Plant and Soil 243: 229–241, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
229
Responses to drought of five Brachiaria species. I. Biomass production, leaf growth, root distribution, water use and forage quality Orlando Guenni1 , Douglas Mar´ın 1 & Zdravko Baruch2,3 1 Facultad
de Agronom´ıa, Instituto de Bot´anica Agr´ıcola, Universidad Central de Venezuela, Apartado 4579, Maracay 2101, Venezuela. 2 Departamento de Estudios Ambientales, Universidad Sim´on Bol´ıvar, Apartado 89000, Caracas 1080, Venezuela. 3 Corresponding author∗ Received 27 February 2001. Accepted in revised form 17 April 2002
Key words: Brachiaria pastures, drought stress, forage quality, growth, root distribution, soil water use, tropical savannas Abstract The introduction of African grasses in Neotropical savannas has been a key factor to improve pasture productivity. We compared the response of five Brachiaria species to controlled drought (DT) in terms of biomass yield and allocation, pattern of root distribution, plant water use, leaf growth, nutrient concentration and dry matter digestibility. The perennial C4 forage grasses studied were B. brizantha (CIAT 6780), B. decumbens (CIAT 606), B. dictyoneura (CIAT 6133), B. humidicola (CIAT 679) and B. mutica. Two DT periods, which mimic short dry spells frequent in the rainy season, were imposed by suspending irrigation until wilting symptoms appeared. They appeared after 14 days in B. brizantha, B. decumbens and B. mutica, and after 28 days in B. humidicola and B. dictyoneura. The impossed drought stress was mild and only the largest grass, B. brizantha, showed reduced (23%) plant yield. The other grasses were able to adjust growth and biomass allocation in response to DT leaving total plant yield relatively unaffected. Brachiaria mutica, had a homogeneous root distribution throughout the soil profile. In the other species more than 80% of root biomass was allocated within the first 30 cm of the soil profile. Brachiaria brizantha and B. decumbens had the lowest proportion of roots below 50 cm. Drought caused a general reduction in root biomass. The shoot:root ratio in B. mutica and B. humidicola increased in response to DT at the expense of a reduction in root yield down to 50 cm depth. Although the total water volume utilized under DT was similar among grasses, the rate of water use was highest (0.25 l day−1 ) in B. brizantha, B. decumbens and B. mutica and lowest (0.13 l day−1 ) in B. humidicola and B. dictyoneura. In all species leaf expansion was reduced by DT but it was rapidly reassumed after rewatering. Drought increased specific leaf mass (SLM) only in B. brizantha compensating for leaf area reduction, but leaf area ratio (LAR) was unaffected in all species. In almost all grasses DT increased leaf N and K concentration and in vitro dry matter digestibility. The results indicate that B. brizantha, B. decumbens and to a lesser extent, B. mutica are better adapted to short dry periods, whereas B. humidicola and B. dictyoneura are better adapted to longer dry periods.
