gravelly loam soil at four different soil water depletion (SWD) levels in containers and in an orchard in southern Florida. The phenological cycles of young trees grown in containers were not as well defined as those of mature trees in an orchard. Shoot extension of trees in the orchard and containers was observed from the first week of March until the third week of December. Two peak flowering periods occurred during the first week of March, and from mid-September to mid-October. The major fruit harvest periods were August and December. Shoot flushing, extension shoot growth, flowering, and fruiting showed little response to irrigation at four SWD levels. This lack of response was likely caused by sufficient soil water due to precipitation and capillary rise from the high water table located about 1–2 m below the soil surface. Regardless of the lack of SWD effects on phenological cycles of carambola, the periodicity of shoot flushing, extension shoot growth, flowering, and fruiting and the intensity of these phenological events elucidated in this study should provide useful guidelines for carambola orchard management in southern Florida.

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J.A. Doty, W.S. Braunworth Jr., S. Tan, P.B. Lombard and R.D. William

Evapotranspiration (ET) of three perennial ryegrass (Lolium perenne L.) cultivars and one cultivar each of colonial bentgrass (Agrostis tenuis L.) and tall fescue (Festuca arundinacea L.) was measured in the field. Soil water depletion was measured with a neutron probe. Under minimal maintenance (i.e., no irrigation and infrequent mowing), ET was not significantly different for five perennial grasses. All grasses used more water than the bare-ground treatment. Soil water uptake was greatest in the upper soil layer (O to 25 cm) and decreased with depth. Few differences in water uptake were noted among grasses within each soil layer.

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Mike Caron and Roger Kjelgren

, we plotted daily average and 3-d running average Hargreaves ET o (cool season grass reference). Soil water depletion was compared in the root ball or in ambient soil for each treatment using two-way analysis of variance (ANOVA) (SigmaStat 2.1; Systat

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Roger Kjelgren, Lixue Wang and Daryl Joyce

water-requiring landscapes. Anisohydric species tolerate greater soil water depletion by maintaining open stomata and generating more negative internal water potential while scavenging soil water under water stress, particularly if deep-rooted ( West et

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Thomas E. Marler and Frederick S. Davies

Growth responses of young `Hamlin' orange [Citrus sinensis (L.) Osbeck] on sour orange (C. aurantium L.) trees to microsprinkler irrigation were studied under field conditions from 1985 to 1987 to determine the most-efficient irrigation rates and duration. Trees were irrigated when available soil water depletion (SWD) reached 20% (high frequency), 45% (moderate frequency), and 65% (low frequency). Trees at the moderate and low levels received 49% and 13%, respectively, as much irrigation water as the high treatment. Canopy volume, trunk cross-sectional area, dry weight, shoot length, leaf area, total root dry weight and volume, and new root dry weight were similar for the high and moderate levels in 2 of 3 years, but were significantly reduced at the low level. Summer and fall growth flushes were delayed or did not occur at the moderate and low levels. More than 90% of root dry weight was within 80 cm of the trunk at the end of the first growing season.

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Milton E. McGiffen Jr., John B. Masiunas and Morris G. Huck

Eastern black nightshade (Solanum ptycanthum) and black (Solanum nigrum) nightshade are difficult to control in tomato, interfering with harvest and decreasing fruit quality and yield. In irrigated tomatoes, soil water depletion was greater as nightshade density increased. However, tomato yield loss due to black nightshade was greatest at the lower weed densities. As density increases, photosynthetic activity (photosynthetic rates, stomatal conductance, intercellular CO2 concentration, and stomatal resistance) of black nightshade is more affected than eastern black nightshade. Photosynthetic activity of tomato is the least affected. In greenhouse experiments where water was denied for approximately a week prior to measurement, tomatoes were more sensitive to water stress than were nightshades. Nightshades were more adapted to drought stress than were tomatoes.

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Roger Kjelgren

Supplemental watering of shade trees in field production nurseries is needed, even in summer-rainfall climates, to achieve maximum growth. Scheduling the timing and amount of supplemental watering makes more efficient use of financial and water resources while maintaining maximum growth. Methods of scheduling supplemental watering based on uniform canopy and rooting in production agriculture must be modified, however, for shade trees in a production setting. Nursery trees are non-uniform in canopy and rooting compared to an agricultural crop. Applying the water budget method can be effective with sprinkler systems if tree water loss and rooting depth can be properly estimated. A measure of reference evapotranspiration and a species-specific multiplier are typically used to estimate water loss. Since species diversity in a field nursery is quite high, however, estimates of both tree transpiration and rooting depth must necessarily be simplified assumptions less accurate than for a uniform agricultural crop. If supplemental water is to be applied with drip irrigation, estimates of tree transpiration and soil water depletion need to be converted to volume units with information on total tree leaf area.

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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.