The Genus Nomocharis

Background: Ziziphus lotus, wild jujube, is a xerophytic shrub of the Rhamnaceae family widely distributed in arid and semi-arid regions of Tunisia, where it occupies most soil types. Phenological patterns of desert plants are strongly affected by the seasonality of water availability and phreatophytes represent a particularly interesting case for studying such relationships.

Aim: This study aims to investigate the relationship between phenological traits and water potential patterns of the wild jujube as a tool for understanding how plants cope with extreme drought.

Methods: Phenophases and predawn (Ψ_pd) and midday (Ψ_md) xylem water potentials of wild jujube were studied monthly (Nov 2007–Oct 2008) at Samaâliate and Oued El Hallouf in southern Tunisia. These sites receive164 mm and 191 mm of annual rainfall, respectively, and differ in slope and soil type.

Results: The Ψ_md decreased progressively and concomitantly with increasing seasonal drought, reaching the lowest values in late summer (down to –3.9 MPa for both sites). Seasonality of Ψ_pd was less pronounced for plants established in Oued El Hallouf (–2.09 MPa) than in Samaâliate (–2.63 MPa) at the end of the dry season. Wild jujube is dormant from October through to March and mature plants flower in May and produce fruits in August.

Conclusions: Our results clearly demonstrate that wild jujube is a drought tolerant species reaching low water potentials during the driest months of summer.     

The partitioning of water uptake between growth forms in a Neotropical savanna: do herbs exploit a third water source niche?

In addition to trees and grasses, the savannas of central Brazil are characterised by a diverse herbaceous dicot flora. Here we tested whether the coexistence of a highly diversified assemblage of species resulted in stratification or strong overlap in the use of soil water resources. We measured oxygen and hydrogen isotope ratios of stem water from herbs, grasses and trees growing side by side, as well as the isotopic composition of water in soil profile, groundwater and rainfall, and predawn (Ψ_pd) and midday (Ψ_md) leaf water potentials. We used a stable isotope mixing model to estimate vertical partitioning of soil water by the three growth forms. Grasses relied on shallow soil water (5–50 cm) and were strongly anisohydric. Ψ_pd and Ψ_md decreased significantly from the wet to the dry season. Trees extracted water from deeper regions of the soil profile (60–120 cm) and were isohydric. Ψ_pd and Ψ_md did not change from the wet to the dry season. Herbs overlapped with grasses in patterns of water extraction in the dry season (between 10 and 40 cm), but they took up water at soil depths intermediate (70–100 cm) to those of trees and grasses during the wet season. They showed seasonal changes in Ψ_pd but not in Ψ_md. We conclude that vertical partitioning of soil water may have contributed to coexistence of these three growth forms and resulted in a more complex pattern of soil water extraction than the two‐compartment model of soil water uptake currently used to explain the structure and function of tropical savanna ecosystems.     

Variations in leaf respiration across different seasons for Mediterranean evergreen species

Leaf respiration (R _L) of evergreen species co-occurring in the Mediterranean maquis developing along the Latium coast was analyzed. The results on the whole showed that the considered evergreen species had the same R _L trend during the year, with the lowest rates [0.83 ± 0.43 μmol(CO₂) m^?2 s^?1, mean value of the considered species] in winter, in response to low air temperatures. Higher R _L were reached in spring [2.44 ± 1.00 μmol(CO₂) m^?2 s^?1, mean value] during the favorable period, and in summer [3.17 ± 0.89 μmol(CO₂) m^?2 s^?1] during drought. The results of the regression analysis showed that 42% of R _L variations depended on mean air temperature and 13% on total monthly rainfall. Among the considered species, C. incanus, was characterized by the highest R _L in drought [4.93 ± 0.27 μmol(CO₂) m^?2 s^?1], low leaf water potential at predawn (Ψ_pd= ?1.08 ± 0.18 MPa) and midday (Ψ_md = ?2.75 ± 0.11 MPa) and low relative water content at predawn (RWC_pd = 80.5 ± 3.4%) and midday (RWC_md = 67.1 ± 4.6%). Compared to C. incanus, the sclerophyllous species (Q. ilex, P. latifolia, P. lentiscus, A. unedo) and the liana (S. aspera), had lower R _L [2.72 ± 0.66 μmol(CO₂) m^?2 s^?1, mean value of the considered species], higher RWC_pd (91.8 ± 1.8%), RWC_md (82.4 ± 3.2%), Ψ_pd (?0.65 ± 0.28 MPa) and Ψ_md (?2.85 ± 1.20 MPa) in drought. The narrow-leaved species (E. multiflora, R. officinalis, and E. arborea) were in the middle. The coefficients, proportional to the respiration increase for each 10°C rise (Q₁₀), ranging from 1.49 (E. arborea) to 1.98 (A. unedo) were indicative of the different sensitivities of the considered species to air temperature variation.     

