Jan Henk Venema

Submergence-Induced Morphological, Anatomical, and Biochemical Responses in a Terrestrial Species Affect Gas Diffusion Resistance and Photosynthetic Performance

Gas exchange between the plant and the environment is severely hampered when plants are submerged, leading to oxygen and energy deficits. A straightforward way to reduce these shortages of oxygen and carbohydrates would be continued photosynthesis under water, but this possibility has received only little attention. Here, we combine several techniques to investigate the consequences of anatomical and biochemical responses of the terrestrial species Rumex palustris to submergence for different aspects of photosynthesis under water. The orientation of the chloroplasts in submergence-acclimated leaves was toward the epidermis instead of the intercellular spaces, indicating that underwater CO2 diffuses through the cuticle and epidermis. Interestingly, both the cuticle thickness and the epidermal cell wall thickness were significantly reduced upon submergence, suggesting a considerable decrease in diffusion resistance. This decrease in diffusion resistance greatly facilitated underwater photosynthesis, as indicated by higher underwater photosynthesis rates in submergence-acclimated leaves at all CO2 concentrations investigated. The increased availability of internal CO2 in these “aquatic” leaves reduced photorespiration, and furthermore reduced excitation pressure of the electron transport system and, thus, the risk of photodamage. Acclimation to submergence also altered photosynthesis biochemistry as reduced Rubisco contents were observed in aquatic leaves, indicating a lower carboxylation capacity. Electron transport capacity was also reduced in these leaves but not as strongly as the reduction in Rubisco, indicating a substantial increase of the ratio between electron transport and carboxylation capacity upon submergence. This novel finding suggests that this ratio may be less conservative than previously thought.

Impact of Suboptimal Temperature on Growth, Photosynthesis, Leaf Pigments and Carbohydrates of Domestic and High-altitude Wild Lycopersicon Species

The impact of near-optimal (25/20 degrees C) and suboptimal (16/14 degrees C) day/night temperatures on growth, photosynthesis, pigment composition and carbohydrate content was compared between domestic and high-altitude wild Lycopersicon species. When related to the relative shoot growth rate (RSGR) measured at optimal temperature, genotypes of the domestic tomato (L. esculentum (L.) Mill. cv. Abunda and cv. Large Red Cherry (LRC) showed a stronger inhibition of RSGR at suboptimal temperature than the high-altitude wild species L. peruvianum Mill. LA 385 and L. hirsutum Humb. & Bonpl. LA 1777. The initiation rare of new leaves was 2.1-fold faster in all species at 25/20 degrees C than at 16/14 degrees C. In contrast to the other genotypes, the leaf area of suboptimally grown Abunda plants was 28 % smaller than the area of leaves that were fully expanded at optimal temperature. In all species, specific leaf area (SLA) at 16/14 degrees C was 17-26 % lower than at 25/20 degrees C. The percentage of leaf dry matter increased in response to growth ar suboptimal temperature. This increase was higher in L. esculentum genotype Abunda (99 %) than in genotype LRC (38 %), and the wild species L. peruvianum (50 %) and L. hirsutum (38 %), which could be attributed to inter- and intra-specific differences in starch accumulation of 16/14 degrees C-grown leaves. Only in both L. esculentum genotypes, net photosynthetic rate at growth irradiance (A(225)) and at light saturation (A(sat)) was 14 to 30 % lower in leaves grown and measured at suboptimal temperature, compared with leaves grown and measured at optimal temperature (25 degrees C). Chlorophyll (Chl) a fluorescence measurements indicated that the decrease of A225 in leaves of suboptimally grown L. esculentum plants was paralleled by a decrease in the quantum yield of photosystem II electron transport (Phi(PSII)), which could be mainly attributed to a decrease in the photochemical quenching component (q(P)). In all species, the nonphotochemical quenching component (NPQ) was 2 to 4-fold higher at 16/14 degrees C. Growth temperature hardly affected Chi content on a leaf area basis, whereas the content of xanthophyll cycle pigments (violaxanthin + antheraxanthin + zeaxanthin) on a Chi basis was ca. 1.5-fold higher in 16/14 degrees C-grown leaves. The epoxidation state of the xanthophyll cycle pool was only slightly lower in suboptimal leaves due to the moderate growth irradiance.

