Effects of Paclobutrazol and its Method of Application on the Growth and Transpiration of Acacia mangium Seedlings

Ten-week-old potted A. mangium seedlings were subjected to a growth retardant, paclobutrazol (PP333), at concentrations of 0, 0.5, 1, 4 and 12 g/l. Four methods of application were tested namely S, soil drenching at week 0; Fl, foliar spray (potting soil protected from chemical) at week 0; F2, foliar spray (potting soil protected from chemical) at week 0 and 6; and S+F, soil and foliar spray (potting soil exposed to chemical spray) at week 0. Plants were harvested after 12 weeks for various growth measurements. Height and leaf area increments, transpiration and stomatal conductance were monitored at weekly intervals. Paclobutrazol was found to be effective in reducing root and shoot growth, transpiration and stomatal conductance of the seedlings. Treated plants had higher root to shoot ratios. The effects of the chemical increased with increasing concentration and frequency of application for the foliar spray. The chemical was most effective when applied through the soil either by drenching (S) or soil and foliar spray (S+F). INTRODUCTION reported to be good, but mortality sets in when Malaysia has a programme to establish plantatransplanting is delayed and the seedlings overtions of fast-growing hardwood species, princig r o w i n s i z e (Abod and Abun 1989). The pally Acacia mangium Willd. for general utility mortalities have often been attributed to unfatimber totalling 500,000 hectares by the year vourable root to shoot ratios resulting in desic2000. The success of such a large-scale plantac a t i o n post-transplanting. Delays in transplanttion establishment depends on an efficientlying often occur because of climatic factors which managed nursery to produce high-quality planta f f e c t t h e timing of site preparation and its ing stock which can survive and grow rapidly synchronization with field planting. This study when outplanted. Presently, potted seedlings w a s conducted in an attempt to develop an about 3 months old and averaging 30 cm in effective manipulative technique for controlling height are transplanted from the nursery to the t h e growth of A. mangium seedlings and improvfield. Survival and growth of these seedlings are i n g t h e i r establishment in the field. S. A. ABOD AND L. T. JENG Researchers have for many years sought reliable, effective and safe methods of controlling shoot growth of tree species using chemical growth retardants. Several new compounds have become available for testing in the last few years, all inhibitors of endogenous gibberellin biosynthesis. One of the most potent and longlasting, the triazole paclobutrazol, has shown great efficacy in reducing height growth of many temperate fruit species and cultivars (Webster and Quinlan 1984). At certain concentrations and spray volumes, paclobutrazol is reported to reduce shoot but not root growth and, in some instances, to improve plant water relations (Rademacher et al 1984; Wang et al 1986; Atkinson 1986). Swietlik and Miller (1983) reported that root length was stimulated by paclobutrazol applications at low to moderate concentrations. Moreover, Steffens et al (1983) showed increases in fibrous root length, root: shoot ratio and unsuberised root diameter of apple (Malus domestica Borkh.) seedlings after treatment with paclobutrazol. Higher concentrations, however, may reduce root growth (Swietlik and Miller 1983). Increases in root: shoot ratios stimulated by plant growth retardant treatment should, in theory, also improve plant water relations and increase tolerance to drought (Turner and Begg 1981). Where growth retardants are applied at concentrations sufficient to control shoot growth it is likely that most of their effects on the water economy of the plant will be attributed to the indirect effect of reduced leaf number and total leaf area (Atkinson and Crisp 1982; Asamoah and Atkinson 1985). However, both Atkinson and Crisp (1982) and Dubravec et al (1986) suggested an additional anti-transpirant effect of paclobutrazol, in addition to the indirect influence on water relations by reduction in leaf area. Several methods have been used for applying paclobutrazol to trees. These include foliar sprays, soil surface sprays, trunk drenches, soil injection in narrow bands, incorporation into potting soil and pressure injection into the vascular system of woody stems. Foliar sprays with paclobutrazol have been reported to give immediate control of apple tree growth in the United Kingdom (Quinlan and Richardson 1984),but several applications are required in a single season to obtain effective growth control (Lever el al 1982). Application as a soil drench is more effective than foliar spray in the long term (Curry and Williams 