Leaf nitrogen, photosynthesis, conductance and transpiration: scaling from leaves to canopies

A model is presented which solves simultaneously for leaf-scale stomatal conductance, CO₂ assimilation and the energy balance as a function of leaf position within canopies of well-watered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogen- and light-use efficiency), and to set bounds on the magnitudes of the parameters. For canopies low in total N, daily assimilation rates are ～10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, G_c should be calculated as G_c=Aσ(f_slg_sl+f_shg_sh), where Δ is leaf area index, f_si and f_sh are the fractions of sunlit and shaded leaves at each level, and g_si and g_sh are the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the ‘big-leaf’ version of the Penman-Monteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation. Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogen- and radiation-use efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2- to 3-fold variation in these efficiencies.     

Regulation of the photosynthetic capacity of primary bean leaves by the red:far-red ratio and photosynthetic photon flux density of incident light

The main objective of the present work was to examine the effects of the red:far-red ratio (R:FR) prevailing during leaf development on the photosynthetic capacity of mature leaves. Plants of Phaseolus vulgaris L. cv. Balin de Albenga were grown from time of emergence in a controlled environment room, 25 ± 3°C, 12-h photoperiod, with different light treatments:a) high photosynthetic photon flux density (PPFD) = 800 μmol m⁻¹ s⁻¹+ high R:FR= 1.3;b) low PPFD= 300 μmol m⁻² s⁻¹+ high R:FR= 1.3; c) high PPFD=800 μmol m⁻² s⁻¹+ low R:FR= 0.7; d) low PPFD= 300 μmol m⁻²s⁻¹+ low R:FR=0.7. With an R:FR ratio of 1.3, a decrease in irradiance during leaf growth reduced photosynthesis when measured at moderate to high PPFD; but when measured at low PPFD, leaves expanded under low irradiance actually had photosynthesis rates higher than those of leaves grown in high irradiance. A low R:FR ratio during development reduced the photosynthetic capacity of the leaves. In leaves expanded under R:FR = 0.7 and high irradiance photosynthesis was reduced by 42 to 89%, depending on the PPFD at which measurements were made, whereas for leaves developed at R:FR = 0.7 and low irradiance photosynthesis decreased by 21 to 24%, compared to leaves under R:FR = 1.3 and similar irradiance. The reduced photosynthetic capacity under R:FR = 0.7 and high irradiance. In natural environments, leaves may experience low R:FR conditions temporarily during their development, and this may affect their future photosynthetic capacity in full sunlight.     

Simple scaling of photosynthesis from leaves to canopies without the errors of big-leaf models

In big-leaf models of canopy photosynthesis, the Rubisco activity per unit ground area is taken as the sum of activities per unit leaf area within the canopy, and electron transport capacity is similarly summed. Such models overestimate rates of photosynthesis and require empirical curvature factors in the response to irradiance. We show that, with any distribution of leaf nitrogen within the canopy (including optimal), the required curvature factors are not constant but vary with canopy leaf area index and leaf nitrogen content. We further show that the underlying reason is the difference between the time-averaged and instantaneous distributions of absorbed irradiance, caused by penetration of sunflecks and the range of leaf angles in canopies. These errors are avoided in models that treat the canopy in terms of a number of layers – the multi-layer models. We present an alternative to the multi-layer model: by separately integrating the sunlit and shaded leaf fractions of the canopy, a single layered sun/shade model is obtained, which is as accurate and simpler. The model is a scaled version of a leaf model as distinct from an integrative approach.     

Three years of free-air CO2 enrichment (POPFACE) only slightly affect profiles of light and leaf characteristics in closed canopies of Populus

