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Recent increase in forest mortality events worldwide and their relationship with drought episodes highlight the importance of understanding tree resilience to a changing climate. Empirical models of forest mortality have been typically used and were focusing on carbon related variables such as growth to predict tree death. Recent efforts have shifted toward a more mechanistic modeling of mortality. Mechanistic approaches use tree traits and climate as inputs to model processes and represent carbon and water fluxes, all necessary to plant life. The advantage of mechanistic approaches is their ability to account for potential adaptation of trees to climate change, but also to physically explore the causes of vulnerability and resilience to droughts. Mechanistically, the atmospheric demand for moisture at the canopy level is communicated to the tree through stomata, creating a water gradient between the leaves and the roots, and resulting in the ascent of sap via the plant hydraulic structure. Depending on the climate (temperature, atmospheric dryness, light, precipitation), different architectures will perform differently at maintaining the gradient. For example, deep roots can access deep water in dry regions and shallow roots can access rare precipitation events whereas larger leaf area increases the atmospheric demand for moisture. In very harsh conditions such as extreme or lasting droughts, the hydraulic structure might suffer from a steep water gradient. Protection against excessive gradients can be achieved either through an investment in a stronger structure (denser wood) or through a regulation of the pulling force at the top of the canopy (closing leaf stomata). Evolution of structures and physiological strategies have resulted in fitness advantages and partially explain the diversity of species architectures across climates. More importantly, this diversity is at the core of the vulnerability and resilience of each species to increased aridity and frequency of extreme events, and therefore its mortality. This dissertation investigates the resilience to droughts of two co-occuring species in common woodlands of New Mexico, USA. This location is of specific interest because drought conditions (high temperature and/or low precipitation) have become more frequent as a result of global warming and because these ecosystems have suffered extensive mortality in the last decades. The two species, Pinus edulis and Juniper monosperma have very different physiological strategies, which allows for an extra level of vulnerability comprehension. To further test their resilience to extreme drought and possibly future climatic conditions, I studied trees that were subject to a six-year rain-reduction experiment. In the first part we develop a mechanistic model of the tree functioning that includes water and carbon fluxes and is based on their respective supply-demand balances. We use this simplified mechanistic model to study the sensitivity of mortality to hydraulic structure variations and to the physiological strategy of each species. We find that for both species death resulted from an irreversible damage of tissues transporting water. Despite P. edulis’s ability to close stomata to reduce the atmospheric demand, they died first because of their vulnerable tissues. In the second part, I specifically investigate P. edulis’s structural response to drought at the canopy level. By dissecting branch anatomy at an annual resolution, I find that during droughts this species increase relatively more leaf area (water demand) compared to transport area (water supply). I suggest that the structural adjustments that occur at the branch level do not contribute to the protection of the tissues transporting water. In the third part, I analyze the anatomy of these tissues in branches of P. edulis. I find that in response to long-lasting drought the trees built tissues more efficient at transporting water but also more vulnerable to future drought. By contrast, a short-intense drought decreases efficiency without changing vulnerability. I hence show that during lasting drought the anatomical adjustment of branch tissues increase the vulnerability of the piñons. This study shows the importance of considering climate responses of structure and physiology together to compare resilience across species. It also shows that adjustments of hydraulic elements in response to drought tend to decrease hydraulic resilience and could favor a run-away scenario. If the population of Pinus edulis - a dominant species of the Southwest US - were to decline, major shift should be expected in related ecosystems.

Utah’s water resources are endangered by low rainfall rates, high per capita water consumption and a strong projected increase of residents. The irrigation of ornamental plant landscaping is estimated to account for 60% of residential water use, and is, therefore, a target of education programs in the effort to promote water conservancy. The water-wise “Yellow Tag” program developed by the Utah Division of Water Resources seeks to provide retail nurseries with free tags for labeling plants which are considered low water use with the objective of promoting water conservation. The objective of this study is to determine consumer preferences for plants labeled with the Yellow Tag. As a means of measuring consumer preference we use willingness-to-pay (WTP). We give participants the choice between daylily, spiderwort and neither. Choice alternatives differ in flower color, purported irrigation need, production location and price. The data for this study was collected through an online survey instrument applied to 463 participants residing in the state of Utah. Our results show that consumers prefer daylilies labeled with the Yellow Tag, and dislike spiderwort labeled with a high irrigation need. Special preference for Yellow Tag labeled ornamentals was found for respondents who are female, living in single houses, are concerned about the price of water and are drought aware. We do not find a preference for flower color or production location. These promising results may encourage Utah governmental and education agencies to continue the Water-Wise program, expand educational programs to increase drought awareness and help retailers optimize their future product mixes.

