Diurnal Course Of Photosynthesis Research Articles

Soil Temperature Prior to Veraison Alters Grapevine Carbon Partitioning, Xylem Sap Hormones, and Fruit Set

To gain a better understanding of environmental effects on grapevines and the physiological regulation of acclimation, we determined the effects of soil temperature (14 or 24°C) between anthesis and veraison on growth, nonstructural carbohydrates, cytokinins, abscisic acid, and leaf function of potted Vitis vinifera cv. Shiraz. Plants of each regime were selected from two groups that had been grown in a glasshouse from three weeks prior to budbreak at an average soil temperature of either 13 or 23°C. Soil temperature between anthesis and veraison affected utilization and restoration of root and trunk nonstructural carbohydrates and changes in biomass of major plant organs. Soil warming promoted shoot growth via utilization of starch reserves, while soil cooling promoted starch storage in both the root and wood and shifted overall biomass partitioning to the roots. A change in soil temperature from warm to cool through flowering was also associated with reduced fruit set. Diurnal courses of photosynthesis, transpiration, and stomatal conductance after fruit set were significantly affected by soil temperature. Phytohormones (cytokinin and abscisic acid) were measured in the xylem sap and leaves at fruit set and veraison. Differences between these two sample types during grapevine development highlight a phytohormone shift likely involved in postveraison fruit ripening. We conclude that soil temperature significantly affects grapevine growth and that the responses are mediated largely by an influence of temperature on mobilization of nonstructural carbohydrates from the roots.

Photosynthetic response of two shrubs to rainfall pulses in desert regions of northwestern China

Pulses of rainfall are particularly pivotal in controlling plant physiological processes in ecosystems controlled by limited water, and the response of desert plants to rainfall is a key to understanding the responses of desert ecosystems to global climatic change. We used a portable photosynthesis system to measure the responses of the diurnal course of photosynthesis, light-response curves, and CO2-response curves of two desert shrubs (Nitraria sphaerocarpa Maxim. and Calligonum mongolicum Turcz) to a rainfall pulse in a desert-oasis ecotone in northwestern China. The photosynthetic parameters, light- and CO2-response curves differed significantly before and after the rainfall pulse. Their maximum net photosynthetic rate (PN) values were 23.27 and 32.92 μmol(CO2) m−2 s−1 for N. sphaerocarpa and C. mongolicum, respectively, with corresponding maximum stomatal conductance (gs) values of 0.47 and 0.39 mol(H2O) m−2 s−1. The PN of N. sphaerocarpa after the rainfall was 1.65 to 1.75 times the value before rainfall, whereas those of C. mongolicum increased to approximately 2 times the prerainfall value, demonstrating the importance of the desert plants response by improving their assimilation rate to precipitation patterns under a future climate.

Thinning effects on jack pine and black spruce photosynthesis in eastern boreal forests of Canada

A decrease in the average diameter of commercially harvested tree species in the Eastern boreal forest of Canada has led to a decrease in availability of quality wood for the forest industry. Commercial thinning has been proposed as a means to increase stem diameter growth. However, little is known about physiological responses underlying species responses to thinning. We assessed the effect of canopy opening on the photosynthetic response of mature jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill.) BSP) trees. Two years after thinning and for each species, light response curves and the diurnal course of photosynthesis were characterized from measurements taken in a completely randomized block experiment on current-year and one-year-old needles of 12 trees from stands subjected to different levels of canopy opening. Soil water content, air and soil temperatures, and needle N concentration were not affected by thinning for either species. However, light availability increased with basal area removed and could explain the significantly positive relationship between thinning intensity and diurnal course of photosynthesis for one-year-old needles of jack pine. Black spruce photosynthesis did not respond to increases in light. Light-saturated rate of net photosynthesis (A), photosynthetic efficiency (α), light compensation point (LCP), and diurnal respiration (R) did not vary with thinning for either of the species. Jack pine and black spruce responses to thinning should be interpreted in light of species autecology.maxd

More Productive Than Maize in the Midwest: How Does Miscanthus Do It?