Modelling is used to predict long‐term forest responses to increased atmospheric CO₂ concentrations. Although productivity models are based on light intercepted by the canopy, very little experimental data are available for closed forest stands. Nevertheless, the relationships between light inside a canopy, leaf area, canopy structure, and individual leaf characteristics may be affected by elevated CO₂, affecting in turn carbon gain. Using a free‐air CO₂ enrichment (FACE) design in a high‐density plantation of Populus spp., we studied the effects of increased CO₂ concentrations on transmittance (τ) of photosynthetic photon flux density (Q_p), on ratios of red/far‐red light (R/FR), on leaf area index (LAI), on leaf inclination, on leaf chlorophyll (chl) and nitrogen (N) concentrations, and on specific leaf area (SLA) in the 2nd and 3rd years of treatment. Continuous measurements of τ were made in addition to canopy height profiles of light and leaf characteristics. Two years of Q_p measurements showed an average decrease of canopy transmittance in the FACE treatment, with very small differences at canopy closure. Results were explained by an unaffected LAI in closed canopies, without a FACE‐induced stimulation of relative crown depth. In agreement, leaf inclination and extinction coefficients for light were similar in control and FACE conditions. Ratios of R/FR were not significantly affected by the FACE treatment, neither were leaf characteristics, with the exception of leaf N, which allows speculation about N limitation. In general, treatment differences in canopy profiles resulted from an initial stimulation of height growth in the FACE treatment. P. × euramericana differed from P. alba and P. nigra, but species did not differ significantly in their response to the FACE treatment. By the time fast‐growing high‐density forest plantations have passed the exponential growth phase and reached canopy closure, the likely effects of elevated atmospheric CO₂ concentration on canopy architecture and absorption of Q_p are minor.     

Leaf carbon gain simulation for tree seedlings acclimatized to microsite light regimes of a temperate pine forest

Leaf carbon gain simulation was performed forQuercus serrata seedlings with previously reported 6 day photosynthetic photon flux density (PPFD) histograms from 20 understorey microsites of a pine forest (Washitani & Tang 1991). This simulation was performed with or without an assumption of the acclimatization of photosynthetic capacity (P_max) to microsite light availability. A constant ratio of respiration rate to P_max, within, the range of 0.07–0.1, was assumed as a constraint. In relatively well illuminated microsites with a diffuse site factor above 0.1, predicted optimal P_max was about 5 μmol m⁻² s⁻¹, with the predicted mean daily net carbon gain being about 50 mmol m⁻² day⁻¹. Each of the predicted optimal P_max and the simulated mean daily net carbon gains with a constant P_max (5 μmol m⁻² s⁻¹) or the predicted optimal P_max was linearly related to the microsite light availability index, diffuse site factor. Simulated net carbon gain was negative at diffuse site factors below 0.04, if the constant of P_max was assumed. The predicted linear relationship between net carbon gain and diffuse site factor could provide an ecophysiological basis for the observed linear dependency of the relative growth rate of biomass ofQ. serrata seedlings on the microsite diffuse site factor (Washitani & Tang 1991).     

Photosynthetic Response to Irradiance in Valeriana jatamansi Jones,a Threatened Understorey Medicinal Herb of Western Himalaya

Net photosynthetic rate (P _N) of Valeriana jatamansi plants, grown under nylon net shade or under different tree canopies, was saturated with photons at 1 000 mol m^–2 s^–1 photosynthetic photon-flux-density (PPFD), whereas open-grown plants were able to photosynthesise even at higher PPFD, e.g. of 2 000 mol m^–2 s^–1. Plants grown under net shade had higher total chlorophyll (Chl) content per unit area of leaf surface. However, Chl a/b ratio was maximal in open-grown plants, but remained unchanged in plants grown in nylon net shade and under different tree canopies. Sun-grown plants had thicker leaves (higher leaf mass per leaf area unit), higher wax content, and higher P _N than shade grown plants. Thus V. jatamansi is able to acclimate to high PPFD and therefore this Himalayan species may be cultivated in open habitat to meet the ever-increasing industrial demand.     

花生幼苗光合特性对弱光的响应

在苗期用黑色遮阳网对丰花1号和丰花2号进行不同遮光处理(不遮光,遮光27%、43%和77%),研究了苗期遮光及恢复对花生叶片光合特性的影响.结果表明： 遮光后,随遮光程度增强,叶片叶绿素含量显著增加,实际光化学效率(Ф_PSⅡ)和最大光化学效率(F_v/F_m)升高,叶绿素a/b和净光合速率(P_n)降低.恢复自然光照后1 d,在高光强下测定时,随前期遮光程度增强,P_n、气孔导度(G_s)下降,细胞间隙CO₂浓度(C_i)升高;在低光强下测定时,P_n显著升高,G_s和C_i下降;低光强与高光强下测定的P_n比值显著升高;恢复自然光照后,随前期遮光程度增强,光补偿点、光饱和点、CO₂补偿点、CO₂饱和点和羧化效率显著降低,表观量子效率显著升高.恢复自然光照后,P_n、Ф_PSⅡ和F_v/F_m先迅速下降,3～5 d后逐渐回升;恢复15 d后,遮光27%处理的各项指标恢复到对照水平,其他处理的恢复程度则因遮光程度和品种而异.丰花1号各处理叶绿素含量、P_n、Ф_PSⅡ均高于丰花2号.苗期遮光提高了花生利用弱光的能力,降低了其利用强光的能力.     

