Differences on photosynthetic limitations between leaf margins and leaf centers under potassium deficiency for Brassica napus L

Nitrogen-potassium balance improves leaf photosynthetic capacity by regulating leaf nitrogen allocation in apple

Nitrogen (N) and potassium (K) are two important mineral nutrients in regulating leaf photosynthesis. However, the influence of N and K interaction on photosynthesis is still not fully understood. Using a hydroponics approach, we studied the effects of different N and K conditions on the physiological characteristics, N allocation and photosynthetic capacity of apple rootstock M9T337. The results showed that high N and low K conditions significantly reduced K content in roots and leaves, resulting in N/K imbalance, and allocated more N in leaves to non-photosynthetic N. Low K conditions increased biochemical limitation (BL), mesophyll limitation (MCL), and stomatal limitation (SL). By setting different N supplies, lowering N levels under low K conditions increased the proportion of water-soluble protein N (Nw) and sodium dodecyl sulfate-soluble proteins (Ns) by balancing N/K and increased the proportion of carboxylation N and electron transfer N. This increased the maximum carboxylation rate and mesophyll conductance, which reduced MCL and BL and alleviated the low K limitation of photosynthesis in apple rootstocks. In general, our results provide new insights into the regulation of photosynthetic capacity by N/K balance, which is conducive to the coordinated supply of N and K nutrients.

Nitrogen supply alleviates seed yield reduction by improving the morphology and carbon metabolism of pod walls in shaded rapeseed

Shading significantly affects rapeseed yield, while reasonable nitrogen (N) application has efficiency gains. However, the functions and mechanisms of N are not fully established for shaded rapeseed plants. Therefore, we conducted a 2-year field experiment to study the effect of N on pod wall morphology and carbon metabolism of shaded rapeseed. Two varieties, three N rates (120 [N1], 240 [N2], and 360 [N3] kg hm-2 ) and two light intensities (100 and 70% light transmission) from 10 to 35 days after the end of flowering were set as experimental parameters. Shading decreased the pod wall chlorophyll content, ribulose 1,5-bisphosphate carboxylase (Rubisco) activity and glucose content at 25 and 35 days after flowering (DAF). Decreased sucrose synthase (SuSy) and sucrose phosphate synthase activity caused by shading reduced sucrose and fructose content. They are responsible for the decline in the 1000-seed weight and a 22.1-37.6% decline in seed yield. More N under shading promoted pod elongation and pigment content, improved chloroplast ultrastructure, increased Rubisco and SuSy activity at 35 DAF, thus contributing to pod wall photosynthesis and fructose and glucose levels in shaded rapeseed plants. Similar trends were observed in pod number, pod weight, and seed weight, while the greatest increase in seed/wall ratio was observed under N2 for shaded rapeseed plants. The results indicated that N can reduce the yield difference between different light conditions and balance partitioning and conversion of photoassimilates in pod wall, but avoid applying an excessive amount of nitrogen.

Decoupling of P from C, N, and K Elements in Cucumber Leaves Caused by Nutrient Imbalance under a Greenhouse Continuous Cropping System

There is insufficient information regarding the stoichiometric variation and coupling status of carbon (C), nitrogen (N), phosphorus (P), and potassium (K) in the leaves of nutrient-enriched greenhouse agroecosystems with increasing planting time. Therefore, we assessed the variation in elemental stoichiometry ratios in soil and cucumber (Cucumis sativus L.) leaves, and the coupling status of elemental utilization in the leaves under continuous cropping systems using natural (only soil; i.e., control soil, CO) and artificial (soil + straw + chicken + urea; i.e., straw mixture soil, ST) soil via monitoring studies for 11 years in a solar greenhouse. Soil organic C, total N, and total P concentrations increased by 63.4%, 72.7%, and 144.3% in the CO, respectively, after 11 years of cultivation (compared to the first year), and by 18.1%, 24.3%, and 117.7% in the ST under continuous cropping conditions, respectively. Total K concentrations remained unchanged in both soils. Moreover, the availability of these soil elements increased to different degrees in both soils after 11 years of planting. Additionally, the leaf P concentration increased by 9.8% in the CO, while leaf N and K concentrations did not change, suggesting decoupling of P utilization from that of N and K in leaves under a continuous cropping system. These findings suggest that imbalanced soil nutrients under continuous cropping conditions results in decoupling of P from N and K in the utilization of leaf nutrients.

