3 Results and discussion
3.5 Climate change and plant interactions with associated
In nature, apart from abiotic factors, plants are also constantly exposed to various biotic factors, such as herbivores and microorganisms. In this last part of my thesis, I have focused on the UV-B- and temperature-induced changes in aspen interaction with naturally occurring insects, rust disease and fungal endophytes (II). The effect of climate change on rust prevalence in dark-leaved willow was also investigated (III). A background summary concerning plant interactions with insects, rust pathogens and fungal endophytes is included in the introductory section of manuscript II.
Interestingly, slow-growing aspen clones with less salicylate were more severely infected by rust fungi than were fast-growing ones (II). This is in line with studies on P. trichocarpa × deltoides (Miranda et al. 2007), but they found that plants infected by rust fungi produced more condensed tannins (instead of salicylates) than did non-infected ones. Therefore, higher salicylate concentration might explain the resistance of fast growing aspen clones to rust fungi. This does not, however, explain the negative effect of warming on rust prevalence. In both willows and aspens, warming decreased the incidences of rust disease and the concentrations of phenolic compounds (II, III, Nybakken et al. 2012). Thus, plant growth may also determine plant susceptibility to rust disease or rust disease may have reduced plant growth. Likewise, warming reduced
39 both the relative abundance of endophytic morphs that harbored aspen leaves and condensed tannin concentration (II), while higher condensed tannin concentrations were associated with lower abundance of endophytic fungi in P. fremontii (Bailey et al. 2005). Therefore, in my study, phenolic concentration may not have mediated the negative effect of warming on the relative abundance of fungal endophytes.
UV-B and temperature treatments had no statistically significant effects on the probability of insect damage in European aspen seedlings. However, slow-growing clones, which contained less salicylates were more prone to insect damage than were fast-growing ones (II). While salicylates may determine the host preference and oviposition of some specialist herbivores, they may also deter generalist herbivores (Boeckler et al. 2011), which were probably more abundant in the field than were the specialists. In addition, the higher leaf nitrogen concentration of slow-growing clones may also have contributed to the incidence of herbivore damage, since nitrogen is often limiting to insects.
39 both the relative abundance of endophytic morphs that harbored aspen leaves and condensed tannin concentration (II), while higher condensed tannin concentrations were associated with lower abundance of endophytic fungi in P. fremontii (Bailey et al. 2005). Therefore, in my study, phenolic concentration may not have mediated the negative effect of warming on the relative abundance of fungal endophytes.
UV-B and temperature treatments had no statistically significant effects on the probability of insect damage in European aspen seedlings. However, slow-growing clones, which contained less salicylates were more prone to insect damage than were fast-growing ones (II). While salicylates may determine the host preference and oviposition of some specialist herbivores, they may also deter generalist herbivores (Boeckler et al. 2011), which were probably more abundant in the field than were the specialists. In addition, the higher leaf nitrogen concentration of slow-growing clones may also have contributed to the incidence of herbivore damage, since nitrogen is often limiting to insects.
40 41
4 Main findings and concluding remarks
Despite their sessile lifestyle, plants have amazing capabilities of acclimating their physiology, morphology and growth in response to environmental conditions. Under varying environmental conditions, resources are optimally allocated among competing processes in order to maximize individual fitness and productivity, resulting in plant acclimation. Natural genetic variation in plant populations may, however, have confounding effects on the patterns of resource allocation. The use of genetically identical plantlets (within clones) facilitated our interpretation of the effects of elevated UV-B and temperature, and mineral nutrient limitations from the resource allocation perspective. In addition, due to their gender differences in reproductive investments, growth and chemical allocation, dioecious plants represent good models for studying trade-offs between life history traits.
To conclude:
As evidenced from European aspen and dark-leaved willow, climate change factors may have additive or counteractive effects on plant growth, physiology and secondary metabolism, depending on plant species.
The differential responses of genders to global change that we demonstrated could result in different competitive abilities in males and females of dioecious plant species. Thus, there is a potential effect of global change on altered sex ratios, and this merits further research.
Within- and between-plant variations in different groups of phenolics were consistent with the main hypotheses proposed to explain their distribution, highlighting the
41
4 Main findings and concluding remarks
Despite their sessile lifestyle, plants have amazing capabilities of acclimating their physiology, morphology and growth in response to environmental conditions. Under varying environmental conditions, resources are optimally allocated among competing processes in order to maximize individual fitness and productivity, resulting in plant acclimation. Natural genetic variation in plant populations may, however, have confounding effects on the patterns of resource allocation. The use of genetically identical plantlets (within clones) facilitated our interpretation of the effects of elevated UV-B and temperature, and mineral nutrient limitations from the resource allocation perspective. In addition, due to their gender differences in reproductive investments, growth and chemical allocation, dioecious plants represent good models for studying trade-offs between life history traits.
To conclude:
As evidenced from European aspen and dark-leaved willow, climate change factors may have additive or counteractive effects on plant growth, physiology and secondary metabolism, depending on plant species.
The differential responses of genders to global change that we demonstrated could result in different competitive abilities in males and females of dioecious plant species. Thus, there is a potential effect of global change on altered sex ratios, and this merits further research.
Within- and between-plant variations in different groups of phenolics were consistent with the main hypotheses proposed to explain their distribution, highlighting the
42
complementarities of plant defense theories in explaining allocation to constitutive phenolics.
The genera-, gender- and genotype-specific responses of Salicaceous species to multiple global change factors may extend to plant-insect and plant-fungi interactions.
This accentuates the importance of implementing realistic and multifactor experiments and taking into account genetic variations when predicting plant responses to global change.
43
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