Influence of magnesium deficiency on rates of leaf expansion, starch and sucrose accumulation, and net assimilation in Phaseolus vulgaris

Twenty one‐day‐old Phaseolus vulgaris‘Saxa’plants were cultured in a growth chamber and the plants supplied with either a complete or a Mg‐free nutrient solution. From 6 days after transfer to the Mg‐free solution, the rate of increase of the area of the second trifoliate leaf was considerably reduced; by day 11 the sucrose concentration in the first trifoliate leaf had increased 6. 2‐fold at the end of the dark period and 4. 6‐fold after the light period as compared with the control plants. Corresponding starch concentrations increased 6. 6‐fold and 2. 9‐fold respectively. After days 5 to 6 the assimilation rates declined in the first trifoliate leaf of the plants showing deficiency, in comparison with the plants fully supplied with nutrients; respiration increased during darkness. The reduction in net assimilation rate was to a great extent reversible after resupply of magnesium. The reduction of magnesium concentration in the deficient plants was much more marked in the expanding leaves than in the mature primary leaves and roots. Sucrose and starch accumulation did not occur when the first trifoliate leaf was partially shaded, although magnesium concentration, as in the unshaded leaves, was reduced to 13% of that of the control plants. The consequences of magnesium deficiency in the expanding first trifoliate leaf are discussed in terms of the possibility of sink limitation.