Differences in regulation of water use, water use efficiency and growth of six
Brachiaria genotypes exposed to combined stress conditions of drought and
V. Hoyos , J. Polanía , C.S. Morales , J. Miles and I.M. Rao
1 1 2 1 1
Introduction The combined stress conditions of terminal drought and Al stress, were more
tolerated by B.decumbens CIAT 606 and B. brizantha CIAT 26110 cv. Toledo, by
Brachiaria grasses are the most widely planted forages in the tropics. Seasonal showing a delay in stomatal closure through an efficient use of the moisture
drought affects both quantity and quality of forage in tropical subhumid stored in the soil during dehydration process (Figure 3b). Genotypes such as B.
environments. Al toxicity affects root development which in turn affects the brizantha CIAT 6294 cv. Marandu and B. ruziziensis 44-02 were found to be
acquisition of nutrients and water. Although Brachiaria grasses are known to differ senstitive to combined stress conditions. Among the 6 genotypes tested B.
in drought and Al resistance (Baruch, 1994; Rao, 2002; Guenni et al., 2002; De brizantha cv. Toledo and B. decumbens CIAT 606 showed lower values of FTSWc
Mattos et al., 2005), there is very limited knowledge on the physiological bases of indicating their superior level of adaptation to drought stress in an Al-toxic soil
adaptation of Brachiaria grasses to drought. B. brizantha CIAT 6780, B. decumbens (Figure 2). Water used vs. Dry weight
CIAT 606, Mulato and Mulato 2 are known to be relatively more adapted to drought
stress. Our objective was to determine the differences in regulation of water use,
water use efficiency and growth among 6 Brachiaria genotypes that were subjected 10.5 Mulato CIAT 36061
to combined stress of drought and Al toxicity.
Materials and methods 9.5
Brachiaria decumbens CIAT 606 cv. Basilisk
Dry weight (g)
Brachiaria brizantha CIAT 6294 cv. Marandú
A greenhouse experiment was conducted to determine differences in regulation of 8.5
Mulato 2 CIAT 36087
water use, WUE (water use efficiency) and shoot growth of 6 Brachiaria genotypes Mean = 8.52 g
(Brachiaria decumbens CIAT 606 cv. Basilisk, Brachiaria ruziziensis 44-02, 8.0
Brachiaria brizantha CIAT 6294 cv. Marandú, Brachiaria brizantha CIAT 26110 cv. 7.5
y = 1.57 + 9.45x Brachiaria brizantha CIAT 26110 cv. Toledo
Toledo, Brachiaria hybrid Mulato (CIAT 36061) and Brachiaria hybrid Mulato 2 (CIAT R2 = 0.46
Mean= 0.74 L
36087) that were subjected to a combined stress of drought and Al toxicity in soil.
6.5 Brachiaria ruziziensis 4402
Two watering levels were maintained (In soil
from Matazul, Meta with 80% Al saturation but 0.55 0.60 0.65 0.70 0.75 0.80 0.85
adequate levels of fertilization): 100% field Water consumed (L)
capacity (FC) as control or well watered and Figure 3. (a) Relationship between total dry matter (shoot + root) production and water use of 6 Brachiaria genotypes subjected to
combined stress conditions of terminal drought and Al toxicity. (b) Contrasting genotypes under both treatments
terminal drought by withholding water after
three weeks of establishment. The amount of
water held at 100% FC is the maximum amount
of water held in soil after free drainage. This
weight was recorded and used to maintain Relationship between the total dry
Figure 1. Plants were grown for three weeks under 100% FC treatment. For inducing terminal matter (shoot + root) production and
well watered conditions to later be subjected to 100%
field capacity (FC) and terminal drought conditions, to drought stress, water supply was simply water consumed showed that cv.
assure only water loss was via transpiration, withheld. The surface of the pot was sealed Mulato was outstanding in consuming
evaporative losses were reduced by covering the soil
surface with plastic. The experiment was set in a with plastic in order to avoid evaporative water the water and producing the shoot dry
completely randomized design with three replications
for a period of 51 days (21 days of establishment, 30 losses (Figure 1). weight while. B. ruziziensis was least
days of treatment)
productive (Figure 3). Based on the
The weight of each pot was recorded every day in order to determine water loss due values of relative reduction in shoot
to transpiration based on weight difference between the days. At the end of the growth under drought stress compared
drying cycle, the transpiration data of the terminal drought pots was normalized by with well watered condition, B.
correcting the transpiration of the stressed plant against that of the control pots decumbens was better adapted to
(100% FC). The experiment was completed when this value reached ~0.1 for each drought stress when combined with Al
pot, which was defined as the endpoint. toxicity in soil (Figure 3b).
