Protection by light against heat stress in leaves of tropical crassulacean acid metabolism plants containing high acid levels

Determining the ecophysiological limits of a narrow niche tropical conifer tree (Podocarpus trinitensis)

Many tropical species live close to their thermal limits within a narrow niche. Here, we investigate the ecophysiological limits of the tropical tree Podocarpus trinitensis, which is endemic to Trinidad and Tobago where most populations exist as isolated stands on hilltops. Five wild stands from a range of elevations were compared in the field with measurements of leaf temperature, canopy cover, stomatal conductance (gs), chlorophyll content and several chlorophyll fluorescence parameters. A parallel greenhouse experiment was used to acclimate seedlings to 'CONTROL' and 'HEAT' treatments (with mid-day air temperatures of 34.5 and 37 °C respectively), after which the above parameters were measured along with photosynthetic light and temperature response curves, leaf morphology and in vitro Fv/Fm thermostability. There was a positive association between improved physiological performance and elevation. In the high elevation sites, leaf temperatures were significantly lower while most of the physiological parameters were higher (gs, chlorophyll content, ɸ PSII, ETRmax and Isat90). In the greenhouse, HEAT and CONTROL plants were similar for most parameters, except leaf temperature (which was coupled with air temperature) and leaf mass per unit area (which was higher in HEAT plants). Temperature response curves showed an optimum temperature for photosynthesis of 30 ± 0.5 °C (TOpt) and in vitro Fv/Fm indicated a critical temperature of 47.4 ± 0.38 °C for HEAT and 48.2 ± 0.24 °C for CONTROL (T50), with no indication of heat acclimation. Podocarpus trinitensis was found to be shade tolerant. In the field, seedlings established under a close canopy (>95% canopy cover) and had a low light saturation point (LCP). In the greenhouse, where more light was available, seedlings retained a low light compensation point, light saturation point (LSP) and maximum photosynthetic rate (Amax). The results suggest that P. trinitensis is moderately heat tolerant with the higher elevation sites being more habitable, but stands are also able to survive near sea level under a closed canopy. The narrow niche, along with the 30 ± 0.5 °C optimum temperature for photosynthesis and the lack of thermal plasticity in critical temperature, suggests that P. trinitensis has little room to acclimate to temperatures higher than those currently experienced.

Anatomical and physiological responses of Aechmea blanchetiana (Bromeliaceae) induced by silicon and sodium chloride stress during in vitro culture

Salt stress is one of the most severe abiotic stresses affecting plant growth and development. The application of silicon (Si) is an alternative that can increase the tolerance of plants to various types of biotic and abiotic stresses. The objective was to evaluate salt stress's effect in vitro and Si's mitigation potential on Aechmea blanchetiana plants. For this purpose, plants already established in vitro were transferred to a culture medium with 0 or 14 µM of Si (CaSiO₃). After growth for 30 days, a stationary liquid medium containing different concentrations of NaCl (0, 100, 200, or 300 µM) was added to the flasks. Anatomical and physiological analyses were performed after growth for 45 days. The plants cultivated with excess NaCl presented reduced root diameter and effective photochemical quantum yield of photosystem II (PSII) (ΦPSII) and increased non-photochemical dissipation of fluorescence (qN). Plants that grew with the presence of Si also had greater content of photosynthetic pigments and activity of the enzymes of the antioxidant system, as well as higher values of maximum quantum yield of PSII (F_V/F_M), photochemical dissipation coefficient of fluorescence (qP) and fresh weight bioaccumulation of roots and shoots. The anatomical, physiological and biochemical responses, and growth induced by Si mitigated the effect of salt stress on the A. blanchetiana plants cultivated in vitro, which can be partly explained by the tolerance of this species to grow in sandbank (Restinga) areas.