1983) mainly because the chemical can persist in the soil for longer periods and is readily absorbed by the roots and translocated acropetally through the xylem to the meristematic regions (Richardson and Quinlan 1986). MATERIAL AND METHODS A. mangium seeds were obtained from plantations in Sabah, Malaysia that were initially estat> lished from seeds imported from Queensland. Australia. Seeds were pretreated in boiling water for 30 seconds, soaked in tap water for 24 hours and then germinated in a greenhouse. Four weeks after germination, each seedling was transplanted into a plastic pot measuring 19 cm in diameter and 14 cm in height. The potting medium was a mixture of soil, sand and peat in a ratio of 7:3:2. Each pot contained 3 kg of the medium To every 100 kg of the potting medium, 2 kg of triple superphosphate fertilizer was added. The experiment was conducted in a green house. Plants were selected 10 weeks after germination. Seedlings with uniform shape and measuring 20 cm tall were chosen from a large number of available plants. Paclobutrazol (PP333) was supplied by Imperial Chemical Indusries (ICI) in aqueous suspension at a concentration of 250 g/1 with an active ingredient content of 22.0% w/w. Its trade name is Cultar and chemical formula [2RS. 3RS) -l-(4-chlorophenyl)] -4, 4-dimethyl-2(1H1,2,4 triazol-1-yl) pentan-3-ol). The chemical was diluted in distilled water to give concentrations of 0.5, 1, 4, and 12 g/1. A surfactant (Du Pom agricultural surfactant) also supplied by ICI was added at a concentration of 2.0 ml/1. The experiment tested the effects of 5 concentrations (inclusive of control at 0 g/I)an(H methods of application as follows: S : Soil drenching at week 0 (i.e. at the start of the treatment) Fl : Foliar spray at week 0 (potting soil protected from chemical) F2 : Foliar spray at week 0 and 6 (potting soil protected from chemical) S+F : Foliar spray at week 0 (potting SOL exposed to chemical) The surfaces of the pots in the Fl and K treatments were covered with plastic sheets to 144 PERTANIKAj. TROP. AGRIC. SCI. VOL. 16 NO. 2, 1993 EFFECTS OF PACLOBUTRAZOL ON GROWTH & TRANSPIRATION OF A. MANGIUM SEEDLINGS shield the soil from the foliar sprays. A knapsack sprayer was used to spray the aerial parts of plants to run-off. A total of 102 plants were used at 6 replicates per treatment. Only 6 unsprayed plants were used as a common control for all treatments. Plants were harvested 12 weeks after treatment and measurements were taken for height (i.e. length of the main stem from the soil to the shoot apex), root collar diameter, number of branches, total leaf area, total root length, root and shoot dry weights. Roots were washed over a sieve of mesh pore size less than 1.0 mm square using pressurized tap water. Total root length was measured by a Comair root length scanner (Abod and Webster 1989; 1991). Leaf area was measured by a portable leaf area meter. Increments in height and leaf area were monitored at weekly intervals from 3 plants selected randomly from each treatment. Measurements of transpiration and stomatal conductance were also made on these plants. Transpiration was measured gravimetrically at weekly intervals and expressed as gram water use (or water loss) per unit leaf area per week. Theplants were watered to field capacity (as determined by a soil moisture meter a voltaic probe manufactured by Plant Cove Ltd. UK) from the base by placing the base of each pot in a shallow dish of water. The pot and soil surface were then enclosed within a polythene bag, sealed around the stem base and weighed. Soil moisture status was monitored using the soil moisture meter and the plants maintained near field capacity by watering via the dish twice each day. The amount of water provided per week per plant was recorded and the pots were re-weighed at the end of each week. By estimating approximate values for plant fresh weight increase per week, from values at planting and at harvest, and using the records of both water use and leaf area, transpiration per week was estimated. Measurements of stomatal conductance, commencing in the first week following spraying were made on the abaxial surface of selected leaves using a steady-state porometer. This was similar to that described by Jones and Norton (1979) which measured the difference in relative humidity between inlet (maintained at zero) and outlet air flowing at a constant rate (2.5 ml/ s) through a chamber enclosing 1.76 cm of leaf surface. Measurements were taken between noon and 1400 hours on one fully-expanded, unshaded leaf at the approximate mid-length of the main shoot of each plant. The experiment was designed to test the effects of paclobutrazol and its method of application in a 4 x 5 factorial using 6 replications in a completely randomized design.