Role of sedoheptulose-1,7 bisphosphatase in low light tolerance of rice (Oryza sativa L.)

Rice grain yield is drastically reduced under low light especially in kharif (wet) season due to cloudy weather during most part of crop growth. Therefore, 50–60% of yield penalty was observed. To overcome this problem, identification of low light tolerant rice genotypes with a high buffering capacity trait such as photosynthetic rate has to be developed. Sedoheptulose-1,7 bisphosphatase, a light-regulated enzyme, plays pivotal role in the Calvin cycle by regenerating the substrate (RuBP) for RuBisCo and therefore, indirectly regulates the influx of CO₂ for this crucial process. We found a potential role of SBPase expression and activity in low light tolerant and susceptible rice genotypes by analyzing its influence on net photosynthetic rate and biomass. We observed a significant relationship of yield with photosynthesis, SBPase expression and activity especially under low light conditions. Two tolerant and two susceptible rice genotypes were used for the present study. Tolerant genotypes exhibited significant but least reduction compared to susceptible genotypes in the expression and activity of SBPase, which was also manifested in its photosynthetic rate and finally in the grain yield under low light. However, susceptible genotypes showed significant reduction in SBPase activity along with photosynthesis and grain yield suggesting that tracking the expression and activity of SBPase could form a simple and reliable method to identify the low light tolerant rice cultivars. The data were analyzed using the Indostat 7.5, Tukey–Kramer method through Microsoft Excel 2019 and PAST4.0 software. The significant association of SBPase activity with the grain yield, net assimilation rate, electron transfer rate, biomass and grain weight were observed under low light stress. These traits should be considered while selecting and breeding for low light tolerant cultivars. Thus, SBPase plays a major role in the low light tolerance mechanism in rice.Electronic supplementary materialThe online version of this article (10.1007/s12298-020-00905-z) contains supplementary material, which is available to authorized users.     

Relationship between leaf photosynthetic function at grain filling stage and yield in super high-yielding hybrid rice ( Oryza sativa . L)

The characteristics of dry matter production before and after heading and the relationships between photosynthesis of flag leaves and dry matter accumulation in panicles were investigated on super high-yielding rice cv. Xieyou 9308 (the yield of up to 12 t/ha) with rice cv. Xieyou 63 as a control. The results showed that (i) the capacity of dry matter production before and after heading in Xieyou 9308, i.e. biomass and daily dry matter production, was remarkably higher than that in Xieyou 63, especially after heading; (ii) CO₂ assimilation capacity in flag leaves in Xieyou 9308, namely Leaf Source Capacity (LSC), was also significantly higher than that in Xieyou 63, and the supply of photosynthate in leaves and the demand of grain filling were completely synchronous in Xieyou 9308, but photosynthetic function in flag leaves in Xieyou 63 declined sharply 20 days after heading and it was not enough to meet the demand of grain filling. These results confirmed that high efficient photosynthetic function in leaves after heading and its complete synchronization with grain filling are the key approaches to super high yield of rice.     

Effect of topdressing on individual leaf photosynthesis at different position in direct-sown rice with non-woven fabric mulch system

Direct sowing with non-woven fabric mulch is the new organic rice cultivation system. We studied the effect of topdressing on individual leaf photosynthesis at different position and grain yield in rice plants cultivated by this system. Leaf photosynthetic rate at the different leaf position per plant (P _N-LP) of the third and fourth to lower leaves was higher when the topdressing amount was increased. Without topdressing or in no-fertilizers plots, the P _N-LP values of lower leaves were very low. The leaf photosynthetic rate per unit leaf area (P _N-LA) decreased gradually as the leaf position became lower. Again, the P _N-LA values of the top-dressed plots at the lower leaves were higher than that of plots without topdressing or without fertilizers. The lower leaves maintained a higher P _N because of a higher rate of nitrogen accumulation due to topdressing. The higher rate of photosynthesis in these leaves resulted in better root activity, which contributed to a better ripening percentage and ultimately higher rice grain yield.     