Phenological comparison between two co-occurring Mediterranean woody species differing in growth form

Halimium atriplicifolium and Thymus vulgaris are two Mediterranean woody species, which differ in growth form and may co-occur under the same climatic constraints. Both possess distinct short (SS) and long shoots (LS). The aims of this work were: (1) to compare their phenological patterns, (2) to relate plant phenology with root depth and summer water potential, and (3) to compare the structure and phenology of SS and LS. Pre-dawn and mid-day shoot water potentials (Ψ_pd and Ψ_md) were assessed at the beginning and at the end of the driest period. SS and LS growth, flowering and fruit setting was followed every month throughout the annual cycle. Leaf shedding was followed with litter traps and leaf demography were monitored separately in SS and LS. Ψ_pd and Ψ_md exhibited a sharper drop in T. vulgaris than in H. atriplicifolium along the summer. Root depth of H. atriplicifolium was more than twice T. vulgaris. Phenophases of H. atriplicifolium occurred between spring and summer, while those of T. vulgaris concentrated in spring. The latter species shed many current-year leaves in September, probably in response to water shortage. In both species, LS growth occurred during a more rainy period than SS growth and during a warmer period in H. atriplicifolium. Leaf area and leaf mass per area were smaller for SS leaves than LS, probably due to water and carbon shortage at the time of SS growth. In conclusion, T. vulgaris suffered from more severe water stress than H. atriplicifilium due to its shallower root system and arrested phenological activity earlier in the summer. The different morphological and phenological traits of LS and SS suggest a specialisation in carbon gain along different time periods of the year.     

Effects of NaCl salinity and water stress on growth and leaf water relations of Asteriscus maritimus plants

Potted plants of Asteriscus maritimus (L.) Less were submitted to water stress (during two consecutive cycles, irrigation water was withheld for 5 days followed by a recovery period of 25 days) and saline stress (150 days of exposure to 0, 70 and 140 mM NaCl daily irrigation) in order to assess the effect on leaf water relations and growth parameters. Plants under saline and water stress conditions showed lower biomass and an early reduction in leaf expansion growth. Both stresses promoted a substantial degree of stomatal regulation; but, in spite of this, the plants showed signs of leaf tissue dehydration, decreases in RWC and Ψ_pd values. However, salt-treated plants, developed a NaCl inclusion mechanisms, underwent osmotic adjustment, which was able to maintain leaf turgor. Under both stress conditions g_l was independent to plant water status in the range between –0.8 and 1.0 MPa. Under water stress conditions, midday leaf water potential showed a threshold value (around −1.1 MPa), below which leaf conductance remained constant. In the salt-treated plants, the gradual closure of the stomata over a wide range of Ψ_md may be important in maintaining some level of photosynthesis.     

Leaf phenology and water potential of five arboreal species in gallery and montane forests in the Chapada Diamantina; Bahia; Brazil

