Transpiration rate (E) and epidermal conductance (gc,) were determined for avocado leaves and floral parts under controlled environmental conditions (28�C; PAR 60 8mole quanta m-2 s-1; VPD 1.14 kPa). E of hydrated flowers was shown to be approximately 60% that of similarly treated leaves. Around 13% of total transpirational water loss from tree canopies could be attributed to floral organs. The mean gc of leaves and flower panicles was similar at 0.028 and 0.023 mm s-1 respectively. The available canopy surface area for water loss increased by c. 90% during flowering. Xylem water potentials in panicle segments with open flowers were lower than those of mature leaves measured on trees in the field. Maximum transpiration rate of leaves measured in the field was 6.7 8g cm-2 s -1 at 0700 hours, declining to 3.0 8g cm-2 s-1 at 1100 hours, while stomatal conductance (gs) on the same leaves fell from 10.2 mm s-1 at 0700 hours to 2.52 mm s-1 at 1100 hours. Scanning Electron Microscope studies of leaves and floral structures highlighted morphological and anatomical features for water conservation. Mature leaves have an epicuticular wax-like layer on the adaxial surface. Stomates were located only on the abaxial surface, which was also covered wlth wax-like deposits. Stomate density was estimated at 73 000 cm-2 on sun leaves. Stomates were also located on abaaial surfaces of flower sepals and petals. All floral structures were densely pubescent, thereby increasing the effective boundary layer depth.
Avocado trees (cv. Fuerte) were protected from root rot caused by Phytophthora cinnamomi by foliar metalaxyl. Protected trees had higher xylem water sprays with phosethyl-A1 or soil applications of potentials and showed faster and more complete recovery from water stress during the evening than un treated root rot affected trees. protected trees had greater fruit yields and there was a correlation between the incidence of the physiological disorder ring neck and xylem water potential in trees.
Translocation of phosphonic acid (H3PO3) in cv. Hass avocado trees was studied after trunk injection with 20% H3PO3, formulated as potassium phosphonate, at three stages of tree phenology during the growing season. Initially, translocation was solely acropetal in the xylem, and H3PO3was detected in the leaves 24 h after treatment. Several days after injection, H3PO3concentration in the bark of trunks and in roots increased, indicating basipetal phloem transport of H3PO3from leaves. The rate of accumulation and the final concentration of H3PO3in the roots were directly related to the sink strength of the shoot at the time of injection. For example, trunk injection at the beginning of spring growth flush, when renewal shoots were strong sinks, resulted in low H3PO3root concentrations (<9 8g gfw-1) which peaked about 45 days after treatment. When potassium phosphonate was injected after the transition of spring-grown shoots from sinks to sources, or at summer shoot maturity, root concentrations of H3PO3increased to >25 8g gfw-1 by 30 days after treatment. These results suggest that strategic timing of injections according to phenological events may greatly improve fungicide efficacy when targeting specific organs for protection.
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