不同光环境下亚热带常绿阔叶树种和落叶阔叶树种幼苗的叶形态和光合生理特征

由于地球环境的演变,亚热带常绿阔叶林中常有落叶阔叶成分的存在,但从生态角度,其存在的机理尚不清楚.通过比较研究常绿阔叶优势树种大头茶与落叶阔叶树种枫香幼苗的叶形态和光合生理特征对不同光环境(旷地、林窗、林下)的响应,尝试解释落叶成分遗留的基础.结果表明:①枫香具有较小的比叶重(LMA)和较高的光合氮利用效率(PNUE),在高光环境中,并未受到光抑制,具有较高的光合可塑性,尤其在林窗,表现出较大头茶高3倍的光合能力(Pnmax),保证了其在较短的生长季节积累更多的光合产物,增强与常绿树种的竞争力;②常绿阔叶树种大头茶具有较大的LMA,在旷地受到严重的光抑制,将更多的氮用于化学防御中.在林窗和林下具有相对高的Pnmax,生长幅度较广,但在林下将更多的氮投入到比生长更为重要的生存消耗中.而枫香在林下响应表现为Rubisco活性降低,光合受阻;③两树种在林窗均表现出较高的光合适应性,具有最大的光合能力(Pnmax)及合理的氮在光合机构中的分配系数,说明林窗是它们更新的最佳环境.但枫香的光合作用更依赖于光照,而大头茶则更依赖于CO2浓度;④总之,落叶阔叶树种以其较高的形态可塑性和对高光的光合生理可塑性能够在常绿阔叶林种生存、生长,并成为常绿阔叶林中固有成分的主要原因之一.     

A new method to measure and calculate light intensity and light quality simultaneously by using portable spectrometer

The influence of light intensity and light quality on plants is highly concerned in the field of plant physiology and ecology. However, the calibrated quantum meter for measurement of light intensity cannot measure light quality, and vice versa. Here we developed an empirical formula to convert light energy to photon flux density, based on the measurement conditions of spectrometer. Under the guide of the formula, a portable spectrometer (AvaSpec-ULS2048×64) was calibrated by using four narrowband light emitting diode (LEDs) in combination with a calibrated quantum meter (LI-190SB). After calibration of the spectrometer, we can calculate photosynthetic photon flux density (PPFD or PAR) and measure spectrum of radiation flux simultaneously. Under natural light conditions, the errors between measured and calculated PPFDs are in the range from -2% to 5%, indicating the reliability of the method. With this new approach, the application of portable spectrometer can be greatly broadened: 1) the light intensity and quality of light source and plant growth light environment can be obtained simultaneously, 2) PPFD can be obtained within any specified wavelength range, and 3) there is no need to use standard light source to obtain the absolute light/radiation flux of a spectrum measured by spectrometer. In conclusion, this method has potential applications for the study of plant physiology and ecology.     

用便携式光谱仪同步测定和计算光强和光质的新方法

外界光强和光质对植物的影响在植物生理生态研究领域中一直受到高度关注。而测定光强的光量子计不能测定光质; 测定光质的光谱仪不能直接测定光强, 两者均不能同步测定光强和光质。该文作者建立了一个基于光谱仪测定条件的能量与光量子的经验转换公式, 用4只不同波长的窄带发光二极管(LED)光源结合光量子计(LI-190SB)对便携式光谱仪(AvaSpec-ULS2048×64)所获得的光谱进行了快速标定, 实现了用便携式光谱仪同步直接测定光量子通量密度和光质的目的。在自然光照条件下, 采用转换公式计算出光量子通量密度(PPFD)与实测的PPFD之间误差在-2%-5%范围内, 证实了这种方法的可靠性。通过这个新方法, 可以极大地拓宽便携式光谱仪的适用范围: 1)实验室内或野外只需用便携式光谱仪即可对光源及植物生长的光强和光质环境进行同步精确测定和计算; 2)可以计算光谱仪测定范围内任意波长区段的光量子通量密度; 3)无需采用标准光源即可获得绝对辐射(光)通量值。因此, 这项技术在植物生理生态研究领域具有广阔的应用前景。     