Potassium modulates central carbon metabolism to participate in regulating CO2 transport and assimilation in Brassica napus leaves

Potassium (K) regulates plant metabolism and enhances plant's ability to adapt to adversity. However, under different K deficiency stress, the net photosynthetic rate (A_n) was reduced, influenced by CO₂ conductance or biochemical capacities. The interplay between metabolome and photosynthetic characteristics under K deficiency stress was analyzed to explore the mechanisms by which K regulates photosynthetic capacity. With increasing K deficiency stress, dominations limiting A_n varied from CO₂ conductance to biochemical limitations. Multivariate analyses indicated that organic acids, amino acids and sedoheptulose-7-bisphosphate were significantly related to A_n, CO₂ conductance and carboxylation rate. Under moderate K deficiency, organic acids were up-regulated. Acidification of subcellular compartments reduced sedoheptulose-1,7-bisphosphatase activity, inducing downregulation of sedoheptulose-7-bisphosphate and hindrance of ribulose bisphosphate regeneration. Moreover, increased CO₂ shortage with increasing K deficiency induced a shift of increased citric acid to amino acid synthesis, causing excessive accumulation of amino acids. In addition, the reduced serine level indicated impaired photorespiration. These two changes triggered more serious reduction in photosynthetic capacity. The intimate, changes in photosynthetic capacities were tightly coupled with shifts in central C metabolism, which provides insights into the methods used to enhance A_n and plant's adaptability to abiotic stresses, through the regulation of C metabolites using molecular technology.

High nitrogen inhibits photosynthetic performance in a shade-tolerant and N-sensitive species Panax notoginseng

Nitrogen (N) is a primary factor limiting leaf photosynthesis. However, the mechanism of high-N-driven inhibition on photosynthetic efficiency and photoprotection is still unclear in the shade-tolerant and N-sensitive species such as Panax notoginseng. Leaf chlorophyll (Chl) content, Ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) activity and content, N allocation in the photosynthetic apparatus, photosynthetic performance and Chl fluorescence were comparatively analyzed in a shade-tolerant and N-sensitive species P. notoginseng grown under the levels of moderate nitrogen (MN) and high nitrogen (HN). The results showed that Rubisco content, Chl content and specific leaf nitrogen (SLN) were greater in the HN individuals. Rubisco activity, net photosynthetic rate (A_net), photosynthetic N use efficiency (PNUE), maximum carboxylation rate (V_cmax) and maximum electron transport rate (J_max) were lower when plants were exposed to HN as compared with ones to MN. A large proportion of leaf N was allocated to the carboxylation component under the levels of MN. More N was only served as a form of N storage and not contributed to photosynthesis in HN individuals. Compared with the MN plants, the maximum quantum yield of photosystem II (F_v/F_m), non-photochemical quenching of PSII (NPQ), effective quantum yield and electron transport rate were obviously reduced in the HN plants. Cycle electron flow (CEF) was considerably enhanced in the MN individuals. There was not a significant difference in maximum photo-oxidation P₇₀₀₊ (P_m) between the HN and MN individuals. Most importantly, the HN individuals showed higher K phase in the fast chlorophyll fluorescence induction kinetic curve (OJIP kinetic curve) than the MN ones. The results obtained suggest that photosynthetic capacity might be primarily inhibited by the inactivated Rubisco in the HN individuals, and HN-induced depression of photoprotection might be caused by the photodamage to the donor side of PSII oxygen-evolving complex.

Genome-Wide Survey and Expression Analysis of the KT/HAK/KUP Family in Brassica napus and Its Potential Roles in the Response to K+ Deficiency

The KT/HAK/KUP (HAK) family is the largest potassium (K⁺) transporter family in plants, which plays key roles in K⁺ uptake and homeostasis, stress resistance, and root and embryo development. However, the HAK family has not yet been characterized in Brassica napus. In this study, 40 putative B. napus HAK genes (BnaHAKs) are identified and divided into four groups (Groups I–III and V) on the basis of phylogenetic analysis. Gene structure analysis revealed 10 conserved intron insertion sites across different groups. Collinearity analysis demonstrated that both allopolyploidization and small-scale duplication events contributed to the large expansion of BnaHAKs. Transcription factor (TF)-binding network construction, cis-element analysis, and microRNA prediction revealed that the expression of BnaHAKs is regulated by multiple factors. Analysis of RNA-sequencing data further revealed extensive expression profiles of the BnaHAKs in groups II, III, and V, with limited expression in group I. Compared with group I, most of the BnaHAKs in groups II, III, and V were more upregulated by hormone induction based on RNA-sequencing data. Reverse transcription-quantitative polymerase reaction analysis revealed that the expression of eight BnaHAKs of groups I and V was markedly upregulated under K⁺-deficiency treatment. Collectively, our results provide valuable information and key candidate genes for further functional studies of BnaHAKs.

