Water is a key limiting factor for plant productivity worldwide. Although many mechanisms that reduce water loss have evolved in plants, water loss from leaves is inevitable, since photosynthesis requires open stomata (i.e. pores) through which carbon dioxide is absorbed and water vapor is lost. Accordingly, in the dark when photosynthesis is inactive, most plants are expected to completely close stomata thus reducing water loss and conserving water in the root zone. However, contrary to this expectation, it has been repeatedly demonstrated that many plant species have substantial nighttime stomatal opening (up to 90% of daytime values) and have significant nighttime water loss under natural conditions (up to 15% of daily water loss). Water spending by plants through high nighttime water loss may benefit nutrient-limited plants at least partly because of increased nutrient supply with flux of water to roots, as suggested by preliminary data and computer simulations. The objective of this research is to investigate the adaptive significance and regulation of nighttime stomatal opening and water loss in plants. Using laboratory, growth chamber, greenhouse, and field experiments three hypotheses will be tested. The experimental approach for each is also listed below.
1) Availability of nutrients and water affect nighttime stomatal opening and water loss. Nutrient availability, plant nutrient status, soil and plant water status, and atmospheric evaporative demand (e.g. relative humidity and temperature) will be manipulated and effects on nighttime stomatal opening will be measured. 2) Benefits of high nighttime water loss for plant growth and seed yield will be found in conditions where nutrients are more limiting than water. Nighttime water loss will be increased or decreased by manipulating atmospheric evaporative demand at night and effects on plant nutrient acquisition, growth, and seed yield will be measured. 3) At the evolutionary scale, selection for high nighttime stomatal opening and water loss may have occurred in habitats with abundant water but low nutrient availability. Closely related plant species native to habitats with differing nutrient and water availability will be assessed for nighttime stomatal opening and water loss under uniform conditions.
Hypotheses 1 and 2 will be tested with four focal species that differ in growth form and stress tolerance: mouse-ear cress (small annual), sunflower (large annual), greasewood (salt-tolerant desert shrub), and cottonwood (tree). These plant species all have high nighttime stomatal opening and water loss, but they can further close stomata at night under water stress. Leaf and whole-plant physiological responses and long-term growth and seed yield effects will be measured. For Hypothesis 3, the four focal species will be compared to many additional species within eight diverse taxonomic groups allowing broader interpretation of results.
Understanding effects of nighttime water loss will shed new light on how plants cope with water and nutrient deficiencies in nature. Knowledge of the magnitude of nighttime water loss and its regulation is important for crop breeding and management, because nighttime water loss increases total crop water use. Additionally, knowledge of nighttime water loss and its regulation will ultimately improve understanding of larger scale processes such as competition, ecosystem water and nutrient fluxes, and uptake of gaseous pollutants by plants. The project will train a diverse group of undergraduate and graduate students, a technician, and a postdoctoral researcher. The Donovan and Richards labs both have strong records of proactively recruiting underrepresented groups and directing inquiry-based undergraduate research.
