Coupling of Proton Fluxes in the Polar Leaves of Potamogeton lucens L

An attempt has been made to quantify the light-induced H⁺ effluxand influx observed in polar leaves of Potamogeton lucens.Theseproton fluxes are spatially separated. The H⁺ efflux, mediatedby a plasmalemma bound H⁺ –ATPase, occurs across theplasmamembrane at the morphological lower epidermis and is accompaniedby an H⁺ influx (or OH^– efflux) at the upper side oftheleaf. As a result, these leaves exhibit a remarkable pH–polarityin the light. The pH near the lower epidermis may drop to avalueas low as 3.5, while a pH of about 10.5 can be observed at theupper epidermis. Obviously this phenomenon requires theco–ordinationof transport processes in the different cell layers of the leaftissue. These observations led to quantitative studies oftherelation between the H⁺ fluxes at either plasmalemma. Thesefluxes were calculated from the pH values recorded at twodistancesfrom the leaf surface. Although the H⁺ influx always exceededthe efflux, a coupling between the transport processesacrosseither plasma membrane became evident from the time–coursesof the two fluxes. Key words: Potamogeton lucens, proton flux, flux coupling, pH–;polarity     

Photosynthesis and Respiration of Maize Seedlings at Suboptimal Temperatures

Growth of maize seedlings is limited by low temperature andceases at about 10 °C. In a search for processes underlyingthis limitation, rates of photosynthesis and dark respirationwere measured in controlled environments at night/day temperaturesof 5/10, 10/15, and 15/20 °C. CO₂ exchange was measuredon whole seedlings including the root system. Two maize hybrids,which differed in dry matter production at low temperature,were tested from the 4th-leaf until the 8th-leaf stage. Grossphotosynthesis, net photosynthesis, and respiration increasedproportionally with leaf area. The response to low temperaturewas independent of seedling stage. The decline of photosynthesisand respiration with decreasing temperature was similar in bothhybrids. At 10 °C, rates of photosynthesis and respirationwere relatively high indicating a minimum temperature for thoseprocesses much lower than 10 °C. Possible reasons for growthlimitation at 10 °C are discussed.