Arboriculture & Urban Forestry 33(6): November 2007 405 In this experiment, an increase in the amino acid proline in Figure 2. Time course recovery of leaf chlorophyll content (SPAD values) of evergreen oak (Quercus ilex L.) placed outdoors after a 21-day drought period. Two milliliters paclobutrazol (PBZ) foliar spray and 5 mL PBZ root drench- treated plant values mean of eight trees, five leaves per tree. Four milliliters PBZ foliar spray and 10 mL PBZ root drench-treated plant values mean of six trees, five leaves per tree. Controls, values mean of mean of four trees, five leaves per tree. = Control; = 2 mL PBZ foliar spray; = 4 mL PBZ foliar spray; ✚ = 5 mL PBZ root drench;= 10 mL PBZ root drench. as carotenoids as well as ROS-scavenging enzymes such as SOD and calatase (CAT; Apel and Hirt 2004). Relatively higher activities of ROS-scavenging enzymes have been re- ported in stress-tolerant genotypes when compared with sus- ceptible ones, suggesting that these antioxidant enzymes plays an important role in plant tolerance against environ- mental stresses such as salinity and drought (Apel and Hirt 2004). In the current study, a significant increase in leaf SOD and CAT activity was observed in both PBZ-treated English and evergreen oak. The function of SOD is to scavenge su- peroxide in the chloroplast and catalyze the first step in the detoxification of active oxygen to H2O2. H2O2 is then scav- enged by catalase resulting in the dismutation of H2O2 to water and oxygen. Likewise, carotenoids (carotenes and xan- thoylls) function as protective photooxidative pigments re- sponsible for the quenching of chlorophyll excited states, singlet oxygen, and interception of deleterious free oxygen and organic radicals caused by drought-induced photoinhibi- tion (Lawlor 2001). Enhanced leaf concentrations of caro- tenes and xanthoylls recorded in PBZ-treated plants before the start of the drought treatment imposed in this investiga- tion would be important contributors to reducing drought- induced photoinhibitory damage to plant cellular membranes and the photosynthetic system (Kraus and Fletcher 1994). plants treated by PBZ was observed. The function of proline is to reduce the osmotic potential so that the water potential gradient favors water uptake and therefore the plant can maintain turgor, for example, under drought or salinity con- ditions. The advantage of osmotic adjustment to the plant is that water potential across cells and tissues is maintained against a fall in water potential during drought, in turn pre- venting or reducing whole plant turgor. In addition, it is sug- gested that proline may also protect protein configurations during dehydration (Fernandez et al. 2006). Consequently, an increase in leaf proline concentration would permit a more favorable water uptake under water stress conditions improv- ing the adaptation of plants to drought conditions. The asso- ciation between increased proline concentration and tolerance to drought has been shown elsewhere (De Ronde et al. 2000; Ain-Lhout et al. 2001; Jiang and Huang 2001; Ehmedov et al. 2002; Yamada et al. 2005). Likewise, exogenously applied proline has been shown to significantly increase the stress tolerance of a broad range of plants (Van Swaaij et al. 1985; Songstad et al. 1990; Santarius 1992). With the exception of these experiments, few studies have observed the direct effect of PBZ on proline accumulation within plant tissue, although a number of examples exist showing a subsequent increase in leaf protein (to include proline) concentrations post PBZ ap- plication (Sankhla et al. 1992; Kraus and Fletcher 1994). The chlorophyll fluorescence PI value is a measure of leaf photosynthetic efficiency and subsequently provides an indi- rect measure of tree vitality (Clark et al. 1998, 2000). A decline in this value is a sensitive indicator of photoinhibitory damage caused by a wide range of environmental stresses such as salinity, drought, and ozone exposure. After exposure to 3 weeks of drought, the PI value in the PBZ-treated trees was substantially higher than non-PBZ-treated controls. Thus, the plants treated with PBZ were found to be from 60% to greater than 100% more efficient photosynthetically. The maintenance of relatively high fluorescence values in PBZ- treated plants under stress has been observed in previous studies (Pinhero and Fletcher 1994). Increased photosynthetic efficiency permits increased metabolic activity within the plant such as synthesis of sugars, amino acids, proteins, nucleic acids, and lipids, essential for the growth and repair of damaged tissue. If the photosynthetic system is impaired, then the carbohydrates required for repair and growth cannot be produced. Higher PI values in PBZ-treated plants indicate less damage to the leaf photosynthetic system in turn manifest by a greater ability to repair damaged tissue, grow, and sur- vive after the cessation of drought (Percival et al. 2006). Leakiness of leaf tissue, determined from electrolyte leak- age, indicated that cellular membrane integrity was damaged more rapidly in non-PBZ-treated controls compared with PBZ-treated plants at the cessation of 3 weeks of drought. ©2007 International Society of Arboriculture
November 2007
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