In the first side-by-side large-scale trials of these two C(4) crops in the U.S. Corn Belt, Miscanthus (Miscanthus x giganteus) was 59% more productive than grain maize (Zea mays). Total productivity is the product of the total solar radiation incident per unit land area and the efficiencies of light interception (epsilon(i)) and its conversion into aboveground biomass (epsilon(ca)). Averaged over two growing seasons, epsilon(ca) did not differ, but epsilon(i) was 61% higher for Miscanthus, which developed a leaf canopy earlier and maintained it later. The diurnal course of photosynthesis was measured on sunlit and shaded leaves of each species on 26 dates. The daily integral of leaf-level photosynthetic CO(2) uptake differed slightly when integrated across two growing seasons but was up to 60% higher in maize in mid-summer. The average leaf area of Miscanthus was double that of maize, with the result that calculated canopy photosynthesis was 44% higher in Miscanthus, corresponding closely to the biomass differences. To determine the basis of differences in mid-season leaf photosynthesis, light and CO(2) responses were analyzed to determine in vivo biochemical limitations. Maize had a higher maximum velocity of phosphoenolpyruvate carboxylation, velocity of phosphoenolpyruvate regeneration, light saturated rate of photosynthesis, and higher maximum quantum efficiency of CO(2) assimilation. These biochemical differences, however, were more than offset by the larger leaf area and its longer duration in Miscanthus. The results indicate that the full potential of C(4) photosynthetic productivity is not achieved by modern temperate maize cultivars.

Response of gas exchange to neighborhood interference in leaves of teak ( Tectona grandis L. f.) in a tropical plantation forest

The growth performance of individual plants in a population can be affected by the plant-plant interaction, which has been well recognized and is called neighborhood interference. Though mechanisms are still unclear, the variations in gas exchange parameters in relation to the neighborhood interference between individual plants are crucial for evaluating the effects of plant-plant interaction. CO 2 assimilation in leaves of teak under natural conditions of dry seasons and wet seasons as well as its response to variations in light flux density and CO 2 concentration in different neighborhood interferences were simultaneously measured with Li-6400 portable photosynthesis system in a 21-year-old tropical plantation forest in Jianfengling, Hainan Island, China. This article dealt with the change rule of neighborhood interference on tree characteristics of gas exchange and its dynamic response to light en-vironments of individual plants. Diurnal courses of photosynthesis of individual leaves were not affected by neighborhood interfer-ence, but net photosynthetic rates showed a negative relationship with the intensity of neighborhood interference. The ratio of daily average P n in weak, moderate, strong and heavy neighborhood interferences was 2.5:2.3:1.7:1.0, and the daily maximum P n in weak interference was 2.8 times that in heavy interference. Leaf transpiration and stomatal conductance decreased with the increasing of interference intensity. Characteristics of photosynthetic light response and CO 2 response changed with the neighborhood interference, and values of leaf gas exchange parameters including A sat, Q sat, α A, CE, V cmax and J max in weak interference were enhanced by 2.7, 1.3, 1.4, 2.7, 1.9 and 2.8 times, respectively, than those in heavy interference. But changes in those parameters partly depended on light environment and CO 2 concentration, and the influence of changes in light environment on weak interference individuals was sig-nificantly stronger than that on heavy interference. Beyond the growth CO 2 concentration, the influence resulting from changes in CO 2 concentration on heavy interference individuals was obviously stronger than that on weak interference. Neighborhood interference can be described as a major means of intraspecific competition of population in a plantation forest with uniform forest structure and consistent management. Carbon assimilation can be affected by the neighborhood interference, and result in divergence in growth performance. Indices of neighborhood interference can be used to evaluate the intraspecific competition, and based on this point, we can make maximum use of resources after stand structure has been well adjusted.