The influence of water potential (Ψ_W) on the leaf phenological behaviors of five arboreal species within gallery and montane forests on the eastern slope of the Chapada Diamantina Mountain Range in Bahia State, NE Brazil, was examined to investigate the following questions: (1) Do seasonal variations in rainfall influence the water potential of these species in their natural habitats (gallery and montane forests)? (2) Do the gallery forest species show smaller daily variations in their water potential and greater nocturnal recuperation than montane species? (3) Can the water potentials of these species be used to identify their particular types of leaf phenologies? The species examined were Tibouchina fissinervia and Clusia nemorosa (evergreen with continual leafdrop); Tapirira guianensis and Vochysia pyramidalis (evergreen with concentrated leafdrop), and Maprounea guianensis (brevi-deciduous) were chosen based on their abundance, importance value indices, and contrasting phenological behaviors. The leaf phenologies of all species were similar in both forest types. Evergreen species with continual leafdrop demonstrated leaf budding and leaf fall occurred at low levels during the entire year. Evergreen species with continual leafdrop as well as brevi-deciduous species demonstrated peaks of leaf fall and leaf budding during the dry period. Brevi-deciduous trees remained without leaves for an average of three days. The major episodes of leaf budding occurred during the dry period in all of the species examined. M. guianensis usually demonstrated low predawn (Ψ_PD) and midday (Ψ_MD) water potentials (Ψ_W). C. nemorosa demonstrated the greatest Ψ_PD and Ψ_MD values and the lowest daily amplitude (Δ_Ψ), suggesting that this plant used CAM during most of the study period. The majority of the species demonstrated small seasonal differences in Ψ_W, and the Ψ_PD values indicated a satisfactory nocturnal recuperation of their water status, even during the dry period. The maximum daily amplitude of Ψ_W was observed in V. pyramidalis, a species dependent on moist habitats for establishment and growth. No large decrease in Ψ_PD was observed during the study period in the species examined here, and all species demonstrated leaf budding during the dry period, suggesting that there were no severe water restrictions in either the gallery or montane forests. However, during dry periods, the species generally demonstrated lower Δ_Ψ. This reflects the restriction of transpiration, which according to phenological data, may occur due to different mechanisms, depending on the species. Our data supports the view that there is no strict relationship between decreasing water status in these plants and leaf fall.     

A quantitative comparison of two extremes in chaparral shrub phenology

We quantitatively compared phenology and water relations of a fully deciduous shrub, Styrax officinalis, and an evergreen shrub, Arctostaphylos glauca, in shared microsites in a sandstone outcrop in southern California during a multi-year drought. Pre-dawn xylem pressure potentials, Ψ_pd, were similar for the two species during most months of 2 years, but occasional differences and watering experiment results suggest S. officinalis may have phreatophytic roots that tap water in deep rock cavities and joint traces, while A. glauca may have primarily shallow roots. Neither species varied in maximum or minimum Ψ_pd between years of very different rainfall totals. Twig elongation and leaf production of S. officinalis began earlier during spring, and its leaves matured more quickly and more synchronously than A. glauca. Leaves lived a mean of 180 days for S. officinalis and 849 days for A. glauca. Leaf life spans varied among years in both species. S. officinalis leaf senescence occurred mostly in August and September and was not discernibly related to Ψ_pd or drought avoidance. A. glauca leaf senescence occurred throughout the year, but especially coincided with leaf production. In A. glauca most senescence occurred at the beginning of a leaf cohort's third growing season, but numbers of retained older leaves increased during 4 years of drought. Timing of twig elongation and leaf and flower production appeared to be related to current rainfall, but amount of twig growth and numbers of leaves and flowers produced appeared to be related to rainfall of the previous year or years for S. officinalis and, more complexly, A. glauca. Because of an interrupted pattern of flower production, number of flowers produced by A. glauca may be responsive to rainfall amounts during two environmental periods. Morphological differences, including much higher above ground allocation, many more leaves/twig, lower allocation to stem mass, and longer duration of leaves/year in A. glauca, are probably responsible for A. glauca having >6 times more above ground biomass per plant than S. officinalis. During the multi-year drought S. officinalis changed little, indicating either superb adaptation or growth pattern rigidity, while A. glauca underwent extensive phenological and morphological changes, indicating either stress or adaptive flexibility.     

Linking the patterns in soil moisture to leaf water potential, stomatal conductance, growth, and mortality of dominant shrubs in the Florida scrub ecosystem