Acclimation of photosynthetic capacity to irradiance in tree canopies in relation to leaf nitrogen concentration and leaf mass per unit area

The observation of acclimation in leaf photosynthetic capacity to differences in growth irradiance has been widely used as support for a hypothesis that enables a simplification of some soil‐vegetation‐atmosphere transfer (SVAT) photosynthesis models. The acclimation hypothesis requires that relative leaf nitrogen concentration declines with relative irradiance from the top of a canopy to the bottom, in 1 : 1 proportion. In combination with a light transmission model it enables a simple estimate of the vertical profile in leaf nitrogen concentration (which is assumed to determine maximum carboxylation capacity), and in combination with estimates of the fraction of absorbed radiation it also leads to simple ‘big‐leaf’ analytical solutions for canopy photosynthesis. We tested how forests deviate from this condition in five tree canopies, including four broadleaf stands, and one needle‐leaf stand: a mixed‐species tropical rain forest, oak (Quercus petraea (Matt.) Liebl), birch (Betula pendula Roth), beech (Fagus sylvatica L.) and Sitka spruce (Picea sitchensis (Bong.) Carr). Each canopy was studied when fully developed (mid‐to‐late summer for temperate stands). Irradiance (Q, µmol m^?2 s^?1) was measured for 20 d using quantum sensors placed throughout the vertical canopy profile. Measurements were made to obtain parameters from leaves adjacent to the radiation sensors: maximum carboxylation and electron transfer capacity (V_a, J_a, µmol m^?2 s^?1), day respiration (R_da, µmol m^?2 s^?1), leaf nitrogen concentration (N_m, mg g^?1) and leaf mass per unit area (L_a, g m^?2). Relative to upper‐canopy values, V_a declined linearly in 1 : 1 proportion with N_a. Relative V_a also declined linearly with relative Q, but with a significant intercept at zero irradiance (P < 0·01). This intercept was strongly related to L_a of the lowest leaves in each canopy (P < 0·01, r² = 0·98, n= 5). For each canopy, daily lnQ was also linearly related with lnV_a(P < 0·05), and the intercept was correlated with the value for photosynthetic capacity per unit nitrogen (PUN: V_a/N_a, µmol g^?1 s^?1) of the lowest leaves in each canopy (P < 0·05). V_a was linearly related with L_a and N_a(P < 0·01), but the slope of the V_a : N_a relationship varied widely among sites. Hence, whilst there was a unique V_a : N_a ratio in each stand, acclimation in V_a to Q varied predictably with L_a of the lowest leaves in each canopy. The specific leaf area, L_m(cm² g^?1), of the canopy‐bottom foliage was also found to predict carboxylation capacity (expressed on a mass basis; V_m, µmol g^?1 s^?1) at all sites (P < 0·01). These results invalidate the hypothesis of full acclimation to irradiance, but suggest that L_a and L_m of the most light‐limited leaves in a canopy are widely applicable indicators of the distribution of photosynthetic capacity with height in forests.     

Longevity and factors influencing photosynthesis in tea leaves

Quadratic relationship between the age of a tea leaf and the net photosynthetic rate (PN) has been found. A progressive increase in PN was recorded for four months. Then the PN slowly declined, yet even seven-month-old tea leaves sustained a low PN. In a tea shoot, the PN increased from the first leaf onwards. Besides the physiological maturity and proximity, photon flux density (PFD) played an important role in reducing the PN. The tea leaf PN was influenced by cultivation procedures which in turn disrupted the quantum of PFD transmitted through the canopy.     

