ASHS 2024 Conference

Increasing drought conditions and variable water availability under climate change impact plant productivity, ecosystem function and the global carbon cycle, with many species-level responses remaining unknown. Variation in response and ability to acclimate to decreased water availability differs among plant species and across biomes. This project utilized a preexisting water deficit trial of horticultural taxa across sites in the Western U.S. to assess the interactions between acclimation to climate and water availability across a growing season. Four focal taxa, Physocarpus ‘Diabolo’, P. ‘Little Devil’, Cercis canadensis and C. occidentalis shared across three locations in Washington, Oregon and Utah were measured for physiological and hydraulic traits on the leaf and stem scale in response to irrigation treatment. The cultivars of Physocarpus are popular landscape shrubs known for their distinctive purple foliage yet understudied physiologically. C. occidentalis and C. canadensis have distinct native ranges, with the former originating west of the Rocky Mountains while the latter is east coast in origin, thus their performance was compared across these western U.S. sites. Full gas exchange, specific leaf area, 13C isotope discrimination, hydraulic conductivity, stomatal conductance, ΦPSII, were analyzed and water use efficiency was calculated each taxon at each location. Impacts of site, treatment, taxa and change across the growing season were analyzed on this suite of traits. Results show distinctions in water use strategy by climatic location (p: 1e-05) and between closely related species and cultivars. Additionally, physiological measurements indicate measurable physiological plasticity across the growing season. These findings indicate the importance of setting on the ability of different plant cultivars to acclimate to water stress, taxa-level differences among horticulturally important species, and overall knowledge of plant drought response, knowledge gaps that are crucial to address in the face of anthropogenic climate change.

Stomatal morphology dictates the maximum stomatal conductance and relates to plant water use efficiency and carbon assimilation rate. Aspects of stomatal morphology, including size characteristics and density, are plastic in some taxa, can respond to environmental stressors, and are thought to be relevant in drought acclimation within an individual. The Climate Ready Landscape Plants (CRLP) trial consists of 6 sites across the Western U.S. that have installed common garden drought experiments that utilized daily ETo to implement three water deficit treatments. Stomatal conductance and stomatal images were collected from 3 cultivars of Hibiscus syriacus and 3 cultivars of Rosa spp. from 4 of these sites: Seattle, WA; Aurora, OR; Davis, CA; and Irvine, CA. Stomatal images were measured to determine stomatal density and size, which was used to calculate gsmax. Differences between stomatal traits were tested between sites and water deficit treatments using ANOVA. Correlation between gsw and gsmax were determined with regression analysis. PCA was used to determine which site characteristics and treatments primarily explain observed differences. Here we aim to test 1) Are stomatal morphological traits plastic across water deficit treatments and sites in multiple Rosa and Hibiscus syriacus varieties? 2) Does measured stomatal conductance (gsw) correlate with morphologically derived anatomical maximums (gsmax)? 3) Do site characteristics across the maritime Western US predict physiological and morphological stomatal traits? Findings reveal important ecological and horticultural considerations in plant stress response to drought and acclimation potential across an environmental and latitudinal gradient. The results can help in plant selection and categorization of species vulnerability, based on ability to manipulate stomatal characteristics in response to water deficit.

Evergreen leaves of Rhododendron species inhabiting temperate/montane climates are typically exposed to both high radiation and freezing temperatures during winter when photosynthetic biochemistry is severely inhibited. This could lead to accumulation of excess energy (radiation) in photosynthetic reaction centers causing photoinhibition or photooxidative damage. Cold-induced ‘thermonasty’, i. e. lamina rolling and petiole curling/drooping, can reduce the amount of leaf area exposed to solar radiation and has been associated with photoprotection in overwintering rhododendrons. The present study was conducted on natural, mature plantings of a cold-hardy and large-leaved thermonastic North American species (R. maximum) during winter freezes. Infrared thermography was used to determine initial sites of ice formation, patterns of ice propagation, and dynamics of the freezing process in leaves to understand the temporal and mechanistic relationship between freezing and thermonasty. Results indicated extracellular freezing in leaves always preceded the initiation or intensification of thermonasty. Ice initially formed in the vascular tissue of the midrib and then propagated into other portions of the vascular system/venation. Ice was never observed to initiate or propagate into palisade, spongy mesophyll, or epidermal tissues. These observations, together with the leaf- and petiole-histology, and a simulation of the rolling effect of dehydrated leaves using a cellulose-based, paper-bilayer system, suggest that thermonasty occurs due to anisotropic contraction of cell wall cellulose fibers of adaxial versus abaxial surface as the cells lose water to ice present in vascular tissues.

