*A*, pooled tension data (with their standard deviations) collected for different P_{i} concentrations at 10 °C (circles, 24 fibres) and at 20 °C (triangles, 13 fibres). Data were collected from each fibre in control solution (no added P_{i}) and in solutions with 3.1, 6.25, 12.5 and 25 mm added P_{i} and normalised to the initial control tension (the control tension after a series was (mean ±s.d.) 0.97 ± 0.06 at 10 °C and 0.95 ± 0.05 at 20 °C). The mean (±s.d.) normalised tension is plotted on the ordinate against [P_{i}] within the fibre on a logarithmic abscissa. For 10 °C, [P_{i}] was taken as 0.735 mm in the control and as 0.735 mm+ added [P_{i}] for others; the corresponding values for 20 °C were 1.17 mm (control) and 1.17 mm+ added [P_{i}] (see Methods). The filled symbols are the normalised tensions obtained with a P_{i} mop (from nine fibres at 10 °C, three of which also contributed data to the full series, and from four fibres at 20 °C, two of which contributed to the full series; they are plotted at fibre generated [P_{i}] - see Methods). The curve through each set of points is the hyperbolic relation fitted to the pooled data. The *P*_{max}, *P*_{min} and *K*_{d} values (mean ±s.e.m.) from the curve fits are 1.06 ± 0.02, 0.34 ± 0.04 and 8.8 ± 1.5 mm for 10 °C and 1.03 ± 0.01, 0.58 ± 0.06 and 14.6 ± 4.4 mm for 20 °C. Data obtained at more than one temperature in a few individual fibres (*n* = 6) also indicated the same trends. The difference in the tension reduction with 3–25 mm added [P_{i}] was significant between the two temperatures (*P* < 0.01). *B*, data similar to *A*, but for 5–7 °C (squares) and for 28–30 °C (diamonds). Curve fits are for pooled data from a total of eight fibres at ≈5 °C (P_{i} mop from four fibres - filled symbol, two of which also contributed to the full series) and from a total of six fibres at ≈30 °C (P_{i} mop from three fibres, all contributing to the full series). The *P*_{max}, *P*_{min} and *K*_{d} values (mean ±s.e.m.) from the curve fits are 1.14 ± 0.04, 0.36 ± 0.07 and 4.2 ± 1.4 mm for ≈5 °C and 1.06 ± 0.02, 0.71 ± 0.04 and 8.3 ± 3.0 mm for ≈30 °C (*P* < 0.001). *C*, predicted tension level (mean ±s.e.m. from curve fit) at excess [P_{i}], i.e. *P*_{min}, at ≈5, 10, 20 and ≈30 °C; dashed line represents the tension level in the control. Note that the maximum P_{i}-induced depression is similar between ≈5 and 10 °C, but is markedly decreased at higher temperatures. Differences are significant (*P* < 0.05) except between 5 and 10 °C and between 20 and 30 °C.

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