Effect of acclimation to suboptimal temperature on chilling-induced photodamage: comparison between a domestic and a high-altitude wild Lycopersicon species

Plants of a domestic (Lycopersicon esculentum [L.] Mill. cv. Abunda) and a high-altitude wild Lycopersicon species (L. peru6ianum Mill. LA 385) were grown at near-optimal (25:20°C) or suboptimal (16:14°C) temperature. Leaf discs from just fully expanded leaves were exposed to an irradiance of 1000 mmol m 2 s 1 at 5°C for 48 h. The effect of growth temperature on the susceptibility to photoinhibition of photosystem II (PSII) and its recovery, degradation of leaf pigments, chlorophyll (Chl) fluorescence quenching and xanthophyll cycle activity were examined. Leaves of L. esculentum and L. peru6ianum plants grown at optimal temperature, were similarly susceptible to photodamage. Suboptimal-grown leaves of both species showed a higher tolerance to photoinhibition than optimal-grown leaves. In both species, recovery of photoinhibited PSII was more complete in leaves grown at suboptimal than at optimal temperature. In contrast to L. esculentum, suboptimal-grown leaves of L. peru6ianum exhibited faster kinetics of recovery from photoinhibition than optimal-grown leaves. Light-induced degradation of leaf pigments in leaves grown at 16:14°C was 2.3- and 2.7-times slower in L. esculentum and L. peru6ianum, respectively, when compared with leaves grown at 25:20°C. Non-photochemical quenching (NPQ) of Chl fluorescence developed faster in leaves of suboptimalgrown plants, and steady-state levels were20% higher than in leaves of optimal-grown plants of both species. An increased pool size of xanthophyll cycle pigments together with a slightly higher conversion state, resulted in a 1.5- (L. esculentum) or 3-fold (L. peru6ianum) higher maximal zeaxanthin content in suboptimal-, as compared with optimal-grown leaves. These results demonstrate that acclimation to suboptimal temperature increased the capacity to resist chilling-induced photodamage in both the domestic and the high-altitude wild Lycopersicon species. However, the acclimatory response was more pronounced in L. peru6ianum than in L. esculentum, indicating a greater ability of the high-altitude wild species to acclimate its photosynthetic apparatus to suboptimal temperatures.

Copper exposure interferes with the regulation of the uptake, distribution and metabolism of sulfate in Chinese cabbage.

Exposure of Chinese cabbage (Brassica pekinensis) to enhanced Cu(2+) concentrations (1-10 microM) resulted in leaf chlorosis, a loss of photosynthetic capacity and lower biomass production at > or = 5 microM. The decrease in pigment content was likely not the consequence of degradation, but due to hindered chloroplast development upon Cu exposure. The Cu content of the root increased with the Cu(2+) concentration (up to 40-fold), though only a minor proportion (4%) was transferred to the shoot. The nitrate uptake by the root was substantially reduced at > or = 5 microM Cu(2+). The nitrogen content of the root was affected little at lower Cu(2+) levels, whereas that in the shoot was decreased at > or = 5 microM Cu(2+). Cu affected the uptake, distribution and metabolism of sulfate in Chinese cabbage. The total sulfur content of the shoot was increased at > or = 2 microM Cu(2+), which could be attributed mainly to an increase in sulfate content. Moreover, there was a strong increase in water-soluble non-protein thiol content in the root and, to a lesser extent, in the shoot at > or = 1 microM, which could only partially be ascribed to a Cu-induced enhancement of the phytochelatin content. The nitrate uptake by the root was substantially reduced at > or = 5 microM Cu(2+), coinciding with a decrease in biomass production. However, the activity of the sulfate transporters in the root was slightly enhanced at 2 and 5 microM Cu(2+), accompanied by enhanced expression of the Group 1 high affinity transporter Sultr1;2, and the Group 4 transporters Sultr4;1 and Sultr4;2. In the shoot, there was an induction of expression of Sultr4;2 at 5 and 10 microM Cu(2+). The expression of APS reductase was affected little in the root and shoot up to 10 microM Cu(2+). The upregulation of the sulfate transporters may be due not only to greater sulfur demand at higher Cu levels, but also the consequence of interference by Cu with the signal transduction pathway regulating the expression and activity of the sulfate transporters.

