Net Photosynthetic Assimilation Research Articles

Phosphorus concentrations in the leaves of defoliated white clover affect abscisic acid formation and transpiration in drying soil

Increased leaf phosphorus (P) concentration improved the water-use efficiency (WUE) and drought tolerance of regularly defoliated white clover plants by decreasing the rate of daily transpiration per unit leaf area in dry soil. Night transpiration was around 17% of the total daily transpiration. The improved control of transpiration in the high-P plants was associated with an increased individual leaf area and WUE that apparently resulted from net photosynthetic assimilation rate being reduced less than the reductions in the transpiration (27% vs 58%). On the other hand, greater transpiration from low-P plants was associated with poor stomatal control of transpirational loss of water, less ABA in the leaves when exposed to dry soil, and thicker and smaller leaf size compared with high-P leaves. The leaf P concentration was positively related with leaf ABA, and negatively with transpiration rates, under dry conditions (P < 0.001). However, leaf ABA was not closely related to the transpiration rate, suggesting that leaf P concentration has a greater influence than ABA on the transpiration rates.

Hormonal and Physiological Responses of Gerbera jamesonii to Flooding Stress

The effects of flooding (3 days) and recovery (3 days) on leaf gas exchange parameters (stomatal conductance, net photosynthetic assimilation rate, and leaf transpiration rate) were studied in five, 1-year-old gerbera (Gerbera jamesonii Bolus) cultivars under greenhouse conditions. Flooding reduced the measured parameters in all cultivars, which had not recovered 3 days after flooding ceased. A more detailed study was conducted with the `Beauty' cultivar, in which leaf water potential (Ψw); root hydraulic resistance (Rh); and hormonal concentrations of ABA, IAA, and CK were measured in leaves and roots. Plants were flooded for 2 days, then allowed to recover for 4 days. After 2 days of flooding, Ψw had decreased and Rh had increased. Leaf ABA content increased and IAA and CK content decreased from the start of flooding and did not change during recovery. In roots a transient increase in root ABA levels occurred during flooding and a sharp decrease was measured during recovery, which was related to root death. Similar patterns were observed in root CK concentrations. The IAA concentration in roots remained constant throughout the experiment. The results suggest that ABA and CK may act as signals of flooding stress. Also, the marked intolerance of gerbera to flooding could be a serious barrier to its culture under anaerobic conditions, and hence careful irrigation management is required. Chemical names used: abscisic acid (ABA); indoleacetic acid (IAA); cytokinin (CK).

Photosynthetic Activity of Four Florist Azalea Cultivars

The photosynthetic activity of four Rhododendron simsii cultivars `Dorothy Gish', `Paloma', `White Gish', and `Gloria' were studied at both the individual leaf level using a portable photosynthesis system (closed), or at the whole-plant level using assimilation chambers (semi-open system). Net photosynthetic assimilation curves in response to light in both systems will be established. The experimental points obtained will be adjusted to a photosynthetic model as described in the literature. The model parameters [original efficiency (α) dark respiration (Rd), maximum photosynthetic capacity to saturated light (Pmax)] will be presented. The evolution of these parameters will be presented as a function of the various stages of development. Also a comparison of the four cultivars will be shown.

Mepiquat Chloride and Temperature Effects on Photosynthesis and Respiration of Fruiting Cotton

The bioregulator, mepiquat chloride (1,1‐dimethylpiperidinum chloride), consistently suppresses rank growth of cotton (Gossypium hirsutum L.), but yield responses are erratic. The objective of this study was to determine cotton plant response to mepiquat chloride (MC) at different temperatures, so that appropriate functions can be provided to the cotton crop simulation model. Cotton plant foliage was sprayed with MC at initial bloom, and maintained at four different day/night temperatures for 49 d. Carbon dioxide fluxes were measured continuously, and used to calculate photosynthetic and respiration rates. Plant components and leaf area were measured at the end of the treatment period. Mainstem growth rates and node addition rates were reduced by MC application at all temperatures. Gross photosynthesis increased linearly when the ambient air temperature increased to 30°C and declined at 40°C. Mepiquat chloride increased the canopy gross photosynthesis (Pg) within 48 h, suggesting a direct effect on photosynthesis. The increased Pg established on Day 2 persisted throughout the season at various temperatures, but the differences decreased at the end of the season. Respiration rates increased linearly as air temperature increased and decreased with plant age when expressed on unit weight basis. Biomass yields of cotton plants harvested 49 d after treatment were closely related to net photosynthetic assimilation accumulated over the same period. We conclude that MC and temperature impact cotton growth, development, respiration, and photosynthetic rates. This information will be useful for predicting optimum timing and amounts of MC application for improved management of cotton canopies.