Relationship between leaf photosynthetic function at grain filling stage and yield in super high-yielding hybrid rice (<Emphasis Type="Italic">Oryza sativa</Emphasis>. L)

The characteristics of dry matter production before and after heading and the relationships between photosynthesis of flag leaves and dry matter accumulation in panicles were investigated on super high-yielding rice cv. Xieyou 9308 (the yield of up to 12 t/ha) with rice cv. Xieyou 63 as a control. The results showed that (i) the capacity of dry matter production before and after heading in Xieyou 9308, i.e. biomass and daily dry matter production, was remarkably higher than that in Xieyou 63, especially after heading; (ii) CO₂ assimilation capacity in flag leaves in Xieyou 9308, namely Leaf Source Capacity (LSC), was also significantly higher than that in Xieyou 63, and the supply of photosynthate in leaves and the demand of grain filling were completely synchronous in Xieyou 9308, but photosynthetic function in flag leaves in Xieyou 63 declined sharply 20 days after heading and it was not enough to meet the demand of grain filling. These results confirmed that high efficient photosynthetic function in leaves after heading and its complete synchronization with grain filling are the key approaches to super high yield of rice.     

Enhanced leaf photosynthesis as a target to increase grain yield: insights from transgenic rice lines with variable Rieske FeS protein content in the cytochrome b6/f complex

Although photosynthesis is the most important source for biomass and grain yield, a lack of correlation between photosynthesis and plant yield among different genotypes of various crop species has been frequently observed. Such observations contribute to the ongoing debate whether enhancing leaf photosynthesis can improve yield potential. Here, transgenic rice plants that contain variable amounts of the Rieske FeS protein _in the cytochrome (cyt) b₆/f complex between 10 and 100% of wild‐type levels have been used to investigate the effect of reductions of these proteins on photosynthesis, plant growth and yield. Reductions of the cyt b₆/f complex did not affect the electron transport rates through photosystem I but decreased electron transport rates through photosystem II, leading to concomitant decreases in CO₂ assimilation rates. There was a strong control of plant growth and grain yield by the rate of leaf photosynthesis, leading to the conclusion that enhancing photosynthesis at the single‐leaf level would be a useful target for improving crop productivity and yield both via conventional breeding and biotechnology. The data here also suggest that changing photosynthetic electron transport rates via manipulation of the cyt b₆/f complex could be a potential target for enhancing photosynthetic capacity in higher plants.     

The potential role of sucrose transport gene expression in the photosynthetic and yield response of rice cultivars to future CO2 concentration

The metabolic basis for observed differences in the yield response of rice to projected carbon dioxide concentrations (CO₂) is unclear. In this study, three rice cultivars, differing in their yield response to elevated CO₂, were grown under ambient and elevated CO₂ conditions, using the free-air CO₂ enrichment technology. Flag leaves of rice were used to determine (1) if manipulative increases in sink strength decreased the soluble sucrose concentration for the ‘weak’ responders and (2), whether the genetic expression of sucrose transporters OsSUT1 and OsSUT2 was associated with an accumulation of soluble sugars and the maintenance of photosynthetic capacity. For the cultivars that showed a weak response to additional CO₂, photosynthetic capacity declined under elevated CO₂ and was associated with an accumulation of soluble sugars. For these cultivars, increasing sink relative to source strength did not increase photosynthesis and no change in OsSUT1 or OsSUT2 expression was observed. In contrast, the ‘strong’ response cultivar did not show an increase in soluble sugars or a decline in photosynthesis but demonstrated significant increases in OsSUT1 and OsSUT2 expression at elevated CO₂. Overall, these data suggest that the expression of the sucrose transport genes OsSUT1 and OsSUT2 may be associated with the maintenance of photosynthetic capacity of the flag leaf during grain fill; and, potentially, greater yield response of rice as atmospheric CO₂ increases.     

Blue light effects on rose photosynthesis and photomorphogenesis

Through its impact on photosynthesis and morphogenesis, light is the environmental factor that most affects plant architecture. Using light rather than chemicals to manage plant architecture could reduce the impact on the environment. However, the understanding of how light modulates plant architecture is still poor and further research is needed. To address this question, we examined the development of two rose cultivars, Rosa hybrida‘Radrazz’ and Rosa chinensis‘Old Blush’, cultivated under two light qualities. Plants were grown from one‐node cuttings for 6 weeks under white or blue light at equal photosynthetic efficiencies. While plant development was totally inhibited in darkness, blue light could sustain full development from bud burst until flowering. Blue light reduced the net CO₂ assimilation rate of fully expanded leaves in both cultivars, despite increasing stomatal conductance and intercellular CO₂ concentrations. In ‘Radrazz’, the reduction in CO₂ assimilation under blue light was related to a decrease in photosynthetic pigment content, while in both cultivars, the chl a/b ratio increased. Surprisingly, blue light could induce the same organogenetic activity of the shoot apical meristem, growth of the metamers and flower development as white light. The normal development of rose plants under blue light reveals the strong adaptive properties of rose plants to their light environment. It also indicates that photomorphogenetic processes can all be triggered by blue wavelengths and that despite a lower assimilation rate, blue light can provide sufficient energy via photosynthesis to sustain normal growth and development in roses.     