Estimating photosynthetic radiation use efficiency using incident light and photosynthesis of individual leaves

It has been theorized that photosynthetic radiation use efficiency (PhRUE) over the course of a day is constant for leaves throughout a canopy if leaf nitrogen content and photosynthetic properties are adapted to local light so that canopy photosynthesis over a day is optimized. To test this hypothesis, 'daily' photosynthesis of individual leaves of Solanum melongena plants was calculated from instantaneous rates of photosynthesis integrated over the daylight hours. Instantaneous photosynthesis was estimated from the photosynthetic responses to photosynthetically active radiation (PAR) and from the incident PAR measured on individual leaves during clear and overcast days. Plants were grown with either abundant or scarce N fertilization. Both net and gross daily photosynthesis of leaves were linearly related to daily incident PAR exposure of individual leaves, which implies constant PhRUE over a day throughout the canopy. The slope of these relationships (i.e. PhRUE) increased with N fertilization. When the relationship was calculated for hourly instead of daily periods, the regressions were curvilinear, implying that PhRUE changed with time of the day and incident radiation. Thus, linearity (i.e. constant PhRUE) was achieved only when data were integrated over the entire day. Using average PAR in place of instantaneous incident PAR increased the slope of the relationship between daily photosynthesis and incident PAR of individual leaves, and the regression became curvilinear. The slope of the relationship between daily gross photosynthesis and incident PAR of individual leaves increased for an overcast compared with a clear day, but the slope remained constant for net photosynthesis. This suggests that net PhRUE of all leaves (and thus of the whole canopy) may be constant when integrated over a day, not only when the incident PAR changes with depth in the canopy, but also when it varies on the same leaf owing to changes in daily incident PAR above the canopy. The slope of the relationship between daily net photosynthesis and incident PAR was also estimated from the photosynthetic light response curve of a leaf at the top of the canopy and from the incident PAR above the canopy, in place of that measured on individual leaves. The slope (i.e. net PhRUE) calculated in this simple way did not differ statistically from that calculated using data from individual leaves.     

Sigma factor SigC is required for heat acclimation of the cyanobacterium Synechocystis sp. strain PCC 6803

The role of the primary-like sigma factor SigC was studied in Synechocystis. Under high temperature stress (48 degrees C) the DeltasigC inactivation strain showed a lower survival rate than the control strain. The DeltasigC strain grew poorly at 43 degrees C in liquid cultures under normal air. However, change to 3% CO(2) enhanced growth of DeltasigC at 43 degrees C. Differences in expression of many genes related to the carbon concentrating mechanisms between the control and the DeltasigC strain were recorded with a genome-wide DNA microarray. We suggest that low solubility of CO2 at high temperature is one of the factors contributing to the poor thermotolerance of the DeltasigC strain.     

中国野生山梨叶片形态及光合特性

以异位保存在国家梨资源圃的48份我国原产山梨品种和2份秋子梨地方品种为材料,比较了野生山梨与地方秋子梨品种间差异,研究了我国野生山梨的光合特征以及光合特性相关指标间的关系,建立了山梨光合及瞬时水分利用特征的线性回归方程.结果表明:地方品种叶片形态特征指标、叶绿素含量、光合特征指标都显著低于野生品种平均值,且低于大部分野生品种的测定值;山梨叶片的比叶面积、叶干物质含量、胞间CO2浓度的变异系数较低,其他8项指标变异系数为0.12～0.41,表现出较高的多样性水平,可见我国野生山梨资源光合特征差异明显;光合特性指标与叶绿素组成(Chl a/b)、叶干物质含量呈显著相关;光合速率与胞间CO2浓度、蒸腾速率、气孔导度呈显著的指数曲线关系,山梨光合速率主要受气孔限制的影响.‘锦州山梨’具有高光合特性,可作为山梨资源光合特征研究利用的良好材料.     

