Higher plants obtain the majority of their sulphur in the form of sulphate which is
taken up directly from the soil. In industrialised areas, this uptake can be
supplemented by the absorption of sulphur-containing pollutants from the air. Due to increasing environmental concerns, levels of atmospheric sulphurous pollutants have fallen dramatically in the last 30 years. This (in conjunction with increased use of sulphur-free straight and compound fertilisers), has led to the appearance of sulphur deficiency symptoms in modern-day crops. The reductions in yield associated with these sulphur deficient crops represents a threat to U.K. agriculture.
Sulphur deficiency symptoms in higher plants appear first in the lower leaves.
These observations have been linked to the slow re-mobilization of sulphate from mature leaf tissue. Radio-tracer studies using 35S-sulphate have indicated that mature leaves contain two populations of kinetically distinguishable cell each of which exchanges sulphate at a different rate.
Using single cell sampling and analysis techniques, the aim of the experiments
described in this thesis was to investigate the behaviour of sulphur when induced to accumulate in and then re-mobilize from mature wheat leaves. This aim was
addressed by inducing plants to accumulate excess sulphur (termed high S plants) which was achieved by both providing extra sulphate and restricting the supply of monovalent anions. Accumulation of sulphur in ageing leaves coincided with decreases in turgor and osmotic pressure and diminished efflux of potassium and influx of calcium.
The withdrawal of sulphate and re-instatement of nitrate supplies to high S
material induced re-mobilization of epidermal and bundle sheath sulphur reserves. There was negligible re-mobilization of accumulated sulphur from mesophyll cells. It was concluded that much of the sulphur amassed by the leaf must effectively be 'immobile' and thence unavailable for re-translocation irrespective of the sulphur-status of the plant.