Changes in photosynthesis and other chloroplast traits in lanceolate leaflet isoline of soybean.

Lanceolate leaflet soybean (Glycine max L. Merrill) has been known to photosynthesize more CO(2) per unit leaf area than normal leaflet soybean. The exact reason for this increase in photosynthetic rate is still unclear. The present study was undertaken to investigate the leaf photosynthetic rate and other physiological traits in relation to chloroplast of lanceolate leaflet soybean. Ontogenic changes in apparent photosynthesis (AP) were related primarily to variations in the amount of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit leaf area, and only secondarily to difference in specific activity of the said enzyme. Moreover, lanceolate leaflet consistently maintained a higher leaf AP, higher Rubisco activity, and more chloroplasts per unit leaf area basis than did normal leaflet soybean throughout leaf ontogeny. However, lanceolate soybean tended to have lower AP and Rubisco activity on a chloroplast basis. The superiority of leaf AP and other leaf physiological traits, expressed on a leaf area basis, in lanceolate leaflet soybean is associated with a corresponding increase in chloroplast number.

Relative sensitivity of photosynthetic assimilation and translocation of carbon to water stress.

The relationship between photosynthesis and translocation rate changes as affected by water stress intensity and stage of plant development was evaluated in cotton and sorghum, representing a C(3) and a C(4) photosynthetic type, respectively. Photosynthetic rates were reduced as midday leaf water potentials declined from -14 to -27 bars in both species. Sorghum maintained higher photosynthesis and translocation rates compared to cotton at comparable leaf water potentials; however, the rate of change per bar decline in water potential was greater in sorghum than in cotton. Photosynthetic rates were reduced with increasing water stress prior to any significant change in translocation rates suggesting that photosynthesis is the more sensitive of the two processes. Severe water stress, corresponding to leaf water potentials of -27 bars, did not completely inhibit either photosynthesis or translocation.