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Root hydraulic conductivity (Lp) and osmotic potential (π) were measured in young, drought-stressed and non-stressed peach (Prunus persica), Olive (Olea europea), Citrumelo (Citrus paradisi x Poncirus trifoliata) and Pistachio (Pistachia integerrima) plants. Drought stress reduced Lp 2.5 to 4.2-fold, depending on species, but π was reduced only in expanded citrumelo leaves and unexpanded olive leaves by 0.34 and 1.4 MPa, respectively. A simulation model of plant water uptake and leaf water relations was constructed to quantify the offsetting effects of reduced Lp and osmotic adjustment (OA) on turgor maintenance. For olive data, a 2.5-fold reduction of Lp caused a linear decrease in turgor pressure difference between stressed and non-stressed plants, such that the effect of OA was totally offset at a leaf water potential (stressed) of ≈ -3.0 MPa. For citrumelo, because the degree of OA was lower, the water potential at which the effects of OA and reduced Lp were offsetting with respect to turgor maintenance was ≈ -0.6 MPa. The analysis suggests that some level of stomatal closure would be necessary to extend the water potential range over which stressed plants maintain higher turgor than non-stressed plants for citrumelo. Conversely, no degree of stomatal closure would be required of stressed olive plants to maintain higher turgor than non-stressed counterparts over a physiologically meaningful range of leaf water potential.
Illiciums, or star-anises, have increased in popularity in the nursery and landscape industries. However, confusion exists as to which taxa are tolerant of high light intensities during production and subsequent establishment in the landscape. We investigated the effect of two light intensity treatments, 45% and 100% full sunlight, on gas-exchange parameters of five Illicium taxa: Illicium anisatum L., I. floridanum Ellis. `Pebblebrook', I. henryi Diels., I. lanceolatum A.C. Sm., and I. parviflorum Michx. Ex. Vent. `Forest Green'. Light-response curves were determined for individual leaves, and mean response parameters calculated. Chlorophyll and total carotenoids were analyzed after extraction in acetone, with total chlorophyll also estimated with a SPAD chlorophyll meter. In general, highest rates of CO2 assimilation (Amax) and lowest rates of dark respiration (Rd) were found in the 45% light treatment for all taxa. Both Illicium anisatum and I. floridanum `Pebblebrook' had substantial reductions in Amax in 100% light, 94% and 81% respectively, compared to plants grown in the 45% light treatment. Illicium henryi failed to survive the 100% light treatment. Illicium lanceolatum and I. parviflorum `Forest Green' were least affected by the 100% light treatment. Severe photooxidative bleaching was noted and confirmed by SPAD and pigment data, although SPAD readings were a poor predictor of total chlorophyll. For taxa of Illicium in our study, photosynthetic gas-exchange parameters and foliage pigment characteristics were improved in the low light treatment, suggesting optimal growth occurs in shaded conditions.
One-year-old rooted cuttings of `Nemaguard' peach [Prunus persica (L.) Batsch.] were irrigated with 0, 1000, or 2000 mg·L-1 salts under greenhouse conditions to study the effect of foliar paclobutrazol (PBZ) application on salt stress response. Salinity reduced growth of nontreated plants by ≈60%, but only by ≈30% for PBZ-treated plants. PBZ-treated plants also had less defoliation and fewer leaves per plant showing salt stress symptoms, and had higher rates of leaf gas exchange than nontreated plants. PBZ application generally reduced Na+ and Cl- contents in leaves, roots, and stems, regardless of salt treatment. Furthermore, total Na+ per plant in PBZ-treated plants was about half that found in nontreated plants, although total Cl- per plant was reduced by PBZ in only one of two salt treatments. The data suggested that PBZ promoted salt stress avoidance in peach by reducing the uptake and accumulation of harmful Na+ and Cl- ions in plant tissues.
We studied the effect of a 200-mg·liter–1 foliar application of paclobutrazol (PBZ) on growth and physiological responses of Prunus persica `Nemaguard' (salt-sensitive) and Olea europea `Manzanillo' (salt-tolerant) to salt stress. One-year-old trees were grown in 3 sand: 3 field soil: 4 pine bark media in 20-cm pots in a greenhouse and were irrigated with nutrient solutions adjusted with 0, 9, 18, or 36 mmol NaCl for peach and 0, 36, 72, 108 mmol NaCI for olive. Dry weight, photosynthesis, and leaf conductance decreased with increasing salinity for both species. However, leaf expansion rate was unaffected by NaCl. PBZ reduced dry weight for peach only, but PBZ increased photosynthesis and reduced leaf expansion rate for both species. Relative water content was decreased by salt but increased by PBZ. PBZ reduced the foliar Na and Cl content in peach but not olive. Olive had less Na in leaves than peach at 36 mmol NaCI, accumulated less C in leaves in all salt treatments, and had higher foliar Na without symptom expression. PBZ may reduce salt stress in sensitive species like peach by reducing foliar Na and Cl accumulation but has less influence on the salinity response of the more salt-tolerant olive.