Intercellular Partial Pressure Of CO2 Research Articles

White LED Lighting Increases the Root Productivity of Panax ginseng C. A. Meyer in a Hydroponic Cultivation System of a Plant Factory.

To identify effective light spectra for increasing the productivity of Panax ginseng, we conducted experiments in a controlled environment using a hydroponic cultivation system in a plant factory. We investigated the effect of single LEDs (red, blue, and yellow) and mixed LEDs (red + blue and red + blue + white). The relationships between four light spectra (red, blue, yellow, and white) and physiological responses (net photosynthetic rate, stomata conductance, transpiration rate, and intercellular CO2 partial pressure), as well as growth responses (shoot and root biomass), were analyzed using multivariate statistical analysis. Among the four physiological response variables, shoot biomass was not increased by any pathway, and root biomass was increased only by the intercellular CO2 partial pressure. Red and yellow light increased shoot biomass, whereas white light promoted an increase in the net photosynthetic rate and enhanced root biomass. In contrast, blue light was less effective than the other light spectra in increasing both shoot and root biomass. Therefore, red and yellow light are the most effective light spectra for increasing shoot biomass and white light is effective for increasing root biomass in a plant factory that uses artificial LED lighting. Furthermore, the intercellular CO2 partial pressure is an important physiological variable for increasing the root biomass of P. ginseng.

Growth under Fluctuating Light Reveals Large Trait Variation in a Panel of Arabidopsis Accessions.

The oxidation of P700 in photosystem I (PSI) is a robust mechanism that suppresses the production of reactive oxygen species. We researched the contribution of photorespiration to the oxidation of P700 in wheat leaves. We analyzed the effects of changes in partial pressures of CO2 and O2 on photosynthetic parameters. The electron flux in photosynthetic linear electron flow (LEF) exhibited a positive linear relationship with an origin of zero against the dissipation rate (vH+) of electrochromic shift (ECS; ΔpH across thylakoid membrane), indicating that cyclic electron flow around PSI did not contribute to H+ usage in photosynthesis/photorespiration. The vH+ showed a positive linear relationship with an origin of zero against the H+ consumption rates in photosynthesis/photorespiration (JgH+). These two linear relationships show that the electron flow in LEF is very efficiently coupled with H+ usage in photosynthesis/photorespiration. Lowering the intercellular partial pressure of CO2 enhanced the oxidation of P700 with the suppression of LEF. Under photorespiratory conditions, the oxidation of P700 and the reduction of the plastoquinone pool were stimulated with a decrease in JgH+, compared to non-photorespiratory conditions. These results indicate that the reduction-induced suppression of electron flow (RISE) suppresses the reduction of oxidized P700 in PSI under photorespiratory conditions. Furthermore, under photorespiratory conditions, ECS was larger and H+ conductance was lower against JgH+ than those under non-photorespiratory conditions. These results indicate that photorespiration enhances RISE and ΔpH formation by lowering H+ conductance, both of which contribute to keeping P700 in a highly oxidized state.

Mitochondrial glutathione peroxidase (OsGPX3) has a crucial role in rice protection against salt stress

Rice is one of the world’s most important crops and an excellent model system for understanding the interaction between genes and environmental changes. However, its productivity is often challenged by abiotic stresses, which results in the accumulation of reactive oxygen species. Glutathione peroxidases are part of the mechanism by which plants face oxidative stress. These enzymes can control redox homeostasis and also play a role in redox signaling. Here, we investigate the role of rice GPX3 in plant responses to salt stress using OsGPX3-RNAi silenced rice plants (GPX3s). Our results indicate that GPX3s plants are more sensitive to salinity showing decreased biomass, CO2 assimilation rate, stomatal conductance, and intercellular CO2 partial pressure. Moreover, these plants present significant damage to photosystem II activity and decline in chlorophyll content. Salt stress induced ROS accumulation in both non-transformed (NT) and GPX3s plants, indicating that GPX3s sensibility to salt stress was not due to the significant impairment in redox equilibrium. Together, these results show GPX3 importance in rice to achieve salt stress tolerance via an independent ROS-scavenger mechanism. Moreover, it also provides new light into the cross-talk between chloroplasts and mitochondria, suggesting a novel role to this enzyme beyond its role as ROS-scavenger.

