The rationale and aims of the project 1.14 Overview
1.15 Studies of field beans
1.16 Studies of bumblebees
1.17 Choice of observational and analytical methods
1.14 Overview
The primary aim of the project described in this thesis was to determine whether cultivars of field bean {Vida faha) show different degrees of attractiveness to bumblebees. This was investigated by means of observations which might indicate whether the bumblebee communities on selected horticultural varieties exhibited significant differences in foraging behaviour, and whether any such differences could be understood in terms of variation between field bean cultivars. It was hoped that this might help to elucidate the roles of nectar secretion patterns, floral morphology and plant architecture in determining the floral choices of different species of bumblebees. The project aimed to consider intraspecific variation in field bean nectar profiles and interspecific variation in bumblebee V.faba foraging. Work was carried out on twelve commercial cultivars, with an additional study of bumblebee communities on a nearby crop of Phacelia tanacetifolia to provide a local point of comparison.
1.15 Studies of field beans
Field beans have been cultivated for at least 4,000 years, and the range of cultivars now available on the market exhibit an extremely wide range of genetic variation. Most commercial varieties are of standard height (see Table 2.1), however, maximum height is ultimately dependent on the fertility of the soil and the prevailing climatic
Iniroduciion pari II: the rationale and aims o f the project
conditions. Many tall cultivars come into flower relatively late in the season. Phenological differences in flowering may be related to plant architecture via differential growth rates and the time taken for stems to reach maximum height. The leaves may be rounded or narrow, and vary in size and density of spacing on the plant. The "standard" colour for field bean flowers is white with a black or dark brown wingspot. In some cultivars, a convergence of pigmented veins on the wing petal may act as nectar guides. Pure white varieties are popular in commercial agriculture because the unpigmented flowers are genetically linked to low tannin seed levels and higher nutritional quality. Under UV light, field bean flowers show further differentiation through patterns of light reflectance which may be visible to bees. The intensity of the floral display ranges from sparse (e.g. the wild type V. faba var. paucijuga) to high (e.g. Scirocco). Late cultivars may not begin to flower until the flowering period of early cultivars is completely over. Clonal differences in patterns of nectar secretion are generally not recognised as of importance for commercial cultivars. High levels of homozygosity may be associated with strong floral fragrance. Variations in the quality of scent of different cultivars may be detected by experienced breeders (G. Ram say, personal com m unication), but were not investigated in the current work.
The sources of phenotypic variation in field beans are summarised below: 1. Plant architecture : •plant height •leaf size •density of foliage •intemode distance 2. Floral characters:
•flower colour and pattern
•presence or absence of wingspot •presence or absence of nectar guides •overall colour
•flower size:
•length from corolla base to tip of standard petal •functional corolla tube depth
3. Floral density:
•number of flowering nodes •number of flowers per node 4. Flowering period: •dates •duration 5. Nectar: •concentration •volume •patterns of secretion 6. Scent: •strength of fragrance •quality of scent
The field bean cultivars selected for the study exhibited a wide range of this variation. The main phenotypic features of each experimental cultivar are listed in Table 2.1.
For the purposes of this study the experimental cultivars were grouped according to plant architecture. This grouping was based on the preliminary reasoning that since the height and foliage density of a plant influence the microclimate immediately surrounding the plant, and since nectar characteristics are significantly affected by ÿ tÉ different architectural configurations might show variation in the characteristics of the standing crop of nectar even if there were little or no genetic basis for nectar differences between cultivars. If, for example, variation in nectar production between field bean cultivars was a heritable trait (e.g. Ryle 1954 for Trifolium pratense ), plant architectural factors might amplify the genetic effect.
Three architectural groups were recognised (see Table 2.1): • Dwarf cultivars;
• Medium cultivars;
• Tall cultivars (including late-flowering varieties).
Three cultivars were selected for each architectural group, plus three more late- flowering tall cultivars, with the assumption that increased within-group genetic variation might help to balance out the between-group genetic variation relating to traits other than plant architecture. Despite the statistical complexities that would inevitably result, studying a range of cultivars from a diverse genetic background
Introduction part II: the rationale and aims o f the project
offered a means of obtaining a more general picture of bumblebee activity on field beans. In addition, it would suggest which varieties were most hardy under the prevailing climatic conditions and therefore suitable for further study projects. It is to be hoped that the number and range of cultivars selected did not provide an obstacle to interpreting relationships between plants and pollinators in a naturally complex floral environment.
Bumblebees visit V.faba for nectar and pollen, but at any one time most bumblebees foraging on field beans appear to be gathering nectar. It was hypothesised that any significant clonal variation in nectar characteristics should be reflected in patterns of flower visitation by bees. There is the further possibility that plant architecture might have a direct effect on bumblebee foraging behaviour; for example through microclimatic factors such as humidity and shade. W hatever the case, it was considered appropriate to base this investigation on a study of the nectar characteristics and secretion patterns of each cultivar. The body of information gathered from such a study is known as a nectar profile, and may provide the major element of the energetic background against which patterns of bumblebee foraging behaviour and abundance can be studied.