Thesis (PhD(Agric))--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Water scarcity is a key limiting factor for viticulture in dry regions. Traditionally drought sensitive varieties have the potential to grow in dry areas, however in most situations, through the use of rootstocks. Drought-tolerant rootstocks are expected to improve grapevine response to water deficit by improving the water uptake and transport and by reducing the water loss in leaves by root-to-shoot signalling. The mechanisms of rootstocks’ tolerance to drought are not yet fully understood. The main aim of this study was to improve the understanding of the rootstock/scion-cultivar interaction in the regulation of grapevine water use and leaf stomatal behaviour. Irrigated field vines without any water constraint were compared to rain-fed grapevines subjected to moderate water constraint. To better manage vine water status, reduce variability, and compare more rootstocks, greenhouse trials were also conducted where plants were well watered or subjected to severe water constraints. Pinotage grapevines (Vitis vinifera L.) grafted onto 110 Richter, 140 Ruggeri and 1103 Paulsen rootstocks were used for field experiments whereas Pinotage grapevines grafted onto 99 Richter, 110 Richter, 140 Ruggeri, 1103 Paulsen and Ramsey were used for greenhouse experiments. Our study suggested the influence of rootstocks on scion-cultivar water status and leaf stomatal size and density and gas exchange of the scion, implying an influence on water uptake and transport and a tight regulation of the stomatal conductance. Our data supported the hypothesis that the influence of rootstock in response to drought seemed to be higher under increasing water deficit up to a point where the plant water status is the main driver of the stomatal conductance and therefore photosynthesis regulation, considering the plant water status thresholds. In addition, the results suggested that stomatal development is affected by light, drought and possibly by rootstocks. Nevertheless, it is still not clear how the rootstock affects stomatal development and the link with scion-cultivar water use. It seems that the transpiration rate of leaves is more related to stomatal size than density. Thus one possible mechanism of Pinotage leaf adaptation to water constraints was structural during leaf growth, with a reduction in pore size to reduce plant water loss. The results showed that the rootstock is regulating the cultivar's stomatal size (anatomical changes during leaf growth) and functioning (stomatal regulation) through a complex signalling process. The effect of light on stomatal development is interesting in the context of canopy microclimate and canopy manipulation (choice of the vine architecture vs canopy size, in the context of climate change versus the possible increase in drought and water scarcity). The use of rootstocks is a long term investment which aims to provide resistance to soil pests and pathogens and to confer to the scion-cultivar drought and salt tolerance. The use of drought tolerant rootstocks is actually one of the most relevant practical solutions in dry terroir – units and in situations where water availability is limited. The understanding of the physiological and genetic mechanisms which govern scion-cultivar drought tolerance/behaviour induced by rootstocks is critical in terms of rootstocks choice in interaction with the scion-cultivar and is critical to assist breeding programs to create/select drought tolerant rootstocks.

Legumes being the most important crops worldwide are limited in terms of adaptability and productivity mainly by the abiotic stresses. In this study, the aim was to understand tolerance mechanisms of lentil cultivars under drought stress by physiological and biochemical analyses. This study was carried out with six Turkish Lentil cultivars (Seyran, Kafkas, Malazgirt, &Ccedil / agil, &Ccedil / ift&ccedil / i, &Ouml / zbek) subjected to drought stresses (10% and 15% PEG) and their physiological and biochemical properties were examined to select drought-tolerant and drought-sensitive cultivars. Drought stress was applied for 5 days to 7 days-grown lentil plants. 12-days old, stressed and control plant shoots and roots were analyzed in terms of physiological and biochemical parameters (length, fresh weight, ion leakage, proline, MDA and H

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The Mississippi River Valley Alluvial Aquifer (MRVAA) aquifer is the principal water source for Mid-South irrigators, but the current rate of withdrawal from the aquifer is not sustainable. Means to increase the sustainability of the MRVAA is to improve corn’s water use efficiency (WUE) while, concurrently, maintaining or improving profitability. The objectives of this experiment were to determine the effects of sensor-based irrigation threshold, planting date, and hybrid on corn grain yield, WUE, and net returns above seed and irrigation cost. Season long irrigation thresholds of -50 kPa full irrigation (FI) and -125 kPa limited irrigation (LI) were used, utilizing furrow irrigation. In conclusion, utilizing a delayed irrigation trigger of -125 kPa decreases grain yield and, ultimately, net returns in years with less naturally occurring rainfall, but increases WUE. Shifting the planting date earlier and using full-season (FS) hybrids, increases grain yield, WUE, and net returns.