Mesophyll conductance does not contribute to greater photosynthetic rate per unit nitrogen in temperate compared with tropical evergreen wet-forest tree leaves.

Globally, trees originating from high-rainfall tropical regions typically exhibit lower rates of light-saturated net CO2 assimilation (A) compared with those from high-rainfall temperate environments, when measured at a common temperature. One factor that has been suggested to contribute towards lower rates of A is lower mesophyll conductance. Using a combination of leaf gas exchange and carbon isotope discrimination measurements, we estimated mesophyll conductance (gm ) of several Australian tropical and temperate wet-forest trees, grown in a common environment. Maximum Rubisco carboxylation capacity, Vcmax , was obtained from CO2 response curves. gm and the drawdown of CO2 across the mesophyll were both relatively constant. Vcmax estimated on the basis of intercellular CO2 partial pressure, Ci , was equivalent to that estimated using chloroplastic CO2 partial pressure, Cc , using 'apparent' and 'true' Rubisco Michaelis-Menten constants, respectively Having ruled out gm as a possible factor in distorting variations in A between these tropical and temperate trees, attention now needs to be focused on obtaining more detailed information about Rubisco in these species.

Long-term trends in leaf level gas exchange mirror tree-ring derived intrinsic water-use efficiency of Pinus cembra at treeline during the last century

The ability of treeline conifers in the Central European Alps to cope with recent climate warming and increasing CO2 concentration is still poorly understood. We determined basal area increment (BAI) and tree ring stable carbon isotope ratios (δ13C) of Pinus cembra trees from 1925 through 2013. Stable isotope ratios and BAI were compared with leaf level gas exchange measurements carried out in situ between 1934 and 2012, and thus, provided new insights into long-term trends of tree-ring derived intrinsic water-use efficiency (iWUE). Mature P. cembra trees at treeline responded to increasing Ca and air temperature with a parallel increase in maximum net CO2 uptake rate at ambient CO2 (Amax) and tree-ring-derived intercellular CO2 partial pressure (Ci). Amax tripled and was positively correlated to BAI and Ci. The latter increased in parallel with ambient CO2 concentration and stomatal conductance. In contrast to the instantaneous gas exchange parameters, δ13C derived iWUE informs about the long-term changes in the carbon water relations. These data showed three changes in the iWUE chronosequences, which could be identified with different long term gas exchange patterns: (1) from stomatal controlled functioning from 1925 to 1981, to a situation where (2) both net CO2 fixation (A) and leaf conductance for water vapour (gw), responded to the environment from 1982 to 1997, and (3) back to a stomata controlled pattern over iWUE from 1998 onwards. This temporal pattern was also mirrored in leaf level gas exchange assessments, suggesting a parallel increase of A and gw of P. cembra at treeline during the last nine decades.

Comparative photosynthetic responses in upland and lowland sugarcane cultivars grown in cool and warm conditions