Coupled Dynamics of Photosynthesis, Transpiration, and Soil Water Balance. Part I: Upscaling from Hourly to Daily Level

The governing equations of soil moisture dynamics, photosynthesis, and transpiration are reviewed and coupled to study the dependence of plant carbon assimilation on soil moisture. The model follows the scheme of the soil‐plant‐atmosphere continuum (SPAC) and uses a simplified model of the atmospheric boundary layer to arrive at an upscaled, parsimonious representation at the daily time scale. The analysis of soil moisture, transpiration, and carbon assimilation dynamics provides an assessment of the role of soil, plant, and boundary layer characteristics on the diurnal courses of photosynthesis and transpiration rates, while the subsequent upscaling at the daily level provides a functional dependence of stomatal conductance on soil moisture that is in good agreement with field experiments. The upscaled dependence of transpiration and carbon assimilation on soil moisture is used in Part II of this paper to explore the impact of soil moisture dynamics on plant conditions when rainfall variability is explicitly considered.

A simple dynamic model of photosynthesis in oak leaves: coupling leaf conductance and photosynthetic carbon fixation by a variable intracellular CO2 pool.

Modelling the diurnal course of photosynthesis in oak leaves (Quercus robur L.) requires appropriate description of the dynamics of leaf photosynthesis of which diurnal variations in leaf conductance and in CO2 assimilation are essential components. We propose and analyse a simple photosynthesis model with three variables: leaf conductance (gs), the CO2 partial pressure inside the leaf (pi), and a pool of Calvin cycle intermediates (aps). The environmental factors light (I) and vapour pressure deficit (VPD) are used to formulate a target function G(I, VPD) from which the actual leaf conductance is calculated. Using this gs value and a CO2 consumption term representing CO2 fixation, a differential equation for pi is derived. Carboxylation corresponds to the sink term of the pi pool and is assumed to be feedback-inhibited by aps. This simple model is shown to produce reasonable to excellent fits to data on the diurnal time courses of photosythesis, pi and gs sampled for oak leaves.

Modelling Diurnal Courses of Photosynthesis and Transpiration of Leaves on the Basis of Stomatal and Non-Stomatal Responses, Including Photoinhibition

A mathematical model for photoinhibition of leaf photosynthesis was developed by formalising the assumptions that (1) the rate of photoinhibition is proportional to irradiance; and (2) the rate of recovery, derived from the formulae for a pseudo first-order process, is proportional to the extent of inhibition. The photoinhibition model to calculate initial photo yield is integrated into a photosynthesis-stomatal conductance (gs) model that combines net photosynthetic rate (PN), transpiration rate (E), and gs, and also the leaf energy balance. The model was run to simulate the diurnal courses of PN, E, gs, photochemical efficiency, i.e., ratio of intercellular CO2 concentration and CO2 concentration over leaf surface (Ci/Cs), and leaf temperature (T1) under different irradiances, air temperature, and humidity separately with fixed time courses of others. When midday depression occurred under high temperature, gs decreased the most and E the least. The duration of midday depression of gs was the longest and that in E the shortest. E increased with increasing vapour pressure deficit (VPD) initially, but when VPD exceeded a certain value, it decreased with increasing VPD; this was caused by a rapid decrease in gs. When air temperature exceeded a certain value, an increase in solar irradiance raised T1 and the degree of midday depression. High solar radiation caused large decrease in initial photon efficiency (α). PN, E, and gs showed reasonable decreases under conditions causing photoinhibition compared with non-photoinhibition condition under high irradiance. The T1 under photoinhibition was higher than that under non-photoinhibition conditions, which was evident under high solar irradiance around noon. The decrease in Ci/Cs at midday implies that stomatal closure is a factor causing midday depression of photosynthesis.