Brassinosteroids regulate grain filling in rice

Genes controlling hormone levels have been used to increase grain yields in wheat (Triticum aestivum) and rice (Oryza sativa). We created transgenic rice plants expressing maize (Zea mays), rice, or Arabidopsis thaliana genes encoding sterol C-22 hydroxylases that control brassinosteroid (BR) hormone levels using a promoter that is active in only the stems, leaves, and roots. The transgenic plants produced more tillers and more seed than wild-type plants. The seed were heavier as well, especially the seed at the bases of the spikes that fill the least. These phenotypic changes brought about 15 to 44% increases in grain yield per plant relative to wild-type plants in greenhouse and field trials. Expression of the Arabidopsis C-22 hydroxylase in the embryos or endosperms themselves had no apparent effect on seed weight. These results suggested that BRs stimulate the flow of assimilate from the source to the sink. Microarray and photosynthesis analysis of transgenic plants revealed evidence of enhanced CO₂ assimilation, enlarged glucose pools in the flag leaves, and increased assimilation of glucose to starch in the seed. These results further suggested that BRs stimulate the flow of assimilate. Plants have not been bred directly for seed filling traits, suggesting that genes that control seed filling could be used to further increase grain yield in crop plants.     

Relationship between leaf photosynthetic function at grain filling stage and yield in super high-yielding hybrid rice (Oryza sativa. L)

The characteristics of dry matter production before and after heading and the relationships between photosynthesis of flag leaves and dry matter accumulation in panicles were investigated on super high-yielding rice cv. Xieyou 9308 (the yield of up to 12 t/ha) with rice cv. Xieyou 63 as a control. The results showed that (i) the capacity of dry matter production before and after heading in Xieyou 9308, i.e. biomass and daily dry matter production, was remarkably higher than that in Xieyou 63, especially after heading; (ii) CO2 assimilation capacity in flag leaves in Xieyou 9308, namely Leaf Source Capacity (LSC), was also significantly higher than that in Xieyou 63, and the supply of photosynthate in leaves and the demand of grain filling were completely synchronous in Xieyou 9308, but photosynthetic function in flag leaves in Xieyou 63 declined sharply 20 days after heading and it was not enough to meet the demand of grain filling. These results confirmed that high efficient photosynthetic function in     

Changes in leaf photosynthesis of transgenic rice with silenced <Emphasis Type="Italic">OsBP-73</Emphasis> gene

In comparison with its wild type (WT), the transgenic (TG) rice with silenced OsBP-73 gene had significantly lower plant height, grain number per panicle, and leaf net photosynthetic rate (P _N). Also, the TG rice showed significantly lower chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), RuBPCO activase, and RuBP contents, photosystem 2 (PS2) photochemical efficiency (F_v/F_m and ΔF/F_m′), apparent quantum yield of carbon assimilation (Φ_c), carboxylation efficiency (CE), photosynthetic electron transport and photophosphorylation rates as well as sucrose phosphate synthase activity, but higher intercellular CO₂ concentration, sucrose, fructose, and glycerate 3-phosphate contents, and non-photochemical quenching of Chl fluorescence (NPQ). Thus the decreased P _N in the TG rice leaves is related to both RuBP carboxylation and RuBP regeneration limitations, and the latter is a predominant limitation to photosynthesis.     

中国北方杂草稻光合与水分生理特性

杂草稻是一类重要的稻属种质资源,具有耐寒、耐旱、耐瘠薄等优良特性.本文以88份中国北方杂草稻资源和4份栽培稻为材料,研究了中国北方杂草稻的光合速率、蒸腾速率、气孔导度等光合与水分生理特性及其相互关系.结果表明: 北方杂草稻资源的光合和水分生理特性存在较大差异,具有丰富的多样性.杂草稻的光合速率变化范围在12.47～28.67 μmol CO₂·m^-2·s^-1,瞬时水分利用效率的变化范围在1.39～3.40 mg·g^-1.光合参数中,胞间CO₂浓度的变异系数最小,气孔导度的变异系数最大.光合速率与蒸腾速率、气孔导度呈极显著的二次曲线关系,光合速率与胞间CO₂浓度呈显著的直线关系,瞬时水分利用效率与蒸腾速率、气孔导度呈极显著的二次曲线关系.可用杂草稻材料的优越性能对栽培稻进行品种改良.     