Potassium and Magnesium Mediate the Light and CO2 Photosynthetic Responses of Grapevines

Potassium (K) and magnesium (Mg) deficiency are common stresses that can impact on grape yield and quality, but their effects on photosynthesis have received little attention. Understanding the diffusional and biochemical limitations to photosynthetic constraints will help to guide improvements in cultural practices. Accordingly, the photosynthetic response of Vitis vinifera cvs. Shiraz and Chardonnay to K or Mg deficiency was assessed under hydroponic conditions using miniature low-nutrient-reserve vines. Photosynthesis was at least partly reduced by a decline in stomatal conductance. Light and CO₂-saturated photosynthesis, maximum rate of ribulose 1.5 bisphospate (RuBP) carboxylation (V_cmax) and maximum rate of electron transport (J_max) all decreased under K and Mg deficiency. Likewise, chlorophyll fluorescence and electron transport were lower under both nutrient deficiencies while dark respiration increased. K deficiency drastically reduced shoot biomass in both cultivars, while root biomass was greatly reduced under both Mg and K deficiency. Taken together, these results indicate that the decrease in biomass was likely due to both stomatal and biochemical limitations in photosynthesis. Optimising photosynthesis through adequate nutrition will thus support increases in biomass with carry-on positive effects on crop yields.

Leaf photosynthesis is mediated by the coordination of nitrogen and potassium: The importance of anatomical-determined mesophyll conductance to CO2 and carboxylation capacity

Both nitrogen (N) and potassium (K) have been widely studied in maintaining efficient photosynthesis and plant growth. However, the mechanisms underlying the photosynthetic response to their interaction remain unclear. This study assessed the effects of N and K supply on photosynthetic limitations and the corresponding changes in anatomical structures in leaves of rice (Oryza sativa L.) plants, grown hydroponically under different levels of N and K in a greenhouse. Results revealed that a suitable leaf N/K ratio (2.99-3.10) maintain a high rate of photosynthesis (A). The A under N and/or K deficiency was primarily limited by mesophyll conductance (g_m) and RuBP carboxylation in biochemical processes. The decline of g_m in N- or K-starved leaves was mostly resulted from low surface area of chloroplasts exposed to intercellular airspaces (S_c) and high mesophyll cell wall thickness. Synergistic effects of N and K on g_m were reflected in leaf anatomical structure, especially their coordinated roles in enhancing S_c. The enhanced photosynthesis in plants with coordinated supply of N and K was caused by the balance of RuBP carboxylation and regeneration. These results highlight the synergistic effect of N and K on leaf photosynthesis, which are mainly reflected in facilitating anatomical-determined g_m and carboxylation capacity.

Leaf photosynthetic capacity is regulated by the interaction of nitrogen and potassium through coordination of CO2 diffusion and carboxylation

Combined application of nitrogen (N) and potassium (K) fertilizer could significantly enhance crop yield. Crop yield and photosynthesis are inseparable. However, the influence of N and K interaction on photosynthesis is still not fully understood. Field and hydroponic experiments were conducted to examine the effects of N and K interaction on leaf photosynthesis characteristics and to explore the mechanisms in the hydroponic experiment. CO₂ conductance and carboxylation characteristic parameters of oilseed leaves were measured under different N and K supplies. Results indicated that detectable increases in leaf area, biomass and net photosynthetic rate (A_n ) were observed under optimal N and K supply in field and hydroponic experiments. The ratio of total CO₂ diffusion conductance to the maximum carboxylation rate (g_tot /V_cmax ) and A_n presented a linear-plateau relationship. Under insufficient N, increased K contributed to the CO₂ transmission capacity and improved the proportion of N used for carboxylation, promoting g_tot /V_cmax . However, the low V_cmax associated with N insufficiency limited the A_n . High N supply obviously accelerated V_cmax , yet K deficiency led to a reduction of g_tot , which restricted V_cmax . Synchronous increases in N and K supplementation ensured the appropriate ratio of N to K content in leaves, which simultaneously facilitated g_tot and V_cmax and preserved a g_tot /V_cmax suitable for guaranteeing CO₂ transmission and carboxylation coordination; the overall effect was increased A_n and leaf area. These results highlight the suitable N and K nutrients to coordinate CO₂ diffusion and carboxylation, thereby enhancing photosynthetic capacity and area to obtain high crop yield.

