Leaf Area, Nonstructural Carbohydrates, and Root Growth Characteristics of Blue Grama Seedlings

Abstract

Establishment of blue grama [Boutelouu gracilis (H.B.K.) Lag ex Steud.] seedlings requires extension of adventitious roots into the soil profile. The objectives of this study were to determine the effects of leaf area and total nonstructural carbohydrates (TNC) on root growth characteristics of blue gmma. Seedlings supported by the seminal root only were treated with 3 days of reduced light and then with 0, 1,2, and 3 days of full sunlight to alter TNC percentage in crowns. Seedlings within each of these treatments were then clipped at a height of 3,6,9, and 12 cm, or left unclipped to alter leaf area. Adventitious root growth was studied during a j-day test. Path coefficients indicating the effects of leaf area on number of roots per seedling, depth of roofs, and root weight per unit length (diameter) were 0.72, 0.47, and 0.77, respectively. The TNC had smaller effects on root growth than did seedling leaf area. Clipping treatments probably reduced root growth because of a deficiency of photosynthetic products. But, the reduction was explained by an adjustment in all components of growth rather than in root depth only. Thus, blue grama seedlings maintained a reasonable rate of root elongation even under severe clipping treatments. Establishment of blue grama [Bouteloua grucilis (H.B.K.) Lag. ex Steud.] seedlings requires the initiation and the extension of adventitious roots (Wilson and Briske 1979). That process depends on seedling leaf area (Wilson 198 I), total nonstructural carbohydrates (TNC), tolerance of dehydration (Briske and Wilson 1980, Khan 1980), and favorable environmental conditions (Hyder et al. 1971). The relationships among clipping treatments, TNC, rate of root elongation, and total dry weight of roots have been studied by several investigators (Booysen and Nelson 1975, Buwai and Trlica 1977, Crider 1955, Hansen 1978, Smith 1974, Youngnerand Nudge 1976). Little information is available, however, on the possible effects of clipping and TNC on other components of root growth (Parker and Sampson 1930). The objectives of this study were to determine effects of seedling leaf area (clipping treatments) and crown TNC percentage (light treatments) on 3 components of root growth in blue grama seedlings: number of roots, root length (depth), and root weight per unit length (diameter). Information on root growth characteristics associated with clipping and environmental stress will aid in seedling establishment and in managing blue grama stands. Materials and Methods The effects of light and clipping treatments on the components of adventitious root growth were investigated on blue grama seedlings under greenhouse conditions during June through September. Air temperatures in the greenhouse varied from 25 to The author, now deceased, was plant physiologist, USDA ARS, Crops Research Laboratory, Colorado State University, Fort Collins 80523. This study was a cooperative investigation of USDA Agricultural Research Service and Colorado State University Experiment Station. Scientific Paper 2862. The technical assistance of C.J. Carren, D.S. Edyvean, D.A. Nason, and B.L. Oskroba during this study is gratefully acknowledged. Manuscript accepted March 7, 1984. 35OC. Maximum midday _fhotosynthetic photon flux density (PPFD) was 1,650 pmole m set-’ in early summer and 850 pmole me2 set-’ in late summer. Plastic pots (15 cm diameter by I5 cm deep) were filled with 1,800 g of sterilized (100°C dry heat for 2 days) sandy loam soil (fine-loamy, mixed, mesic Aridic Argiustoll). The soil was surface irrigated with 250 ml of water, and 25 ‘Lovington’blue grama seeds were planted at a depth of 2 mm in the moist soil. Seeds were then covered with 2.5 cm of air-dry soil. Seedlings emerged through the dry soil layer within about 5 days. Pots were weighed on alternate days and the amount of water needed to bring the lower 1,800-g of soil to field capacity was placed in a petri dish. Water moved into the soil through holes in the bottom of the pot. The subirrigation procedure maintained a moist subsoil and a dry soil surface which promoted seedling growth but prevented growth of adventitious roots. Thus, seedlings were supported by the seminal root only during this phase of the study. At 3 weeks after planting, pots were thinned to 8 vigorous, well-spaced seedlings. At 5 weeks, seedlings were exposed to 3 days of shade and then to 0, 1,2. or 3 days of sunlight to create various levels of total nonstructural carbohydrates in seedling crowns. Average midday PPFD in shade was 240 pmole m-’ sec.