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Photosynthesis in Crop Profiles, Measured by Phytometers

G. J. Leach and D. J. Watson
Journal of Applied Ecology
Vol. 5, No. 2 (Aug., 1968), pp. 381-408
DOI: 10.2307/2401568
Stable URL: http://www.jstor.org/stable/2401568
Page Count: 28
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Photosynthesis in Crop Profiles, Measured by Phytometers
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Abstract

The net assimilation rates (E) of phytometers were determined at different positions in field crops of barley, winter and spring wheat, potato, sugar beet and kale, to study how the microclimate affecting photosynthesis changes with depth in crop profiles. The phytometers consisted of sugar-beet seedlings grown in the glasshouse in solution culture, selected for uniformity, and transferred to the crops for 1 week. The distributions of leaf area index (L) in horizontal layers of the crops, defined by the positions of the phytometers, were also determined, and the light intensity incident on the phytometers was measured in wheat and kale. Phytometer E decreased with depth in the crop much more rapidly in kale than in spring-sown barley or wheat, and at intermediate rates in the other crops. In cereals and kale, leaf area concentration (L per 10 cm) was greatest near the top of the crop, and small near the ground; in sugar beet it was largest near the middle of the profile, decreasing towards both the top and bottom; in potatoes it increased throughout the profile from top to bottom. Much of the variation between crop species in the rate of change of phytometer E with depth can be explained by differences in the distribution of crop L. When phytometer E was plotted against crop L above the phytometer, the relations were nearly identical for cereals, potato and sugar beet, but phytometer E decreased more rapidly with increase in crop L in kale than in the other crops, suggesting that unit L of kale intercepted more light. The extinction coefficients of light by leaf area index were not constant throughout the crop profiles. In wheat, they increased with depth, but in kale they were largest near the top of the crop. In the upper part of the crop profiles, where there was most leaf area, the extinction coefficients were larger in kale than in wheat. A linear regression of phytometer E on mean daily visible radiation (R) received by the phytometers at all depths in wheat and kale crops accounted for nearly 90% of the variance of E. Separate regressions calculated for wheat and kale were not significantly different. In the range of R found in the crop profiles, the values of E were less than those of phytometers grown under neutral shades receiving R of equal energy. Light penetrating into crop profiles presumably includes a proportion transmitted through leaves, not absorbed by the chloroplast pigments and hence not active in photosynthesis, that increases with depth, and this may explain why the light in crop profiles is less efficient photosynthetically than daylight.

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