Drought imposed at different times on four spring cultivars of each of two oilseed rape species (Brassica campestris and B. napus) markedly influenced seed yield and its components and other characteristics of the mature plant. In general, drought reduced those plant and yield components associated with plant size, whereas other characters were variable, depending on the species and treatments. Seed yield was lowest in both species when drought was applied either from stem elongation or from the time of anthesis. However, considering grain yields in terms of the amount of water added in each treatment, B. napus cultivars were most sensitive at anthesis, with the exception of Masoweicki which was most sensitive at pod-filling. In B. campestris, Span was most sensitive when drought stress was first imposed at stem elongation, whilst other cultivars were sensitive at later reproductive stages. Principal component analyses reduced the complex interrelationships between all variables excluding yield to two major factors in each species and treatment. In general, the first factor, which explained most of the variation in the parameters measured, was composed of variables associated with plant size, whereas the second was composed of variables which were independent of plant size. The relative importance of these two factors as yield determinants varied substantially with the different species and treatments. In B. napus the first factor was the major determinant of yield ill the control and in the drought treatment imposed from stem elongation. In B. campestris the second factor was a significant determinant of yield in all treatments and the major factor influencing yield in the control and stem elongation treatments. Increased root weight was associated with increased yield in the drought treatments in both species, but this was primarily due to its association with plant size. When the influence of plant size was accounted for, it was observed that a smaller root weight relative to the above-ground plant weight and a greater tap-root weight relative to lateral root weight was associated with higher yield in both species in all drought treatments. As well as these latter characters, seed weight per pod, and harvest index were the most important group of characters contributing to yield in both species in all treatments when variation in plant size was accounted for.

Variation between and within Species of Rapeseed (Brassica campestris and B. napus) in Response to Drought Stress. I Sensitivity at Different Stages of Development

Two rapeseed species and cultivars within each of these species differed significantly with respect to the influence of variation in sowing date on growth, development and yield on two different soil types. Soil moisture stress, particularly after anthesis, was the major environmental factor affecting these processes. Grain yield declined markedly with later sowings in both species, and B. napus, despite its later maturity, was more tolerant of severe soil moisture deficits since its grain yield was consistently higher than B. campestris in the more stressed environments. The major distinguishing feature between species contributing most to this difference in yield was the pattern of dry matter accumulation. In B. campestris most of the dry weight of the plant was accumulated after anthesis when drought was most severe, whereas in B. napus dry weight accumulation occurred before anthesis. This resulted in a greater contribution of reserves accumulated by anthesis to grain-filling in B. napus. Most of the variation in seed yield resulted from differences in sowing dates and soil types. When these environmental effects were excluded, the main determinants of genotypic variation in yield were the numbers of pods and branches and harvest index in both species, growth rate in the post-anthesis phase in B. campestris, and plant weight and root/shoot ratio at anthesis in B. napus. Selection strategies for yield improvement in rapeseed growing in drought-stressed environments are discussed. _____________________ *Part I, Aust. J. Agric. Res., 29: 469 (1978).

Variation between and within species of rapeseed (Brassica campestris and B. napus) in response to drought stress. II.* Growth and development under natural drought stresses

The inheritance of petal (flower) colour and seed colour in Brassica rapa was investigated using two creamy-white flowered, yellow-seeded yellow sarson (an ecotype from Indian subcontinent) lines, two yellow-flowered, partially yellow-seeded Canadian cultivars and one yellow-flowered, brown-seeded rapid cycling accession, and their F1, F2, F3 and backcross populations. A joint segregation of these two characters was examined in the F2 population. Petal colour was found to be under monogenic control, where the yellow petal colour gene is dominant over the creamy-white petal colour gene. The seed colour was found to be under digenic control and the yellow seed colour (due to a transparent coat) genes of yellow sarson are recessive to the brown/partially yellow seed colour genes of the Canadian B. rapa cvs.‘Candle’ and ‘Tobin’. The genes governing the petal colour and seed colour are inherited independently. A distorted segregation for petal colour was found in the backcross populations of yellow sarson × F1 crosses, but not in the reciprocal backcrosses, i.e. F1× yellow sarson. The possible reason is discussed in the light of genetic diversity of the parental genotypes.

