The Adaptation of Tropical Perennial Grasses to Abiotic Constraints in Sandy Soils

Abstract

The inclusion of perennial forage grasses in agricultural systems has the potential to improve livestock production, ecosystem health and climate resilience. However, the establishment of forage grasses in many ecosystems is challenged by abiotic stresses. Perennial forage grass species are often grown in environments with limited water availability following establishment and rely on accessing water deep in the soil profile to survive. In sandy soils with rapid surface drying and hardpan soils, root growth to greater soil depths enables forage grass species to survive soil surface drying following establishment and is assisted by increased root growth rate and greater ability to penetrate compacted soils. In addition, the presence of large rhizomes in rhizomatous grasses promotes post-harvest regrowth rate and post-establishment survival or drought resistance without relying on root access to water at depth. Therefore, characterisation of these traits in perennial forages grass species will provide an objective means for species selection, based on the local abiotic constraints. Five glasshouse experiments were conducted to identify the mechanisms of perennial grass adaptation to abiotic constraints in ecosystems with rapid surface-drying soils and hardpan soils. The first experiment assessed variations in vertical root growth rates between tropical perennial forage grass species, and characterised traits associated with higher vertical root growth rates. Tropical forage grasses, namely Urochloa (basionym: Brachiaria) brizantha Mekong Briz, Urochloa decumbens cv. Basilisk, Urochloa humidicola cv. Tully, Urochloa hybrid cv. Mulato II, Urochloa mosambicensis cv. Nixon, Megathyrsus maximus (basionym: Panicum maximum) cv. Tanzânia, and Setaria sphacelata cv. Solander, were established in large rhizotrons that facilitated measurement of the rate of root depth development, the rate of root length development, photosynthesis and morphological traits. Rapid vertical root growth with narrow root angle, high photosynthetic rate, high ratio of root length to leaf area and high percentage of fibrous roots were apparent mechanisms that enabled M. maximus and U. mosambicensis to establish deep roots faster than other forage grass species. M. maximus and U. mosambicensis were identified as species with exploitative growth strategy. Wide root angles and a higher proportion of shallow root distribution to 10 cm depth were associated with decreased vertical root growth rate. The exponential rate of root depth development per growing degree day increased with average root diameter in U. humidicola and Urochloa hybrid Mulato II, indicating a conservative growth strategy. The second and third experiments investigated traits correlated with vertical root growth rates, the stability of variation between grasses in root and shoot growth in summer and winter, and relationships between vertical root growth rates and post-establishment drought resistance, using 12 bermudagrass ecotypes (Cynodon spp.) from varied Australian climatic zones. Previous field experiments using these ecotypes found that drought resistance was promoted by water extraction; however, relationships between vertical root growth rates and drought resistance are poorly understood. The 12 ecotypes were established in large rhizotrons during experiments in mild winter (17 to 24 °C mean temperature) and summer (19 to 38 °C mean temperature). A proportion of root length became inactive due to seasonal root death in winter conditions, and vertical root growth rate during winter significantly decreased as this proportion increased. During summer, vertical root growth rate significantly increased with a greater tiller appearance rate but significantly decreased with increased root distribution to 10 cm depth. Despite the inconsistency of variation between ecotypes in shoot growth, the genotypic rank of root length, root dry weight, vertical root growth rate and leaf area were consistent in both seasons. Positive correlations between vertical root growth rate measured in both seasons and drought resistance were found, suggesting that increased vertical root growth rate promotes active roots for extracting water at the greater depth of soil profiles in association with post-establishment drought resistance. The fourth experiment was conducted to examine variations in root penetration in forage grass species and characterise forage grass species with a high root penetration capability using wax layers to measure root penetration. U. brizantha, U. decumbens, U. humidicola, U. hybrid cv. Mulato II, U. mosambicensis, U. ruziziensis, M. maximus, and S. sphacelata, Panicum coloratum cv. Makarikariense, Paspalum scrobiculatum cv. BA96 10 were evaluated. Root diameters were determined for each species from seedlings grown in growth pouches. Increased root penetration at high resistance was associated with larger root diameter and increased vertical root growth rate. The results indicate that M. maximus can avoid water stress during soil surface drying better than other forage species by accessing profile moisture due to its greater vertical root growth rate and capability of root penetration. The fifth experiment analysed variation in rhizome growth and correlated traits to examine relationships between rhizome growth and aridity index, rainfall and evapotranspiration, using bermudagrass ecotypes collected from environments with varying aridity indices. A total of 142 ecotypes collected from regions in Australia with varied aridity indices were grown in pots for 14 weeks during the summer in South Australia to measure rhizomes and plant traits. Rhizome growth between ecotypes from arid (aridity indices less than 0.65) and non-arid environments was not significantly different. Bermudagrasses with the largest rhizomes were amongst those ecotypes that originated from arid environments. Moreover, rhizome growth of bermudagrass ecotypes from arid environments had a positive response to environments with more humid climate conditions in winter, while the rest of the year is dry. Amongst bermudagrass ecotypes from both regions, increased rhizome growth during establishment was associated with greater leaf width and decreased internode length. Through the combined results of the five experiments presented within this thesis, several mechanisms that enable perennial grass adaptation to abiotic constraints in ecosystems with rapid surface-drying soils and hardpan soils were identified. Increased vertical root growth rate was associated with narrow root angle, greater leaf area and greater shoot growth. Increased root penetration at high resistance was associated with large root diameter and increased vertical growth rate. Post-establishment drought resistance is promoted by greater vertical root growth rate. Bermudagrass ecotypes with the greatest rhizome growth originated from arid regions. In relation to plant traits, greater leaf width and decreased internode length were characteristics of bermudagrasses with large rhizomes. This research provides new insights on the beneficial characteristics of vertical root growth in perennial forage grasses that can be well-adapted to sandy soils with rapid surface drying and hardpan soils. For field application, M. maximus appears to be well-adapted to sandy soils because it had rapid vertical root growth and great capability of root penetration that can be associated with drought resistance and high yield. Caution is advised when recommending M. maximus because it expressed an exploitative growth strategy that is associated with high nutrient requirement to sustain production. Furthermore, results from bermudagrass ecotypes showed that drought-resistant perennial grasses did not have specific traits that differed from drought-susceptible grasses. These findings indicate that drought resistance in perennial grasses may involve complex trait interplay. Therefore, the drought resistance and high yield of forage species such as U. humidicola and U. hybrid cv. Mulato II may be associated with other characteristics, but not the vertical root growth rate. These species that expressed a conservative growth strategy may be well-adapted to a wide range of agronomic conditions.