402 Percival and AlBalushi: Paclobutrazol-induced Drought Tolerance in Containerized Oak Table 3. The effects of 21 days of drought on chlorophyll fluorescence (PI), photosynthetic CO2 chlorophyll content, and necrosis of containerized English oak (Quercus robur L.).zy PI Pn Control 2 mL (0.06 fl oz) PBZ foliar spray 4 mL (0.12 fl oz) PBZ foliar spray 5 mL (0.15 fl oz) PBZ root drench 10 mL (0.3 fl oz) PBZ root drench 0.18 1.15* 2.02* 1.68* 2.41* 1.87 3.38* 3.42* 3.52* 3.58* Chlorophyll content 30.9 38.9* 41.1* 48.3* 42.2* Leaf necrosis 4.1 1.8* 2.1* 1.9* 2.0* fixation (Pn), leaf Membrane integrity 27.2 9.8* 8.9* 13.2* 9.5* Survival (%) 20 100* 100* 100* 100* zMeasurements were made immediately at the cessation of the drought treatment when trees were placed under glasshouse conditions (22°C±2°C [71.6°F± 35.6°F], 16 hr light/8 hr dark photoperiod and minimum 250 mol/m−2/s−1 photosynthetically active radiation at the tree crown. yPBZ-treated plants: PI and leaf chlorophyll content, values mean of ten trees, five leaves per tree. Pn and electrolyte leakage values mean of ten trees, two leaves per tree. Leaf necrosis values mean of ten trees. Controls: PI and leaf chlorophyll content, values mean of two trees, five leaves per tree. Pn and electrolyte leakage values mean of two trees, two leaves per tree. Leaf necrosis values mean of two trees. *Significantly different from controls according to least significant difference at P < 0.05; ns not significantly different from control value. PBZ paclobutrazol. photosynthetic system and greater structural integrity of the chlorophyll molecule at the cessation of the drought period. Leaf electrolyte leakage as a measure of drought-induced damage to cellular membrane integrity was reduced by 49% to 67% in the case of PBZ-treated English oak and by 28% to 48% in the case of PBZ-treated evergreen oak compared with control values (Tables 3 and 4). Similarly, visual leaf necrosis was reduced by 51% to 56% in the case of PBZ-treated En- glish oak and by 21% to 37% in the case of PBZ-treated evergreen oak compared with control values (Tables 3 and 4). In most cases, these reductions were significant (P < 0.05) from controls (Tables 3 and 4). Effects of Paclobutrazol on Drought Recovery Rates The pattern of recovery over the next 9 weeks after 3 weeks of drought on PI, leaf necrosis, chlorophyll content, and cel- lular electrolyte leakage is shown for English and evergreen oak (Tables 5 and 6). In addition, the pattern of recovery is shown diagrammatically for both species with respect to leaf chlorophyll content (Figures 1 and 2). Irrespective of treat- ment (with or without PBZ), all four parameters began to recover in surviving trees after the start of watering. Conse- quently, the pattern of recovery was quantified by quadratic regression analysis to compare the rate of recovery over the next 9 weeks (Tables 5 and 6). Irrespective of species, all PBZ-treated trees were the most capable of recovery as re- flected by higher regression values with respect to PI and leaf chlorophyll recovery and lower regression values with re- spect to leaf necrosis and cellular electrolyte leakage (Tables 5 and 6). Such responses in PBZ-treated trees are associated with enhanced recovery of leaf photosynthetic integrity, im- proved photosynthetic efficiency, reduced degradation of the chlorophyll molecule structure, and enhanced membrane in- Table 4. The effects of 21 days of drought on chlorophyll fluorescence (PI), photosynthetic CO2 chlorophyll content, and necrosis of containerized evergreen oak (Quercus ilex L.).zy PI Pn Control 2 mL (0.06 fl oz) PBZ foliar spray 4 mL (0.12 fl oz) PBZ foliar spray 5 mL (0.15 fl oz) PBZ root drench 10 mL (0.3 fl oz) PBZ root drench 1.70 2.27ns 2.57ns 2.42ns 3.34* 1.59 2.83* 1.98ns 2.07ns 1.78ns Chlorophyll content 50.6 95.8* 61.0ns 68.7ns 84.3* Leaf necrosis 3.8 2.7ns 2.8ns 3.0ns 2.4ns fixation (Pn), leaf Membrane integrity 25.4 18.6ns 17.4ns 16.0ns 12.3* Survival (%) 40 80* 60ns 80* 60ns zMeasurements were made immediately at the cessation of the drought treatment when trees were placed under glasshouse conditions (22°C±2°C [71.6°F± 35.6°F], 16 hr light/8 hr dark photoperiod and minimum 250 mol/m−2/s−1 photosynthetically active radiation at the tree crown). yTwo milliliters PBZ foliar spray and 5 mL (0.15 fl oz) PBZ root drench-treated plants: PI and leaf chlorophyll content, values mean of eight trees, five leaves per tree. Pn and electrolyte leakage values mean of eight trees, two leaves per tree. Leaf necrosis values mean of ten trees. Four milliliters PBZ foliar spray and 10 mL (0.3 fl oz) PBZ root drench-treated plants: PI and leaf chlorophyll content, values mean of six trees, five leaves per tree. Pn and electrolyte leakage values mean of six trees, two leaves per tree. Leaf necrosis values mean of ten trees. Controls: PI and leaf chlorophyll content, values mean of four trees, five leaves per tree. Pn and electrolyte leakage values mean of four trees, two leaves per tree. *Significantly different from controls according to least significant difference at P < 0.05; ns not significantly different from control value. PBZ paclobutrazol. ©2007 International Society of Arboriculture
November 2007
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