Effects of the decomposing liquid of Cladophora oligoclona on Hydrilla verticillata turion germination and seedling growth

Excessive proliferation of filamentous green algae (FGA) has been considered an important factor resulting in the poor growth or even decline of submerged macrophytes. However, there is a lack of detailed information regarding the effect of decaying FGA on submerged macrophytes. This study aimed to investigate whether the decomposing liquid from Cladophora oligoclona negatively affects Hydrilla verticillata turion germination and seedling growth. The results showed that the highest concentrations of decomposing liquid treatments inhibited the turion germination rate, which was the lowest than other treatments, at only 84%. The chlorophyll a fluorescence (JIP test) and physiological indicators (chlorophyll a content, soluble sugars, Ca²⁺/Mg²⁺-ATPase and PAL activity) were also measured. The chlorophyll a content in the highest concentration (40% of original decomposing liquid) treatment group decreased by 43.53% than that of the control; however, soluble sugars, Ca²⁺/Mg²⁺-ATPase, and PAL activity increased by 172.46%, 271.19%, and 26.43% respectively. The overall results indicated that FGA decay has a considerable effect on submerged macrophyte turion germination and seedling growth, which could inhibit their expansion and reproduction. This study emphasized the need to focus on effects of FGA decomposition on the early growth stages of submerged macrophytes and offered technological guidance for submerged vegetation restoration in lakes and shallow waters.

Co-regulation of photosynthetic processes under potassium deficiency across CO2 levels in soybean: mechanisms of limitations and adaptations

Plants photosynthesis-related traits are co-regulated to capture light and CO2 to optimize the rate of CO2 assimilation (A). The rising CO2 often benefits, but potassium (K) deficiency adversely affects A that contributes to the majority of plant biomass. To evaluate mechanisms of photosynthetic limitations and adaptations, soybean was grown under controlled conditions with an adequate (control, 5.0 mM) and two K-deficient (moderate, 0.50 and severe, 0.02 mM) levels under ambient (aCO2; 400 µmol mol-1) and elevated CO2 (eCO2; 800 µmol mol-1). Results showed that under severe K deficiency, pigments, leaf absorption, processes of light and dark reactions, and CO2 diffusion through stomata and mesophyll were down co-regulated with A while light compensation point increased and photorespiration, alternative electron fluxes, and respiration were up-regulated. However, under moderate K deficiency, these traits were well co-regulated with the sustained A without any obvious limitations amid ≈ 50% reduction in leaf K level. Primary mechanism of K limitation to A was either biochemical processes (Lb ≈ 60%) under control and moderate K deficiency or the CO2 diffusion limitations (DL ≈ 70%) with greater impacts of mesophyll than stomatal pathways under severe K deficiency. The eCO2 increased DL while lessened the Lb under K deficiency. Adaptation strategies to severe K deficiency included an enhanced K utilization efficiency (KUE), and reduction of photosystem II excitation pressure by decreasing photosynthetic pigments, light absorption, and photochemical quenching while increasing photorespiration and alternative electron fluxes. The eCO2 also stimulated A and KUE when K deficiency was not severe. Thus, plants responded to K deficiency by a coordinated regulation of photosynthetic processes to optimize A, and eCO2 failed to alleviate the DL in severely K-deficient plants.

Synergistic Effects of Nitrogen and Potassium on Quantitative Limitations to Photosynthesis in Rice ( Oryza sativa L.)

The inhibition of the net CO₂ assimilation ( A) during photosynthesis is one of the major physiological effects of both nitrogen (N) and potassium (K) deficiencies on rice growth. Whether the reduction in A arises from a limitation in either the diffusion and biochemical fixation of CO₂ or photochemical energy conversion is still debated in relation to N and K deficiencies. In this study, the gas exchange parameters of rice under different N and K levels were evaluated and limitations within the photosynthetic carbon capture process were quantified. A was increased by 17.3 and 12.1% for the supply of N and K, respectively. The suitable N/K ratio should be maintained from 1.42 to 1.50. The limitation results indicated that A is primarily limited by the biochemical process. The stomatal conductance ( L_S), mesophyll conductance ( L_M), and biochemical ( L_B) limitations were regulated by 26.6-79.9, 24.4-54.1, and 44.1-75.2%, respectively, with the N and K supply.