Inheritance of petal colour and its independent segregation from seed colour in Brassica rapa

Forage rape. B. napus, is self-compatible, the work described illustrates the introgression of functional incompatibility alleles into B. napus from turnips, B. campestris, in two generations. By grading seed, produced by backcrossing turnip/rape hybrids to rape, the frequency of 2n=38 ‘semi articial’ B. napus recovered, amply justifies the cytological screening involved in such breeding programmes.

The introgression of S alleles into forage rape, Brassica napus L. from turnip. Brassica campestris L. ssp. Rapifera

Publisher Summary This chapter examines homomorphic self-incompatibility. To escape inbreeding, angiosperms have developed well-organized systems such as dichogamy and self-incompatibility. Plants having self-incompatibility cannot achieve fertilization with the sperm of self-pollen, although they produce functional male and female organs. Self-incompatibility involves a recognition reaction between self and non-self in higher plants. In higher plants, recognition reactions are only known in the pollination system, involving self- and interspecific incompatibility, and in plant–pathogen interactions. The homomorphic type of self-incompatibility includes sporophytic and gametophytic systems, which are defined by the action of S genes. In the sporophytic system, the behavior of pollen tubes is determined by the phenotype of the sporophyte, by which the pollen is produced. The self-incompatibility system is composed of three stages: (1) germination of the pollen grain, (2) ability of the pollen tube to penetrate the cuticle of the stigmatic papilla, and (3) interaction of substances secreted by the pollen tube with products of the stigmatic cytoplasm. A signal transduction system appears to be involved in sporophytic incompatibility, as SRK (S-receptor kinase) genes are found. Such genes have not been found to be involved in gametophytic incompatibility.

A Review of Recent Studies on Homomorphic Self-Incompatibility

Self-incompatibility in Brassica campestris c.v. Arlo is controlled by a single locus sporophytic system. The identity and expression of the S alleles were determined in eight inbred and two hybrid families. It was found that co-dominance of alleles is more frequent in the stigma, whereas dominance relations between pairs of alleles predominate in the pollen. A linear order of dominance was established between six S alleles and alleles high, intermediate and low in the dominance series were recognized. In considering the variation in the expression of compatibility and the segregation ratios in inbred, F1, F2 and backcross progenies, the presence of a specific S allele conditioning self-fertility, or a single dominant self-compatibility factor independent of the S locus could not be established. Instead, self-compatibility in this cultivar was ascribed to the segregation of a polygenic complex which is capable of modifying the incompatibility reaction to the point of self-fertility, or to a reduction in the strength of the reaction due to the presence of S alleles low in the dominance series.

The genetics of self-incompatibility in Brassica campestris L. ssp. Oleifera Metzg. I. Characteristics of S-locus. Control of self-incompatibility

Continuous variation in the expression of self-incompatibility, from complete self-incompatibility to self-compatibility, was observed among plants of the spring turnip rape cultivar Arlo. Significance of the variation between inbred progenies of plants from the base population suggested that a polygenic system had some influence on the expression of self-compatibility. The responses to one generation of selection within some of these progenies supported this conclusion. Further evidence of the operation of some polygenic modifying complex was provided by an analysis of a diallel cross among seven of the inbred lines. The analysis detected significant additive genetic variation among the parental lines, considerable non-allelic interaction and some dominance of an ambi-directional type.

N. Thurling

Papers

The genetics of self-incompatibility in Brassica campestris L. ssp. Oleifera Metzg. I. Characteristics of S-locus. Control of self-incompatibility

The genetics of self-incompatibility in Brassica campestris L. ssp. Oleifera Metzg. II. Genotypic and environmental modification of S locus control