The C 4 plant sugarcane (Saccharum sp. Linnaeus) is a chilling-sensitive crop grown at low latitudes for refined sugar and bioenergy. As a leading source of sugar and feedstock for bioethanol production, there is interest in cultivating it at higher latitudes and elevations where sub-optimal temperatures are common. To assess its potential for photosynthetic acclimation to temperature, we compared photosynthetic temperature responses of lowland and upland Hawaiian sugarcane cultivars grown at 32/26 and 21/18 °C day/night temperature. The temperature response of net CO2 assimilation rate (A) of the cultivars was similar in both treatments, with the exception that short-term exposure to 45 °C greatly inhibited A in the 21/18 °C-grown plants of both varieties when subsequently measured below 21 °C. Plants grown at 32/26 °C exhibited a modest inhibition of A below 21 °C following a 45 °C exposure. Little variation between the cultivars was observed in their responses to intercellular CO2 partial pressure and light at varying temperatures. We thus conclude sugarcane does not exhibit a strong thermal acclimation response at moderately sub-optimal to supra-optimal temperatures, but will acclimate to warm growth conditions in a manner that increased tolerance of heat; however, the heat intolerance was manifested as reduced performance at cooler temperatures. This could be problematic in growing regions such as the southeastern USA and China where springtime conditions can include hot days followed by strong cold fronts that brings sub-optimal temperatures.

Lotus corniculatus L. response to carbon dioxide concentration and radiation level variations

Carbon dioxide concentration and light conditions may greatly vary between mountainous and lowland areas determining the photosynthetic performance of plants species. This paper aimed to evaluate the photosynthetic responses of Lotus corniculatus, growing in a mountain and a lowland grassland, under low and high radiation and CO2 concentration. Net photosynthetic rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration were measured while the water-use efficiency and the ratio of variable to maximal fluorescence were calculated. Photosynthetic response curves to different levels of radiation and intercellular CO2 partial pressure were estimated. Our results showed that high radiation and CO2 concentration enhanced water-use efficiency of plants at both sites, enabling them to use more efficiently the available water reserves under drought conditions. The increase of radiation and CO2 concentration would enhance the photosynthetic performance of the mountainous population of L. corniculatus, which overall seems to express higher phenotypic plasticity.

Responses of photosynthetic parameters to differences in winter temperatures throughout a temperature gradient in two evergreen tree species

The temperature dependence of photosynthetic parameters has been a focus of interest during recent years owing to its profound implications in the new climate scenario. Many studies have addressed the short-term responses of photosynthetic parameters to temperature change. Less attention has been given to the intraspecific variability in the biochemical parameters of photosynthesis in response to differences in growth temperature. This study explores the effects of winter harshness on the leaf traits of two evergreen tree species (Quercus ilex and Q. suber). Leaf mass per unit area (LMA) and the concentrations of fiber, nitrogen (N), soluble protein, chlorophyll and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) were determined in both species throughout a temperature gradient. Several photosynthetic parameters [maximum carboxylation rate (V cmax), maximum light-driven electron flux (J max), day respiration rate (R d) and relative stomatal limitation to photosynthesis] were assessed by measuring leaf response curves of net CO2 assimilation versus intercellular CO2 partial pressure. LMA and structural carbohydrate concentrations increased with the decrease in winter temperatures, whereas N concentrations did not show definite patterns. Chlorophyll, soluble proteins, Rubisco, V cmax and J max declined with the decrease in winter temperatures, whereas R d at a set common temperature (25 °C) was higher at colder sites. Our results suggest that an increase in LMA and in the concentration of structural carbohydrates in cold environments is associated with a reduced N allocation to the photosynthetic machinery, which leads to reduced photosynthetic capacity.

Photosynthetic response of Bromus inermis in grasslands of different altitudes

In Greece, grasslands constitute an important component of montane areas. The aim of the present study was to evaluate the photosynthetic responses of the forage species Bromus inermis at two montane grasslands (Filippaioi, Grammatiko) and a lowland grassland under differential irradiation and CO2 concentration levels. In each area, gas exchange parameters were calculated under: a) ambient conditions along with the measurement of variable to maximal fluorescence, b) low (300 µmol m-2 s-1) and high (1400 µmol m-2 s-1) radiation, and c) ambient (380 µmol mol-1) and high (800 µmol mol-1) CO2 concentrations. In addition, assimilation response curves to different levels of irradiation and intercellular CO2 partial pressure were recorded. The photosynthetic machinery of B. inermis responded better to changes in irradiation in the montane area of Filippaioi, while it was more sensitive to changes in CO2 concentration in the lowland area. Photosynthesis in the montane area of Grammatiko could be subject to novel ecological pressures shaped by lower grazing intensity. No effect of altitude on physiological responses of B. inermis was found. Instead, plants from each area may use different adaptive mechanisms to persist and thrive in the environmental conditions of each area.

