Distinct Cold Acclimation of Productivity Traits in Arabidopsis thaliana Ecotypes.

Improvement of crop climate resilience will require an understanding of whole-plant adaptation to specific local environments. This review places features of plant form and function related to photosynthetic productivity, as well as associated gene-expression patterns, into the context of the adaptation of Arabidopsis thaliana ecotypes to local environments with different climates in Sweden and Italy. The growth of plants under common cool conditions resulted in a proportionally greater emphasis on the maintenance of photosynthetic activity in the Swedish ecotype. This is compared to a greater emphasis on downregulation of light-harvesting antenna size and upregulation of a host of antioxidant enzymes in the Italian ecotype under these conditions. This differential response is discussed in the context of the climatic patterns of the ecotypes' native habitats with substantial opportunity for photosynthetic productivity under mild temperatures in Italy but not in Sweden. The Swedish ecotype's response is likened to pushing forward at full speed with productivity under low temperature versus the Italian ecotype's response of staying safe from harm (maintaining redox homeostasis) while letting productivity decline when temperatures are transiently cold. It is concluded that either strategy can offer directions for the development of climate-resilient crops for specific locations of cultivation.

Foliar sieve elements: Nexus of the leaf.

In this review, a central position of foliar sieve elements in linking leaf structure and function is explored. Results from studies involving plants grown under, and acclimated to, different growth regimes are used to identify significant, linear relationships between features of minor vein sieve elements and those of 1) leaf photosynthetic capacity that drives sugar synthesis, 2) overall leaf structure that serves as the platform for sugar production, 3) phloem components that facilitate the loading of sugars (companion & phloem parenchyma cells), and 4) the tracheary elements that import water to support photosynthesis (and stomatal opening) as well as mass flow of sugars out of the leaf. Despite comprising only a small fraction of physical space within the leaf, sieve elements represent a hub through which multiple functions of the leaf intersect. As the conduits for export of energy-rich carbohydrates, essential mineral nutrients, and information carriers, sieve elements play a central role in fueling and orchestrating development and function of the plant as well as, by extension, of natural and human communities that depend on plants as producers and partners in the global carbon cycle.

Photosynthesis and foliar vascular adjustments to growth light intensity in summer annual species with symplastic and apoplastic phloem loading.

Concomitant adjustments in photosynthetic capacity and size, composition, and/or density of minor foliar veins in response to growth environment were previously described primarily for winter annuals that load sugars into foliar phloem apoplastically. Here, common trends, differences associated with phloem-loading mechanism, and species-dependent differences are identified for summer annuals (loading sugars either symplastically [cucumber, pumpkin, and basil] or apoplastically [tomato and sunflower]) that were grown in low and high light. Photosynthetic capacity per leaf area was significantly positively correlated with leaf-level volume of phloem-loading cells (LCs), sugar-export conduits (sieve elements), and water conduits (tracheary elements) irrespective of phloem-loading mechanism. The relative contribution to leaf-level volume of LC numbers versus individual LC size was greater in apoplastic loaders than in symplastic loaders. Species-dependent differences included different vein density within each loading group and either greater or lower numbers of cells per minor vein (especially of tracheary elements in the symplastic loaders basil versus cucumber, respectively), which may be due to genetic adaptation to different environmental conditions. These results indicate considerable plasticity in foliar vascular features in summer annuals as well as some loading-mechanism-dependent trends.

Associations between the acclimation of phloem-cell wall ingrowths in minor veins and maximal photosynthesis rate.

The companion cells (CCs) and/or phloem parenchyma cells (PCs) in foliar minor veins of some species exhibit invaginations that are amplified when plants develop in high light (HL) compared to low light (LL). Leaves of plants that develop under HL also exhibit greater maximal rates of photosynthesis compared to those that develop under LL, suggesting that the increased membrane area of CCs and PCs of HL-acclimated leaves may provide for greater levels of transport proteins facilitating enhanced sugar export. Furthermore, the degree of wall invagination in PCs (Arabidopsis thaliana) or CCs (pea) of fully expanded LL-acclimated leaves increased to the same level as that present in HL-acclimated leaves 7 days following transfer to HL, and maximal photosynthesis rates of transferred leaves of both species likewise increased to the same level as in HL-acclimated leaves. In contrast, transfer of Senecio vulgaris from LL to HL resulted in increased wall invagination in CCs, but not PCs, and such leaves furthermore exhibited only partial upregulation of photosynthetic capacity following LL to HL transfer. Moreover, a significant linear relationship existed between the level of cell wall ingrowths and maximal photosynthesis rates across all three species and growth light regimes. A positive linear relationship between these two parameters was also present for two ecotypes (Sweden, Italy) of the winter annual A. thaliana in response to growth at different temperatures, with significantly greater levels of PC wall ingrowths and higher rates of photosynthesis in leaves that developed at cooler versus warmer temperatures. Treatment of LL-acclimated plants with the stress hormone methyl jasmonate also resulted in increased levels of wall ingrowths in PCs of A. thaliana and S. vulgaris but not in CCs of pea and S. vulgaris. The possible role of PC wall ingrowths in sugar export versus as physical barriers to the movement of pathogens warrants further attention.

