Fungal pathogens as selective forces in plant populations and communities*

Pathogens are potent selective forces whose importance in shaping the size and structure of individual plant populations and whole communities has been underestimated. Even in situations where host and pathogen have been associated over long periods of time, pathogens regularly affect host fitness by reducing fecundity and increasing mortality either directly or indirectly through reductions in competitive ability. The genetic consequences of such disease-induced reductions in fitness are profound. On a broad geographic scale, race-specific resistance generally occurs more frequently in regions characterized by environments favourable for disease development. Within such areas, however, the distribution of resistant plant genotypes is often very patchy. This probably reflects the importance of extinction and colonization events in the continuing co-evolutionary dynamics of host-pathogen associations. At a demographic level, pathogen-induced reductions in host fitness may lead to changes in the size of populations. In turn, this may lead to changes in the relative diversity of whole communities. Documentation of this scale of interaction is poor, but the devastating consequences of the introduction of pathogens into alien environments provides a salutary reminder of their power to change plant communities radically.     

Fruit abscission and ethylene production of four blackberry cultivars (Rubus spp.)

In all cultivars the force required to harvest fruit declined during ripening. The fruit retention strength (FRS) of ripe fruit varied between cultivars with ‘Ashton Cross’ and ‘Chehalem’ being easier to remove than ‘Bedford Giant’ or ‘Oregon Thornless’. The cultivars ‘Ashton Cross’ and ‘Oregon Thornless’ showed no increase in ethylene production during fruit ripening whereas ‘Bedford Giant’ and ‘Chehalem’ had increased rates of ethylene production (EPR) in the ripe fruit. Exogenous ethylene accelerated abscission, ethylene production and pigment changes in ‘Ashton Cross’ fruit at all stages of development. 1-amino-cyclopropane-1-carboxylic acid (ACC) supplied to fruit at all stages of development was converted to ethylene at levels in excess of those found naturally. The differences between cultivars are discussed with reference to the role of ethylene in both machine harvesting and post-harvest storage of blackberry fruit.     

Temporal patterns of reproduction and outcrossing in weedy populations of Echium plantagineum

Single and multilocus estimates of outcrossing rates were made in three populations of Echium plantagineum. Despite spatial separation, variations in population size (though not density) and reproductive output, no statistically significant difference was detected in outcrossing rates between the populations. Similarly, only slight differences in outcrossing rates were detected within populations when estimates were based on seed collected from flowers open at different times in the flowering season. The earliest flowers tended to have a lower estimated rate of outcrossing. In all cases, multilocus outcrossing rates were high, ranging from 0.81 to 1.05.
In all three populations flower production extended over a period of more than 2 months but the majority of seed was produced by flowers that opened during the first third of the (lowering season. This was largely caused by a high rate of flower production during the early part of the season and not changes in the number of seeds set per flower. The average number of seeds produced per flower varied both between individuals within populations, and between the different populations, but neither of these differences were significant.
Controlled pollination of flowers with self or outcross pollen, applied either singly or together, failed to detect any differences in the likelihood that either type of pollen would give rise to fertile seeds.     

The peel of plantain and cooking banana fruits

The peel of the plantain and cooking banana fruit protects the edible pulp from the surrounding environment. The peel of those cultivars examined contained 85–90% water and between 28 and 60 mg dry weight cm^-2 surface area. The ratio of fruit pulp to peel fresh weights differed between cultivars (1.18-2.28). The surface area of the fruit can be determined from the fresh weight using regression equations for individual cultivars or for all cultivars combined. The stomatal density was generally higher at the fruit tips than at the mid region. Significant differences in stomatal length and density were identified between cultivars although no trends existed between plantains and cooking bananas. There was little difference in the quantity of epicuticular wax on plantains whereas there were differing amounts on cooking bananas. Differences in wax composition between cultivars and for wax extracted with hot or cold chloroform were identified. Removal of the epicuticular wax with chloroform accelerated the rate of weight loss. The use of hot chloroform increased both the amount of wax removed and also the rate of weight loss. The effect of removing the epicuticular wax on water loss is discussed and illustrated with scanning electron micrographs.     

The Role of Ethylene in the Shedding of Red Raspberry Fruit

The production of ethylene by red raspberry (Rubus idaeus L.cv. Glen Clova) fruit increased climacterically during development.The concentration of ethylene within green fruit was low butincreased substantially as fruit abscission and ripening commenced.The receptacle contained higher concentrations than the drupeletsat all stages measured. In the mature ripening fruit the ethyleneconcentrations were found to be physiologically significant,and would accelerate the abscission of large green non-abscisingfruit if supplied as a fumigant. The addition of ethylene toripe fruit did not accelerate abscission, probably because saturatinglevels occurred naturally within these fruit. Reduction of ethylenesynthesis rates using the inhibitor of ethylene production aminoethoxyvinylglycine(AVG) reduced the rate of abscission zone weakening which occursin detached large green fruit. The rate of ethylene productionwas found to be dependent on the supply of the precursor l-aminocyclopropane-l-carboxylicacid (ACC). This only accumulated to any extent in those ripefruit with high rates of ethylene production. Rubus idaeus, raspberry, abscission, fruit ripening, ethylene, aminocyclopropane-l-carboxylic acid     

Epidemiology of damping-off disease (Pythium irregulare) in relation to density of Lepidium sativum seedlings

Controlled experiments on post-emergence damping-off, using small populations of garden cress seedlings (Lepidium sativum) inoculated with Pythium irregulare, demonstrate that planting density of the host population plays an important role in determining the rate of multiplication and the rate of advance of the disease. At high seedling densities the disease is transmitted readily between host plants, but at lower densities the greater distance between adjacent plants reduces the probability of successful transmissions, and this is reflected in the parameters of multiplication and advance. A simple negative relationship was found between the mean distance separating adjacent plants and both rate of advance of disease front and rate of multiplication of disease in a randomly inoculated seedling stand.     

Epidemiology of PytMum-induced damping-off in mixed species seedling stands

Controlled experiments on post-emergence damping-off, using mixtures of susceptible (Lepidium sativum) and resistant (Lolium rigidum) seedlings inoculated with Pythium irregulare, demonstrated that rates of multiplication and advance of the disease were determined mainly by the net density of susceptible seedlings and were little affected by the resistant plants present.