Leah S. Dudley: Abiotic stress

ABIOTIC ENVIRONMENTAL INFLUENCES ON PLANTS (WATER STRESS):

My research focuses on the effects of water stress on plants. I have used several systems to explore this topic. This work has ranged from characterizing phenotypic responses to evolutionary ones.

Male catkin, Salix glauca

Site: CO, Rocky Mts., Pennsylvania Mt., krummholz, Salix glauca

Female catkin, Salix glauca

As a graduate student working with Dr. Candace Galen, I investigated differences between sexes of a willow shrub, Salix glauca. Dioecy, the separation of male and female function into separate individuals, occurs in 10% of flowering plant species. This separation of sexual function sets up a situation in which each sex requires a suite of resources that differ from the other. Male function is fulfilled by flower and pollen production; pollen is highly concentrated in nitrogen. Female function, on the other hand, is fulfilled by ovule, seed and fruit production all of which use relatively high amounts of carbon. Because of the heavy investment by females into reproduction, it is likely that they maintain, overall, a conservative growth strategy relative to fast-growing males. If this is the case, then females are likely to survive more frequently then males under stressful situations. My work in the Colorado Rocky Mountains at high altitude with female and male S. glauca reinforces this idea. Sexes differed in resources allocated to reproduction (nitrogen and/or phosphorous). A biased sex-ratio in several populations reflect potential microhabitat differences. Furthermore, female cloned plants under dry-down treatment experienced higher water stress compared to male plants.

"Ecological correlates of secondary sexual dimorphism in Salix glauca (Salicaceae)"
doi: 10.3732/ajb.93.12.1775
Am. J. Bot. December 2006 vol. 93 no. 12 1775-1783
The ecological causation hypothesis for secondary sexual dimorphism was tested in Salix glauca, a dioecious willow shrub. Plants growing in a Colorado Rocky Mountain (USA) krummholz mosaic of mesic and xeric patches were monitored for four consecutive years. Ecological causation is predicated on unique resource demands associated with sexual function. In S. glauca, seeds have twofold higher N and P concentrations compared to pollen. P, but not N, allocation costs differed between sexes at plant and flower scales. Ecological causation also predicts spatial segregation of sexes along underlying habitat gradients. In five populations of S. glauca, sexes displayed significant spatial segregation. The theory also predicts that sexes differ in performance across gradients of environmental stress or resource availability. Consistent with this hypothesis, females had lower drought tolerance than males under years of extreme aridity. Furthermore, over 10 years, annual shoot growth for females was greatest in mesic habitat patches, while males grew at a consistent rate regardless of habitat aridity. Because current shoot growth is correlated with future catkin production, habitat specialization likely provides a fitness payoff in females. Overall, this long-term study provides some of the strongest evidence to date for ecological causation of secondary sexual dimorphism in plants.

"Stage-Dependent Patterns of Drought Tolerance and Gas Exchange Vary between Sexes in the Alpine Willow, Salix glauca"
Article Stable URL: http://www.jstor.org/stable/40210836
Leah S. Dudley and Candace Galen. Oecologia Vol. 153, No. 1 (Aug., 2007), pp. 1-9
Females and males of sexually dimorphic species have distinct resource demands due to differential allocation to reproduction. Sexual allocation theory predicts that functional traits will diverge between sexes to support these demands. However, such dimorphism may be masked by the impact of current reproduction on sourcesink interactions between vegetative and reproductive organs. We ask whether natural selection has led to genetic dimorphism in homologous physiological traits between sexes of the dioecious willow shrub, Salix glauca. In a common garden experiment we compared physiological responses to drought stress by male and female ramets in the absence of confounding demands from reproductive structures. Ramets experienced similar pre-dawn leaf water status (Ψ₁) as parental genets in flower within the natural population, indicating that experimental dry-down mirrored environmental conditions in nature. Male and female ramets achieved similar instantaneous water use efficiency, based on the ratio of carbon gain to water loss, under wet and dry conditions. However, female ramets experienced greater water stress (i.e., more negative Ψ₁) than males under dry conditions. Lower Ψ₁ for female ramets may partly reflect the maintenance of conductance under drought; males, in contrast, maintain Ψ₁ under drought by reducing conductance. Differences between sexes in terms of conductance and leaf water status of the vegetative ramets were absent in a concomitant comparison of parental flowering plants. Our results show (1) genetic divergence in physiology between sexes of S. glauca occurs in the absence of gender-specific reproductive sinks, (2) males are the more physiologically plastic sex with respect to water use, and (3) paradoxically, divergence in water relations between sexes is not detectable at sexual maturity under natural conditions.

Kageneckia angustifolia male flower

Site: Andes, La Parva, Kageneckia angustifolia

Kageneckia angustifolia female flower and fruit

I explored further how plant sexes may diverge with regard to water use by examining whether male and female mountain olive trees (Kageneckia angustifolia) differ in respect to physiology over an altitudinal gradient during my postdoctoral work in Chile with Dr. Mary Arroyo at the Institute of Ecology and Biodiversity. Not only did I learn Spanish there, but I also learned that sexes differed in their physiology regardless of population or elevation. I measured photosynthetic light response curves on male and female leaves of K. angustifolia. These curves describe the photosynthetic efficiency of plants under different light regimes. While there was a lot of variation among light curves, females exhibited higher maximum photosynthesis but lower photosystem II efficiency (ФF_II) than males regardless of elevation, indicating a less efficient but higher capacity in females to capture light and transform it to photosynthate.