The temporal and species dynamics of photosynthetic acclimation in flag leaves of rice (Oryza sativa) and wheat (Triticum aestivum) under elevated carbon dioxide

In this study, we tested for the temporal occurrence of photosynthetic acclimation to elevated [CO₂] in the flag leaf of two important cereal crops, rice and wheat. In order to characterize the temporal onset of acclimation and the basis for any observed decline in photosynthetic rate, we characterized net photosynthesis, g_s, g_m, C_i/C_a, C_i/C_c, V_cmax, J_max, cell wall thickness, content of Rubisco, cytochrome (Cyt) f, N, chlorophyll and carbohydrate, mRNA expression for rbcL and petA, activity for Rubisco, sucrose phosphate synthase (SPS) and sucrose synthase (SS) at full flag expansion, mid‐anthesis and the late grain‐filling stage. No acclimation was observed for either crop at full flag leaf expansion. However, at the mid‐anthesis stage, photosynthetic acclimation in rice was associated with RuBP carboxylation and regeneration limitations, while wheat only had the carboxylation limitation. By grain maturation, the decline of Rubisco content and activity had contributed to RuBP carboxylation limitation of photosynthesis in both crops at elevated [CO₂]; however, the sharp decrease of Rubisco enzyme activity played a more important role in wheat. Although an increase in non‐structural carbohydrates did occur during these later stages, it was not consistently associated with changes in SPS and SS or photosynthetic acclimation. Rather, over time elevated [CO₂] appeared to enhance the rate of N degradation and senescence so that by late‐grain fill, photosynthetic acclimation to elevated [CO₂] in the flag leaf of either species was complete. These data suggest that the basis for photosynthetic acclimation with elevated [CO₂] may be more closely associated with enhanced rates of senescence, and, as a consequence, may be temporally dynamic, with significant species variation.     

太阳辐射减弱对水稻光合生理特性和中微量元素积累的影响

近年来雾霾等大气环境污染问题突出,气溶胶导致到达地表的太阳辐射强度降低,对农作物生长造成了较大影响。为了探讨太阳辐射强度降低对粮食作物生理特性和矿质元素营养的影响,本研究以‘南粳5055’水稻品种为供试材料,采用田间随机区组设计,通过不同程度遮光处理模拟弱光环境(CK,不遮光对照;Y₁、Y₂遮光率分别为19%、45%),研究了在水稻关键生育期(拔节期、抽穗期、灌浆期)内叶绿素含量(SPAD)、叶面积指数(LAI)、叶片净光合速率、产量和籽粒中、微量金属元素(Ca、Mg、Fe、Zn、Mn、Cu)含量的响应特征。结果表明: 全生育期内遮光处理抑制了水稻光合作用产物的合成,降低了叶面积指数;但在生育前期对叶绿素含量无显著影响,在生育后期则显著增加。两种遮光处理下水稻的千粒重较CK分别降低了14.4%和18.4%,结实率降低了4.3%和12.9%,从而导致水稻产量下降,且随着遮光程度增加,水稻减产率加大,产量分别下降了58.5%和66.4%。遮光处理籽粒糙米和颖壳中金属元素含量升高,尤其微量元素含量显著上升。可见,遮光对水稻生长产生了不利影响,最终使其减产,并使Cu、Mn等重金属元素含量增加,这可能造成污染风险进而对人体健康造成威胁,因此,太阳辐射减弱对作物产量和品质的影响需要综合评价。     

Sugar accumulation,photosynthesis and growth of two indica rice varieties in response to salt stress