Effects of Light Spectral Quality on Morphogenesis and Source–Sink Relations in Radish Plants

The accumulation of dry matter and the content of major phytohormones in the aboveground and underground plant parts, as well as light curves and the diurnal course of photosynthesis in the leaves were studied in radish (Raphanus sativusL.) plants of different ages that were grown under red (RL) or blue (BL) light. As seen from the rapid increase in plant biomass, the development of storage organs (hypocotyl or tap root) started on the 14th day after the emergence of seedling of the BL plants and on the 21st day for the RL plants. Conversely, RL stimulated biomass accumulation in the aboveground parts (petioles and stems) already in the early stages of plant development. Light spectral quality only slightly affected the activity and the diurnal course of photosynthesis. The GA content was ten times higher in the aboveground parts of the RL plants than those of the BL plants. The hypocotyl of the BL plants contained much higher amounts of cytokinins and IAA than that of the RL plants. The specific responses of the source–sink relations to the light quality were related to the distribution of various phytohormones between the aboveground and underground parts of the plants: RL increased the content of gibberellins (GA) in the aboveground parts of plants, thus increasing their sink activity, whereas BL stimulated the synthesis of cytokinins and IAA in the hypocotyl and enhanced its development. Light quality-specific morphogenetic responses were reversed when plants were treated with exogenous GA or paclobutrazol, an inhibitor of GA synthesis. The treatment of the BL plants with exogenous GA stimulated petiole and hypocotyl elongation and induced stem formation. The treatment of the BL plants with paclobutrazol led to shortened petioles, the flattening of the storage organ, and the disappearance of the stem.

A nitrogen sensitive model of leaf carbon dioxide and water vapour gas exchange: application to 13 key species from differently managed mountain grassland ecosystems

During the EU-project ECOMONT (Project No. ENV4-CT95-0179), which focuses on effects of land-use changes on mountain ecosystems, an experimental and modelling strategy at the leaf level was developed, which centres on the intimate relationship between leaf nitrogen content and net photosynthesis. Leaf nitrogen content reflects nutrient availability, shows characteristic seasonal dynamics and is of great importance for the analysis of intraspecific variability of gas exchange. The fully parameterised leaf model is capable of predicting net photosynthesis and stomatal conductance for any combination of microclimatic variables as well as any leaf nitrogen content. At the ECOMONT pilot research area Monte Bondone (Trentino/Italy) three grassland sites differing in land-use, a hay meadow, a pasture and an area abandoned since 35 years were selected as study sites. Leaf gas exchange characteristics and nitrogen contents of 13 species were studied and used for parameterising nitrogen sensitive leaf models. Independent data sets, diurnal courses of photosynthesis and stomatal conductance under the prevailing environmental conditions, were used for validation. A sensitivity analysis was performed in order to test the ability of the model to account for changes in leaf nitrogen content, varying leaf nitrogen content together with the environmental driving variables. These leaf models provide the physiological basis for scaling-up gas exchange from the leaf to the whole plant and canopy level and further to the landscape level.

Photosynthesis and conductance of spring‐wheat leaves: field response to continuous free‐air atmospheric CO2 enrichment

Spring wheat was grown from emergence to grain maturity in two partial pressures of CO2 (pCO2): ambient air of nominally 37 Pa and air enriched with CO2 to 55 Pa using a free‐air CO2 enrichment (FACE) apparatus. This experiment was the first of its kind to be conducted within a cereal field without the modifications or disturbance of microclimate and rooting environment that accompanied previous studies. It provided a unique opportunity to examine the hypothesis that continuous exposure of wheat to elevated pCO2 will lead to acclimatory loss of photosynthetic capacity. The diurnal courses of photosynthesis and conductance for upper canopy leaves were followed throughout the development of the crop and compared to model‐predicted rates of photosynthesis. The seasonal average of midday photosynthesis rates was 28% greater in plants exposed to elevated pCO2 than in contols and the seasonal average of the daily integrals of photosynthesis was 21% greater in elevated pCO2 than in ambient air. The mean conductance at midday was reduced by 36%. The observed enhancement of photosynthesis in elevated pCO2 agreed closely with that predicted from a mechanistic biochemical model that assumed no acclimation of photosynthetic capacity. Measured values fell below predicted only in the flag leaves in the mid afternoon before the onset of grain‐filling and over the whole diurnal course at the end of grain‐filling. The loss of enhancement at this final stage was attributed to the earlier senescence of flag leaves in elevated pCO2. In contrast to some controlled‐environment and field‐enclosure studies, this field‐scale study of wheat using free‐air CO2 enrichment found little evidence of acclimatory loss of photosynthetic capacity with growth in elevated pCO2 and a significant and substantial increase in leaf photosynthesis throughout the life of the crop.