Patterns in soil moisture availability affect plant survival, growth and fecundity. Here we link patterns in soil moisture to physiological and demographic consequences in Florida scrub plants. We use data on different temporal scales to (1) determine critical soil moisture content that leads to loss of turgor in leaves during predawn measurements of leaf water status (Ψ _crit), (2) describe the temporal patterns in the distribution of Ψ _crit, (3) analyze the strength of relationship between rainfall and soil moisture content based on 8 years of data, (4) predict soil moisture content for 75 years of rainfall data, and (5) evaluate morphological, physiological and demographic consequences of spring 2006 drought on dominant shrubs in Florida scrub ecosystem in the light of water-uptake depth as determined by stable isotope analysis (δ¹⁸O). Based on 1998–2006 data, the soil moisture content at 50 cm depth explained significant variation in predawn leaf water potential of two dominant shrubs, Quercus chapmanii and Ceratiola ericoides (r ²?=?0.69). During 8 years of data collection, leaves attained Ψ _crit only during the peak drought of 2000 when the soil moisture fell below 1% by volume at 50 and 90 cm depth. Precipitation explained a significant variation in soil moisture content (r ²?=?0.62). The patterns in predicted soil moisture for 75 year period, suggested that the frequency of drought occurrence has not increased in time. In spring 2006, the soil reached critical soil moisture levels, with consequences for plant growth and physiological responses. Overall, 24% of plants showed no drought-induced damage, 51% showed damage up to 50%, 21% had intense leaf shedding and 2% of all plants died. Over the drought and recovery period (May–October 2006), relative height growth was significantly lower in plants with greater die-back. All species showed a significant depression in stomatal conductance, while all but deep-rooted palms Sabal etonia and Serenoa repens showed significantly lower predawn (Ψ _pd) and mid-day (Ψ _md) leaf water potential in dry compared to wet season. Plants experiencing less severe die-back exhibited greater stomatal conductance, suggesting a strong relationship between physiology and morphology. Based on results we suggest that the restoration efforts in Florida scrub should consider the soil moisture requirements of key species.     

近30年北京自然历的主要物候期、物候季节变化及归因

根据中国物候观测网资料并结合气象观测数据, 重新编制了北京颐和园地区1981-2010年的自然历。通过与原自然历比较, 揭示了北京物候季节变化特征, 分析了1963年以来物候季节变化的可能原因。研究发现: 与原自然历相比, 1981-2010年北京的春、夏季开始时间分别提前了2天和5天, 秋、冬季开始时间分别推迟了1天和4天; 夏、秋季长度分别延长了6天和3天, 春、冬季长度则分别缩短了3天和6天; 各个物候期的平均日期、最早日期、最晚日期在春、夏季以提前为主, 在秋、冬季以推迟为主; 且春、秋、冬季节内部分物候期次序也出现了不同程度的变化。春、夏、冬季开始日期前的气温变化和秋季开始日期前的日照时数变化可能是北京颐和园地区物候季节变化的主要原因; 不同物种、不同物候期对气温变化的响应程度不同, 导致了物候季节内各种物候现象出现的先后顺序发生变化。     

Leaf isoprene emission declines in Quercus pubescens seedlings experiencing drought – Any implication of soluble sugars and mitochondrial respiration?

Previous studies suggest that the sensitivity of leaf mitochondrial respiration and the pool of soluble sugars to water stress could influence the response of leaf isoprene emission to drought by affecting the availability of extra-chloroplastic carbon for isoprene synthesis. We measured rates of isoprene emission and CO₂ exchange, and the concentration of nonstructural carbohydrates in leaves of Quercus pubescens Willd. seedlings subjected to either normal watering (control plants, C) or drought (droughted plants, D). Stopping of watering caused predawn leaf water potential (Ψ_pd) to decline between −2.3 and −5.1 MPa among D plants, whereas Ψ_pd remained higher than −0.45 MPa in C plants. Isoprene emission (I_s), net CO₂ assimilation (A_n) and dark mitochondrial respiration (R_d) decreased with increasing water deficit, with declines in these variables relative to the respective means of C plants being A_n > I_s > R_d. This resulted in positive pairwise correlations between the three variables. The concentration of nonstructural carbohydrates did not change between treatments, but the concentration of soluble sugars increased and that of starch decreased in D plants as compared with C plants. As a consequence, there was a negative correlation between I_s and the concentration of soluble sugars, which supports a limited use of cytosolic sugars in sustaining isoprene synthesis at high to severe water stress. Our data also indicate that competition between I_s and R_d for the same carbon substrates had little importance for isoprene emission at high to severe water stress, as compared to the overall constraint on isoprene metabolism probably imposed by the shortage of photosynthetic carbon, energy and reducing equivalents.     

