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- Author or Editor: Frederick S. Davies x
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.
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.
The effects of flooding calcareous soil on physiology and growth have been studied for several subtropical and tropical fruit crops including avocado (Persea americana Mill.), mango (Mangifera indica L.), carambola (Averrhoa carambola L.), and several Annona species. In calcareous soils that have a high pH, short-term flooding can actually be beneficial to subtropical and tropical fruit crops by increasing the solubility of particle-bound nutrient elements such as Fe, Mn and Mg due to flooding-induced decreases in soil pH. Additionally, flooding reduces the redox potential in the soil, resulting in Fe being reduced from Fe3+ to Fe2+, which is the cation metabolized by plants. As with other woody perennial crops, one of the early physiological responses of subtropical and tropical fruit trees to flooding is a decrease in stomatal conductance and net CO2 assimilation. If the flooding period is prolonged, lack of O2 (anoxia) in the soil results in a reduction of root and shoot growth, wilting, decreased nutrient uptake and eventual death. The flooding duration required to cause tree mortality varies among species, among cultivars within species, and with environmental conditions, particularly temperature. Several tropical and subtropical fruit crops have anatomical or morphological adaptations to tolerate prolonged flooding, such as development of hypertrophied stem lenticels, adventitious rooting or formation of porous aerenchyma tissue. For grafted trees, flooding-tolerance is conferred by the rootstock and not the scion. Therefore there is a possibility to increase flood tolerance of subtropical and tropical fruit crops by identifying or developing flood-tolerant rootstocks.
The effect of flooding on container-grown `Tommy Atkins' mango (Mangifera indica L.) trees on two rootstock, and on container-grown seedling `Peach' mango trees, was investigated by evaluating vegetative growth, net gas exchange, and leaf water potential. In general, flooding simultaneously reduced net CO2 assimilation (A) and stomatal conductance (gs) after 2 to 3 days. However, flooding did not affect leaf water potential, shoot extension growth, or shoot dry weight, but stem radial growth and root dry weight were reduced, resulting in larger shoot: root ratios for flooded trees. Mortality of flooded trees ranged from 0% to 45% and was not related to-rootstock scion combination. Hypertrophied lenticels were observed on trees that survived flooding but not on trees that died. The reductions in gas exchange, vegetative growth, and the variable tree mortality indicate that mango is not highly flood-tolerant but appears to possess certain adaptations to flooded soil conditions.
Thermal properties of tree wraps commonly used for freeze protection of young citrus trees were measured in the laboratory using a newly developed method to determine which factors are most important in wrap design and performance. Thermal diffusivity was lowest for wet fiberglass and styrofoam wraps with water containers attached to their inner surface, intermediate for dry fiberglass, and highest for thin-walled polyethylene and polystyrene wraps. Thermal diffusivity was inversely related to freeze protection capacity observed under field conditions for the tree wraps tested. Addition of water, either throughout the wrap material or in containers inside wraps, decreased thermal diffusivity three- and ten-fold, respectively. Minimum trunk temperatures of 2-year-old ‘Hamlin’ orange trees [Citrus sinensis (L.) Osb.] were up to 2°C lower under ventilated tree wraps compared to similar nonventilated wraps on mild freeze nights. An ideal tree wrap should have low thermal diffusivity and lack free airspaces and ventilation holes, while allowing for gas exchange and expansion of the tree trunk.
The effectiveness of intermittent and constant microsprinkler irrigation, compared with no irrigation, for freeze protection of young ‘Hamlin’ orange trees [Citrus sinensis (L.) Osb.], was studied during radiative and advective freezes in 198586. All tree trunks were wrapped with fiberglass insulation to reduce evaporative cooling. Intermittent irrigation of 5 min on and 10 or 15 min off maintained trunk temperatures under the wrap above critical values during radiative and advective freezes at air temperatures of — 7.1°C. However, trunk temperatures above the wrap decreased to below air temperatures due to evaporative cooling when using intermittent treatments during advective conditions. Trunk temperatures were highest for constant irrigation, intermediate for intermittent, and lowest for unirrigated wrapped trees; yet, survival was similar for all treatments. Nevertheless, intermittent irrigation provided acceptable freeze protection to the lower trunk using one-fourth as much water as constant irrigation and may allow citrus growers more flexibility in using irrigation for freeze protection of young citrus trees.
The influence of root temperature on whole-plant water relations and cold hardiness in seedlings of 2 citrus rootstocks—rough lemon (Citrus jambhiri Lush.) and Carrizo citrange [C. sinensis (L.) Osbeck × Poncirus trifoliata (L.) Raf.]—and ‘Valencia’ scions on both rootstocks was examined. Plants were exposed to root temperatures of 5°, 10°, or 15°C for 5–8 weeks, while shoots were exposed to a nonacclimating air temperature of 30°. Root temperatures of 5° decreased leaf xylem water potential and increased cold hardiness. Statistical differences in diffusive resistance and transpiration were observed only at the 5° root temperature. Root temperature did not significantly alter leaf relative water content in either seedlings or budded plants. A decrease in soil and root temperature alone, without a simultaneous reduction in air temperature, can provide an effective cold-acclimating environment for citrus.
Two-year-old ‘Hamlin’ orange trees [Citrus sinensis (L.) Osb.],on Poncirus trifoliata (L.) Raf. rootstock were used to study the effects of various microsprinkler irrigation rates and spray patterns on trunk temperatures and microclimate during freeze conditions. Irrigation treatments consisted of a factorial combination of two spray patterns (90° and 360°) × three application rates (38, 57, and 87 liters hr−1), plus an unirrigated control. All trunks were wrapped with fiberglass insulation. Trunk temperatures were 1° to 5°C higher for irrigated than unirrigated trees, regardless of spray pattern or application rate and were 1° to 2°C higher with the 90° than 360° spray pattern. The 57 and 87 liter·hr−1 rates produced the highest trunk temperatures, although 38 liter·hr−1 maintained trunk temperatures above −2.5°C with air temperatures as low as − 12°C. Outgoing radiation was higher for irrigated than unirrigated treatments during advective freeze conditions only, but air temperature and vapor pressure were unaffected by microsprinker irrigation.
Some morphological aspects of navel oranges [Citrus sinensis (L.) Osbeck] were studied in 1980 and 1981 on healthy fruit and fruit affected by secondary-fruit yellowing (SFY), stylar-end decay (SED), and fruit splitting, major causes of summer and summer-fall fruit drop in Florida. Fruit affected by SED or splitting were generally larger and had larger styler-end apertures, and those affected by SED had a higher frequency of rind protrusions into primary-fruit locules than healthy fruit. Healthy fruit harvested from the south top of trees were heavier and had larger secondary fruit, stylar-end apertures, and thicker peel than fruit from the northern-bottom canopy position. Some of these characteristics are associated with incidence of SFY, SED, and fruit splitting.
Detached shoots of double-flowered peach [Prunus persica (L.) Batsch] selections Fla. 6-1 and Fla. 0-5 were successfully opened in floral solutions containing 1 to 10% sucrose in deionized water. Addition of 8-hydroxyquinoline citrate (8-HQC), gibberellic acid (GA3), or 6-benzylaminopurine (BA) to solutions did not extend vase life. Solution uptake rate decreased over the 8-day life of the shoots and was influenced by solution molarity. Xylem plugging by pectic-type materials increased with time in solution. Addition of 1% ethanol to the floral solution hastened time of first opening, decreased the extent of xylem plugging, and extended vase life. Ethanol at 2% extended vase life and increased solution uptake rate over solutions containing sucrose alone.