Alfalfa was grown hydroponicalHy in 0, 0.6, and 4.8 millimolar K in order to determine the influence of tissue level of K on photosynthesis, dark respiration, photorespiration, stomatal and mesophyll resistance to CO2, photosystem I and II activity, and synthesis and activity of ribulose 1,5-bisphosphate carboxylase (RuBPc).A severe (0.0 millimolar) and mild (0.6 millimolar) K deficiency, compared to plants grown at 4.8 millimolar K, produced a significant decrease in photosynthesis and photorespiration, but an increase in dark respiration.Both deficient K levels increased hydrophyllic resistance to C02, but only the severe deficiency increased stomatal resistance. Photosystem I and II activity of isolated chloroplasts was not affected by K deficiency. The apparent activity of a crude RuBPc preparation was significantly reduced in severely deficient plants. Activity of the enzyme could not be restored to normal rates by the addition of K to the reaction medium.The specific activity of RuBPc isolated from severely K-deficient and K- K deficiency has been shown to reduce the rate of photosynthesis in numerous higher plants (2,10,12,13,18,19). Cooper et al. (3) attributed the decrease in photosynthesis of K-deficient alfalfa to a decrease in stomatal number and aperture size. Reduction in photosynthesis of K-deficient corn was attributed to increased stomatal resistance (4, 9). A combination of increased stomatal and mesophyll resistance to CO2 was thought to reduce photosynthesis in K-deficient sugarbeets (20, 21). Since K is the major solute in turgid guard cells (6), it is reasonable to suggest that K deficiency will result in stomatal closure. In the presence of Na, low K increased stomatal resistance less, but mesophyll resistance remained high (21). These results suggest that the role of K in regulating photosynthesis may be within mesophyll cells. (NO3)2 was 6.5, 6.2, and 4.6 and that of CaCl2 was 0, 0.3, and 2.4 mm for the 0, 0.6, and 4.8 mm K solutions, respectively. One plant per crock was established and the crocks were arranged in a completely randomized block design with eight replications. Crocks were subirrigated every 2 h during the day and every 4 h at night. Nutrient solutions were changed every 2 weeks. The temperature in the greenhouse was 25/20 ± 5 C day/night, and the average daily solar radiation was approximately 310 cal cm-2 day-'. Plants were cut back to a 5-cm stubble when they reached the 'ho bloom stage and had attained approximately 20 days of regrowth (early bud stage) at the time of measurement.Gas Exchange. Photosynthesis, transpiration, dark respiration, and stomatal and mesophyll resistance to CO2 diffusion were measured on 20 individual attached leaves (fourth fully expanded leaf from the top) per treatment using an air-sealed leaf chamber (26)

Field experiments with Upland cotton (Gossypium hirsutum L.) were initiated to determine the effect of boll removal on the distribution of 14C‐assimilates 24 hours following removal. The mainstem leaf subtending the sympodium supplied the first and second boll positions with assimilates. In the absence of bolls, assimilates in the mainstem leaf moved basipetally from the leaf subtending the first position. The majority of assimilates were translocated to the first position boll. Removal of the first position boll resulted in assimilate movement to the second position parts of the plant when the second position was 10 days old. The majority of the assimilates from the second position leaf, being further out on the sympodium, remained in the sympodium, as long as at least one fruiting body was present. Removal of all the bolls from the sympodium resulted in translocation primarily in a basipetal direction.

The photosynthetic responses of four alfalfa (Medicago sadva L.) cultivars to 10 and 22 C air temperatures were examined and the relationship between the photosynthetic response at 10 C and the fatty acid composition of the chloroplast membranes was determined. Chilling-resistant cultivars exhibited moderate reductions in photosynthesis at 10 C, compared to 22 C, and contained a significantly greater percentage of polyunsaturated fatty acids in the chloroplast membrane and a greater double bond index than the chilling-sensitive cultivars. The chiiUng-sensitive cultivars exhibited severe reductions in photosyntbesis at 10 C, compared to 22 C. The reduction in pbotosynthesis at 10 C is shown to be negatively correlated (r --0.94) with the double bond index of the chloroplast membranes of the cultivars observed.The results support the hypothesis that reduced photosynthesis due to chiling temperatures is influenced by the unsaturated fatty acid composition of the chloroplast membrane which affect temperature-induced phase changes in chloroplast membrane lipids.The inability of chilling-sensitive plants to maintain physiological activity at chilling temperatures has been correlated with a marked increase in the activation energy of respiratory enzymes (5, 10) and a marked increase in the activation energy of photoreduction of NADP+ (7, 1 1). The observed increase in activation energy of these systems is related to temperature-induced phase changes in the lipids of the respective membranes (9-11). This phase change has been detected by a change in the mobility of spin labels and occurs at precisely the same temperature as the change in activation energy (8,9). No change in activation energy was detected in respiratory enzymes (5, 10) or NADP+ photoreduction (8, 11) of chilling-resistant plants, nor was a phase change in the lipids detected by spin labeling (8,9 Chloroplast Extraction. Chloroplasts were isolated from depetiolated leaves of each plant, three replications, in extraction buffer (0.4 M sucrose, 0.05 M Tricine, 0.01 M NaCl, pH 7.8) (1). The leaves were homogenized in a VirTis "23" homogenizer for 10 sec at maximum speed and 30 sec at 0.5-speed in 40 ml of extraction buffer. The suspension was filtered through four layers of cheesecloth and the filtrate centrifuged for 90 sec at 500g. The pellet was discarded and the supernatant recentrifuged for 7 min at 1,000g. The resulting pellet was resuspended in 20 ml of extraction buffer, centrifuged for 90 sec at 500g, and the pellet discarded. The supernatant was recentrifuged for 10 min at 2,000g and the resulting pellet dissolved in 25 ml of methanol and 5 ml of methylene chloride. This solution was directly analyzed for fatty acid composition. The chloroplast pellet was found to contain 90%o intact chloroplasts, as judged by light microscopy.Chloroplast Fatty Acid Assay. Methyl esters were prepared by direct micromethanolysis using boron trichloride gas (3). Qualitative and quantitative analyses were performed according to established gas-liquid chromato...

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Most studies involving photosynthetic efficiency have compared species or cultivars that were grown under fixed day/night temperature regimes. Researchers attempting to improve photosynthetic efficiency and productivity of a crop may also need to consider genotype response to adverse factors, such as temporary chilling, which have been shown to reduce subsequent photosynthetic rates.Three alfalfa (Medicago sativa L.) cultlvars, representing a range of environmental adaptation, were grown in a greenhouse at 22/18 C (day/night), and then were exposed for one night to 5 or 18 C to determine the effect of low temperature on physiological processes the following morning. The CO2 exchange rate (CER) of all three cultivars was reduced following a 5 C night. A greater reduction in CER occurred in ‘Mesa‐Sirsa’ and ‘Saranac’ (69.1 and 60.2% less than 18 C, respectively) than in ‘Kane’ (36.7% less than 18 C). Transpiration rates were significantly reduced following chilling exposure of Mesa‐Sirsa only (75.5% less than 18 C). This reduction in transpiration was associated with a 50% increase in leaf diffusive resistance and a 4.3 bar decrease in xylem water p6tential. Increased leaf diffusive resistance was not observed in Saranac and Kane following a chilling night. A greater leaf carbohydrate concentration and reduced l4C‐photosynthate translocation from leaf tissue was noted in all cultivars following a 5 C night.These data suggest a feedback inhibition of CER in alfalfa following one 5 C chilling night. A water stress inducing an increased leaf diffusion resistance and decreased transpiration was found in Mesa‐Sirsa only.