Introduction A key component for the improvement of animal productivity in Neotropical savannas has been the introduction and cultivation of African grasses. During the last 30 years, improved forage species of the genera Andropogon, Cynodon, Panicum and Brachiaria ∗ FAX No: +58-2-9063064. E-mail:
[email protected]
have replaced native and low quality pastures in the Orinoco savannas of South America. This new technology is based on their high biomass production and forage quality in acid soils (Lascano, 1991; Rao et al., 1993). At present, Brachiaria pastures in Venezuela cover > 3 million Ha (Pizarro et al., 1998). The genus Brachiaria of the sub-family Panicoideae is represented by annual and perennial species with stoloniferous and caespitose life forms (Bog-
230 dan, 1977; Roche et al., 1990; Skerman and Riveros, 1992). There is also a wide genetic variability among and within species of this genus in response to water stress. Empirically B. mutica (Forsk.) Stapf is recognized for its adaptation to lowland, seasonally flooded areas, whereas B. brizantha (A. Rich) Stapf, B. humidicola (Rendle) Schweickt, B. decumbens Stapf and B. dictyoneura (Fig. & de Not.) Stapf are better adapted to drier habitats of the savanna (Humphreys, 1987; Pizarro et al., 1998; Skerman and Riveros, 1992; Thomas and Grof, 1986). Although the potential of Brachiaria for increasing pasture and animal productivity is well documented (Lascano, 1991; Pizarro et al., 1998), little is known in relation to their underlying mechanisms for drought adaptation (Fisher and Kerridge, 1998). Forage species and cultivars that use quickly the water stored in the soil profile at the beginning of the dry season will have lower dry matter yields than those with a better control of transpiration (Jones et al., 1980; Ray et al., 1997). Drought stress reduces plant biomass and leaf area affecting the root:shoot ratio of many tropical grasses. These changes are species or cultivar specific and interact with other environmental limitations or management practices (Baruch, 1994; Baruch et al., 1989; Coughenour et al., 1985; Páez et al., 1995; Simoes and Baruch, 1991). Reductions in dry matter yields will also depend on the degree of acclimation to water stress (Baruch and Fisher, 1991; Kang et al., 2000; Ludlow, 1980; Sharp and Davies, 1985). Plant traits related to improved water use and nutrient uptake under drought include: low shoot:root ratio, high root length density, deep root penetration and high root hair density (Boot, 1989; Hochman and Helyar, 1989). However, in some cases no clear correlation between these root traits and water extraction ability has been found (Hamblin and Tennant, 1987; Petrie and Hall, 1992b). Drought effects have also been related to forage quality. When water stress is moderate, forage nutrient content and digestibility may be improved. This phenomenon has been associated with delayed leaf ontogeny (Wilson, 1983; Humphreys, 1991). This study is part of a research program that aims to identify and compare different strategies that may contribute to production and survival under drought in widely used species of Brachiaria. This would assist in the selection of drought resistant species and contribute to improve sustainable animal production in Neotropical savannas.
Materials and methods All species under study are perennial C4 (PEP-CK type) grasses (Bogdan, 1977; Renvoize et al., 1998; Skerman and Riveros, 1992). Brachiaria brizantha (CIAT 6780) is a caespitose or bunch grass whereas B. humidicola (CIAT 679) and B. dictyoneura (CIAT 6133 cv. Llanero) are mostly stoloniferous. Brachiaria mutica and B. decumbens (CIAT 606) produce both erect shoots and stolons. Brachiaria mutica was included as a control because of its reputation as being less tolerant to drought (Bodgan, 1977; Baruch, 1994). Although B. dictyoneura has been recently identified as a B. humidicola type (Maass, 1998), in this work it was treated as a separate species due to its differential plant architecture with longer leaf blades, higher leaf to stem ratio and less stoloniferous habit. Three to five sections of culms or stolons of similar size were planted and thinned to one per pot after rooting. Plants were grown in 32 L PVC (polyvinyl chloride) pipes (0.2 m diameter × 1 m high), with 45 kg of dry Mollisol soil. Coarse sand was added on the top of the containers to prevent soil evaporation. Soil water potentials were obtained from a curve of gravimetric soil water content against water tension, measured with a tension plate apparatus. From this curve, soil water content (dry weight basis) at field capacity (−0.03 MPa) was 19 and 4.3% at permanent wilting point (−1.5 MPa). Bulk density in soil columns was 1.35 ± 0.02 g cm−3 (n=5) which was used to calculate volumetric soil water content. To facilitate root extraction from the intact soil column at the end of the experiment, all pipes were lined with plastic bags and filled with dry soil. At the bottom, a mix of sand and gravel allowed drainage. A gypsum block, used for soil water potential estimation (Bouyoucos, 1954), was inserted across the soil column of four cylinders per treatment at 75, 45 and 25 cm depth. Each humidity sensor was calibrated in the laboratory using the same soil of the experiment. Values of electrical resistance () measured from the gypsum block were then transformed to their corresponding values of soil water potential (−MPa). All soil columns were watered to saturation and after 7 days of growth, plants were fertilized with the equivalent of 50, 22, 40, 20 and 20 kg ha−1 of N, P, K, Mg and S, respectively. After 3 weeks all plants were clipped at 5 cm above soil surface to stimulate regrowth. A second cut was done 8 weeks later. Soil columns were then water-saturated several times and after 19 days of re-growth, plants were fertilized with