Low Light Facilitates Cyclic Electron Flows around PSI to Assist PSII against High Temperature Stress

Photosystem II (PSII) of grapevine leaves is easily damaged under heat stress, but no such injury is observed when the leaves are heated in low light. To elucidate the mechanisms, we compared the photosynthetic characteristics of grapevine seedlings under heat treatments (42 °C) for 4 h in the dark or low light (200 μmol m^-2 s^-1). At 42 °C in the dark, the PSII maximum quantum yield (Fv/Fm) decreased significantly with the increase in time but did not change much in low light. The JIP (chlorophyll a fluorescence rise kinetics) test results showed that low light significantly alleviated the damage to the oxygen evolving complexes (OECs; the K-step was less visible) by heat stress. Further, in the presence of de novo D1 protein synthesis inhibitor chloramphenicol, Fv/Fm did not differ significantly between dark and light treatments under heat stress. The 50% re-reduction (RR50) of P700⁺ on cessation of far-red illumination was faster after light treatment than that in the dark. After exposure to 25 °C in a low light for 15 min, Y(NO) (the constitutive non-regulatory non-photochemical quenching) treated by heat stress and darkness was higher than that by heat stress and light. Overall, our results suggested that enhanced CEFs around PSI in low light could assist PSII against heat damage by maintaining the rate of PSII repair and inhibiting the non-radiative charge recombination in PSII reaction centers.

Acclimation to water stress improves tolerance to heat and freezing in a common alpine grass

Alpine plants in Australia are increasingly exposed to more frequent drought and heatwaves, with significant consequences for physiological stress responses. Acclimation is a critical feature that allows plants to improve tolerance to environmental extremes by directly altering their physiology or morphology. Yet it is unclear how plant performance, tolerance, and recovery are affected when heat and water stress co-occur, and whether prior exposure affects responses to subsequent climate extremes. We grew a common alpine grass species under high or low watering treatments for three weeks before exposure to either none, one, or two heat stress events. We determined photosynthetic heat and freezing tolerance (LT₅₀, mean temperature causing 50% irreversible damage to photosystem II) and growth. Physiological adjustments to low watering, including more negative water potentials and reduced growth, were also characterised by improved tolerance to high and low-temperature extremes. Shifts to higher heat tolerance were also evident with increasing exposure to heat stress events, though freezing tolerance was not affected. Acclimation effects were mostly short-term, however; prior exposure to heat and/or water stress had little to no effect on growth and thermal tolerance following the six-week recovery period. We conclude that rapid acclimation to water and heat stress that co-occur during summer enhances the capacity of alpine plants to tolerate increasingly frequent temperature extremes.

Leaf heat tolerance of 147 tropical forest species varies with elevation and leaf functional traits, but not with phylogeny

Exceeding thermal thresholds causes irreversible damage and ultimately loss of leaves. The lowland tropics are among the warmest forested biomes, but little is known about heat tolerance of tropical forest plants. We surveyed leaf heat tolerance of sun-exposed leaves from 147 tropical lowland and pre-montane forest species by determining the temperatures at which potential photosystem II efficiency based on chlorophyll a fluorescence started to decrease (T_Crit ) and had decreased by 50% (T₅₀ ). T_Crit averaged 46.7°C (5th-95th percentile: 43.5°C-49.7°C) and T₅₀ averaged 49.9°C (47.8°C-52.5°C). Heat tolerance partially adjusted to site temperature; T_Crit and T₅₀ decreased with elevation by 0.40°C and 0.26°C per 100 m, respectively, while mean annual temperature decreased by 0.63°C per 100 m. The phylogenetic signal in heat tolerance was weak, suggesting that heat tolerance is more strongly controlled by environment than by evolutionary legacies. T_Crit increased with the estimated thermal time constant of the leaves, indicating that species with thermally buffered leaves maintain higher heat tolerance. Among lowland species, T₅₀ increased with leaf mass per area, suggesting that in species with structurally more costly leaves the risk of leaf loss during hot spells is reduced. These results provide insight in variation in heat tolerance at local and regional scales.