-‘. After shade and light treatments, the pots were separated into 5 groups and seedlings were clipped at heights of 3,6,9, or 12 cm. The fifth group of seedlings was left unclipped. Four seedlings in each pot were randomly sampled to determine the weight of shoot removed by clipping and the weight of shoot remaining after clipping. Leaf-blade area remaining after clipping was also determined. The lower 3-cm portion of stem base was dried at 60°C and used for determination of percent TNC (Association of Official Agricultural Chemists 1965, Heinze and Murneek 1940, Smith et al. 1964). Measurements before the root growth test were based on a composite sample of 12 seedlings harvested from 3 pots. Immediately after clipping, the remaining seedlings in each pot were surface irrigated to promote growth of adventitious roots during a 3-day test. Shade, light, and clipping treatments were scheduled so that the root growth test could be started on the same day for all treatments. After 3 days with a moist soil surface, seedlings in each pot were harvested, adventitious roots were counted, the length of each root (main axis) was measured, and roots were oven-dried and weighed. Root weight per unit length was estimated from the total length of the main axis of all roots in a sample divided by the dry weight of the sample. There was little or no branching of adventitious roots at this stage of development. Leaf blades were removed and leaf area was measured. Measurements after the root growth test were based on a composite sample of 6 seedlings harvested from 3 pots. The study was conducted as a randomized complete-block. Factor I represented 4 light treatments and factor 2 represented 5 clipping treatments. The study included 6 replications which represented different dates of growth in the greenhouse. Analysis of variance was used to test for differences among treatments. Path 514 JOURNAL OF RANGE MANAGEMENT 37(6), November 1984 Table 1. Effects of days of sunlight after shade and height of clipping on leaf am per seedling (cm’) before and after the 3&y root growth test. Days of sunlight 3 Clipping height (cm) 6 9 12 Unclipped control Mean Before root growth test 0 1.55 1 1.34 2 1.59 3 1.31 Mean I .4s Pooled standard error (si) = 0.59 After root growth test 0 3.32 1 3.13 2 3.74 3 3.94 Mean 3.692 Pooled standard error (si) = 0.67 4.51 6.86 8.52 10.95 4.15 6.64 8.82 11.58 4.23 6.61 8.49 11.43 4.04 6.48 8.72 II.32 4.25 6.65 8.63 11.32 6.83 10.20 10.95 13.07 6.08 10.17 11.32 12.24 7.00 9.30 13.06 12.58 7.57 9.40 12.18 12.24 6.87 9.71 II.88 12.53 6.49’ 6.51 6.46 6.37 8.87’ 8.71 9.13 9.06 ‘Within each sampling period (before or after root growth test), differences among light treatments were not significant (KO.05). 2Within each sampling period (before or after root growth test), differences among clipping treatments were significant (KO.01). Table 2. Effects of days of sunlight after shade and height of clipping on the average number of adventitious roots and the length of longest adventitious root (cm) produced per seedling during the 3-day test. Clipping height (cm) Days of sunlight 3 6 9 12 Unclipped control Mean Number of roots per seedling 0 11.3 18.4 21.4 24.0 27.3 20.5’ 1 13.0 18.3 21.2 23.2 25.0 20.1 2 13.9 20.6 22.2 21.4 26.6 22.2 3 14.8 22.2 24.3 21.4 30.2 23.8 Mean 13.22 19.9 22.3 25.5 27.3 Pooled standard error (si) = 1.3 Length of longest root per seedling 0 5.63 I 5.80 2 6.67 3 6.40 Mean 6.122 Pooled standard error (Q q 0.25 7.48 8.97 8.90 9.85 8.17’ 7.72 8.83 9.92 9.83 8.42 8.20 9.22 9.62 9.98 8.74 8.53 9.13 9.65 9.87 8.72 7.98 9.04 9.52 9.88 ‘Differences among light treatments in number of roots and length of longest root per seedling were significant (cUO.01). zDiffcrences among clipping treatments in number of roots and length of longest root per seeding were significant (KO.01). Table 3. Effects of days of sunlight after shade and height of clipping on the weight per unit length of adventitious roots (Ccg/cm) and the total weight of adventitious roots (mg/seedllng) produced during the May test.