Photosynthesis of Sugarbeets under N and P Stress: Field Measurements and Carbon Balance

Advances in crop management are limited by our inability to recognize changes in growth resulting from short-term fluctuations in plant environment. Simple, rapid, and nondestructive methods are needed to indicate daily and hourly rates of growth under field conditions. Because plant carbon balance is one possible approach to this problem, CO? gas exchange parameters were measured on Beta vulgaris leaves of field-grown plants to learn whether or not they might signal the onset of nitrogen and P stresses. The sugarbeets were grown on field plots of Portneuf silt loam (Xerollic Calciothid) soil and allowed to develop severe N and P stresses. Carbon dioxide exchange and stomatal resistance of intact plant leaves were measured with small leaf chambers during 1-min periods. Leaf water potentials were also measured in the field with a hydraulic press. The CO? compensation points, dark respiration, and osmotic pressures of excised leaf tissue were measured in the laboratory by standard methods. Photosynthesis per unit leaf area was reduced in advanced stages of N deficiency; however, none of the measurements satisfactorily indicated the onset of N or P stress because of heterogeneity between leaves. Calculations using the daily carbon balance of an average plant predicted the relative effects on root yield of 1) photosynthetic rate per unit leaf area, 2) photorespiration, 3) stomatal resistance to gas diffusion, 4) mesophyll resistance to carbon dioxide transfer, 5) size of the seedling tops in the spring, 6) partitioning of photosynthate between tops and roots near the end of the season, and 7) date of full plant cover. While all these factors are important, advances that can be made by manipulating most of them are limited. Increasing photosynthesis by reducing mesophyll resistance appears to offer the greatest potential for large yield increases