Reciprocal grafting between clones with contrasting drought tolerance suggests a key role of abscisic acid in coffee acclimation to drought stress

The role of abscisic acid (ABA) in drought tolerance of Coffea canephora is unknown. To determine whether ABA is associated with drought tolerance and if the use of tolerant rootstocks could increase ABA and drought tolerance, we performed reciprocal grafting experiments between clones with contrasting tolerance to drought (clone 109, sensitive; and clone 120, tolerant). Plants were grown in large (120 L) pots in a greenhouse and subjected to drought stress by withholding irrigation. The non-grafted 120 plants and graft treatments with 120 as a rootstock showed a slower reduction of predawn leaf water potential (Ψ_pd) and a lower negative carbon isotopic composition ratio compared with the other grafting combinations in response to drought. The same 120 graft treatments also showed higher leaf ABA concentrations, lower levels of electrolyte leakage, and lower activities of ascorbate peroxidase and catalase under moderate (Ψ_pd?=???1.0 or ??1.5 MPa) and severe (Ψ_pd?=???3.0 MPa) drought. Root ABA concentrations were higher in plants with the 120 rootstocks regardless of watering regime. The 120 shoots could also contribute to drought tolerance because treatment with 120/109 rootstock/scion combination showed postponed dehydration, higher leaf ABA concentration, and lower leaf electrolyte leakage compared with the sensitive clone. We conclude that both the shoot and root systems of the tolerant clone can increase the concentrations of ABA in leaves in response to drought. This further suggests that ABA is associated with a delayed onset of severe water deficit and decreased oxidative damage in C. canephora.     

Water relations and leaf chemistry of Chrysothamnus nauseosus ssp. consimilis (Asteraceae) and Sarcobatus vermiculatus (Chenopodiaceae)

At Mono Lake, California, we investigated field water relations, leaf and xylem chemistry, and gas exchange for two shrub species that commonly co-occur on marginally saline soils, and have similar life histories and rooting patterns. Both species had highest root length densities close to the surface and have large tap roots that probably reach ground water at 3.4-5.0 m on the study site. The species differed greatly in leaf water relations and leaf chemistry. Sarcobatus vermiculatus had a seasonal minimum predawn xylem pressure potential (ψ_pd) of -2.7 MPa and a midday potential (ψ_md) of -4.1 MPa. These were significantly lower than for Chrysothamnus nauseosus, which had a minimum ψ_pd of -1.0 MPa and ψ_md of -2.2 MPa. Sarcobatus had leaf Na of up to 9.1 % and K up to 2.7 % of dry mass, and these were significantly higher than for Chrysothamnus which had seasonal maxima of 0.4% leaf Na and 2.4 % leaf K. The molar ratios of leaf K/Na, Ca/Na, and Mg/Na were substantially lower for Sarcobatus than for Chrysothamnus. Xylem ionic contents indicated that both species excluded some Na at the root, but that Chrysothamnus was excluding much more than Sarcobatus. The higher Na content of Sarcobatus leaves was associated with greater leaf succulence, lower calculated osmotic potential, and lower xylem pressure potentials. Despite large differences in water relations and leaf chemistry, these species maintained similar diurnal patterns and rates of photosynthesis and stomatal conductance to water vapor diffusion. Sarcobatus ψ_pd may not reflect soil moisture availability due to root osmotic and hydraulic properties.     

Leaf gas exchange and multiple enzymatic and non-enzymatic antioxidant strategies related to drought tolerance in two oil palm hybrids

Key message

The drought tolerance in young oil palm plants is related to greater efficiency in preventing oxidative damage by activating enzymatic and non-enzymatic antioxidant strategies simultaneously.