Photosynthetic duration, carboxylation efficiency and stomatal limitation of sun and shade leaves of different ages in field-grown grapevine ( Vitis vinifera L.)

The relationship of photosynthesis (A) of grapevine ( Vitis vinifera L.) sun and shade leaves of primary and secondary (lateral) shoots to insertion level was investigated over two seasons in the field. The leaf plastochron index (LPI) was used to denote leaf position on the shoot. Additionally, laboratory and field measurements of the response of A to CO 2 were conducted. An empirical model was developed to estimate carboxylation efficiency (CE) and stomatal limitations (1) of A. In sun leaves, the relationship of A to LPI changed little until the end of the season (October), whereas stomatal conductance (g) and the intercellular partial pressure of CO 2 (c i ) increased. Leaves acclimated to low light and leaves older than LPI 5 had 30 % lower A and were operating at a slightly higher ci as sun leaves. During September and October, lateral leaves had highest rates of CO 2 assimilation and CE. In mid-October, A and g decreased rapidly and simultaneously for all leaf types, leaf positions and both treatments (sun and shade). Photosynthesis responded similar to individual leaf age as to leaf position. A was linearly related to c i up to non-limiting conductances for sun and shade leaves, for all ages and at all times during the season. The CE and 1 were highest at the beginning of the season and strongly dependent on leaf position. Stomatal limitation declined continuously from about 55 % at the beginning to about 23 and 18 % for sun and shade leaves, respectively, at the end of the season.

Estimation of leaf and canopy photosynthesis of pot chrysanthemum and its implication on intensive canopy management

Variation in leaf net photosynthetic rate (A) with the changes in incoming radiation at each leaf of pot chrysanthemum under different leaf temperatures (T1) were used to model the canopy net photosynthesis. Model parameterisation when A was Rubisco-limited, RuBp-limited, and triose-phosphate-utilization limited was made under constant or variable levels of intercellular partial pressure of CO2, photosynthetically active radiation (PAR; Q) and T1. The minimum A obtained from above approaches were considered as the estimate of A through the biochemical model and the A/Q responses developed were in close agreement with the measured A/Q responses at different Tl. Top, middle and bottom leaf area with 23%, 48% and 29% of the leaf area of a plant contributed 30–32%, 47–51% and 18–20% of the canopy photosynthesis, respectively. The peak rate of net photosynthesis was reached when CO2 enriched to 1000ppm, PAR at 1000–1400μmolm−2s−1 and Tl at 20 and 28.8°C. Therefore, artificial lighting alone at lower temperatures is not adequate to achieve a higher canopy photosynthesis in pot chrysanthemum as lower leaf layers are not contributing to A and combination of CO2 enrichment with artificial lighting at higher temperatures are required to achieve the maximum production.

Artefactual responses of mesophyll conductance to CO2 and irradiance estimated with the variable J and online isotope discrimination methods

Studies with the variable J method have reported that mesophyll conductance (gm ) rapidly decreases with increasing intercellular CO2 partial pressures (Ci ) or decreasing irradiance. Similar responses have been suggested with the online isotope discrimination method, although with less consistency. Here we show that even when the true gm is constant, the variable J method can produce an artefactual dependence of gm on Ci or irradiance similar to those reported in previous studies for any of the following factors: day respiration and chloroplastic CO2 photocompensation point are estimated with Laisk method; Ci or electron transport rate is positively biased; net photosynthetic rate is negatively biased; insufficient NADPH is assumed while insufficient ATP limits RuBP regeneration. The isotopic method produces similar artefacts if fractionation of carboxylation or Ci is positively biased or Δ(13) negatively biased. A non-zero chloroplastic resistance to CO2 movement results in a qualitatively different dependence of gm on Ci or irradiance and this dependence is only sensitive at low Ci . We thus cannot rule out the possibility that previously reported dependence of gm on Ci or irradiance is a methodological artefact. Recommendations are made to take advantage of sensitivities of the variable J and isotopic methods for estimating gm .