Site: Sierra Nevada, Sawmill Road,
Clarkia xantiana ssp parviflora

Site: Sierra Nevada, Camp 3
Clarkia xantiana ssp xantiana

Site: Sierra Nevada, Democrat Springs
Clarkia unguiculata

In my current work on a co-authored NSF grant, I am investigating the correlated evolution of mating system and water use strategies among closely related species in the annual herbaceous genus Clarkia. This research stems from observations and patterns of selfing taxa compared to closely related outcrossing ones. For example, selfing taxa tend to have distributions that are characterized by extreme environments, have smaller petals, and to flower earlier than their outcrossing sister taxa. We have shown in previous greenhouse work that for two pairs of sister taxa (C. exilis vs C. unguiculata and C. xantiana ssp parviflora vs C. xantiana ssp xantiana), selfers flower earlier and flowers develop more rapidly than their outcrossing counterpart. We are currently using both field and greenhouse studies in order test the hypothesis that self-fertilization may evolve indirectly as a response to direct selection on correlated traits (such as physiological rates and development rates). If so, this would challenge the widely held view that selfing evolves primarily as an adaptation that ensures reproductive success where pollinators are scarce. To address this hypothesis, we’ve measured several physiological parameters (e.g. photosynthesis, transpiration, fluorescence) on field grown plants in multiple populations of each taxon, using an infrared gas exchange analyzer (LiCor6400). We’ve obtained indicators of water and light stress using leaf water potential measurements and antioxidant constituents. In addition we’re measuring components of fitness such as biomass and fruit production so that we may measure the strength and direction of phenotypic selection on these physiological parameters in all four taxa. We are complimenting these field surveys with an artificial selection experiment aimed to determine whether there is a strong genetic correlation between mating system, flowering time, and floral traits associated with mating system. Beginning with the outcrossing taxa in this approach, we are selecting (and breeding) early flowering and selfing genotypes in greenhouse-grown plants for several generations. We will then determine whether genotypes selected to flower early have physiological and floral developmental rates that are similar to those genotypes selected to self.

Pollinated stigma, Clarkia unguiculata

"Physiological performance and mating system in Clarkia (Onagraceae): Does phenotypic selection predict divergence between sister species?"
doi: 10.3732/ajb.1100387
Am. J. Bot. March 2012 vol. 99 no. 3 488-507
Premise of the study: The evolution of self-fertilization often occurs in association with other floral, life history, and fitness-related traits. A previous study found that field populations of Clarkia exilis (a predominantly autogamous selfer) and its sister species, Clarkia unguiculata (a facultative outcrosser) differ in mean photosynthetic rates and instantaneous water use efficiency (WUEi). Here, we investigate the strength and direction of selection on these traits in multiple populations of each taxon to determine whether natural selection may contribute to the phenotypic differences between them.
Methods: In spring 2008, we measured instantaneous gas exchange rates in nine populations during vegetative growth (Early) and/or during flowering (Late). We conducted selection gradient analyses and estimated selection differentials within populations and across pooled conspecific populations to evaluate the strength, direction, and consistency of selection on each trait early and late in the season.
Key results: The direction and relative strength of selection on photosynthetic rates in these taxa corresponds to the phenotypic difference between them; C. exilis has higher photosynthetic rates than C. unguiculata, as well as stronger, more consistent selection favoring rapid photosynthesis throughout the growing season. Patterns of selection on transpiration, WUEi, and the timing of flowering progression are less consistent with phenotypic differences (or lack thereof) between taxa.
Conclusions: We detected several examples where selection was consistent with the phenotypic divergence between sister taxa, but there were also numerous instances that were equivocal or in which selection did not predict the realized phenotypic difference between taxa.

Herkogamous flower with receptive stigma,
Clarkia xantiana ssp xantiana

"Physiological Performance in Clarkia Sister Taxa with Contrasting Mating Systems: Do Early-Flowering Autogamous Taxa Avoid Water Stress Relative to Their Pollinator-Dependent Counterparts?"
DOI: 10.1086/656305
Susan J. Mazer, Leah S. Dudley, Alisa A. Hove, Simon K. Emms, Amy S. Verhoeven. International Journal of Plant Sciences, 2010; 171 (9): 1029
http://www.sciencedaily.com/releases/2010/11/101110212919.htm
Consistent differences in the physiological performance of wild populations of closely related plant taxa may be the result of environmentally induced phenotypic plasticity or adaptive evolution (or a combination of the two). Here we report the results of a field study of physiological and fitness-related traits in geographically proximate sister taxa in the annual wildflower genus Clarkia (Onagraceae) and interpret the differences between them in light of their ecological and reproductive differences. Within two pairs of taxa, the predominantly autogamous (self-fertilizing) taxon flowers and completes its life cycle before its pollinator-dependent (predominantly outcrossing) counterpart growing in sympatry or at similar elevations in the southern Sierra Nevada. Selfers generally exhibited higher rates of photosynthesis and transpiration than their outcrossing sister taxa, and, except for the earliest-flowering (autogamous) taxon, both photosynthetic and transpiration rates tended to decline as the season progressed. Within taxa, high photosynthetic rates were positively correlated with lifetime fruit production, and selfers had lifetime fruit production equivalent to or higher than that of outcrossers, despite the fact that the latter had higher aboveground stem biomass. These patterns are consistent with the hypothesis that natural selection has favored higher gas exchange rates in selfers to allow them to achieve their faster life cycles and so escape seasonal late-spring drought. An alternative explanation is that the differences in gas exchange rates represent environmentally induced plastic responses to the cooler temperatures and higher soil moisture content in early spring. Further experimental work is necessary to distinguish between these hypotheses.