Abstract

Drought is a major environmental constraint limiting growth and yield of oil palm trees. In this study, two oil palm hybrids (BRS Manicoré and BRS C 2501) were grown in large containers and subjected to a water deficit during 57 days. Leaf gas exchange analysis was combined with an in-depth assessment of the antioxidant system over the drought imposition. Under drought, leaf water potential at predawn (Ψ _pd) decreased similarly in both hybrids. In parallel, there were decreases in the net CO₂ assimilation rate (A), chlorophyll concentrations and Rubisco total activity. Overall, these decreases were more pronounced in BRS C 2501 than in BRS Manicoré. BRS C 2501 plants triggered more markedly its enzymatic antioxidant system earlier (Ψ _pd = ?2.1 MPa) than did BRS Manicoré, but these responses were accompanied by higher concentrations of H₂O₂ and malondialdehyde in BRS C 2510 than in BRS Manicoré. With the progress of drought stress (Ψ _pd = ?2.9 MPa and below), BRS Manicoré was better able to cope with oxidative stress through a more robust antioxidant system. In addition, significant decreases in drought-induced NAD⁺-malate dehydrogenase activities were only observed in stressed BRS C 2501 plants. Regardless of watering regimes, the total carotenoid, ascorbate and glutathione concentrations were higher in BRS Manicoré than in BRS C 2501. In conclusion, BRS Manicoré is better able to tolerate drought than BRS C 2501 by triggering multiple antioxidant strategies involved both in reactive oxygen species scavenging and dissipation of excess energy and/or reducing equivalents particularly under severe drought stress.

     

Seasonal variation in plant hydraulic traits of two co-occurring desert shrubs, Tamarix ramosissima and Haloxylon ammodendron, with different rooting patterns

Effective hydraulic responses to varying soil moisture and evaporative demand are crucial to plant survival in arid ecosystems. This study was carried out during two growing seasons (2004?C2005) on two typical desert shrub species, Tamarix ramosissima and Haloxylon ammodendron, co-occurring in the Gurbantonggut Desert (Central Asia), to investigate their hydraulic responses to seasonal variations in water availability. The root distribution was studied by excavating the intact root systems. Leaf-specific apparent hydraulic conductance (K _l) for the two species was calculated based on leaf water potential (?? _l) and transpiration rate (T _r), which were monitored during the growing seasons. T. ramosissima had a deeper taproot (3.1 vs. 2.6 m) and a larger root surface area (3.02 vs. 1.28 m²) than H. ammodendron. Combined with a higher ?? _l, this meant that it maintained a better water status in general. For the deep-rooting T. ramosissima, the seasonal pattern of its predawn leaf water potential (?? _pd) was in high accordance with the seasonal changes in soil moisture at a depth of 2.6?C2.8 m, which was largely influenced by the upflow of groundwater through capillary ascent, and barely responded to rain events. For the shallow-rooting H. ammodendron, the seasonal pattern of ?? _pd was closely related to soil moisture in the upper layer at a depth of 0?C0.2 and 0.6?C0.8 m, which was recharged periodically by rain events and responded acutely to rainfall above 5 mm. The two species differed in their maximum transpiration rates (T _rmax) and ?? _pd: in T. ramosissima, T _rmax gradually dropped with decreasing ?? _pd; in H. ammodendron, T _rmax showed no significant response to ?? _pd. For T. ramosissima, the major water resource was groundwater and vadose zone water, and ?? _l contributed significantly to transpiration regulation. For H. ammodendron, the primary water resource was precipitation input, which was not sufficient to keep the ?? _l for this species outside the critical range of leaf shedding in summer. Thus, for these two representative species that share the same habitat, contrasting response strategies to water limitation were observed in relation to water acquisition and root distribution characteristics.     

Drought tolerance and antioxidant enzymatic activity in transgenic ‘Swingle’ citrumelo plants over-accumulating proline

In this study we investigated the effects of the high endogenous proline level on water relations, gas exchange and antioxidant enzymatic activity in leaves of transgenic ‘Swingle’ citrumelo rootstocks transformed with the P5CSF129A gene coding for the key-enzyme for proline synthesis, under water deficit. Leaf total water, osmotic and pressure potentials, stomatal conductance, photosynthetic rates and xylem sap flow were evaluated in non-transformed control and transgenic plants during water deficit treatment. Malondialdehyde (MDA) content, catalase (CAT; EC 1.11.1.6), superoxide dismutase (SOD; EC 1.15.1.1) and ascorbate peroxidase (APX; EC 1.11.1.11) activities were quantified in leaves collected based on their total water potential, representing the following conditions: irrigated (Ψ_w = −1.3 MPa), moderate stress (Ψ_w = −2.3 to −2.5 MPa), severe stress (Ψ_w = −3.8 to −3.9 MPa) and recovery (24 h after re-irrigation: Ψ_w = −1.3 to −1.9 MPa). Osmotic adjustment was observed in transgenic plants until 11 days after withholding water, while pressure potential in non-transformed controls was close to zero after nine days of water deprivation. This superior maintenance of turgor pressure in leaves of transgenic plants led to higher stomatal conductance, photosynthetic and transpiration rates when compared to non-transgenic plants. Drought caused a significant decrease in APX and SOD activities in control plants, followed by an increase after re-watering. On the other hand, CAT was more active in control than in transgenic plants under irrigated condition and both stress levels. Our results suggest that transgenic plants were able to cope with water deficit better than non-transformed controls since the high endogenous proline level acted not only by mediating osmotic adjustment, but also by contributing to gas exchange parameters and ameliorating deleterious effects of drought-induced oxidative stress.     

