The growth and inorganic ion composition of K-sufficient lettuce shoots were measured after withholding K or K and Na from transplanting in sand culture experiments in the glasshouse. Initial reductions in shoot growth occurred shortly after transplanting in both K-deficient treatments when the K concentrations were still well above that known to limit enzymic activity. These early growth effects coincided with a rapid decline in total ionic strength within the plants caused by the fall in K concentration which could not be compensated for by increases in the uptake of Na, Mg or Ca, even when adequate concentrations of all of these ions were available. Further rapid reductions in growth rate occurred when the concentration of K declined below about 600 mmol kg-' in both K-deficient treatments. These results support the hypothesis of a dual mechanism for the effect of K concentration on growth during K deficiency. The primary effect was caused by a rapid decline in ionic concentration shortly after the K became unavailable which severely affected the mechanisms controlling turgor pressure within the plant. The secondary effect was caused by a reduction in enzymic activity and only became important when the K status of the plant was relatively low.

Rapid tests have been developed for estimating concentrations of P and K in plant sap in the field or glasshouse. The tests are based on the Hach Orthophosphate Method (for simple colorimetric determinations in solution) and the Merckoquant K TestStrips (for spot testing on paper). Measurements of phosphate concentration in sap samples (containing 0 to 160 μg ml -1 P) by the new method were in good agreement with those by a laboratory spectrophotometric procedure, but the method can be readily adapted to estimate sap concentrations of up to 326 μg ml -1 P. Measurements with K test strips on sap from K-deficient plants were in broad agreement with corresponding flame photometric determinations over the range 1000 to 2000 μg ml -1 K, but deviated considerably at lower concentrations. These errors were apparently caused by increases in the concentration of brown pigments and of putrescine and agmatine in the sap of more highly K-deficient plants which interfered with the test strip measurements. Sap samples with K concentrations greater than 2000 μg ml -1 K were also determined satisfactorily after dilution. It was concluded that both of the new methods performed satisfactorily over concentration ranges 1463 1464 BURNS AND HOTSBY likely to correspond to subclinical deficiencies of P or K for most species and their use could streamline rapid sap testing procedures in the field. INTRODUCTIONIn recent years there have been marked increases in the use of rapid tests both for medical diagnoses of various clinical conditions and for monitoring the chemical composition of industrial samples. The popularity of these tests originates largely from their greater convenience and the need for less experienced operators compared with conventional methods of measurement. They allow large numbers of samples to be screened relatively easily, often with only minor losses of accuracy. Similar rapid tes;s have been developed for measuring nitrate concentrations in plant sap and have proved particularly useful for monitoring changes in crop N status during growth and for detecting the onset of N deficiency 1 » 2 .

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Glasshouse experiments were carried out to examine the way in which potassium was redistributed within lettuce plants during development and alleviation of K deficiency, with a view to providing a more objective method of selecting the best leaf for sap analysis. The changes in distribution of K between leaves were measured by analysis of samples of petiole sap and were compared with the corresponding changes in overall K status of the plants determined by analysis of the total shoot dry matter. Young expanding leaves were most sensitive to alterations in external K supply: their sap concentrations changed more quickly than in any of the older leaves, but because of the inherently lower concentrations in these immature leaves, the overall size of these changes was ultimately less than in fully developed ones. Statistical analyses also showed that measurements of K in the sap from young expanding leaves were more precise than from mature ones. Relative changes in K concentration in the sap of these expanding leaves also occurred initially more quickly than in the total shoot dry matter, but measurements on the latter were considerably more precise. It was concluded that for sap K analysis the best results were obtained using an immature leaf. K e y w o r d s

Changes in the concentration of Na in the shoot dry matter and in the petiole sap of individual leaves of lettuce were measured in order to determine the extent to which Na was able to replace K within the plants during K deficiency. The results were used to estimate changes in the distribution of the total ionic concentration between leaves and to make deductions about likely variations in osmotic potential within the shoots and their effect on plant growth. Measurements of Na concentration revealed both a pronounced delay in the uptake of additional amounts of Na and in its translocation within the K-deficient plants. Sodium concentrations increased most rapidly in the young expanding leaves during the onset of K deficiency but the changes in its concentration were ultimately larger in the middle to older ones. Elevated Na concentrations in the younger leaves of K-deficient plants also declined more rapidly than those in any of the other leaves following reintroduction of the K supply. Although the pattern of these changes broadly mirrored the corresponding changes in K concentration within the shoots there were significant reductions in the combined K+Na concentrations during the onset and development of K deficiency. The deficit in K+Na was initially greatest in the young leaves but subsequently disappeared more quickly when either the uptake of Na was increased or the K supply was resumed. Since the combined concentrations of K+Na were directly related to the changes in total ionic concentrations within the shoots, the results suggest that during K deficiency, plants adopt a strategy which reduces the effects of a decline in ionic strength at the most actively growing sites in order to minimise the damage to normal growth processes from changes in the internal osmotic pressure.