Collaborative Research: Linking Abiotic Stress to Gender Specific Fitness in a Desert Bryophyte

Biological soil crusts provide stability and fertility to desert soils. Recent recommendations for management from soil scientists include as a top priority the protection of soil crusts in order to preserve the health of desert ecosystems. The organism of study, the moss Syntrichia caninervis, is a dominant element of soils in the arid U.S. southwest. It exhibits the widespread bryophyte pattern of male rarity, presenting one of the most biased population sex ratios in all of land plants, ranging from 6!f :I r! to 1!f: 0 r! (entirely female). In this predominantly clonal species, initial evidence points to male plants as less tolerant of stress than female plants, a pattern that could apply broadly to dioecious bryophytes. The guiding hypothesis of this research is rooted in the cost of sexual reproduction, where sexual reproduction in female plants constitutes a lower cost (despite rearing offspring) than in male plants. This cost differential between the sexes is reflected in reduced growth and survival in male plants relative to female plants, and in an odd twist, neither sex reproduces sexually to any degree while still incurring the costs thereof. The central objectives of the research are to understand how responses to stress are manifest in gender specific pathways, and to link short term stress response to asexual fitness and survival. The principal prediction is that male plants will exhibit a reduced tolerance to abiotic stress relative to female plants, and this reduced tolerance will be assayed by using (i) several measures of asexual fitness in bryophytes, including mortality, time to emergence, shoot production, protonemal growth rate, and biomass produced, and (ii) two measures of short-term stress - chlorophyll fluorescence and gas exchange. Specific transient stresses to be experimentally imposed include heat stress (wet and dry), desiccation stress (varying periods of desiccation), desiccation tolerance (cycles of drying and wetting periods), and a simulated summer rain event stress that combines wet heat, dry heat, and desiccation tolerance. The summer rain experiment will simulate arguably the most stressful event experienced by desert bryophytes. These realistic stresses will be employed on both field-collected plants and also lab-regenerant plants (plants undergoing active growth under controlled environments), thereby testing the effects of stress on both establishment, colonization, and clonality. Stress response will be measured at three scales of resolution: regeneration capability --fitness (days to weeks), level ofCOz fixation (seconds to minutes), and chlorophyll fluorescence «I second). By using these three measures and comparing the response patterns we can identify when and how stress is manifested from light capturing processes to regeneration, thus allowing for an evaluation of the effect of stress on a major component of the desert crust. This approach characterizes plant stress response by integrating the regeneration response (including life history events) with the photosynthetic response. Intellectual merits of the planned research include gaining a better understanding of one of the longstanding puzzles in bryophytes, namely female-biased adult sex ratios in organisms where at meiosis a balanced ratio is expected. In addition, very little is known, from a regenerational perspective, of how bryophytes tolerate the stresses of temperature and desiccation. The intersection of stress biology and gender ratios in this group of organisms has not been approached experimentally until now, with regeneration a novel approach. Broader impacts of this research include (i) an excellent opportunity to recruit and train undergraduates from underrepresented groups in ecophysiology, (ii) studying a central organism of the biological soil crust that carries out many essential functions determining the health ofthe desert ecosystem, and (iii) this research encompasses climate change by exploring the effects on the biological crust of the predicted climate patterns in the arid southwest of rising temperatures and increased summer rainfall