The adaptation of perennial species to winter freezing temperatures is crucial for their reproductive success and has led to the evolution of diverse survival strategies to mitigate freezing damage. Bud survival is essential for species reproduction and fruit production, as buds carry the dormant flower primordia that will bloom in the next growing season. We studied two freezing survival mechanisms: deep supercooling (DS) and extraorgan freezing (EOF). Deep supercooling involves physical or structural changes that prevent ice nucleation in florets and meristems by sequestering small amounts of water. When the critical nucleating temperature for this sequestered water is reached, ice propagation is rapid, and cellular damage is lethal. Extraorgan freezing causes a gradual dehydration of inner bud tissues, driven by the vapor pressure deficit from extracellular ice formed in bud scales. Despite existing knowledge, the survival benefits of species undergoing deep supercooling, considered a limited strategy compared to extraorgan or extracellular freezing, remain unclear. Similarly, how adaptation to freezing impacts reproductive traits in woody species is not well understood. We focused on the Prunus genus for its dual survival strategies and productive and ornamental value. This study, conducted on six Prunus species at the Arnold Arboretum in Boston, MA, spanned three developmental stages: leaf drop in fall, dormancy in winter, and pre-bud swell in spring. Data encompassed phenology, vascular tissue development, flower primordia size, differential thermal analysis, controlled freezing tests, and characteristics of flowers, fruits, and seeds. Results indicated that DS Prunus species delay vascular tissue development and grow larger flower primordia from fall compared to EOF species. Conversely, EOF species bloom later, producing more and smaller flowers and fruits in a shorter time than DS species. In summary, in the Prunus genus, DS species appear to trade a lower temperature threshold for pre-forming fewer, larger flower primordia per bud, enabling earlier blooming and more efficient use of the growing season to develop larger fruits in contrast to EOF species.

Heat stress is one of the most significant uncontrollable abiotic factors that affect potato plant growth, development, and tuber yield. While short-term acute heat stress experiments have produced considerable insights into the effects of heat stress on potato, there is a lack of information on the mechanisms involved in heat stress adaptation. Our recent studies demonstrate that under prolonged heat stress (35/25°C, day/night, for 3 weeks), newly developed leaves can maintain health and adapt to heat stress by modifying anatomy and physiology. Whereas, the leaves developed prior to heat stress (20/15 °C, day/night) on the same plant suffer (chloroses, senescence) from heat stress. We compared the gene expression in the youngest, fully expanded terminal leaflets developed under control and heat stress in two genotypes, Solanum tuberosum L. ‘Atlantic’ (ATL) and Solanum microdontum Bitter (MCD). As expected, several heat shock proteins (HSP) genes were upregulated in both genotypes. In addition, several desaturase genes were downregulated suggesting an increase in the saturation of membrane lipids may provide membrane integrity under heat stress. Our parallel physiological and anatomical studies have shown that adaptation to heat stress involves increase in stomatal density, lowering of leaf temperature via increased transpiration and maintenance of photosynthesis. Consistent with these results we found significantly regulated genes involved in ABA biosynthesis, photosynthesis, cell growth, expansion and patterning. These data offer insight into potential genes involved in heat tolerance in potato that may be useful in breeding for heat-tolerant potato varieties.

Phosynthetically active radiation (PAR; 400 – 700 nm) is widely acknowledged as essential for photosynthesis in plants. However, recent research has revealed the significant contribution of far-red photons (FR; 700 – 750 nm) to photosynthetic processes, particularly when present alongside PAR. While previous studies have primarily focused on whole-plant gas exchange, limited research exists on leaf-level replication of these findings. In this preliminary study, we investigated leaf gas exchange in five field-grown crop species using A/Ci curves. We exposed the leaves to equal proportions of blue, green, and red light at a photon flux density of 1000 µmol·m-2·s-1 and replaced varying percentages (0%, 15%, or 30%) of these photons with FR. Our hypothesis, based on previous whole-plant studies, was that all species would exhibit similar photosynthetic rates (Pn) across different FR treatments. Contrary to our hypothesis, we observed species- and cultivar-specific variations in leaf-level Pn with FR treatments. For instance, strawberry and green leaf lettuce exhibited decreased Pn with increasing FR, while apple and Swiss chard showed increased Pn. Red leaf lettuce maintained consistent Pn levels. Despite these differences, the overall trends across CO2 concentrations remained consistent regardless of FR levels. Considering that direct sunlight naturally contains FR equivalent to approximately 18% of PAR, and our crops were grown in open-field conditions, our findings suggest a species-specific capacity to utilize FR in photosynthesis. These findings are preliminary, but data is being collected to examine species responses throughout a full growing season.

In wildlands, such as the prairies of the Northern Great Plains, environmental degradation has created the need for ecological restoration of native plants on the landscape. These ecological restorations require native seed. However, many seed-based restoration efforts fail in that they do not produce the desired vegetation. Lack of species-specific information on germination characteristics and dormancy of native seed could be contributing to these failures. Therefore, restoration practitioners and other users of native seed need germination and dormancy information for native species to improve outcomes. Our objectives in this study were to examine germination characteristics and seed treatments that best promote germination in plant species native to the Northern Great Plains and define dormancy classes for each of our study species. To meet these objectives and promote success in seed-based restoration, we conducted a germination experiment for 15 high-priority native forbs. Seeds were treated with four pretreatments (scarification, smoke, fertilizer, and a control), three stratification lengths (2, 4, and 8 weeks), and different temperature regimes. We examined the influence of each factor to determine the means of breaking dormancy and best planting practices. Our data indicated that a scarification treatment before planting Gaillardia aristata increased germination by over 19%. This data suggests that while the majority of our G. aristata seeds are non-dormant, a percentage are physiologically dormant. Our data also shows that Penstemon albidus is strongly influenced by temperature conditions, and the species requires a period of cold stratification to increase overall germination. This information will be used to develop best planting practices for government agencies and aid seed producers and distributors by offering seed storage and planting instructions matching the phenology of native plant species. This rigorous germination experiment can also be used as a model for other priority species and can be adapted to different ecoregions.