After reaching the endpoint, the plants were harvested and the total plant (shoot +
root) dry weights were recorded. To obtain a value for soil moisture, a fraction of
transpirable soil water (FTSW) was also calculated from:
daily pot weight - final pot weight
1) Daily FTSW
initial pot weight - final pot weight B. decumbens B. ruziziensis B. brizantha
The results from these two variables were fitted to the following equation to obtain a
curve explaining the behavior of the plant during terminal drought stress:
1 A * exp B * FTSW Conclusions
Among the 6 Brachiaria genotypes that were subjected to a combined stress of
Full stomatal closure of the genotypes was observed on a mean value of 13.5 days drought and Al toxicity in soil, B. decumbens CIAT 606 and B. brizantha CIAT
and a FTSW (fraction of transpirable soil water) of 30%, with B. decumbens (15.25 26110 cv. Toledo were found to be superior in their ability to tolerate the
days) and B. brizantha CIAT 26110 cv. Toledo (14.75 days) showing higher values combined stress conditions of terminal drought and Al toxicity. The superior
and Mulato 2 CIAT 36087 (13 days) and Mulato CIAT 36061 (11.5 days) showing performance of these two genotypes was attributed to a delay in stomatal
lower values. The genotypes that showed lowest values of FTSW were B. brizantha closure combined with efficient use of the moisture stored in the soil during the
CIAT 26110 cv. Toledo (14%) and B. decumbens CIAT 606 (17%), while the higher dehydration process. Two genotypes, B. brizantha CIAT 6294 cv. Marandu and B.
values were observed with B.ruziziensis 44-02 (46%) and B. brizantha CIAT 6294 cv. ruziziensis 44-02 were found to be sensitive to the combined stress conditions
Marandu (50%, Figure 2). due to early stomatal closure that impacted their ability to use water to produce
Individual curves the shoot biomass. Two Brachiaria hybrids, cv. Mulato (CIAT 36061) and cv.
1.2 0 Mulato 2 (CIAT 36087) showed greater demand for water with their higher
1 growth rate and an intermediate type of response with moderate ability to adjust
1.0 3 to the decreasing soil moisture.
Normalized Transpiration Ratio
0.9 4 Figure 2. Relationship between the
5 normalized transpiration ratio (NTR) and
0.8 6 the daily values of the fraction of
0.7 7 transpirable soil water (FTSW) for 6
genotypes of Brachiaria during terminal
drought stress and Al toxic conditions.
The solid line represents the fit of the
0.5 10 data to equation 2. The inflection point
Brachiaria decumbens CIAT 606 11
0.4 of each line is the point at which
Brachiaria ruziziensis 4402 12 stomata begin to close. BARUCH, Z. 1994. Responses to drought and flooding in tropical forage grasses. I. Biomass allocation, leaf growth and mineral
0.3 Brachiaria brizantha CIAT 6294 cv. Marandú 13 nutrients. Plant and Soil. Nº 164. p. 87-96.
Brachiaria brizantha CIAT 26110 cv. Toledo 14 DE MATTOS, J.L.; GOMIDE, J.A.; MARTÍNEZ y HUAMAN, C.A. 2005. Crescimento de espécies do gênero Brachiaria, sob déficit hídrico.
Mulato CIAT 36061 15 em casa de vegetação. Revista Brasilera de Zootecnia. Vol. 34, No 3. p. 746-754.
0.1 Mulato 2 CIAT 36087 16
GUENNI, O.; BARUCH, Z. and MARIN, D. 2002. Responses to drought of five Brachiaria species. II. Water relations and leaf gas
exchange. Plant and Soil. No 243. p. 229-241.
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 RAO, I.M. 2002. Adapting tropical forages to low-fertility soils. Proceedings of the XIX international grassland congress – Grassland
ecosystems: an outlook into the 21st century. p. 249. ISBN 85-7133-010-7.
Fraction of Transpirable Soil Water
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