Responses to drought of five Brachiaria species. II. Water relations and leaf gas exchange

• Published in: Plant and soil 2004, Vol.258 (1-2), p.249-260
• Main Authors: Guenni, O, Baruch, Z, Marin, D
• Format: Article
• Language: English
• Subjects: Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agricultural and forest meteorology; Agronomy. Soil science and plant productions; Animal and plant ecology; Animal, plant and microbial ecology; Autoecology; Biological and medical sciences; Brachiaria; Brachiaria dictyoneura; C4 plants; Dehydration; Drought; Economic plant physiology; forage grasses; Fundamental and applied biological sciences. Psychology; Gas exchange; General agronomy. Plant production; Grasses; leaf water potential; Leaves; Moisture content; osmoregulation; Pasture; Photosynthesis; Plant growth; plant response; plant stress; plant-water relations; Plants; Plants and fungi; savanna soils; Savannas; Soil water; Solutes; Stomatal conductance; Turgor pressure; Urochloa brizantha; Urochloa decumbens; Urochloa humidicola; Urochloa mutica; Water balance and requirements. Evapotranspiration; Water content; Water potential; Water relations; Water relations, transpiration, stomata; Water stress; Wilting
• ISSN: 0032-079X; 1573-5036
• DOI: 10.1023/B:PLSO.0000016555.53797.58

Neotropical savannas are exposed to recurrent dry periods of varied duration, and forage grasses must be able to cope with such temporal stresses to maintain productive pastures. This study compared leaf water relations and net photosynthesis under drought of five perennial Brachiaria species: the tufted B. brizantha (CIAT 6780), the semi-stoloniferous B. decumbens (CIAT 606) and B. mutica, and the stoloniferous B. humidicola (CIAT 679) and B. dictyoneura (CIAT 6133). Plants of the five grasses were grown in large pots and subjected to drought by suspending watering until first wilting symptoms (14 days for B. brizantha, B. decumbens and B. mutica, and 29 days for B. humidicola and B. dictyoneura). Afterwards, they were re-watered and a second soil dry cycle was imposed. Time trends in leaf water potential (Ψ₁), relative water content (RWC), osmotic potential at full turgor ($\psi _0^{100}$), stomatal conductance (Gs) and net photosynthesis (A) of stressed (DT) plants were compared to those of well-irrigated (CT) plants. Predawn Ψ₁ in DT plants decreased to a minimum of — 1.5 and — 2.0 MPa in B. brizantha and B. mutica, compared to — 2.5 to — 3.0 MPa in B. decumbens, B. humidicola and B. dictyoneura. RWC decreased up to 50% in B. brizantha, compared to 75% in the other species. In B. humidicola, B. dictyoneura and in a lesser extent, B. decumbens, leaves of DT plants adjusted osmotically, by an apparent accumulation of nutrient solutes, at a rather constant ratio of turgid to dry weight of the tissue. Calculated osmotic adjustment ranged between 0.38 (B. decumbens) to 0.87 MPa (B. humidicola). This adjustment in $\psi _0^{100}$ was in some cases maintained 7 days after re-watering. In B. brizantha and B. mutica, Gs and A were significantly affected by drought, with maximum reduction percentages at the second drought period of 65 and 80%, respectively. The corresponding reduction in B. decumbens was 53 and 55%, respectively; whereas in B. humidicola and B. dictyoneura Gs and A were reduced less than 20%. In all species, re-watering allowed for the water relations (except $\psi _0^{100}$) and photosynthetic activity of leaves of DT plants to reach values comparable to those of CT plants. Results are discussed in term of root morphology and soil water extraction pattern, as well as leaf traits that may contribute to withstand drought under moderate soil water stress.