Performance of Hylocereus (Cactaceae) species and interspecific hybrids under high-temperature stress

High temperatures limit the successful cultivation of the Hylocereus species on a global basis. We aimed to investigate the degree of heat tolerance in three species, namely, the diploids Hylocereus undatus and H. monacanthus, and the tetraploid H. megalanthus, and nine of their interspecific-interploid hybrids. Rooted cuttings were exposed to heat stress (45/35 °C) or control conditions (25/20 °C) for eight days. Initially, the plants were screened for their tolerance to heat stress and ranked into four heat tolerance categories: good tolerance, moderate tolerance, low tolerance, or sensitive, according to the decrease in the maximum quantum efficiency of photosystem II (F_v/F_m) and visual stem damage. The physiological and biochemical performances of the parental species and of three hybrids representing three different heat-tolerance categories were further analyzed in depth. H. megalanthus (classified as heat sensitive) showed a 65% decrease in F_v/F_m and severe visual stem damage, along with a marked reduction in total chlorophyll content, a large increase in malondialdehyde, and inhibition of catalase activity. H. undatus and H. monacanthus, (classified as low-tolerance species) exhibited slight stem "liquification." The good-tolerance hybrid Z-16 exhibited the best performance under heat stress (21% decrease in F_v/F_m) and the absence of stem damage, coupled with a small decrease in total chlorophyll content, a slight increase in malondialdehyde, high antioxidant activity, and proline accumulation progressing with time. Our findings revealed that most of the hybrids performed better than their parental species, indicating that our breeding programs can provide Hylocereus cultivars suitable for cultivation in heat-challenging regions.

The Kalanchoë genome provides insights into convergent evolution and building blocks of crassulacean acid metabolism

Crassulacean acid metabolism (CAM) is a water-use efficient adaptation of photosynthesis that has evolved independently many times in diverse lineages of flowering plants. We hypothesize that convergent evolution of protein sequence and temporal gene expression underpins the independent emergences of CAM from C3 photosynthesis. To test this hypothesis, we generate a de novo genome assembly and genome-wide transcript expression data for Kalanchoë fedtschenkoi, an obligate CAM species within the core eudicots with a relatively small genome (~260 Mb). Our comparative analyses identify signatures of convergence in protein sequence and re-scheduling of diel transcript expression of genes involved in nocturnal CO2 fixation, stomatal movement, heat tolerance, circadian clock, and carbohydrate metabolism in K. fedtschenkoi and other CAM species in comparison with non-CAM species. These findings provide new insights into molecular convergence and building blocks of CAM and will facilitate CAM-into-C3 photosynthesis engineering to enhance water-use efficiency in crops.

Secrets of succulence

Succulent plants are iconic components of the florae of many terrestrial ecosystems, but despite having caused fascination and prompted investigation for centuries, they still harbour many secrets in terms of physiological function and evolution. Tackling these mysteries is important, as this will not only provide insights into the dynamics and details of the convergent evolution of a major adaptive syndrome, but also inform efforts to conserve endangered biodiversity and utilize the unique physiological characteristics of succulents for biofuel and biomass production. Here I review advances in the phylogeny and organismal biology of succulent plants, and discuss how insights from recent work in the wider fields of plant hydraulics and photosynthetic physiology may relate to succulents. The potential for the exploration of mechanistic relationships between anatomical structure and physiological function to improve our understanding of the constraints that have shaped the evolution of succulence is highlighted. Finally, attention is drawn to how new methodologies and technologies provide exciting opportunities to address the wide range of outstanding questions in succulent plant biology.

Photosynthetic acclimation to warming in tropical forest tree seedlings

Tropical forests have a mitigating effect on man-made climate change by acting as a carbon sink. For that effect to continue, tropical trees will have to acclimate to rising temperatures, but it is currently unknown whether they have this capacity. We grew seedlings of three tropical tree species over a range of temperature regimes (TGrowth = 25, 30, 35 °C) and measured the temperature response of photosynthetic CO2 uptake. All species showed signs of acclimation: the temperature-response curves shifted, such that the temperature at which photosynthesis peaked (TOpt) increased with increasing TGrowth. However, although TOpt shifted, it did not reach TGrowth at high temperature, and this difference between TOpt and TGrowth increased with increasing TGrowth, indicating that plants were operating at supra-optimal temperatures for photosynthesis when grown at high temperatures. The high-temperature CO2 compensation point did not increase with TGrowth. Hence, temperature-response curves narrowed with increasing TGrowth. TOpt correlated with the ratio of the RuBP regeneration capacity over the RuBP carboxylation capacity, suggesting that at high TGrowth photosynthetic electron transport rate associated with RuBP regeneration had greater control over net photosynthesis. The results show that although photosynthesis of tropical trees can acclimate to moderate warming, carbon gain decreases with more severe warming.