Further characterization of ribosome binding to thylakoid membranes.

Previous work indicated more polysomes bound to pea (Pisum sativum cv Progress No. 9) thylakoids in light than in the dark, in vivo (LE Fish, AT Jagendorf 1982 Plant Physiol 69: 814-825). With isolated intact chloroplasts incubated in darkness, addition of MgATP had no effect but 24 to 74% more RNA was thylakoid-bound at pH 8.3 than at pH 7. Thus, the major effect of light on ribosome-binding in vivo may be due to higher stroma pH. In isolated pea chloroplasts, initiation inhibitors (pactamycin and kanamycin) decreased the extent of RNA binding, and elongation inhibitors (lincomycin and streptomycin) increased it. Thus, cycling of ribosomes is controlled by translation, initiation, and termination. Bound RNA accounted for 19 to 24% of the total chloroplast RNA and the incorporation of [(3)H]leucine into thylakoids was proportional to the amount of this bound RNA. These data support the concept that stroma ribosomes are recruited into thylakoid polysomes, which are active in synthesizing thylakoid proteins.

The relationship between the activity and the activation state of RuBP carboxylase and carbon exchange rate as affected by sink and developmental changes.

The purpose of this study was to explore if sink manipulations which affect leaf carbon exchange rate (CER) are mediated by ribulose 1,5-bisphosphate (RuBP) carboxylase activity. Tomato leaf (Lycopersicon esculentum Mill. cv. Vendor) RuBP carboxylase was assayed using a rapid extraction method. Over a diurnal period, leaf CER fluctuated independent of carboxylase activity. Differences in leaf CER induced by fruit pruning in one leaf-one cluster plants were not accompanied by changes in carboxylase activity.During leaf expansion, carboxylase activity and percent enzyme in the active form paralleled the increase and then decrease in leaf carbon exchange rate. Differences in leaf CER induced by root warming at ambient air temperature, were accompanied by parallel changes in carboxylase activity.These results suggest that modifications in leaf CER are not mediated exclusively through changes in carboxylase activity, but rather that modifications in carboxylase activity coincide with overall changes in leaf physiology and morphology in response to sink demand.

Effect of altering the root-zone temperature on growth, translocation, carbon exchange rate, and leaf starch accumulation in the tomato.

Tomato seedlings (Lycopersicon esculentum Mill. cv Vendor) were grown hydroponically with their root systems maintained at a constant temperature for a 2-week period commencing with the appearance of the first true leaf. Based on fresh and dry weight and leaf area, the optimal root-zone temperature for seedling growth was 30 degrees C. The carbon exchange rate of the leaves was also found to increase with rising root-zone temperature up to 30 degrees C. However, a more complex relationship seems to exist between root-zone temperature and the accumulation of (14)C-labeled assimilates in the roots; inasmuch as there is no enhancement in this accumulation at the most growth promoting root-zone temperatures (22-30 degrees C).