Cold deacclimation mechanisms and reacclimation potential in flower buds of blackcurrant (Ribes nigrum).

As a consequence of global climate change, cold acclimation and deacclimation cycles are becoming increasingly frequent during winter in temperate regions. However, little is known about plant deacclimation and in particular reacclimation mechanisms, although deacclimation resistance and the ability to reacclimate may have wide-ranging consequences regarding plant productivity in a changing climate. Here, we report time-dependent responses of freezing tolerance, respiration rates, metabolite contents (high-resolution magic angle spinning NMR) and fatty acid levels (gas chromatography) in flower buds of two ecodormant Ribes nigrum cultivars exposed to three different deacclimation temperatures followed by a reacclimation treatment at 4°C. The data reveal that despite differences in the progression of deacclimation, the capacity of blackcurrant flower buds to reharden in late winter is virtually non-existing, implying that increasingly irregular temperature patterns is critical for blackcurrant fruit yield. The early phase of deacclimation is associated with a transient increase in respiration and decreasing contents of amino acids, tricarboxylic acid (TCA) cycle intermediates and sugars, indicating an increased need for carbon sources and respiratory energy production for the activation of growth. Decreasing sugar levels may additionally cause loss of freezing tolerance. Deacclimation also involves desaturation of membrane lipids, which likely also contributes to decreased freezing tolerance but may also reflect biosynthesis of signaling molecules stimulating growth and floral organ differentiation. These data provide new insights into the under-researched deacclimation mechanisms and the ability of blackcurrant to reacclimate following different advancements of deacclimation and contribute to our understanding of plant responses to increasingly irregular temperature patterns.

Temperature Variation under Continuous Light Restores Tomato Leaf Photosynthesis and Maintains the Diurnal Pattern in Stomatal Conductance.

The response of tomato plants (Solanum lycopersicum L. cv. Aromata) to continuous light (CL) in relation to photosynthesis, abscisic acid (ABA) and reactive oxygen species (ROS) was investigated to improve the understanding of the development and/or alleviation of CL-induced leaf injury in constant and diurnal temperature fluctuations with similar daily light integral and daily mean temperature. The plants were grown in three photoperiodic treatments for 15 days; One treatment with a 16/8 h light/dark period and a light/dark temperature of 27/17°C (Control), two CL treatments with 24 h photoperiods, one with a constant temperature of 24°C (CLCT) and the other one with variable temperature of 27/17°C for 16/8 ho, respectively (CLVT). A diurnal pattern of stomatal conductance (gs ) and [ABA] was observed in the plants grown in the control and CLVT conditions, while the plants in CLCT conditions experienced a significant decrease in stomatal conductance aligned with an increase in ABA. The net photosynthesis (A) was significantly reduced in CLCT, aligned with a significant decrease in the maximum rate of Rubisco carboxylation (Vcmax), the maximum rate of electron transport (Jmax) and mesophyll diffusion conductance to CO2 (gm) in comparison to the control and CLVT. An increased production of H2O2 and O2•- linked with increased activities of antioxidative enzymes was seen in both CL treatments, but despite of this, leaf injuries were only observed in the CLCT treatment. The results suggest that the diurnal temperature fluctuations alleviated the CL injury symptoms, probably because the diurnal cycles of cellular mechanisms were maintained. The ROS were shown not to be directly involved in CL-induced leaf injury, since both ROS production and scavenging was highest in CLVT without leaf chlorotic symptoms.

Continuous light increases growth, daily carbon gain, antioxidants, and alters carbohydrate metabolism in a cultivated and a wild tomato species.

