Seedlings of Fiskeby V soybean (Glycine max (L.) Merr.) were grown under a controlled environment from germination to cotyledon abscission. From shortly after emergence, attached cotyledon photosynthetic and transpiration rates were assessed within a miniature glass cuvette. The approaches taken were varying the ambient CO2 concentration, withholding external nutrient supplies and source/sink manipulations. There were overall losses in dry matter from the seedlings up until the peak in cotyledon photosynthetic activity on day ten. Photosynthetic and transpiration rates were less than reported values for leaves and were associated with low stomatal and residual conductances. The decline in photosynthetic activity up until these organs abscissed was primarily due to decreasing mesophyll conductance rather than impaired stomatal function. The degree of regulation of carbon fixation and water loss was assessed by measurement of the gains of the feedback loops involving photosynthesis, stomatal conductance and the internal CO2 concentration (Ci). The gain of the assimilation loop was greater than that of the conductance loop. It was concluded that in a seedling well supplied with water, the cotyledons would gain the maximum amount of dry matter possible at the expense of water economy. The omission of selected elements from the nutrient media supplied from day seven produced varying effects on cotyledon photosynthetic activity. Plants without an external supply NO3 of had increased export of dry matter from the cotyledons with a reduction in the photosynthetic rate. However, increased control of Ci by the conductance loop produced a significant increase in water use efficiency. Plants without an external supply of K had impaired cotyledon stomatal function, but remained photosynthetically active longer. The distribution of radioactivity after photoassimilation of 14CO2 by the cotyledons indicated preferential export to the root sink. Steam girdling either the epicotyls, hypocotyls or both, significantly altered the translocation pattern, but did not in the short term affect cotyledon photosynthetic activity.
|Date of Award||1984|