Genetic improvement of lucerne for anthracnose (Colletotrichum trifolii) resistance

Anthracnose, caused by Colletotrichum trifolii, is one of the most serious diseases influencing lucerne persistence and productivity in eastern Australia. The disease is largely controlled by plant resistance; however, new pathotypes of C. trifolii have developed in Australia, seriously limiting the productive life of susceptible cultivars. This paper describes an incompletely recessive and quantitatively inherited resistance to C. trifolii identified in a clone (W116) from cv. Sequel. S-1, F-1, F-2 and backcross populations of W116 and D (highly susceptible clone) were studied for their reaction to C. trifolii race 1. Resistance was found to be quantitatively inherited, and quantitative trait loci associated with resistance and susceptibility were identified in a backcross population (D x W116) x D using random amplified polymorphic DNA and amplified fragment length polymorphic markers. A multi-locus region on linkage group 4 was found to contribute significantly to the resistance phenotype. The application of DNA markers to allow exploitation of this quantitatively inherited resistance in lucerne breeding is discussed.

Genotypic variation for cold tolerance during reproductive development in rice: Screening with cold air and cold water

Rice (Oryza sativa L.) plants are susceptible to low temperature during the young microspore stage, which occurs 10-12 days before heading. Low temperature at this time increases spikelet sterility which can cause massive yield loss. Increasing the cold tolerance of cultivars can reduce yield variability in temperate rice-growing environments. Two experiments were conducted in cold air screenings and two were conducted in cold water screenings to examine genotypic variation for cold tolerance, explore flowering traits related to spikelet sterility, and investigate whether the results reflect the level of cold tolerance determined previously in the field. Cold air screenings imposed day/night temperatures of 27 degrees C/13 degrees C, 25 degrees C/15 degrees C and 32 degrees C/25 degrees C following particle initiation until 50% heading, while cold water screenings maintained a relatively constant 19 degrees C. The variation in the commencement of low air temperature treatment did not have an effect on the level of spikelet sterility, indicating that exposure to low temperature during the young microspore stage was more important than the duration of exposure. Spikelet sterility of common cultivars showed a significant correlation between cold air and cold water screenings (r(2) = 0.63, p < 0.01), cold air and field screenings (r(2) = 0.52, p < 0.01) and cold water and field screenings (r(2) = 0.53, p < 0.01), indicating that cold air and cold water can be used for screening genotypes for low temperature tolerance. HSC55, M 103 and Jyoudeki were identified as cold tolerant and Doongara, Sasanishiki and Nipponbare as susceptible cultivars. There was a significant negative relationship between spikelet sterility and both the number of engorged pollen grains per anther and anther area only after imposing cold air and cold water treatment hence, it was concluded that these flowering traits were facultative in nature. In addition, cultivars originating from Australia and California were inefficient at producing filled grain with similar sized anthers containing a similar number of engorged pollen grains as cultivars from other origins. One suggested reason for this poor conversion to filled grain of cultivars from Australia and California may be associated with their small stigma area, particularly when exposed to low temperature conditions. (c) 2006 Elsevier B.V. All rights reserved.

Improving drought tolerance in rainfed lowland rice: An example from Thailand

A large portion of the world's poor farm in rainfed systems where the water supply is unpredictable and droughts are common. In Thailand there are approximately 6.2 million ha of rain fed lowland rice, which account for 67% of the country's total rice-growing area. This rice system is often characterised by too much and too little water in the same season. Farmers' estimates of their annual losses to drought are as high as 45% in the upper parts of the toposequence. In contrast to irrigated rice systems, gains from crop improvement of rainfed rice have been modest, in part because there has been little effort to breed and select for drought tolerance for the target rainfed environments. The crop improvement strategy being used in Thailand considers three mechanisms that influence yield in the drought prone targets: yield potential as an important mechanism for mild drought (where yield loss is less than 50%), drought escape (appropriate phenology) and drought tolerance traits of leaf water potential, sterility, flower delay and drought response index for more severe drought conditions. Genotypes are exposed to managed drought environments for selection of drought tolerant genotypes. A marker assisted selection (MAS) scheme has been developed and applied for selection of progenies in the backcrossing program. The plant breeding program uses rapid generation advance techniques that enable early yield testing in the target population of environments (TPE) through inter-station (multi-location yield testing) and on-farm trials. A farmer participatory approach has been used to identify the TPE for the breeding program. Four terrace paddy levels have been identified, upper (drought), middle (drought prone to favorable) and lower (flooded). This paper reports the change in the breeding program for the drought prone tainted lowland rice environments of North and Northeast Thailand by incorporating our knowledge on adaptation and on response of rice to drought. (c) 2005 Elsevier B.V. All rights reserved.