CO2농도 및 온도 상승이 백합나무의 생리적 특성에 미치는 영향

This study aimed to investigate the growth and physiological characters of Liriodendron tulipifera seedlings in responses to two different levels of elevated air temperature and CO2 concentration. The seedlings were grown in environment-controlled growth chambers with two combinations of air temperature and CO2 conditions: (1) 22 o C + ambient CO2 380 µmol mol �1 and (2) 27 o C + 770 µmol mol �1 . Physiological characters such as growth, photosynthesis, and water use efficiency, were monitored for 85 days. The seedlings under the elevated treatment showed a greater amount of growth in tree height, compared with those under the control. Regarding the characteristics of assimilatory organs, the elevated treatment resulted in a greater amount of total leaf area, leaf unfolding, and dry weight per leaf area. No significant differences were found in photosynthesis capacity between the two treatments. The increase in water use efficiency with increased intercellular CO2 partial pressure appeared overall lower in the seedling under the elevated treatment, compared with those under the control. The total leaf area of the seedlings under the elevated treatment was larger than that under the control, indicating a higher amount of photosynthesis. In addition, an increase of root growth was noted under the elevated treatment. A resistance mechanism of water stress may be attributed to a higher amount of organ growth as well as the tree height under the elevated treatment than the control.

Pressure effect on rate of production of glucose-equivalent in plant cells

The rate of glucose equivalent production in C4 green plants is investigated as a function of the intercellular partial pressure of CO2, so as to find the precise physical chemistry of photosynthesis. Expressions are first formulated for the dependence of photochemical efficiency and of rubisco activation on pressure. Then a pressure-dependent rate law is derived. The latter is successfully tested for two specific C4 plants, namely, Panicum antidotale and Panicum coloratum.

Limitations to photosynthesis at different temperatures in the leaves of Citrus limon

The response of CO2 assimilation rate (A) to the intercellular partial pressure of CO2 (Ci) was measured on intact lemon leaves over a range of temperatures (10 to 40ºC). The A/Ci response shows how change in the leaf temperature alters the activity of ribulose-1,5-bisphosphate (RuBP) carboxylase-oxygenase (Rubisco) and RuBP regeneration via electron transport. The rate of A reached a maximum of 7.9 to 8.9 µmol m-2 s-1 between 25 and 30ºC, while dark respiration (Rd) increased with temperature from 0.4 µmol m-2 s-1 at 10ºC to 1.4 µmol m-2 s-1 at 40ºC. The maximum rates of carboxylation (Vc,max) and the maximum rates of electron transport (Jmax) both increased over this temperature range from 7.5 to 142 µmol m-2 s-1 and from 23.5 to 152 µmol m-2 s-1, respectively. These temperature responses showed that A can be limited by either process depending on the leaf temperature, when Ci or stomatal conductance are not limiting. The decrease in A associated with higher temperatures is in part a response to the greater increase in the rate of oxygenation of RuBP compared with carboxylation and Rd at higher temperatures. Although A can in theory be limited at higher Ci by the rate of triose-phosphate utilization, this limitation was not evident in lemon leaves.

