Spectral reflectance measurements were taken from immature and mature leaves of `Rio Red' grapefruit `McCarty' grapefruit `Minneola' tangelo, `Satsuma' mandarin, `Dancy' tangerine, `Nagami' oval kumquat, and `Valencia' orange from 1330 to 1530 hr 1 day at the Florida Citrus Arboretum, Division of Plant Industry, Winter Haven, Fla. A PS-1000 spectrometer was used with fiber optic cables and a lens source (visible range of the spectrum 400-800 sun) coupled with a tungsten halogen light source. A data acquisition card was connected to a notebook computer with a SpectraScope computer program for processing and data storage. Immature and mature leaves of `Minneola' tangelo had greater percentage reflectance in the 500-800 sun range than the other cultivars and leaf ages measured. More detailed information was obtained with the PS-1000 than with conventional spectrometers. The slope of the citrus spectral curves in the 800 nm range was not as sharp as conventional spectrometers, but had a much higher reflectance value than those obtained with a different spectrometer. The system used here was convenient to transport and use in the field and produced clear, interpretable data.
C.H. Blazquez, H.N. Nigg, L.E. Hedley, L.E. Ramos and S.E. Simpson
Kenneth A. Shackel, H. Ahmadi, W. Biasi, R. Buchner, D. Goldhamer, S. Gurusinghe, J. Hasey, D. Kester, B. Krueger, B. Lampinen, G. McGourty, W. Micke, E. Mitcham, B. Olson, K. Pelletrau, H. Philips, D. Ramos, L. Schwankl, S. Sibbett, R. Snyder, S. Southwick, M. Stevenson, M. Thorpe, S. Weinbaum and J. Yeager
To be useful for indicating plant water needs, any measure of plant stress should be closely related to some of the known short- and medium-term plant stress responses, such as stomatal closure and reduced rates of expansive growth. Midday stem water potential has proven to be a useful index of stress in a number of fruit tree species. Day-to-day fluctuations in stem water potential under well-irrigated conditions are well correlated with midday vapor-pressure deficit, and, hence, a nonstressed baseline can be predicted. Measuring stem water potential helped explain the results of a 3-year deficit irrigation study in mature prunes, which showed that deficit irrigation could have either positive or negative impacts on tree productivity, depending on soil conditions. Mild to moderate water stress was economically beneficial. In almond, stem water potential was closely related to overall tree growth as measured by increases in trunk cross-sectional area. In cherry, stem water potential was correlated with leaf stomatal conductance and rates of shoot growth, with shoot growth essentially stopping once stem water potential dropped to between −1.5 to −1.7 MPa. In pear, fruit size and other fruit quality attributes (soluble solids, color) were all closely associated with stem water potential. In many of these field studies, systematic tree-to-tree differences in water status were large enough to obscure irrigation treatment effects. Hence, in the absence of a plant-based measure of water stress, it may be difficult to determine whether the lack of an irrigation treatment effect indicates the lack of a physiological response to plant water status, or rather is due to treatment ineffectiveness in influencing plant water status. These data indicate that stem water potential can be used to quantify stress reliably and guide irrigation decisions on a site-specific basis.