Spatial variability of 13C isotopic discrimination in corn

Water stress influences photosynthesis induced isotopic C-13 discrimination (Delta) in C-3 and C-4 plants. In C-4 plants, Delta increases with increasing water stress, while in C-3 plants the opposite is true. The amount of Delta that occurs is a function of plant type, interactions between climatic conditions, plant available water, water stress, photosynthesis capacity, plant water demand, and yield. The objective of this study was to determine if Delta measured in corn (Zea mays) tissue collected from summit, shoulder, backslope, and toeslope areas contains spatial structure. Corn grain or fodder samples collected from gridpoints located at five sites were analyzed for total nitrogen (N) and Delta on a Europa 20:20 ratio mass spectrometer. At sites where crop growth was not N limited, Delta spatial dependence was described using linear models. Spatial dependence most likely was generated by environmental and physical factors that interacted to influence plant available water. Spatial dependence indicates that the basic physiological response of com to water stress, i.e., stomata] closure, can be measured at the plant and integrated to the landscape scale. A relationship between relative yields and Delta suggests that Delta provides an index forwater stress under non nutrient limiting conditions. However, because Delta can be influenced by many factors, Delta as a direct measure of water stress must be used with caution.