Modelling the Dynamical Components of the Sugar Beet Crop

Many models have been constructed to describe the growth ofthe sugar beet crop up to harvesting. In general, these modelshave a complex physiological basis, requiring a large numberof parameters yet relying on empirical functions with no mechanisticbasis to partition assimilates within the crop. An importantfactor in considering the growth of the crop, both from an economicand environmental point of view, is the response of the cropto varying amounts of available nitrogen in the soil. In thispaper, a model is described for crop growth using soil nitrogencontent and solar radiation as driving functions. The parsimoniousapproach to construction resulted in a 14 parameter model, sevenof which are associated with the driving variables. This issubstantially fewer than for other crop models. The model containsa new dynamical way of describing partitioning of assimilatesbetween shoot, storage root and fibrous roots. The partitioningmodel is derived from observations on the effect of soil nitrogenon crop growth. Interception of light is determined by foliagecover, which makes the model suitable for use with data collectedfrom satellite imaging. The model fits well to three independentdata sets with estimated parameters lying within biologicallyreasonable bounds. The model is used to test the sensitivityof yield to changes in soil nitrogen. Modelling; partitioning; parameter estimation; sugar beet; Beta vulgarisL.; nitrogen; crop growth dynamics     

The invertebrates of heather and heathland

WEBB, N. R., 1989. The invertebrates of heather and heathland. The southern heathlands of Great Britain have long been renowned amongst entomologists. The invertebrates fall into two groups: species which feed on heathland plants–about 40 in the case of Calluna–and a second group which depends on the particular physical conditions provided by heathland, such as hot dry sandy areas or the micro-topography of Sphagnum bogs. The ecology of representative examples will be described, and the effects of fragmentation and isolation of the heathlands on the distribution and abundance of invertebrates discussed.     

Oxygen Diffusion Pathways and Nitrogen Fixation in Legume Root Nodules

The theoretical analysis presented in this paper suggests thatthe nature of the diffusion pathway from the surface of a noduleto the infected zone depends on the morphology of the nodule;in particular the cross-sectional area of the intercellularspaces in the inner cortex. If the diffusion barrier containscontinuous pathways there will be no pressure difference betweenthe atmosphere and the infected zone. The conditions under whichthe intercellular spaces of the inner cortex could be air-filled,water-filled, or a combination of both are explored. An experimentto resolve this issue is suggested. Information obtained usingcryo-scanning electron microscopy and oxygen electrode datahave been used to illustrate various points. Expressions arederived for the diffusion resistance of the nodule and its componentparts. To provoke further discussion a simple mechanism forthe control of diffusion is described in general terms. Oxygen, diffusion resistance, nodule, nitrogen fixation, soybean, Glycine max. (L.) Merr. cv. Fiskeby V, Trifolium repens L. cv. Blanca, lucerne, Medicago saliva L. cv. Europe