Water deficit in field-grown Gossypium hirsutum primarily limits net photosynthesis by decreasing stomatal conductance,increasing photorespiration,and increasing the ratio of dark respiration to gross photosynthesis

Much effort has been expended to improve irrigation efficiency and drought tolerance of agronomic crops; however, a clear understanding of the physiological mechanisms that interact to decrease source strength and drive yield loss has not been attained. To elucidate the underlying mechanisms contributing to inhibition of net carbon assimilation under drought stress, three cultivars of Gossypium hirsutum were grown in the field under contrasting irrigation regimes during the 2012 and 2013 growing season near Camilla, Georgia, USA. Physiological measurements were conducted on three sample dates during each growing season (providing a broad range of plant water status) and included, predawn and midday leaf water potential (Ψ_PD and Ψ_MD), gross and net photosynthesis, dark respiration, photorespiration, and chlorophyll a fluorescence. End-of-season lint yield was also determined. Ψ_PD ranged from −0.31 to −0.95 MPa, and Ψ_MD ranged from −1.02 to −2.67 MPa, depending upon irrigation regime and sample date. G. hirsutum responded to water deficit by decreasing stomatal conductance, increasing photorespiration, and increasing the ratio of dark respiration to gross photosynthesis, thereby limiting P_N and decreasing lint yield (lint yield declines observed during the 2012 growing season only). Conversely, even extreme water deficit, causing a 54% decline in P_N, did not negatively affect actual quantum yield, maximum quantum yield, or photosynthetic electron transport. It is concluded that P_N is primarily limited in drought-stressed G. hirsutum by decreased stomatal conductance, along with increases in respiratory and photorespiratory carbon losses, not inhibition or down-regulation of electron transport through photosystem II. It is further concluded that Ψ_PD is a reliable indicator of drought stress and the need for irrigation in field-grown cotton.     

Stem growth and phenology of two tropical trees in contrasting soil conditions

Background and aims

Phenological variations in tropical forests are usually explained by climate. Nevertheless, considering that soil water availability and nutrient content also influence plant water status and metabolism, soil conditions may also be important in the regulation of plant reproductive and vegetative activities over time. We investigated whether phenological patterns and stem growth differ in trees growing in two types of soil that display contrasting water and nutrient availability, namely, Gleysol (moist and nutrient-poor) and Cambisol (drier and nutrient-rich).

Methods

Phenological observations (flushing, leaf fall, flowering and fruiting) and stem diameter growth were recorded for 120 trees fitted with fixed dendrometer bands, at 15 days intervals, for 1 year. Two species of contrasting deciduousness were investigated: Senna multijuga (semi-deciduous) and Citharexylum myrianthum (deciduous).

Results

Both species were seasonal in all phenophases, regardless of soil type. However, frequency, mean date and intensity of phenophases varied according to soil type. Girth increment of C. myrianthum was four times greater in Cambisol than in Gleysol, whereas the type of soil had no significant effect on that of S. multijuga.

Conclusions

These results show that soil characteristics also play an important role in determining phenological patterns and growth and must be considered when analysing phenological patterns in tropical forests.     

