UC Riverside

In many plants from arid and semi-arid ecosystems in both temperate and tropical regions, green photosynthetic stems appear to have evolved as a response to water shortage at least during one period of the year. My work addresses the question of whether plants with photosynthetic stems respond differently to drought. In my first chapter I studied the coordination between photosynthesis and hydraulics in plants with photosynthetic stems in southern California. This topic is critical because photosynthetic stems have been proposed to be more tolerant to drought than leaves, and given the predictions of global climate change, plants in arid ecosystems may face die-back and eventual local extinction if they cannot cope with the detrimental effects of drought. Using drought survival traits, I can predict what species are more likely to survive extreme droughts. My second chapter addresses the question of how carbon and water dynamics differ in green versus non-green-stemmed plants. It is obvious that having photosynthetic stems increases the carbon gain of the whole plant, but what are the costs associated with it? They might have higher water costs than plants without green stems, raising the question of the existence of trade-offs in carbon and water resources in plants with photosynthetic stems. I found that plants with green stems have the same carbon gain as plants without green stems, and both had the same water-use efficiency. However, cuticular conductance was higher in green stems than leaves and brown stems of non-green-stemmed species, raising questions about the possible costs of having green stems in terms of water balance. In my third chapter I studied photosynthetic and hydraulic traits of plants with photosynthetic stems in a subtropical arid ecosystem in Mexico. I found that green stems had higher photosynthetic capacity than fleshy ones, but that fleshy stems had higher hydraulic capacity and water storage capacity. The results show that different photosynthetic stem syndromes can have different physiological outputs. Overall, green stem physiology resembles leaf physiology and the advantages of extra carbon gain are offset by water costs.