Changes in photosynthetic pigment composition and absorbed energy allocation during salt stress and CAM induction in Mesembryanthemum crystallinum.

Mesembryanthemum crystallinum L. undergoes a transition from the C3 photosynthetic pathway to crassulacean acid metabolism (CAM) in response to increasing salinity. As a consequence, growth is greatly reduced and less light energy is utilised in carbon fixation, leading to an increase in dissipation of thermal energy to remove potentially dangerous excess excitation energy. The pigment composition of plants grown for 4 weeks at 20 mm (low) and 400 mm (high) NaCl was sampled, and photochemical performance, tissue acidity and growth were sampled at 2 and 4 weeks. High-salt-grown plants, which switched to CAM, accumulated only 25% of the fresh weight of low-salt-grown plants, which maintained C3 photosynthesis. Predawn Fv / Fm and de-epoxidation of violaxanthin [(A + Z) / (V + A + Z)] was similar between plants after 2 and 4 weeks, revealing no sustained depression in PSII efficiency under the high-salt treatment. However, at midday under high photosynthetic photon flux densities (PPFD) high-salt plants displayed lower PSII efficiency, higher (A + Z) / (V + A + Z) and greater allocation of energy to thermal dissipation over photochemistry than low-salt plants. Pigment contents were similar between treatments for the first 3 weeks, but after 4 weeks high-salt plants had accumulated significantly less chlorophyll and lutein than low-salt plants. However, V + A + Z content did not differ. High-salt treatment, leading to CAM photosynthesis and substantial reduction in growth, was associated with increased allocation of energy to xanthophyll cycle-dependent energy dissipation at high light and adjustment of thylakoid pigment composition.

Effects of lincomycin on PSII efficiency, non-photochemical quenching, D1 protein and xanthophyll cycle during photoinhibition and recovery.

Leaves of Parthenocissus quinquefolia (L.) Planch. (Virginia creeper) were treated with lincomycin (an inhibitor of chloroplast-encoded protein synthesis), subjected to a high-light treatment and allowed to recover in low light. While lincomycin-treated leaves had similar characteristics as controls after a 1 h exposure to high light, total D1 levels in lincomycin-treated leaves were half those in controls at the end of the recovery period. In addition, lincomycin delayed recovery of maximal PSII efficiency of open centers (ratio of variable to maximal chlorophyll fluorescence, F v / F m) and of estimated PSII photochemistry rate upon return to low light subsequent to the high-light treatment. Furthermore, lincomycin treatment slowed the removal of zeaxanthin (Z) and antheraxanthin (A) during recovery in low light, and the level of thermal energy dissipation (non-photochemical fluorescence quenching, NPQ) remained elevated. In lincomycin-treated leaves infiltrated with the uncoupler nigericin immediately after high-light exposure, thermal energy dissipation, sustained with lincomycin alone, declined quickly to control levels. In summary, lincomycin treatment affected not only D1 protein turnover but also xanthophyll-cycle operation and thermal-energy dissipation. The latter effect was apparently a result of the maintenance of a high trans-thylakoid proton gradient. Similar effects were also seen subsequent to short-term exposures to high light in lincomycin-treated Spinacia oleracea L. (spinach) leaves. In contrast, lincomycin treatments under low-light levels did not induce Z formation or NPQ. These results suggest that lincomycin has the potential to lower PSII efficiency (F v / F m) through inhibition of NPQ relaxation and Z + A removal subsequent to high-light exposures.

Seasonal differences in xanthophyll cycle characteristics and antioxidants in Mahonia repens growing in different light environments.

We investigated differences between summer and winter in photosynthesis, xanthophyll cycle-dependent energy dissipation, and antioxidant systems in populations of Mahonia repens (Lindley) Don growing in the eastern foothills of the Colorado Rocky Mountains in deep shade, full exposure, and under a single-layered canopy of Pinus ponderosa (partially shaded). In summer, increasing growth irradiance (from deep shade to partial shade to full exposure) was associated with increased xanthophyll cycle-dependent energy dissipation in PSII and an increased capacity to detoxify reactive reduced oxygen species, as measured by increases in the activities of ascorbate peroxidase, superoxide scavenging, glutathione reductase, and monodehydroascorbate reductase, as well as increases in leaf ascorbate and glutathione content. Leaves of exposed and partially shaded plants exhibited decreased capacities for photosynthetic O2 evolution in winter compared to summer, while in the deeply shaded plants this parameter did not differ seasonally. Seasonal differences in the levels of antioxidants generally exhibited an inverse response to photosynthesis, being higher in winter compared to summer in the exposed and partially shaded populations, but remaining unchanged in the deeply shaded population. In addition, total pool size and conversion state of the xanthophyll cycle were higher in winter than in summer in all populations. These trends suggest that both xanthophyll cycle-dependent energy dissipation in PSII and the capacity to detoxify reactive reduced oxygen species responded to the level of excess light absorption.