Simulation of diurnal transpiration and photosynthesis of a water stressed soybean crop

The diurnal course of photosynthesis and transpiration of different plants can exhibit a dissymmetric behaviour between the morning and the afternoon, a photosynthetic midday depression and a transpiration ‘plateau’. A field experiment conducted in a meditenanean climate allows us to identify these features for a soybean crop undergoing water stress. The experimental data also show that, before and after the depression and plateau times, photosynthetic and transpiration fluxes are similar from one day to another. The duration of the period of depression and plateau increases as the soil dries out. Two soil-vegetation-atmosphere transfer models are used to simulate these features. Both are able to capture the midday depression and the plateau because they impose a direct relationship between leaf water potential, stomatal conductance and photosynthesis. When the leaf water potential reaches a `critical' value, feedback mechanisms tend to regulate the leaf water potential such that the transpiration becomes nearly constant. The stomatal conductance and the photosynthesis keep decreasing in response to an increase of water vapour deficit. The dissymmetry depends on the dissymmetry of the diumal course of saturation deficit in the atmosphere, and on the evolution of the water properties of the soil during the day. We conclude: that, even if they do not include formulations of photosynthesis and stomata] conductance based on hormonal regulations of stomatal conductance and inter-regulations between photosynthesis and conductance, such models remain useful for the simulation of canopy processes.

Diurnal courses of photosynthesis, transpiration, and diffusive conductance in the single-leaf of the rice plants grown in the paddy field under submerged condition.

It had been reported by the authors that leaf stomatal aperture in the rice plant under submerged condition decreased in fine midday with high evaporative demand and that close relation was found between diffusive conductance and photosynthetic rate in rice leaves. From these results it had been predicted that photosynthesis would reach the maximum early in the morning and then decrease gradually toward the afternoon with increase in light intensity and vapour pressure deficit. This study was conducted to ascertain this prediction by measuring the diurnal courses of photosynthetic rate (P), transpiration rate (T), P/T ratio (water use efficiency) and diffusive conductance in the single-leaf of the rice plant in paddy field under submerged condition on fine and cloudy days and to clarify the factors determining the diurnal courses of photosynthesis in a fine day using the simultaneous measurement system of photosynthesis and transpiration developed by KOCH, SCHULTZ and LANGE (Siemens Co. Ltd.). The photosynthetic rate on a fine day increased with increase in light intensity, reached the maximum early in the morning and then gradually decreased down to 75% of the maximum rate toward afternoon even under sufficient light intensity and this was accompanied by a decrease of diffusive conductance (Figs. 2A and 4). The photosynthetic rate in the morning was higher than that in the afternoon under the same light intensity more than 600 μE/m2/sec (Fig. 3A). Transpiration rate increased toward the midday with the increase of light intensity and vapor pressure deficit, and reached the maximum at about 13:00 PM. Water use efficiency was higher early in the morning and late in the evening and lower in the midday (Fig. 2A). The photosynthetic rate, transpiration rate and diffusive conductance on a cloudy day changed according to the change in light intensity (Figs. 2B and 3B). Therefore, light intensity was the dominant factor determining diurnal courses of photosynthetic rate and transpiration rate on a cloudy day. It was found that the time of the daily maximum of diffusive conductance was different from the time of that of photosynthetic rate in their diurnal changes, that is, diffusive conductance had already started to decrease even before the photosynthetic rate reached the maximum early in the morning. Under sufficient light intensity with artificially reflected sunlight by the mirror when diffusive conductance reached the maximum early in the morning, the time when both photosynthetic rate and diffusive conductance reached the maximum coincided. The maximum rater of the photosynthesis was higher than that of the day without the reflected light (Fig. 5). This result suggested that leaf photosynthetic capacity was not always fully realized in diurnal courses of photosynthesis on a fine day. To examine the factor reducing the photosynthetic rate on a fine day, diurnal courses of photosynthetic rate was measured under lower vapor pressure deficit by humidifying air in the chamber. The photosynthetic rate was much higher under lower vapor pressure deficit at least up to noon compared with that under the same vapor pressure deficit as outside (Fig. 6). This fact showed that water stress related to high vapor pressure deficit was the main factor for the decrease of photosynthesis in the midday even though the effects of photosynthate accumulation in the leaf blade and photoinhibition could not be neglected. From these results it was clarified that leaf photosynthetic rate in the rice plants under submerged condition decreased to some extent in the midday on fine days and they could not utilize high solar energy fully for dry matter production. [the rest omitted]