Roots withstanding their environment: Exploiting root system architecture responses to abiotic stress to improve crop tolerance

To face future challenges in crop production dictated by global climate changes, breeders and plant researchers collaborate to develop productive crops that are able to withstand a wide range of biotic and abiotic stresses. However, crop selection is often focused on shoot performance alone, as observation of root properties is more complex and asks for artificial and extensive phenotyping platforms. In addition, most root research focuses on development, while a direct link to the functionality of plasticity in root development for tolerance is often lacking. In this paper we review the currently known root system architecture (RSA) responses in Arabidopsis and a number of crop species to a range of abiotic stresses, including nutrient limitation, drought, salinity, flooding, and extreme temperatures. For each of these stresses, the key molecular and cellular mechanisms underlying the RSA response are highlighted. To explore the relevance for crop selection, we especially review and discuss studies linking root architectural responses to stress tolerance. This will provide a first step toward understanding the relevance of adaptive root development for a plant’s response to its environment. We suggest that functional evidence on the role of root plasticity will support breeders in their efforts to include root properties in their current selection pipeline for abiotic stress tolerance, aimed to improve the robustness of crops.

Correlation between spatial (3D) structure of pea and bean thylakoid membranes and arrangement of chlorophyll-protein complexes

BackgroundThe thylakoid system in plant chloroplasts is organized into two distinct domains: grana arranged in stacks of appressed membranes and non-appressed membranes consisting of stroma thylakoids and margins of granal stacks. It is argued that the reason for the development of appressed membranes in plants is that their photosynthetic apparatus need to cope with and survive ever-changing environmental conditions. It is not known however, why different plant species have different arrangements of grana within their chloroplasts. It is important to elucidate whether a different arrangement and distribution of appressed and non-appressed thylakoids in chloroplasts are linked with different qualitative and/or quantitative organization of chlorophyll-protein (CP) complexes in the thylakoid membranes and whether this arrangement influences the photosynthetic efficiency.ResultsOur results from TEM and in situ CLSM strongly indicate the existence of different arrangements of pea and bean thylakoid membranes. In pea, larger appressed thylakoids are regularly arranged within chloroplasts as uniformly distributed red fluorescent bodies, while irregular appressed thylakoid membranes within bean chloroplasts correspond to smaller and less distinguished fluorescent areas in CLSM images. 3D models of pea chloroplasts show a distinct spatial separation of stacked thylakoids from stromal spaces whereas spatial division of stroma and thylakoid areas in bean chloroplasts are more complex. Structural differences influenced the PSII photochemistry, however without significant changes in photosynthetic efficiency. Qualitative and quantitative analysis of chlorophyll-protein complexes as well as spectroscopic investigations indicated a similar proportion between PSI and PSII core complexes in pea and bean thylakoids, but higher abundance of LHCII antenna in pea ones. Furthermore, distinct differences in size and arrangements of LHCII-PSII and LHCI-PSI supercomplexes between species are suggested.ConclusionsBased on proteomic and spectroscopic investigations we postulate that the differences in the chloroplast structure between the analyzed species are a consequence of quantitative proportions between the individual CP complexes and its arrangement inside membranes. Such a structure of membranes induced the formation of large stacked domains in pea, or smaller heterogeneous regions in bean thylakoids. Presented 3D models of chloroplasts showed that stacked areas are noticeably irregular with variable thickness, merging with each other and not always parallel to each other.

Low-temperature-related growth and photosynthetic performance of alloplasmic tomato (Lycopersicon esculentum Mill.) with chloroplasts from L. hirsutum Humb. & Bonpl.

Growth and photosynthetic performance were analyzed in alloplasmic tomato at a high- (25/17 °C; HTR) and low-temperature regime (12/6 °C; LTR) in order to establish the role of cytoplasmic variation on low-temperature tolerance of tomato (Lycopersicon esculentum Mill.). Four alloplasmic tomato lines, containing the nuclear genome of tomato and the plastome of L. hirsutum LA 1777 Humb. & Bonpl., an accession collected at high-altitude in Peru, were reciprocally crossed with 11 tomato entries with a high inbreeding level and a wide genetic variation, resulting in a set of 44 reciprocal crosses. Irrespective of growth temperature, alloplasmic families with alien chloroplasts of L. hirsutum (h) were on average characterized by a high shoot biomass, a large leaf area, and a low specific leaf area in comparison with their euplasmic counterparts. These results do not directly point to an advantageous effect of h-chloroplasts on biomass accumulation at low temperature but rather towards a small general beneficial effect on growth and/or distribution of assimilates. Significant chloroplast-related differences in photosynthetic performance, however, were not detected at both temperature regimes, indicating that h-chloroplasts can properly function in a variable nuclear background of L. esculentum. It is concluded that chloroplast substitution is not an effective method for breeding tomato plants with improved low-temperature tolerance.