Sugar, a final product of photosynthesis, is reported to be involved in the defense mechanisms of plants against abiotic stresses such as salinity, water deficiency, extreme temperature and mineral toxicity. Elements involved in photosynthesis, sugar content, water oxidation, net photosynthetic rate, activity of enzyme and gene expression have therefore been studied in Homjan (HJ), salt-tolerant, and Pathumthani 1 (PT1), salt-sensitive, varieties of rice. Fructose-1,6-biphosphatase (FBP) and fructokinase (FK) genes were rapidly expressed in HJ rice when exposed to salt stress for 1–6 h and to a greater degree than in PT1 rice. An increase in FBP enzyme activity was found in both roots and leaves of the salt-tolerant variety after exposure to salt stress. A high level of sugar and a delay in chlorophyll degradation were found in salt-tolerant rice. The total sugar content in leaf and root tissues of salt-tolerant rice was 2.47 and 2.85 times higher, respectively, than in the salt-sensitive variety. Meanwhile, less chlorophyll degradation was detected. Salt stress may promote sugar accumulation, thus preventing the degradation of chlorophyll. Water oxidation by the light reaction of photosynthesis in the salt-tolerant variety was greater than that in the salt-sensitive variety, indicated by a high maximum quantum yield of PSII (F _v/F _m) and quantum efficiency of PSII (Φ_PSII) with low nonphotochemical quenching (NPQ), leading to a high net photosynthetic rate. In addition, the overall growth performances in the salt-tolerant variety were higher than those in the salt-sensitive variety. The FBP gene expression and enzyme activity, sugar accumulation, pigment stabilization, water oxidation and net photosynthetic rate parameters in HJ rice should be further investigated as multivariate salt-tolerant indices for the classification of salt tolerance in rice breeding programs.     

不同秧苗素质和移栽密度条件下臭氧胁迫对水稻光合作用、物质生产和产量的影响

近地层臭氧浓度升高使水稻生长受抑进而使产量下降,但这种影响是否因不同栽培条件而异尚不清楚。2011年依托先进的稻田臭氧FACE(Free Air gas Concentration Enrichment)技术平台,以汕优63为供试材料,臭氧设置大气臭氧浓度(Ambient)和高臭氧浓度(比Ambient高50%),秧苗素质设置弱苗(移栽时无分蘖)和壮苗(移栽时带两个分蘖),移栽密度设置低密度(16穴/m2)、中密度(24穴/m2)和高密度(32穴/m2),研究不同秧苗素质和移栽密度条件下臭氧胁迫对水稻生长和产量的影响。结果表明:高浓度臭氧使水稻结实期叶片SPAD值、净光合速率、气孔导度和蒸腾速率明显下降,但胞间CO2浓度和叶温无显著变化。高浓度臭氧对水稻拔节前物质生产量没有影响,但使拔节至抽穗期、抽穗至成熟期物质生产量平均分别降低13%和29%,进而使成熟期生物产量和籽粒产量均显著下降。方差分析表明,臭氧与秧苗素质间没有互作效应,但臭氧与移栽密度的互作对最终产量的影响达显著水平。以上结果表明,臭氧胁迫使水稻生长后期光合受阻,导致物质生产和产量显著下降;适当增加移栽密度可能会减少臭氧胁迫下水稻产量的损失。     

Influence of bacterial leaf blight on the photosynthetic characteristics of resistant and susceptible rice

The effect of crop disease on photosynthetic characteristics is important for disease control. Two varieties, Shenzhou 98 and Neiwuyou 8015 with resistance and susceptibility to bacterial leaf blight (BLB), respectively, were selected, and the responses of the net photosynthetic rate (P_N) to active photon flux density (PPFD) and intercellular carbon dioxide concentration (C_i), as well as chlorophyll fluorescence, pigments and stomatal resistance (SR), were measured. The results showed that BLB infection greatly decreased the maximum photosynthetic rate (P_max), light saturation point (LSP), carboxylation efficiency (CE), maximal fluorescence (F_m) and actual photochemical efficiency of PSII ( Φ _PSII) but increased the light compensation point (LCP) and dark respiratory rate (R_D), which suggested that the performance of rice photosynthesis was decreased by BLB infection. The BLB infection had a lower effect on resistant rice Shenzhou 98 than on susceptible rice Neiwuyou 8015. The reduction of pigment and increased SR caused by BLB infection may have resulted in the decline in the photosynthetic rate. Significant effects of the BLB infection were observed on chlorophyll fluorescence F_m and Φ_PSII in resistant and susceptible rice. These parameters may be useful for noninvasive monitoring of plant disease considering the negative effect caused by other stresses.     