Effect of vapor pressure deficit on photosynthesis of rice leaves with reference to light and CO2 utilization efficiency.

In our previous studies on diurnal courses of photosynthesis, transpiration and diffusive conductance of the leaf in the rice plants grown in the paddy field, it was recoginized that the photosynthetic rate decreased with the decreasse of diffusive conductance due to high vapor pressure deficit under sufficient light intensity in the fine midday. It has been pointed out that water stress affects photosynthetic rate through the decrease of both CO2 supply to mesophyll through stomata and photosynthetic activity in mesophyll. This study was conducted to examine whether high vapor pressure deficit reduced photosynthetic activity by measuring light and CO2 utilization efficiencies at low quantum flux density and CO2 concentration under different vapor pressure deficit, respectively. The results were as follows : 1) Transpiration rate increased rapidly, and photosynthetic rate, diffusive conductance and intercellular CO2 concentration (Ci) decreased gradually with changing vapor pressure deficit from low to high (Figs. 1 and 2). 2) Light utilization efficiency, which was shown by the degree of inclination of light dependent straight line of photosynthesis under low quantum flux density, decreased under high vapor pressure deficit (Figs. 3 and 4). And also CO2 utilization efficiency, shown by the degree of inclination of the straight line expressing the relation between photosynthetic rate and low intercellular CO2 concentration, decreased under high vapor pressure deficit (Figs. 5, 6 and 7). As light and CO2 utilization efficiencies were thought to indicate photochemical and carboxylation activity of photosynthesis, respectively, it was suggested that not only diffusive conductance but also photosynthetic activity related to photochemical and carboxylation activities were affected by high vapor pressure deficit. 3) Intercellular CO2 concentration decreased with the decrease of photosynthetic rate and diffusive conductance under high vapor pressure deficit (Fig. 1). Furthermore, it was found that the degree of inclination of the straight line, expressing the relation between ambient CO2 and intercellular CO2 concentrations, was smaller under high vapor pressure deficit than that under low vapor pressure deficit (Fig. 9). These facts indicated that the decreasing of diffusive conductance was more effective on the decrease of photosynthesis under high vapor pressure deficit than the decreasing of photosynthetic activity. 4) The three following equations were obtained on the basis of the relation between intercellular CO2 concentration and photosynthetic rate (Fig. 6) and on the fact that transpiration rate was proportional to diffusive conductance under constant vapor pressure deficit. In this case, GAASTRA's concept was also considered. P=(44.99×C)/(C+0.377) Ci=(350×C+18.86)/(C+0.377) T=3.43×C The figure (Fig.8) drawn by using these equations showed that the relations between diffusive conductance (C) and photosynthetic rate (P) and between diffusive conductance and intercellular CO2 concentration (Ci) were hyperbollic, while the relation between diffusive conductance and transpiration rate (T) was linear. Thus, if diffusive conductance were decreased without decreasing the photosynthetic activity in mesophyll, water use efficiency (photosynthetic rate to transpiration rate ratio) could be increased.