The photosynthetic and structural differences between leaves and siliques of Brassica napus exposed to potassium deficiency

BACKGROUND

Most studies of photosynthesis in chlorenchymas under potassium (K) deficiency focus exclusively on leaves; however, little information is available on the physiological role of K on reproductive structures, which play a critical role in plant carbon gain. Brassica napus L., a natural organ-succession species, was used to compare the morphological, anatomical and photo-physiological differences between leaves and siliques exposed to K-deficiency.

RESULTS

Compared to leaves, siliques displayed considerably lower CO2 assimilation rates (A) under K-deficient (-K) or sufficient conditions (+K), limited by decreased stomatal conductance (g s), apparent quantum yield (α) and carboxylation efficiency (CE), as well as the ratio of the maximum rate of electron transport (J max) and the maximum rate of ribulose 1,5-bisphosphate (RuBP) carboxylation (V cmax). The estimated J max, V cmax and α of siliques were considerably lower than the theoretical value calculated on the basis of a similar ratio between these parameters and chlorophyll concentration (i.e. J max/Chl, V cmax/Chl and α/Chl) to leaves, of which the gaps between estimated- and theoretical-J max was the largest. In addition, the average ratio of J max to V cmax was 16.1% lower than that of leaves, indicating that the weakened electron transport was insufficient to meet the requirements for carbon assimilation. Siliques contained larger but fewer stoma, tightly packed cross-section with larger cells and fewer intercellular air spaces, fewer and smaller chloroplasts and thin grana lamellae, which might be linked to the reduction in light capture and CO2 diffusion. K-deficiency significantly decreased leaf and silique A under the combination of down-regulated stomatal size and g s, chloroplast number, α, V cmax and J max, while the CO2 diffusion distance between chloroplast and cell wall (D chl-cw) was enhanced. Siliques were more sensitive than leaves to K-starvation, exhibiting smaller reductions in tissue K and parameters such as g s, V cmax, J max and D chl-cw.

CONCLUSION

Siliques had substantially smaller A than leaves, which was attributed to less efficient functioning of the photosynthetic apparatus, especially the integrated limitations of biochemical processes (J max and V cmax) and α; however, siliques were slightly less sensitive to K deficiency.

Effects of fertilization on crop production and nutrient-supplying capacity under rice-oilseed rape rotation system

Incredible accomplishments have been achieved in agricultural production in China, but many demanding challenges for ensuring food security and environmental sustainability remain. Field experiments were conducted from 2011-2013 at three different sites, including Honghu, Shayang, and Jingzhou in China, to determine the effects of fertilization on enhancing crop productivity and indigenous nutrient-supplying capacity (INuS) in a rice (Oryza sativa L.)-rapeseed (Brassica napus L.) rotation. Four mineral fertilizer treatments (NPK, NP, NK and PK) were applied in a randomized complete block design with three replicates. Crop yields were increased by 19-41% (rice) and 61-76% (rapeseed) during the two years of rice-rapeseed rotation under NPK fertilization compared to PK fertilization across the study sites. Yield responses to fertilization were ranked NPK > NP > NK > PK, illustrating that N deficiency was the most limiting condition in a rice-rapeseed rotation, followed by P and K deficiencies. The highest and lowest N, P and K accumulations were observed under NPK and PK fertilization, respectively. The INuS of the soil decreased to a significant extent and affected rice-rapeseed rotation productivity at each site under NP, NK, and PK fertilization when compared to NPK. Based on the study results, a balanced nutrient application using NPK fertilization is a key management strategy for enhancing rice-rapeseed productivity and environmental safety.

