Adaptation of Indian mustard (Brassica juncea L.) to short season dryland Mediterranean-type environments.

Abstract

Indian mustard (Brassica juncea L.) has recently been identified as a potential and profitable alternative oilseed crop in the grain growing regions of Australia. To date, no research has been reported on adaptation of mustard in water limited Mediterranean-type environments in south Western Australia. Experiments presented in this thesis were undertaken to study adaptation of mustard in the Mediterranean-type environments in south Western Australia, with the hypothesis that mustard would be better adapted to these environments due to its reputation for drought tolerance. Experiments were conducted with three main aims. Firstly, to identify the effects of genotype, environment (times of sowing/seasons/sites) and genotype x environment interaction on the phenology, growth, dry matter production, seed yield, oil and protein contents of mustard and canola. Secondly, to identify phenological, morphological and physiological characters responsible for adaptation and yield improvement of mustard in these environments. Thirdly, to study the response of mustard to soil moisture deficits, especially in the post-flowering period, in comparison to canola. Adaptation of six mustard breeding lines/cultivars varying in maturity, height and oil quality and three canola cultivars varying in maturity were tested at a medium rainfall site (Northam) in the 1999 growing season. These genotypes were sown at four times after the break of the season and their phenology, growth, morphology, dry matter production and partitioning, radiation absorption, seed yield and its components, and seed oil and protein concentrations were measured. Adaptation of mustard to short season, low rainfall areas was tested, in the 2000 and 2001 growing seasons, at three sites (Merredin. Mullewa and Newdegate), by sowing seven genotypes of mustard and canola at three times after the break of the season.Seed yield, oil and protein concentrations were measured at all three sites and detailed measurements of phenology, morphology, dry matter production and partitioning, radiation absorption, seed yield and its components, and seed oil and protein concentrations were taken only at Merredin. The effects of post-flowering soil moisture stress on mustard and canola was studied in detail using rainout shelters at Merredin in the 2001 growing season. Measurements of water use, leaf water potential, osmotic potential, osmotic adjustment, relative water content, and leaf diffusive conductance were taken together with morphology, dry matter production and partitioning, radiation absorption, seed yield and its components, and seed oil and protein concentration. Mustard produced seed yields similar to canola at a medium rainfall site at Northam in south Western Australia. Early sowing (May) was more suitable for mid and late maturing genotypes and mid sowing (early June) was optimum for early maturing genotypes at this site. Dry matter production and seed yield was highest in early sowing due to balanced pre-anthesis and post-anthesis development of the crop and its ability to avoid terminal drought. Very late sowing (after July) significantly reduced the dry matter production, seed yield and oil concentration of mustard and canola due to poor establishment, reduced post-anthesis duration, soil moisture and high temperature stresses which occurred at the end of the season. Mustard did not produce significantly higher dry matter and seed yield compared to canola at the medium rainfall site, Northam. Seed yield and oil concentration of mustard and canola in low rainfall environments (Merredin, Mullcwa and Newdegate) were higher when sown early in the season (May). Longer growing duration and post-anthesis duration were favourable for higher yields.Higher rainfall during the post-anthesis phase, warmer pre-anthesis phase and cooler post-anthesis phase were associated with higher seed yield in these environments. As shown by the Principal Component Analysis and the Finlay Wilkinson Analysis, adaptation of mustard genotypes to low rainfall environments was better compared to canola genotypes. Mustard genotypes, 887.1.6.1, 82 No 2298 demonstrated their general adaptability by producing the highest mean seed yield across all environments and showing average phenotypic stability across all environments. The low yielding canola genotype, Oscar was best adapted to high yielding environments and showed below average phenotypic stability. Low yielding mustard genotypes, JM 25 and JM 33 were best adapted to low yielding environments and showed above average phenotypic stability. Early flowering and developmental plasticity had a significant contribution to yield potential and its stability. All mustard genotypes were more tolerant to soil moisture and high temperature stresses and exhibited early vigour compared to canola varieties. Mustards produced significantly higher dry matter compared to canola under soil moisture and high temperature stresses. Yield reduction due to late sowing VI was greater in canola compared to mustards. Greater dry matter production of mustards under severe soil moisture stress was related to their higher water use and radiation use, which in turn was related to their superior osmotic adjustment.Osmotic adjustment improved dry matter production in mustards as it allowed stomata to remain partially open at progressively lower leaf water potentials and maintained higher stomatal conductance, maintained leaf area and reduced the rate of leaf senescence by increasing both avoidance and tolerance of dehydration and thereby increased radiation use, increased water use by stomatal adjustment, and increased soil moisture uptake by producing deeper roots. Mustard exhibited many agronomic advantages over canola, such as vigorous seedling growth, quick ground covering ability, early vigour, and the feasibility of direct harvesting due to non-shattering pods. Despite all these advantages currently available mustard genotypes do not have the ability to out yield canola due to their lower efficiency of conversion of dry matter to seeds, as indicated by lower harvest indices, and inferior yield component structure. Further breeding in mustard is required to modify its morphology and yield component structure. Mustard plants with more pods and pods with more seeds would produce higher yields. Shorter, compact plant stature and reduced branching would improve harvest index in mustard. Furthermore, development of mustard genotypes with high oil quality and concentration similar to canola would improve its market value as an oil seed crop.