As a C4 grass, bermudagrasses are widely used as turf and pasture in tropical and subtropical environments. There have been no studies of drought resistance within large collections of bermudagrass. Recently, over 1,000 ecotypes of bermudagrass have been collected from different Australian climatic zones. Water is a critical resource in Australian agriculture and its conservation will be paramount as available water resources are likely to come under intense competition in the future. The study reported here focused on the assessment of this bermudagrass collection for drought resistance using field and lysimeter experiments. The major objective was to screen the collection and understand the mechanisms for drought resistance, particularly, the contributions of underground traits and soil water extraction patterns. Additional objectives were to: determine the relationship between drought resistance and the climatic zone they originated from; compare bermudagrass with other widely used Australian warm-season turfgrasses in relation to drought resistance, water use and water use efficiency; and evaluate the performance of drought resistant genotypes for other key turfgrass attributes of sod strength, regrowth rate after harvest and frost tolerance.
To address the overall objectives, two shallow lysimeter and 11 field experiments were conducted at several Southeast Queensland locations and one location in Perth, Western Australia. In these studies turf quality (0-9) under drought, GC50 and survival period (SP) were used as the criteria of drought resistance. GC50 and SP were defined as the drought period required when green cover declined to 50% and 0%, respectively.
Study 1: Cultivars of bermudagrass were compared with other warm-season species for drought resistance in shallow lysimeters.
The experiments showed that bermudagrass cultivars were more drought resistant than other species including the Queensland blue couches (Digitaria didactyla Willd), the seashore paspalums (Paspalum vaginatum Swartz.) and St Augustinegrasses (Stenotaphrum secundatum (Walt.) Kuntze) as indicated by longer SP (32 days vs 25 days). Longer SP was also correlated to lower evapotranspiration (r = -0.72~-0.96*) and lower stomatal conductance as indicated by higher canopy temperature (r = 0.72~0.83*) in the early stage of drought, higher water use efficiency during drought (WUE) (r = 0.72*) and higher relative WUE (Drought WUE/Irrigated WUE) (r = 0.72~0.90*).
Study 2: Field vs lysimeter experiments were compared by using 8 bermudagrass genotypes which had been studied for drought resistance in shallow lysimeters in 2008 with the same genotypes grown under a permanent rainout shelter in the field. These 8 genotypes were also used to evaluate field water extraction patterns and drought resistance.
The most drought resistant genotypes had longer GC50 by up to 120 days and extracted more soil water up to 50 mm during the whole drought period than drought sensitive ones. For all genotypes, the pattern of water use as a percentage of total water use was similar across depth and time. The maintenance of green cover and higher soil water extraction were associated with higher stomatal conductance, photosynthetic rate and relative water content under drought.
Study 3: Over 400 bermudagrass ecotypes from different climatic zones and cultivars were screened for drought resistance in the field under ambient conditions at Gatton and a subset of 72 were tested under an automatic rainout shelter.
Large genotypic variation under drought stress was found with turf quality ranging from 0.9 to 9.0 and relative water content ranging from 59.2% to 99.8%. More than 200 ecotypes performed better for drought resistance than best commercial cultivar, Wintergreen. This study suggested that the best drought resistance could be selected from Mediterranean climates.
Study 4: A subset of 12-18 ecotypes from the 400 of study 3 were subsequently selected based on contrasting drought performance and used to investigate genotypic variation of water extraction patterns and underground traits under an automatic rainout shelter at Gatton, Queensland and in the field at Perth, Western Australia.
Surprisingly, there was a similar ranking (r = 0.70*) of drought resistance at these climatically different locations that also contrasted for soil type, Gatton (clay soil) and Perth (sandy soil). Drought resistance was correlated to soil water extraction at 30-170 cm depths during the whole drought period (r = 0.72~0.97**), rhizome dry matter before and after drought treatment (r = 0.78~0.93**) and average root diameter after drought stress (r = 0.82~0.94**), but not correlated to average root diameter before drought. Interestingly, genotypic variation of root length density before or after water deficit was small and had no association with water extraction or drought resistance. Mediterranean bermudagrasses were the more drought resistant and were characterised by much larger rhizomes than others (0.385 kg m-2 vs 0.061 kg m-2). These results suggested the Australian Mediterranean zone with dry summer and sandy soil may have contributed to the evolution of bermudagrass with improved drought resistance.
Study 5: The same genotypes in Study 4 were used to test their agronomic performance in a commercial turfgrass production system.
Significant genotypic variation of sod strength and post-harvest regrowth rate was observed. Sod strength had no significant relationship with sod dry weight, stolon diameter, internode length, branch number and stolon strength. However, post-harvest regrowth rate was correlated to rhizome dry matter (r = 0.80**), which indicated rhizome was an important trait not only contributing to drought resistance but also to high productivity of turfgrass production system.
This study has demonstrated a large genotypic variation of drought resistance among Australian bermudagrass ecotypes originating from different climatic zones. Superior drought resistant genotypes had capacity to extract more soil water. Root length density did not contribute to water uptake while water extraction was correlated to rhizome dry weight before drought and root diameter after drought treatment. A group of Australian Mediterranean bermudagrasses were superior drought resistance with much larger rhizomes that may help to adapt to drought stress.