Long‐term exposure of Norway spruce, Picea abies (L.) Karst., to ozone in open‐top chambers

summaryRates of net photosynthesis were measured on Norway spruce, Picea abies (L.) Karst. exposed to three different concentrations of ozone for three seasons (1985–7) in open‐top chambers. The treatments were non‐filtered air + ozone (NF +), non‐filtered air (NF) and charcoal‐filtered air (CF). Ozone was added daily between 11.00–18.00 local time. The study was performed during September and October, 1987. The rate of net photosynthesis was determined using an infrared gas analyser (IRGA) connected to a small cuvette, which was equipped with an artificial light source for unilateral illumination. The measurements were made on shoots of three different age classes: current year, 1 year old and 2 years old. Measurements took place at saturating light intensity (1150 μmol m1s1), at a temperature of 16.6 ±0.7°C, and at a relative humidity of 67.5 ± 5.3%. In current‐year shoots the rate of net photosynthesis expressed on a needle‐area basis was lowest in the CF treatment and highest in the NF treatment. In 1‐ and 2‐year‐old shoots, a decreased rate of the net photosynthetic assimilation rate, expressed on a needle area basis, was observed in trees from the NF and NF+ treatment compared with that of trees from the CF treatment. The effect increased with increasing ozone concentration and shoot age. A significant increase in the rate of respiration, expressed on a dry‐weight basis, was observed in current year shoots from trees exposed to NF+. A significant decrease of the leaf conductance to carbon dioxide was obtained in 2‐year‐old shoots exposed to NF and NF+. A decrease in the content of chlorophyll was observed in 1‐ and 2‐year‐old needles from trees exposed to NF+.

Influences of Leaf Temperature on Photosynthetic Carbon Metabolism in Wheat

Net photosynthetic assimilation rate (A), extractable activities of three photosynthetic enzymes, and the concentrations of six metabolites were determined for wheat (Tricum aestivum L.) leaves as leaf temperature was varied under photorespiring (350 microliters per liter CO(2) and 21% O(2)) and under nonphotorespiring conditions (800 microliters per liter CO(2) and 2% O(2)). The extractable activity of ribulose-1,5-bisphosphate carboxylase (Rubisco) and fructose-1,6-bisphosphatase declined with increasing leaf temperature from 15 to 45 degrees C. Leaf concentrations of ribulose-1,5-bisphosphate (RuBP) declined slightly between 15 and 25 degrees C but increased to a level which is 4 to 5 times the binding site concentration of Rubisco at leaf temperatures of 35 and 45 degrees C. Leaf concentrations of 3-phosphoglycerate, fructose-6-phosphate, and glucose-6-phosphate all declined with increasing leaf temperature. Outside of the limitations imposed by photorespiration, it is proposed that under high light and at suboptimal temperatures, A is limited by rate of utilization of triose phosphate; at optimal temperatures, by the availability of substrate (CO(2) and RuBP) under photorespiring conditions or utilization of triose phosphate under nonphotorespiring conditions; and at supraoptimal temperatures, by the activation state of Rubisco.

Temperature Dependence of Whole-Leaf Photosynthesis in Eucalyptus pauciflora Sieb. Ex Spreng

The temperature dependence of net photosynthetic assimilation of CO2 by snowgum (Eucalyptus pauciflora Sieb. ex Spreng.) was investigated. CO2 assimilation was divided into its component processes, stomatal and biochemical. The biochemical limitation was investigated with gas-exchange techniques and found to conform well to a recent model of C3 photosynthesis. In line with the model, net assimilation was further divided into ribulose 1,5-bisphosphate (RuP2) regeneration or electron-transport/photophosphorylation limitation, limitation by RuP2 carboxylase/oxygenase (Rubisco; EC 4.1.1.39) activity, together with loss of CO2 in non-photorespiratory respiration. The estimated temperature dependence of electron transport agreed well with one published for uncoupled electron transport, while the estimated temperature dependence of the catalytic activity of Rubisco was slightly less than that reported from biochemical determinations. The estimated rate of non-photorespiratory respiration was about 0.6 times the rate of respiration at night and appeared to have the same temperature dependence. With this information the temperature dependence of the biochemical limitation was modelled. Stomatal conductance was assumed to follow the theory of constant marginal water cost of carbon assimilation (δE/δA) and net assimilation rate at ambient concentration of CO2 was predicted. It was concluded that, for a given Rubisco activity and RuP2-regeneration capacity, both temperature optimum and net assimilation rate at the optimum temperature increased with increasing stomatal conductance.

The Close Relationship between Net Photosynthesis and Crop Yield

Crop yield is closely related to the net photosynthetic assimilation of CO2 throughout an entire season, but instantaneous measurements of photosynthesis may be misleading. Increasing the rates of net photosynthesis and translocation and enlarging the storage capacity by selection and breeding may bring about large increases in yield, especially in C3 species. (Accepted for publication 23 April 1982)