Minimising cold damage during reproductive development among temperate rice genotypes. II. Genotypic variation and flowering traits related to cold tolerance screening

Low temperature during microspore development increases spikelet sterility and reduces grain yield in rice (Oryza sativa L.). The objectives of this study were to determine genotypic variation in spikelet sterility in the field in response to low-temperature and then to examine the use of physio-morphological traits at flowering to screen for cold tolerance. Multiple-sown field experiments were conducted over 4 consecutive years in the rice-growing region of Australia to increase the likelihood of encountering low-temperature during microspore development. More than 50 cultivars of various origins were evaluated, with 7 cultivars common to all 4 years. The average minimum temperature for 9 days during microspore development was used as a covariate in the analysis to compare cultivars at a similar temperature. The low-temperature conditions in Year 4 identified cold-tolerant cultivars such as Hayayuki and HSC55 and susceptible cultivars such as Sasanishiki and Doongara. After low temperature conditions, spikelet sterility was negatively correlated with the number of engorged pollen grains, anther length, anther area, anther width, and stigma area. The number of engorged pollen grains and anther length were found to be facultative traits as their relationships with spikelet sterility were identified only after cold water exposure and did not exist under non-stressed conditions.

Toposequential effects on water balance and productivity in rainfed lowland rice ecosystem in Southern Laos

In the Mekong region, most paddies in rainfed lowland rice (Oryza sativa L.) lie in a sequence on gentle sloping land, and grain yield (GY) often depends on the toposequence position. There is, however, lack of information on toposequential effects on field water supply in rainfed lowland rice and how that influences GY. A total of eight field experiments were carried out on sandy, coarse-textured soils in Southern Laos (Champassak Province and Savannakhet Province) over three wet seasons (2000-2002). Components of the water balance, including downward and lateral water movement (D and L, respectively), were quantified at three different positions along toposequences (top, middle and bottom). GY, days-to-flower (DTF) and rainfall were measured, and the water productivity (WP) was determined. In most experiments, standing water disappeared first in the top position and gradually in lower positions. This was associated with the observation that when there was standing water in the field, the higher position had larger D in both the provinces and also larger L in Champassak Province. However, in one experiment, water loss appeared later in the higher position, as the result of lower L, apparently due to some water inputs other than rainfall occurring at this position. Despite larger D plus L at the top position, seasonal sum of D and L were not much affected by the toposequence position, as the daily rate of D plus L became minimal when the standing water was lost earlier in the top position. Lower GY was associated with earlier disappearance of standing water from the field. Relatively low GY was expected in the top toposequence position. This was clearly shown in the toposequence of Phonthong, Champassak Province, as the timing of standing water disappearance relative to flowering was earlier in the top position. Variation in GY across the toposequence positions was coupled with the WP variation, and both GY and WP tended to decline with increased DTF. Therefore, variation in productivity of rainfed lowland rice across toposequence positions depends mainly on the field water status around flowering time. (c) 2005 Elsevier B.V. All rights reserved.

Use of drought response index for identification of drought tolerant genotypes in rainfed lowland rice

Drought is a major constraint for rice production in the rainfed lowlands in Southeast Asia and Eastern India. The breeding programs for tainted lowland rice in these regions focus on adaptation to a range of drought conditions. However, a method of selection of drought tolerant genotypes has not been established and is considered to be one of the constraints faced by rice breeders. Drought response index (DRI) is based on grain yield adjusted for variation in potential yield and flowering date, and has been used recently, but its consistency among drought environments and hence its usefulness is not certain. In order to establish a selection method and subsequently to identify donor parents for drought resistance breeding, a series of experiments with 15 contrasting genotypes was conducted under well-watered and managed drought conditions at two sites for 5 years in Cambodia. Water level in the field was recorded and used to estimate the relative water level (WLREL) around flowering as an index of the severity of water deficit at the time of flowering for each entry. This was used to determine if DRI or yield reduction was due to drought tolerance or related to the amount of available water at flowering, i.e. drought escape. Grain yield reduction due to drought ranged from 12 to 46%. The drought occurred mainly during the reproductive phase, while four experiments had water stress from the early vegetative stage. There was significant variation for water availability around flowering among the nine experiments and this was associated with variation in mean yield reduction. Genotypic variation in DRI was consistent among most experiments, and genotypic mean DRI ranged from -0.54 to 0.47 (LSD 5% = 0.47). Genotypic variation in DRI was not related to WLREL around flowering in the nine environments. It is concluded that selection for DRI under drought conditions would allow breeders to identify donor lines with high drought tolerance as an important component of breeding better adapted varieties for the rainfed lowlands; two genotypes were identified with high DRI and low yield reduction and were subsequently used in the breeding program in Cambodia. (c) 2006 Elsevier B.V. All rights reserved.