Genetic variation and possible origins of weedy rice found in California

Control of weeds in cultivated crops is a pivotal component in successful crop production allowing higher yield and higher quality. In rice‐growing regions worldwide, weedy rice (Oryza sativa f. spontanea Rosh.) is a weed related to cultivated rice which infests rice fields. With populations across the globe evolving a suite of phenotypic traits characteristic of weeds and of cultivated rice, varying hypotheses exist on the origin of weedy rice. Here, we investigated the genetic diversity and possible origin of weedy rice in California using 98 simple sequence repeat (SSR) markers and an Rc gene‐specific marker. By employing phylogenetic clustering analysis, we show that four to five genetically distinct biotypes of weedy rice exist in California. Analysis of population structure and genetic distance among individuals reveals diverse evolutionary origins of California weedy rice biotypes, with ancestry derived from indica, aus, and japonica cultivated rice as well as possible contributions from weedy rice from the southern United States and wild rice. Because this diverse parentage primarily consists of weedy, wild, and cultivated rice not found in California, most existing weedy rice biotypes likely originated outside California.     

Effect of awns and drought on the supply of photosynthate and its distribution within wheat ears

The presence of awns doubled the net photosynthetic rate of wheat ears and also increased the proportion of ¹⁴CO₂ assimilated by the ear that moved to the grain. The effect of water supply on photosynthesis and movement of assimilates was greater for leaves than ears, so that drought increased the proportion of assimilate contributed by ear photosynthesis to grain filling from 13% to 24% in the awnless ears, and from 34% to 43% in the awned ears. ¹⁴C assimilated by the ears was most important to the economy of the upper spikelets and to the distal florets in each spikelet, whereas flag leaf assimilate went mainly to the spikelets in the lower half of the ear, and to the proximal florets. Awns increased grain yield in the dry but not in the irrigated treatment, despite the large contribution of awned ears to grain filling. Either the supply of assimilate did not limit grain yield when water supply was not limiting, or there were compensating disadvantages to awns. However, they did not seem to have any adverse effect on the development of the upper florets, nor did they reduce grain number per ear.     

All roads lead to weediness: Patterns of genomic divergence reveal extensive recurrent weedy rice origins from South Asian Oryza

Weedy rice (Oryza spp.), a weedy relative of cultivated rice (O. sativa), infests and persists in cultivated rice fields worldwide. Many weedy rice populations have evolved similar adaptive traits, considered part of the ‘agricultural weed syndrome’, making this an ideal model to study the genetic basis of parallel evolution. Understanding parallel evolution hinges on accurate knowledge of the genetic background and origins of existing weedy rice groups. Using population structure analyses of South Asian and US weedy rice, we show that weeds in South Asia have highly heterogeneous genetic backgrounds, with ancestry contributions both from cultivated varieties (aus and indica) and wild rice. Moreover, the two main groups of weedy rice in the USA, which are also related to aus and indica cultivars, constitute a separate origin from that of Asian weeds. Weedy rice populations in South Asia largely converge on presence of red pericarps and awns and on ease of shattering. Genomewide divergence scans between weed groups from the USA and South Asia, and their crop relatives are enriched for loci involved in metabolic processes. Some candidate genes related to iconic weedy traits and competitiveness are highly divergent between some weed‐crop pairs, but are not shared among all weed‐crop comparisons. Our results show that weedy rice is an extreme example of recurrent evolution, and suggest that most populations are evolving their weedy traits through different genetic mechanisms.     

Investigation of the CO(2) Dependence of Quantum Yield and Respiration in Eucalyptus pauciflora

In leaves of C₃ plants, the rate of nonphotorespiratory respiration appears to be higher in darkness than in the light. This change from a high to a low rate of carbon loss with increasing photon flux density leads to an increase in the apparent quantum yield of photosynthetic CO₂ assimilation at low photon flux densities (Kok effect). The mechanism of this suppression of nonphotorespiratory respiration is not understood, but biochemical evidence and the observation that a Kok effect is often not observed under low O₂, has led to the suggestion that photorespiration might be involved in some way. This hypothesis was tested with snowgum (Eucalyptus pauciflora Sieb. ex Spreng.) using gas exchange methods. The test was based on the assumption that if photorespiration were involved, then it would be expected that the intercellular partial pressure of CO₂ would also have an influence on the Kok effect. Under normal atmospheric levels of CO₂ and O₂, a Kok effect was found. Changing the intercellular partial pressure of CO₂, however, did not affect the estimate of nonphotorespiratory respiraton, and it was concluded that its decrease with increasing photon flux density did not involve photorespiration. Concurrent measurements showed that the quantum yield of net assimilation of CO₂ increased with increasing intercellular partial pressure of CO₂, and this increase agreed closely with predictions based on recent models of photosynthesis.