231 N (50 kg ha−1 ) to prevent any potential nutrient deficiency and the experiment was initiated. Drought was imposed by suspending watering (drought treatment, DT) in six plants of each species until leaves showed the first wilting symptoms. After 7 days of rewatering, plants were subjected to a second drought period, re-watered for another 7 days and harvested. This watering schedule mimics common short wet– dry cycles at the transition from dry to rainy seasons in Neotropical savannas. Another six plants per species were kept continuously watered (control treatment, CT). At harvest, DT and CT plants were separated into the different components and the whole root system collected from the soil column which was divided into segments of at the following depths: 0–10, 10–30, 30– 50 and 50–85 cm. Leaf area (LA) was determined in all plants using a leaf area meter (CI-202; CID, Inc., USA). In all harvested plants yield components were dried and weighed to determine dry matter and estimate specific leaf mass (SLM) by using a random leaf sample and calculating its weight:area ratio. The leaf area ratio (LAR) per plant was estimated from the leaf blade area:total plant weight relation. To calculate the shoot to root (S:R) ratio, crown biomass (± 2–3 cm above and below plant base) was excluded in calculations of the S:R ratio due to the differential growth habit of the species used in this study. Sub-samples of leaves from four plants per treatment were analyzed for N, P and K concentration by the Kjeldahl, molybdenum-blue and spectrophotometric atomic absorption methods, respectively (Jackson, 1976). In addition, lignin and cellulose content (Van Soest, 1967) and the in vitro digestibility of the leaf dry matter (Tilley and Terry, 1963) were determined. Throughout the experiment, soil water potential was measured weekly at 25, 45 and 75 cm in four cylinders per species of both treatments. Assuming that most of the water lost from the soil column was the result of plant transpiration, the total volume of soil water extracted by the roots of a DT plant during a particular drought period, was estimated as the difference in volumetric soil water content between the beginning and the end of each drought period. Mean soil water extraction rate (WER) was calculated as the total mean volume of water evapotranspired from the soil column divided by the mean duration of the drought period. Total plant-available water (TPAW) was calculated from the difference between the upper (field capacity) and lower (permanent wilting point) limit of soil water content (Nable et al., 1999). An
Table 1. Two-way GLM test for statistical differences in plant components dry weight (g) among species and between treatments. Differences were tested with the Tukey’s HSD method
Plant component
Source of variation Species (s) Treatment (t)
s ×t
Shoot Culms Green leaves Dead leaves Crown Inflorescences Roots (log10 (roots +1))∗∗ Total
0.0001∗ 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001
ns ns ns 0.002 ns ns ns ns
0.0001 ns ns ns 0.004 ns 0.0001 0.0001
∗ α value for the F -test; ∗∗ logarithmic transformation; ns, not significant
estimate of leaf growth was obtained by measuring for several periods of five to 12 consecutive days the length of eight leaf-blades in four plants per species and treatment. Leaf blade length was measured during both, the drought and re-watering periods. Leaf-blade elongation rate (LER) was calculated as the slope of the simple linear regression model: Leaf length = a + LER × time. Leaf water potential ( ) was measured at predawn throughout the experiment with a pressure bomb (PMS 1000, PMS Inst., Co., USA). The experiment was conducted in a greenhouse (mean minimum and maximum air temperatures: 22 and 39◦ C, respectively; mean minimum and maximum air relative humidity: 37 and 92%, respectively) Mean maximum photosynthetically active photon flux density (PPFD)was 1170 ± 370 µmol m−2 s−1 , about 65% of the total incident PPFD outside the greenhouse. All pots were arranged into a complete randomized block design. The UNIVARIATE and the ANOVA or GLM procedures respectively (SAS Institute, 1989) were used to test for normality and significance of all measured variables. Under nonnormality conditions, data were either log10 (x+1) or arcsine transformed. Comparisons of means among species and between treatments were carried out by the Tukey’s test at P
