The effects of inbreeding and stress on plant performance

The aim of this thesis was to enhance our understanding of the combined effects of inbreeding and environmental stress on plant performance. Inbreeding, the mating among close relatives, reduces the fitness offspring in most organisms. However, the magnitude of the resulting fitness reduction (inbreeding depression, ID) often differs among environments. It is often assumed that ID is higher under more stressful conditions. However, some studies found that ID was reduced under stress, and it has been proposed that ID does not change with stress intensity but is higher in environments that increase phenotypic variation (phenotypic variation hypothesis). I conducted a series of experiments and compared the effects of various types of stress and inbreeding on plants to answer the following questions: (1) Does inbreeding depression (ID) differ among environmental conditions? (2) Does ID generally increase or decrease with the intensity of stress? (3) Does ID increase in environments which increase phenotypic variation? Chapter II and III investigate the interacting effects of inbreeding and stress on performance and plasticity of the perennial herb Silene vulgaris. Seedlings derived from self- and cross-pollinations were clonally propagated, and replicates of each of the genotypes were grown under eight stress treatments in a greenhouse. These included a control, drought, copper addition, simulated herbivory and two levels of nutrient deficiency and of shade. Inbreeding depression differed among stress treatments and decreased with stress intensity (Chapter II). This decrease of ID with stress was particularly strong for stress types to which the species may have become adapted in its population of origin (drought, nutrient deficiency), whereas ID did not change with increasing shade. To test if the results from the experiment can be transferred to the situation in natural populations, I planted selfed and cross-pollinated S. vulgaris into a common garden and into a more stressful field site. In contrast to the greenhouse experiment, ID was higher in the field site than in the common garden. However, the phenotypic variation hypothesis explained both the higher ID in the field compared to the common garden, and the decrease of ID with stress intensity in the greenhouse. Inbreeding also affected the response of various traits of S. vulgaris other than biomass to stress (Chapter III). Offspring from self-pollination were less plastic in some important functional traits, like stem length, leaf area, specific leaf area and chlorophyll content. Plants changed their allocation patterns in response to specific stresses like shading and nutrient deficiency as predicted by optimal partitioning theory, but these allocation responses were not affected by inbreeding. A second study system was the hemiparasite Rhinanthus alectorolophus (Chapter IV). Selfed and open-pollinated offspring from two populations were grown with single individuals of 13 host species known to differ in their quality for the parasite, thus forming a gradient in stress intensity. In a second experiment parasite seedlings were grown with four-species mixtures of the same 13 hosts differing in the number of legumes and of functional groups (grasses, legumes and non-leguminous forbs). Inbreeding reduced the performance of the hemiparasite R. alectorolophus. Inbreeding depression was strongest for parasites grown with good hosts and decreased with stress intensity, i.e. with declining host quality. When grown with mixtures of four host species, ID decreased with the number of host functional groups, suggesting a buffering of the effects of deleterious alleles by host functional diversity. In conclusion, the results of both studies suggest that in contrast to the predominant expectation, ID does not generally increase with stress intensity. In both studies, the magnitude of ID depended on the stress treatment. However, when ID changed with stress intensity, it was lower under more stressful conditions, which supports the alternative hypothesis that cross-pollinated plants are more capable of using favorable conditions than selfed plants. Differences in phenotypic variation explained some of the differences in ID among treatments in S. vulgaris, but not in R. alectorolophus. The phenotypic variation hypothesis thus does not provide a general explanation for environment-dependent inbreeding depression, but may be useful for understanding the mechanisms contributing to differences in ID. We further conclude from the experiments that stressful conditions will not generally exacerbate the negative effects of inbreeding for small and fragmented populations. However, inbred plants may be less able to cope with changing conditions because of reduced phenotypic plasticity. ID may increase under unpredictable, fluctuating conditions including multiple environ¬mental stresses which are characteristic of many natural environments