Altitudinal variation in photosynthetic capacity, diffusional conductance and δ13C of butterfly bush (Buddleja davidii) plants growing at high elevations

In this study, we have examined several physiological, biochemical and morphological features of Buddleja davidii plants growing at 1300 m above sea level (a.s.l.) and 3400 m a.s.l., respectively, to identify coordinated changes in leaf properties in response to reduced CO2 partial pressure (Pa). Our results confirmed previous findings that foliar δ13C, photosynthetic capacity and foliar N concentration on a leaf area basis increased, whereas stomatal conductance (gs) decreased with elevation. The net CO2 assimilation rate (Amax), maximum rate of electron transport (Jmax) and respiration increased significantly with elevation, although no differences were found in carboxylation efficiency of Rubisco (Vcmax). Consequently, also the Jmax to Vcmax ratio was significantly increased by elevation, indicating that the functional balance between Ribulose‐1,5‐biphosphate (RuBP) consumption and RuBP regeneration changes as elevation increases. Our results also indicated a homeostatic response of CO2 transfer conductance inside the leaf (mesophyll conductance, gm) to increasing elevation. In fact, with elevation, gm also increased compensating for the strong decrease in gs and, thus, in the Pi (intercellular partial pressure of CO2) to Pa ratio, leading to similar chloroplast partial pressure of CO2 (Pc) to Pa ratio at different elevations. Because there were no differences in Vcmax, also A measured at similar PPFD and leaf temperature did not differ statistically with elevation. As a consequence, a clear relationship was found between A and gm, and between A and the sum of gs and gm. These data suggest that the higher dry mass δ13C of leaves at the higher elevation, indicative of lower long‐term Pc/Pa ratio, cannot be attributed to changes either in diffusional resistances or in carboxylation efficiency. We speculate that because temperature significantly decreases as the elevation increases, it dramatically affects CO2 diffusion and hence Pc/Pa and, consequently, is the primary factor influencing 13C discrimination at high elevation.

Inclusion of photoinhibition in simulation of carbon dynamics of an alpine meadow on the Qinghai‐Tibetan Plateau

The Qinghai‐Tibetan Plateau is the highest plateau in the world. Covered mainly by alpine grasslands, the plateau plays an important role in the region's carbon budget. The plateau is characterized by high irradiance, low atmospheric pressure, and low temperature. To examine the carbon dynamics of grassland ecosystems on the plateau, we developed a new model, the carbon dynamics model for alpine grasslands (CDMag). CDMag is composed of a gross photosynthesis (Ac) submodel including photoinhibition simulation, an ecosystem respiration (ER) submodel that considers both autotrophic and heterotrophic respiration, and an intercellular CO2 partial pressure and canopy conductance submodel. The photoinhibition simulation is included to describe the gross production of alpine ecosystems in response to the high‐irradiance environment. A multilayered irradiance transfer scheme is adopted to estimate the irradiance on sunlit and shaded leaves within grassy canopies, and a coupled canopy conductance–net photosynthesis function is integrated into the Ac submodel. CDMag was used to simulate the carbon dynamics of the most extensively distributed alpine grassland, a Kobresia meadow ecosystem on the Qinghai‐Tibetan Plateau. CDMag reproduces well the diurnal and seasonal variation of net ecosystem exchange (NEE) and nighttime ER based on data obtained from 2001 to 2003 using the eddy‐covariance method. The mean measured NEE was about 1.62 μmol m−2 s−1 for the measured periods of the 3 years; the mean of the deviations between measured and simulated NEE was −0.16 to 0.33 μmol m−2 s−1. Owing to photoinhibition, daily gross photosynthesis lost about 5.3–5.6%. The losses were mainly contributed by the 7.5–7.8% reduction in Ac of sunlit leaves of the upper canopy layer. In addition, the light‐use efficiency of the alpine meadow had seasonal dynamics, with the highest values in the middle of the growing season. We have generalized CDMag for other alpine grasslands on the entire plateau.

