COUPLING OF TRANSPIRATION TO THE ATMOSPHERE IN HORTICULTURAL CROPS: THE OMEGA FACTOR

The concept of coupling between leaves and atmosphere is described in terms of the water vapour saturation deficit. Leaves are considered to be well-coupled to the atmosphere if the atmospheric saturation deficit is imposed on the leaf surface without local adjustment: they are poorly-coupled to the atmosphere if the surface saturation deficit finds its own value by local equilibration. Transpiration is considered to be made up of an imposed component, driven by the atmospheric saturation deficit and regulated by stomatal conductance, and an equilibrium component driven by the receipt of net radiation. The balance between these two components depends on the degree of coupling of leaves to atmosphere and is expressed by a dimensionless decoupling coefficient, the "omega factor" (range 0 to 1.0): the sensitivity of transpiration to a fractional change in stomatal conductance depends on the size of the omega factor. An analogous set of concepts is developed for transpiration from crop canopies and applied to horticultural crops.

Many horticultural crops are very poorly-coupled to the atmosphere (large omega close to unity) because they are not very tall, or they are surrounded by shelter, or they are in glasshouses. As a result, transpiration from the crops is likely to depend strongly on radiation receipt and to be rather insensitive to changes in stomatal conductance. Well-exposed, quite tall crops, like most orchard crops, are generally well-coupled to the atmosphere (small omega close to zero), with transpiration proceeding largely at the imposed rate. Only in these crops is transpiration likely to respond sensitively to small changes in stomatal conductance.

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