Photosynthesis in crop profiles, measured by phytometers.
Using sugar-beet seedlings as phytometers, NAR, LAI and light intensity were measured at various levels in field-grown spring barley, winter wheat, spring wheat, potatoes, sugar-beet and thousand-headed kale (Brassica olerácea var. millecapitata). Phytometer NAR decreased with depth in crop most rapidly in kale and least rapidly in spring-sown barley or wheat. In cereals and kale, leaf area concentration (LAI/10 cm) was greatest near the top of the crop, and small near the ground; in sugar-beet it was greatest near the middle of the crop profile, decreasing towards both the top and bottom, while in potatoes it increased throughout the profile from top to bottom. Much of the variation between crop species in rate of change of phytometer NAR with depth can be explained by differences in the distribution of crop LAI. When phytometer NAR was plotted against crop LAI above the phytometer, the relations were nearly identical for cereals, potato and sugar beet, but phytometer NAR decreased more rapidly with increase in crop LAI in kale than in the other crops, suggesting that unit LAI of kale intercepted more light. The extinction coefficients of light by LAI increased with depth in wheat, but in kale they were greatest near the top of the crop. A linear regression of phytometer NAR 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 NAR. Separate regressions calculated for wheat and kale were not significantly different. In the range of R found in the crop profiles, the values of NAR 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, but not active in photosynthesis; this may explain why the light in crop profiles is less efficient photo-synthetically than daylight.-F.A.S.