The effect of internal Co2 conductance on leaf carbon isotope ratio

We estimated internal CO2 conductance (gi ) for C3 plant species of different life forms (annual herbs, deciduous trees and evergreen trees) grown in a variety of environments to examine the effect of g i on their leaf carbon isotope ratio (i13C). The purpose of this study was to test the validity of using i13C as an index of photosynthetic water use efficiency (WUE). When comparing deciduous tree species grown in contrasting light environments, there was a strong positive relationship between i13C and WUE. Similarly, i13C was positively correlated to WUE when comparing the different species of evergreen trees. However, the difference in WUE between evergreen and deciduous tree species did not relate to that in leaf i13C. In addition, WUE was similar between highland and lowland herbaceous plants, although the former had a much higher i13C. The positive relationship between i13C and WUE did not hold across different life forms and different altitudes when differences in gi did not relate to those in stomatal conductance, resulting in independence of chloroplast CO2 partial pressure from intercellular CO2 partial pressure.

Moderate shade can increase net gas exchange and reduce photoinhibition in citrus leaves.

Daily variations in net gas exchange, chlorophyll a fluorescence and water relations of mature, sun-acclimated grapefruit (Citrus paradisi Macfady.) and orange (Citrus sinensis L. Osbeck) leaves were determined in tree canopies either shaded with 50% shade screens or left unshaded (sunlit). Mean daily maximum photosynthetic photon flux density (PPFD) under shade varied from 500 to 700 micromol m-2 s-1 and was sufficient to achieve maximum net CO2 assimilation rates (A CO2). Responses of grapefruit and orange leaves to shading were remarkably similar. At midday, on bright clear days, the temperatures of sunlit leaves were 2-6 degrees C above air temperature and 1-4 degrees C above the temperatures of shaded leaves. Although midday depressions of stomatal conductance (gs) and A CO2 were observed in both sunlit and shaded leaves, shaded leaves had lower leaf-to-air vapor pressure differences (D) along with higher gs, A CO2 and leaf water-use efficiency than sunlit leaves. Estimated stomatal limitation to A CO2 was generally less than 25% and did not differ between shaded and sunlit leaves. Leaf intercellular CO2 partial pressure was not altered by shade treatment and did not change substantially with increasing D. Radiation and high temperature stress-induced non-stomatal limitation to A CO2 in sunlit leaves was greater than 40%. Reversible photoinhibition of photosystem II efficiency was more pronounced in sunlit than in shaded leaves. Thus, non-stomatal factors play a major role in regulating A CO2 of citrus leaves during radiation and high temperature stress.

Kaolin Particle Film Applications Can Increase Photosynthesis and Water Use Efficiency of `Ruby Red' Grapefruit Leaves

Effects of foliar sprays of a kaolin clay particle film (Surround WP) on leaf temperature (Tlf), net gas exchange, chlorophyll fluorescence and water relations of sun-exposed leaves on field-grown grapefruit trees (Citrus paradisi L.) were studied during Summer and Fall 2001. Trees were sprayed twice a week for 3 weeks with aqueous suspensions of kaolin (Surround) at 60 g·L-1. Physiological effects of kaolin application were most prominent around midday on warm sunny days than in mornings, evenings or cloudy days. Kaolin sprays increased leaf whiteness (62%), reduced midday leaf temperature (Tlf; ≈3 °C) and leaf to air vapor pressure differences (VPD; ≈20%) compared to water-sprayed control leaves. Midday reductions in Tlf and VPD were accompanied by increased stomatal conductance (gs) and net CO2 assimilation rates (ACO2) of kaolin sprayed leaves, suggesting that gs might have limited ACO2 in water-sprayed control leaves. Midday photoinhibition of photosynthesis was 30% lower in kaolin-sprayed leaves than in control leaves. Midday water use efficiency (WUE) of kaolin-sprayed leaves was 25% higher than that of control leaves. However, leaf transpiration and whole-tree water use were not affected by kaolin film sprays. Increased WUE was therefore, due to higher ACO2. Leaf intercellular CO2 partial pressures (Ci) were similar in control and kaolin-sprayed leaves indicating that stomatal conductance was not the major cause of reduced ACO2. These results demonstrate that kaolin sprays could potentially increase grapefruit leaf carbon uptake efficiency under high radiation and temperature stress.