Effect of Leaf Area, Incident Radiation, and Moisture Stress on Reflectance of Near Infrared Radiation from a Corn Canopy

Plant moisture stress is a major limiting factor in crop production. Remote sensing of moisture stress in crops would allow definition of the environmental conditions causing stress and timely use of management practices to avoid it. The reflectance of solar radiation, however, may be more affected by plant cover and meteorological factors than by moisture stress. The objectives of this research were to monitor continuously incident near infrared radiation (IR ↓) and reflectance (IR ↑) from corn (Zea mays L.) and to relate the reflectance ratio (IR ↑ /IR ↓) to green leaf area index (LAI), IR ↓ , and moisture stress indices. The experiment was conducted at West Lafayette, Indiana on a Typic Argiaquoll (Chalmers silt loam) in 1973 and 1974 and on dune sand in 1975 and 1976. Under non-moisture stress (ns) conditions, LA1 was used in a function (FLAI) to identify the ratio of green plant cover (RGPC). Linear regressions showed that as FLAI or RGPC increased from 0 to 1, (IR ↑ /IR ↓)- increased an average of 9%. For each cal cm⁻² hr⁻¹ increase in IR ↓, (IR ↑ / IR ↓)ₙₛ decreased an average of 0.6% for every hour ending at 0900 to 1800 EST. Differences (DIFR) between measured IR ↑ /IR ↓ and that predicted with the ns regressions for each hour were regressed on a characteristic soil moisture potential (Ψₛₘ) for the dune sand and on a ratio of calculated actual to potential evapotranspiration (ET/ PET) for the Typic Argiaquoll. Correlations of DIFR on Ψₛₘ or ET/ PET were low. Differences in reflectance first appeared to increase with increasing moisture stress, and then decreased with further increase in moisture stress, as leaf rolling reduced the RGPC. These findings suggest one reason for different IR reflectance-plant moisture stress results reported in the literature, and clearly show that without accurate measures of LA1 (or RGPC) and IR ↓ plant moisture stress cannot he detected with measurements of IR ↑