Quantitative limitations to photosynthesis in K deficient sunflower and their implications on water-use efficiency

Potassium (K) is crucial for crop growth and is strongly related to stress tolerance and water-use efficiency (WUE). A major physiological effect of K deficiency is the inhibition of net CO₂ assimilation (A_N) during photosynthesis. Whether this reduction originates from limitations either to photochemical energy conversion or biochemical CO₂ fixation or from a limitation to CO₂ diffusion through stomata and the leaf mesophyll is debated. In this study, limitations to photosynthetic carbon gain of sunflower (Helianthus annuus L.) under K deficiency and PEG- induced water deficit were quantified and their implications on plant- and leaf-scale WUE (WUE_P, WUE_L) were evaluated. Results show that neither maximum quantum use efficiency (F_v/F_m) nor in-vivo RubisCo activity were directly affected by K deficiency and that the observed impairment of A_N was primarily due to decreased CO₂ mesophyll conductance (g_m). K deficiency additionally impaired leaf area development which, together with reduced A_N, resulted in inhibition of plant growth and a reduction of WUE_P. Contrastingly, WUE_L was not affected by K supply which indicated no inhibition of stomatal control. PEG-stress further impeded A_N by stomatal closure and resulted in enhanced WUE_L and high oxidative stress. It can be concluded from this study that reduction of g_m is a major response of leaves to K deficiency, possibly due to changes in leaf anatomy, which negatively affects A_N and contributes to the typical symptoms like oxidative stress, growth inhibition and reduced WUE_P.

Potassium Starvation Limits Soybean Growth More than the Photosynthetic Processes across CO2 Levels

Elevated carbon dioxide (eCO₂) often enhances plant photosynthesis, growth, and productivity. However, under nutrient-limited conditions the beneficial effects of high CO₂ are often diminished. To evaluate the combined effects of potassium (K) deficiency and eCO₂ on soybean photosynthesis, growth, biomass partitioning, and yields, plants were grown under controlled environment conditions with an adequate (control, 5.0 mM) and two deficient (0.50 and 0.02 mM) levels of K under ambient CO₂ (aCO₂; 400 μmol mol^-1) and eCO₂ (800 μmol mol^-1). Results showed that K deficiency limited soybean growth traits more than photosynthetic processes. An ~54% reduction in leaf K concentration under 0.5 mM K vs. the control caused about 45% less leaf area, biomass, and yield without decreasing photosynthetic rate (P_net). In fact, the steady photochemical quenching, efficiency, and quantum yield of photosystem II, chlorophyll concentration (TChl), and stomatal conductance under 0.5 mM K supported the stable P_net. Biomass decline was primarily attributed to the reduced plant size and leaf area, and decreased pod numbers and seed yield in K-deficient plants. Under severe K deficiency (0.02 mM K), photosynthetic processes declined concomitantly with growth and productivity. Increased specific leaf weight, biomass partitioning to the leaves, decreased photochemical quenching and TChl, and smaller plant size to reduce the nutrient demands appeared to be the means by which plants adjusted to the severe K starvation. Increased K utilization efficiency indicated the ability of K-deficient plants to better utilize the tissue-available K for biomass accumulation, except under severe K starvation. The enhancement of soybean growth by eCO₂ was dependent on the levels of K, leading to a K × CO₂ interaction for traits such as leaf area, biomass, and yield. A lack of eCO₂-mediated growth and photosynthesis stimulation under severe K deficiency underscored the importance of optimum K fertilization for maximum crop productivity under eCO₂. Thus, eCO₂ compensated, at least partially, for the reduced soybean growth and seed yield under 0.5 mM K supply, but severe K deficiency completely suppressed the eCO₂-enhanced seed yield.

Anatomical variation of mesophyll conductance under potassium deficiency has a vital role in determining leaf photosynthesis

Leaves exposed to potassium (K) deficiency usually present decreased mesophyll conductance (g_m ) and photosynthesis (A). The relative contributions of leaf anatomical traits in determining g_m have been quantified; however, anatomical variabilities related to low g_m under K starvation remain imperfectly known. A one-dimensional model was used to quantify anatomical controls of the entire CO₂ diffusion pathway resistance within a leaf on two Brassica napus L. cultivars in response to K deficiency. Leaf photosynthesis of both cultivars was significantly decreased under K deficiency in parallel with down-regulated g_m . The mesophyll conductance limitation contributed to more than one-half of A decline. The decreased internal air space in K-starved leaves was associated with the increase of gas-phase resistance. Potassium deficiency reduced liquid-phase conductance by decreasing the exposed surface area of chloroplasts per unit leaf area (S_c /S), and enlarging the resistance of the cytoplasm that can be interpreted by the increasing distance of chloroplast from cell wall, and between adjacent chloroplasts. Additionally, the discrepancies of A between two cultivars were in part because of g_m variations, ascribing to an altered S_c /S. These results emphasize the important role of K on the regulation of g_m by enhancing S_c /S and reducing cytoplasm resistance.