The history of net photosynthesis and transpiration per unit leaf area was determined for intact soybean leaves from their unfolding to senescence during flowering and pod filling on untreated (podded) and partially depodded plants growing in a glasshouse. Leaf diffusive resistances to CO2 were calculated and a water use efficiency parameter was derived (net mass of carbon dioxide fixed per unit mass of water transpired per millibar vapour pressure deficit). Net photosynthesis and transpiration behaved similarly through all stages of leaf development. A number of peaks were evident in these parameters. The first was associated with leaf expansion and occurred when the leaf reached its maximum area. The second peak coincided with flowering of the plant and later peaks occurred during pod filling. Stomatal and mesophyll resistances also exhibited similar behaviour during the life of the leaf; the possible causes of this linkage are discussed. Water use efficiency increased rapidly up to the time of full lamina expansion. Thereafter, it rose slowly or remained stable until leaf senescence approached, when the efficiency declined. Net photosynthesis and transpiration of leaves were very similar in both podded and partially depodded plants. It appears that to prevent a shortage of assimilate during flowering and pod filling, photosynthesis may be maintained or increased in some leaves and the response is not related to the number of pods available for filling. The increases in photosynthesis were correlated with both higher stomatal and mesophyll conductances. Mechanisms by which the plant may control leaf photosynthesis are discussed.