Cultivated tomato species develop leaf injury while grown in continuous light (CL). Growth, photosynthesis, carbohydrate metabolism and antioxidative enzyme activities of a cultivated (Solanum lycopersicum L. 'Aromata') and a wild tomato species (Solanum pimpinellifolium L.) were compared in this study aiming to analyze the species-specific differences and thermoperiod effects in responses to CL. The species were subjected to three photoperiodic treatments for 12 days in climate chambers: 16-h photoperiod with a light/dark temperature of 26/16°C (P16D10 or control); CL with a constant temperature of 23°C (P24D0); CL with a variable temperature of 26/16°C (P24D10). The results showed that both species grown in CL had higher dry matter production due to the continuous photosynthesis and a subsequent increase in carbon gain. In S. lycopersicum, the rate of photosynthesis and the maximum photochemical efficiency of photosystem II declined in CL with the development of leaf chlorosis, reduction in the leaf chlorophyll content and a higher activity of antioxidative enzymes. The normal diurnal patterns of starch and sugar were only present under control conditions. The results demonstrated that CL conditions mainly affected the photosynthetic apparatus of a cultivated species (S. lycopersicum), and to a less degree to the wild species (S. pimpinellifolium). The negative effects of the CL could be alleviated by diurnal temperature variations, but the physiological mechanisms behind these are less clear. The results also show that the genetic potential for reducing the negative effects of CL does exist in the tomato germplasm.

Pore size regulates operating stomatal conductance, while stomatal densities drive the partitioning of conductance between leaf sides.

BACKGROUND AND AIMS: Leaf gas exchange is influenced by stomatal size, density, distribution between the leaf adaxial and abaxial sides, as well as by pore dimensions. This study aims to quantify which of these traits mainly underlie genetic differences in operating stomatal conductance (gs) and addresses possible links between anatomical traits and regulation of pore width. METHODS: Stomatal responsiveness to desiccation, gs-related anatomical traits of each leaf side and estimated gs (based on these traits) were determined for 54 introgression lines (ILs) generated by introgressing segments of Solanum pennelli into the S. lycopersicum 'M82'. A quantitative trait locus (QTL) analysis for stomatal traits was also performed. KEY RESULTS: A wide genetic variation in stomatal responsiveness to desiccation was observed, a large part of which was explained by stomatal length. Operating gs ranged over a factor of five between ILs. The pore area per stomatal area varied 8-fold among ILs (2-16 %), and was the main determinant of differences in operating gs between ILs. Operating gs was primarily positioned on the abaxial surface (60-83 %), due to higher abaxial stomatal density and, secondarily, to larger abaxial pore area. An analysis revealed 64 QTLs for stomatal traits in the ILs, most of which were in the direction of S. pennellii. CONCLUSIONS: The data indicate that operating and maximum gs of non-stressed leaves maintained under stable conditions deviate considerably (by 45-91 %), because stomatal size inadequately reflects operating pore area (R(2) = 0·46). Furthermore, it was found that variation between ILs in both stomatal sensitivity to desiccation and operating gs is associated with features of individual stoma. In contrast, genotypic variation in gs partitioning depends on the distribution of stomata between the leaf adaxial and abaxial epidermis.

Threshold response of stomatal closing ability to leaf abscisic acid concentration during growth.

Leaf abscisic acid concentration ([ABA]) during growth influences morpho-physiological traits associated with the plant's ability to cope with stress. A dose-response curve between [ABA] during growth and the leaf's ability to regulate water loss during desiccation or rehydrate upon re-watering was obtained. Rosa hybrida plants were grown at two relative air humidities (RHs, 60% or 90%) under different soil water potentials (-0.01, -0.06, or -0.08MPa) or upon grafting onto the rootstock of a cultivar sustaining [ABA] at elevated RH. Measurements included [ABA], stomatal anatomical features, stomatal responsiveness to desiccation, and the ability of leaves, desiccated to varying degrees, to recover their weight (rehydrate) following re-watering. Transpiration efficiency (plant mass per transpired water) was also determined. Soil water deficit resulted in a lower transpiration rate and higher transpiration efficiency at both RHs. The lowest [ABA] was observed in well-watered plants grown at high RH. [ABA] was increased by soil water deficit or grafting, at both RHs. The growth environment-induced changes in stomatal size were mediated by [ABA]. When [ABA] was increased from the level of (well-watered) high RH-grown plants to the value of (well-watered) plants grown at moderate RH, stomatal responsiveness was proportionally improved. A further increase in [ABA] did not affect stomatal responsiveness to desiccation. [ABA] was positively related to the ability of dehydrated leaves to rehydrate. The data indicate a growth [ABA]-related threshold for stomatal sensitivity to desiccation, which was not apparent either for stomatal size or for recovery (rehydration) upon re-watering.