Field Water Relations of Sonoran Desert Annuals

Field water relations were examined in winter and summer annuals of the Sonoran Desert. Both groups were characterized by low belowground biomass allocation. Winter annuals had large interspecific variation in water relations parameters, but their leaves generally had high conductances of water vapor and high water potentials during early growth periods. These high values were very short—lived and were reduced by peak flowering periods. Summer annuals were measured midway through the growing season; their leaves showed less interspecific variability in water vapor conductances and had values similar to the winter annuals for this stage of the growing season. Midday and diurnal courses of photosynthesis in the winter annuals exhibited lower rates than values previously reported for plants grown under unlimited water and nutrient regimes in glasshouses. This was probably due to the decreased leaf water potentials and increased vapor pressure deficits present in field situation. Both the winter and the summer annuals showed a dichotomy among species in leaf water potentials, associated with either variations in soil water availability or the ability of leaves to adjust osmotically to decreased soil water availability. Although winter and summer annuals are ephemeral, some species are capable of tolerating low leaf water potentials and therefore are not drought evading in the traditional sense.

Studies on the Dry Matter Production of the Sweet Potato : VIII. The internal factors influence on photosynthetic activity of sweet potato leaf.

In the previous paper, it was recognized that intimate correlations, positive or negative, were found among factors concerning photosynthetic activity, such as potassium per cent, nitrogen per cent., respiratory rate and carbohydrate content in the leaf, but it was not blear that which one of these factors showed a true correlation with photosynthetic activity. Therefore, the present study has been attempt to elucidate the factors showing a true correlation with that. Various experiments shown in table 2 and table 4 were carried out and photosynthetic rate, three major nutrient elements content and carbohydrate content in the leaf were determined. 1) Partial correlation coefficients were calculated between the factors and photosynthetic rate. A high significant partial correlation was found only between carbohydrate content and photosynthetic rate, all the other correlations proving insignificant. 2) In order to evaluate the influence of carbohydrate accumulation on photosynthetic rate, the diurnal changes of both photosynthesis and carbohydrate content in the leaf were observed. The photosynthetic rate showed no marked diurnal fluctuation through morning to afternoon, while the carbonhydrate content showed higher values in the afternoon than in the morning. It is considered from this that carbohydrates accumulation does not exert, at least, direct inhibitory influence upon photosynthetic rate. On the other hand, in cases such as plants were cultured under differential nutrient conditions or at different growth stages, those leaves highly active in their photosynthesis were shown to be low in carbohydrate content, and those less active leaves were shown to be high in carbohydrate content. From these it was considered that the velocity of removing photosynthates from leaves might be more important factor for photosynthesis rather than carbohydrate accumulation itself. 3) The growth of tuber which is the largest acceptor (sink) of the photosynthates in the sweet potato plant at later growth stage, was inhibited by exposing tubers to light. The treated plant was depressed in the photosynthetic rate and increased in starch content of leaves owing, presumably, to the restricted translocation of photosynthates from leaves. Farthermore, in another experiment, there was a parallel relation found between the net assimilation rate and the dry-weight increase of tubers. 4) All the above mentioned results suggest the following hypothesis : since the accumulation of photosynthates in the leaf does not bring any great change in the diurnal course of photosynthesis of sweet potato leaves, it is not accumulation of photosynthates in the leaf but the rate of movement of photosynthates from leaf (source) that is essential in controlling photosynthetic rate from inside. According to this hypothesis, to promote the growth of tuber means to increase the rate of translocation of photosynthates from leaves, and this, in turn, will give an accelerating influence to photosynthetic activity. Thus, the fact that potassium content in the leaf had a very intimate correlation with photosynthetic rate can be explained primarily by the nutritional effect of potassium for promoting the tuber growth.