Age Effects of Cotton Leaves on Light Reflectance, Transmittance, and Absorptance and on Water Content and Thickness

Cotton plants (Gossypium hirsutum L.) were grown in the growth chamber, greenhouse, and field to study effects of leaf matttration on spectral properties, water content, and thickness of single leaves. In the 500- to 750-nm region and at 1,650 and 2,200 nm, leaf age had a variable effect on reflectance. Over the 750- to 1,350-nm wavelength interval (WLI), leaf reflectance was a maximum at node 8, and gradually decreased from node 8 to node 13. At the 1000-nm wavelength, greenhouse- and field-grown leaves had reflectances of approximately 51.0% compared with 46.0% for growth chamber-grown leaves. For all environments~ middle-aged leaves had highest reflectances. Field- and growth chamber-grown leaves of all nodes except 3 from the apex had higher absorptance at the 550-nm wavelength than greenhouse-grown leaves. Middle-aged leaves from all three environments had maximum absorptance. Greenhouse-grown leaves had approximatdy two to four times higher absorptance at the 1000-nm wavelength than field- and growth chamber-grown leaves. For greenhouse leaves, absorptance was least at node 8. Leaf age had little influence on absorptance of field- and growth chamber-grown leaves. Growth chamber-grown leaves had the highest and fieldgrown leaves the lowest water content. In the growth chamber and field, leaf water content increased with leaf age. Leaf thickness generally increased with leaf maturation in all three environments, but growth chamber-grown leaves were thinnest. Because growth chamber-grown plants developed shadetype leaves, measurements on these leaves differed markdly from measurements made on leaves of field- and greenhouse-grown plants. Thus growth chamber results should not be extrapolated to field conditions, unless it is known that the environmental conditions in the growth chamber closely simulate field conditions.