Coupled response of stomatal and mesophyll conductance to light enhances photosynthesis of shade leaves under sunflecks

Light gradients within tree canopies play a major role in the distribution of plant resources that define the photosynthetic capacity of sun and shade leaves. However, the biochemical and diffusional constraints on gas exchange in sun and shade leaves in response to light remain poorly quantified, but critical for predicting canopy carbon and water exchange. To investigate the CO₂ diffusion pathway of sun and shade leaves, leaf gas exchange was coupled with concurrent measurements of carbon isotope discrimination to measure net leaf photosynthesis (A_n), stomatal conductance (g_s) and mesophyll conductance (g_m) in Eucalyptus tereticornis trees grown in climate controlled whole‐tree chambers. Compared to sun leaves, shade leaves had lower A_n, g_m, leaf nitrogen and photosynthetic capacity (A_max) but g_s was similar. When light intensity was temporarily increased for shade leaves to match that of sun leaves, both g_s and g_m increased, and A_n increased to values greater than sun leaves. We show that dynamic physiological responses of shade leaves to altered light environments have implications for up‐scaling leaf level measurements and predicting whole canopy carbon gain. Despite exhibiting reduced photosynthetic capacity, the rapid up‐regulation of g_m with increased light enables shade leaves to respond quickly to sunflecks.     

Leaf internal diffusion conductance limits photosynthesis more strongly in older leaves of Mediterranean evergreen broad-leaved species

Leaf age-dependent changes in structure, nitrogen content, internal mesophyll diffusion conductance (g_m), the capacity for photosynthetic electron transport (J_max) and the maximum carboxylase activity of Rubisco (V_cmax) were investigated in mature non-senescent leaves of Laurus nobilis L., Olea europea L. and Quercus ilex L. to test the hypothesis that the relative significance of biochemical and diffusion limitations of photosynthesis changes with leaf age. The leaf life-span was up to 3 years in L. nobilis and O. europea and 6 years in Q. ilex. Increases in leaf age resulted in enhanced leaf dry mass per unit area (M_A), larger leaf dry to fresh mass ratio, and lower nitrogen contents per dry mass (N_M) in all species, and lower nitrogen contents per area (N_A) in L. nobilis and Q. ilex. Older leaves had lower g_m, J_max and V_cmax. Due to the age-dependent increase in M_A, mass-based g_m, J_max and V_cmax declined more strongly (7- to 10-fold) with age than area-based (5- to 7-fold) characteristics. Diffusion conductance was positively associated with foliage photosynthetic potentials. However, this correlation was curvilinear, leading to lower ratio of chloroplastic to internal CO₂ concentration (C_c/C_i) and larger drawdown of CO₂ from leaf internal air space to chloroplasts (Δ_C) in older leaves with lower g_m. Overall the age-dependent decreases in photosynthetic potentials were associated with decreases in N_M and in the fraction of N in photosynthetic proteins, whereas decreases in g_m were associated with increases in M_A and the fraction of cell walls. These age-dependent modifications altered the functional scaling of foliage photosynthetic potentials with M_A, N_M, and N_A. The species primarily differed in the rate of age-dependent modifications in foliage structural and functional characteristics, but also in the degree of age-dependent changes in various variables. Stomatal openness was weakly associated with leaf age, but due to species differences in stomatal openness, the distribution of total diffusion limitation between stomata and mesophyll varied among species. These data collectively demonstrate that in Mediterranean evergreens, structural limitations of photosynthesis strongly interact with biochemical limitations. Age-dependent changes in g_m and photosynthetic capacities do not occur in a co-ordinated manner in these species such that mesophyll diffusion constraints curb photosynthesis more in older than in younger leaves.     

A feedback control system for the precise and continuous regulation of photosynthetic photon flux density

A simple and inexpensive feedback control system that provides continuous and precise control of photosynthetic photon flux density (PPFD) in a whole plant cuvette is described. A ‘Plexiglass’ tank is interposed between a light source and cuvette and PPFD changed by varying the level of dyed liquid in the tank. The amount of liquid pumped into or drained from the tank is a function of the difference (error) between a defined set point value of PPFD and that measured in the cuvette. The set point can be varied as a function of time, can follow the output of a quantum sensor measuring ambient PPFD or can be driven by values of PPFD read from a data file. Within the 0.4 to 0.64 μm waveband, the dye acts as a neutral density filter so that there is no change in spectral distribution with PPFD. Photosynthetic photon flux density in the cuvette was controlled to better than 20 μmol m⁻²s⁻¹ when the set point was varied from 200 to 1100 μmol m⁻²s⁻¹ over 3 min. When the set point was held constant or changed less rapidly, errors did not exceed 5 μmol m⁻²s⁻¹. Net photosynthesis of Western redcedar (Thuja plicata Donn.) seedlings held at 18 °C closely followed rapid changes in PPFD.