Foliar abscisic acid content underlies genotypic variation in stomatal responsiveness after growth at high relative air humidity.

BACKGROUND AND AIMS: Stomata formed at high relative air humidity (RH) respond less to abscisic acid (ABA), an effect that varies widely between cultivars. This study tested the hypotheses that this genotypic variation in stomatal responsiveness originates from differential impairment in intermediates of the ABA signalling pathway during closure and differences in leaf ABA concentration during growth. METHODS: Stomatal anatomical features and stomatal responsiveness to desiccation, feeding with ABA, three transduction elements of its signalling pathway (H2O2, NO, Ca(2+)) and elicitors of these elements were determined in four rose cultivars grown at moderate (60 %) and high (90 %) RH. Leaf ABA concentration was assessed throughout the photoperiod and following mild desiccation (10 % leaf weight loss). KEY RESULTS: Stomatal responsiveness to desiccation and ABA feeding was little affected by high RH in two cultivars, whereas it was considerably attenuated in two other cultivars (thus termed sensitive). Leaf ABA concentration was lower in plants grown at high RH, an effect that was more pronounced in the sensitive cultivars. Mild desiccation triggered an increase in leaf ABA concentration and equalized differences between leaves grown at moderate and high RH. High RH impaired stomatal responses to all transduction elements, but cultivar differences were not observed. CONCLUSIONS: High RH resulted in decreased leaf ABA concentration during growth as a result of lack of water deficit, since desiccation induced ABA accumulation. Sensitive cultivars underwent a larger decrease in leaf ABA concentration rather than having a higher ABA concentration threshold for inducing stomatal functioning. However, cultivar differences in stomatal closure following ABA feeding were not apparent in response to H2O2 and downstream elements, indicating that signalling events prior to H2O2 generation are involved in the observed genotypic variation.

Smaller stomata require less severe leaf drying to close: a case study in Rosa hydrida.

Stomata formed at high relative air humidity (RH) close less as leaf dries; an effect that varies depending on the genotype. We here quantified the contribution of each stomatal response characteristic to the higher water loss of high RH-grown plants, and assessed the relationship between response characteristics and intraspecific variation in stomatal size. Stomatal size (length multiplied by width), density and responsiveness to desiccation, as well as pore dimensions were analyzed in ten rose cultivars grown at moderate (60%) or high (85%) RH. Leaf morphological components and transpiration at growth conditions were also assessed. High growth RH resulted in thinner (11%) leaves with larger area. A strong positive genetic correlation of daytime and nighttime transpiration at either RH was observed. Stomatal size determined pore area (r=0.7) and varied by a factor of two, as a result of proportional changes in length and width. Size and density of stomata were not related. Following desiccation, high RH resulted in a significantly lower (6-19%) decline of transpiration in three cultivars, whereas the relative water content (RWC) of high RH-expanded leaflets was lower (29-297%) in seven cultivars. The lower RWC of these leaflets was caused by (a) higher (33-72%) stable transpiration and/or (b) lower (12-143%) RWC at which this stable transpiration occurred, depending on the cultivar. Stomatal size was significantly correlated with both characteristics (r=0.5 and -0.7, respectively). These results indicate that stomatal size explains much of the intraspecific variation in the regulation of transpiration upon water deprivation on rose.