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Spatial Orientation of Wheat Leaves¹

A model is presented for characterizing and summarizing leaf spatial orientation in wheat (Triticum aestivum L. em Thell) canopies. The model assumes a random azimuth we found to be valid for wheat. The inputs to the model are based on measurements of the angle to the horizontal of the basal and distal segments of each leaf midrib and a visual classification of the leaf into twist categories based on the number of twists along the leaf. Each leaf is considered to be made up of small, sequential, planar segments, and the relative distribution of area along the length of the midrib must be determined on a few leaves to fit the model to a particular genotype. With these relatively few and simple measurements to "calibrate" each genotype and two measurements of angle on each leaf, the spatial orientation of the entire leaf surface can be determined.

The spatial orientation can be expressed in various ways. The customary cumulative angle distribution for all the leaf tissue can be obtained from the model. Other statistics have proven useful in our studies. The mean basal angle, MBA, (the mean for all leaves of the angle between the plane of the base of the midrib and the horizon) is useful for following changes in time of the angular display of individual leaves or of leaves at a particular insertion level. The planar equivalent angle, PEA, (the angle equivalent to that which all leaves would have if they were rigid planes and set at an angle which would give the same horizontal projection as that found in the canopy) is useful to express the leaf angle of a canopy as a single number.

In addition to the measurements listed above, the relative distribution of area among leaf insertion levels and the leaf area index are required to utilize the orientation model in photosynthetic simulation.

The orientation of leaves of the wheat cultivar ‘Era’ and an upright-leaved mutant of Era were compared in controlled environments and in field plantings. Canopies became increasingly more planophile with time because leaves drooped as they aged. Leaf drooping was characterized by a change in the angle of the base of the leaf blade with the leaf sheath.

Key Words: Crop simulation • Photosynthesis

² Former research assistant (and aspirant, FNRS, Belgium; present address: Laboratoire de'Ecologie Végétale, Université de Louvain, Louvain-la-Neuve, Belgium) and professor (present address: Dep. of Crop Science, Oregon State Univ., Corvallis, OR 97331), respectively.

Received for publication May 5, 1976.