Temporary drought can selectively suppress Schoenoplectus mucronatus in rice

Outdoor pot experiments were conducted in California to quantify differences in rice and Schoenoplectus mucronatus susceptibility to drought and to identify morphological and physiological traits that would favor rice over S. mucronatus under drought. Plants were grown in flooded soil for approximately 5 weeks, and then subjected to different drought periods after which pots were re-flooded. Chlorophyll fluorescence assays revealed that rice and S. mucronatus Fv/Fm first became <0.8 after leaf water potential (Ψ_leaf) had decreased to approximately −4 MPa and −2 MPa, respectively. Thus, by suffering less photosynthetic damage from drought, rice had better recovery after re-flooding than S. mucronatus. When drought reduced Ψ_leaf to −3 MPa, S. mucronatus re-growth was nearly suppressed but that of rice was unaffected. Rice plants depleted soil moisture 1.6 faster than S. mucronatus due to larger and deeper roots and a high water-spending strategy (when Ψ_leaf decreased from approximately −0.5 MPa to −2.5 MPa, ¹³δ increased from −27.8 to −27.4 and from −28.1 to −26.0 for rice and S. mucronatus, respectively). Rice under interspecific competition sustained its Ψ_leaf by extracting more water from greater depths, while causing severe moisture stress and photosynthetic damage to S. mucronatus. Thus temporary drought enhanced rice competitiveness over S. mucronatus, supporting the concept of using brief drought as a tool for S. mucronatus suppression in rice. The Ψ_leaf developed by the end of the drought period predicted rice yields (R² = 0.77, P < 0.0001) and the capacity of S. mucronatus to recover from drought upon irrigation resumption (R² = 0.62, P < 0.001). Brief (8-10 d) drought imposed on 5-week-old rice did not significantly depress late-season rice biomass growth or grain yields, while S. mucronatus never fully recovered from drought. Rice yields were only reduced after Ψ_leaf reached values below approximately −2.5 MPa. Longer drought (∼20 d) delayed maturity and reduced rice yields by approximately 60-80%. The dry-down approach could help suppress weeds similar to S. mucronatus in organic rice where premium prices can compensate for lower grain yield.     

Phenological responses of plants to climate change in an urban environment

Global climate change is likely to alter the phenological patterns of plants due to the controlling effects of climate on plant ontogeny, especially in an urbanized environment. We studied relationships between various phenophases (i.e., seasonal biological events) and interannual variations of air temperature in three woody plant species (Prunus davidiana, Hibiscus syriacus, and Cercis chinensis) in the Beijing Metropolis, China, based on phenological data for the period 1962–2004 and meteorological data for the period 1951–2004. Analysis of phenology and climate data indicated significant changes in spring and autumn phenophases and temperatures. Changes in phenophases were observed for all the three species, consistent with patterns of rising air temperatures in the Beijing Metropolis. The changing phenology in the three plant species was reflected mainly as advances of the spring phenophases and delays in the autumn phenophases, but with strong variations among species and phenophases in response to different temperature indices. Most phenophases (both spring and autumn phenophases) had significant relationships with temperatures of the preceding months. There existed large inter- and intra-specific variations, however, in the responses of phenology to climate change. It is clear that the urban heat island effect from 1978 onwards is a dominant cause of the observed phenological changes. Differences in phenological responses to climate change may cause uncertain ecological consequences, with implications for ecosystem stability and function in urban environments.     

Phenophases alter the soil respiration–temperature relationship in an oak-dominated forest

Soil respiration (SR) represents a major component of forest ecosystem respiration and is influenced seasonally by environmental factors such as temperature, soil moisture, root respiration, and litter fall. Changes in these environmental factors correspond with shifts in plant phenology. In this study, we examined the relationship between canopy phenophases (pre-growth, growth, pre-dormancy, and dormancy) and SR sensitivity to changes in soil temperature (T_S). SR was measured 53 times over 550 days within an oak forest in northwest Ohio, USA. Annual estimates of SR were calculated with a Q₁₀ model based on T_S on a phenological (PT), or annual timescale (AT), or T_S and soil volumetric water content (VWC) on a phenological (PTM) or annual (ATM) timescale. We found significant (p<0.01) difference in apparent Q₁₀ from year 2004 (1.23) and year 2005 (2.76) during the growth phenophase. Accounting for moisture-sensitivity increased model performance compared to temperature-only models: the error was −17% for the ATM model and −6% for the PTM model. The annual models consistently underestimated SR in summer and overestimated it in winter. These biases were reduced by delineating SR by tree phenophases and accounting for variation in soil moisture. Even though the bias of annual models in winter SR was small in absolute scale, the relative error was about 91%, and may thus have significant implications for regional and continental C balance estimates.