Coppicing affects growth, root:shoot relations and ecophysiology of potted Quercus rubra seedlings

Two experiments were conducted to examine the response of Quercus rubra L. seedlings to coppicing. In a greenhouse experiment, growth, biomass distribution, leaf gas exchange, and water and carbohydrate relations were measured for 1-year-old seedlings that were either coppiced when dormant at the time of planting or left intact as controls. Coppicing induced sprouting from the base of the stem, and, in general, the physiology of sprouts and controls was similar. However, the relative growth rate (RGR) of sprouts was 9% higher than that of controls, allowing sprouts to compensate fully for the initial mass lost to coppicing. In a second experiment, in an outdoor cold frame, growth, biomass distribution, leaf gas exchange and plant water relations were measured on 1-year-old seedlings that were either coppiced at the time of planting (dormant-coppiced), coppiced soon after bud break (active-coppiced) or left intact (controls). Dormant coppicing again had little impact on seedling physiology, and dormant-coppiced plants again compensated for initial mass loss with a higher RGR. In contrast, active-coppiced seedlings did not compensate for initial mass loss, as their RGR did not differ from that of controls. By the tenth week of the study, leaf gas exchange rates of active-coppiced sprouts were higher than those of dormant-coppiced and control seedlings. Active-coppiced sprouts also had a greater soil-to-leaf hydraulic conductivity (expressed on a leaf area basis) and a lower ratio of leaf area to root surface area than did controls. Across treatments, photosynthetic rate and stomatal conductance were positively correlated with soil-to-leaf hydraulic conductivity, and gas exchange rates and hydraulic conductivity were negatively related to leaf:root area ratio. Thus, the removal of actively growing shoots may have altered subsequent leaf gas exchange largely through coppice-induced changes in leaf-root balance.     

Compensatory photosynthesis as a response to partial debudding in ezo spruce, <Emphasis Type="Italic">Picea jezoensis</Emphasis> seedlings

The effects of partial debudding on photosynthesis, stomatal conductance and nitrogen contents of 1-year-old needles and newly grown needles were studied in Picea jezoensis (Sieb. et Zucc.). Seventy-five percent of the buds of P.jezoensis seedlings were removed soon after bud break. Gas exchange was measured three times for 1-year-old needles and twice for current-year needles. The photosynthetic rates of 1-year-old needles were greater in debudded seedlings than in control seedlings, and the difference increased as the growing season progressed. This greater photosynthetic rate in debudded seedlings was accompanied by greater stomatal conductance. However, neither the photosynthetic rates nor the stomatal conductance of current-year needles differed between debudded and control seedlings after the needles had fully expanded. Debudding also had no effect on mass-based nitrogen contents in either the 1-year-old or the current-year needles. Area-based nitrogen in the 1-year-old needles did not differ between debudded and control seedlings, but was greater in debudded seedlings than in control seedlings in current-year needles. These results suggest that the enhanced photosynthetic rate is more likely a result of an increased root/leaf ratio that reduces the stomatal limitation of photosynthetic rate than a result of altered sink-source relationships or increased leaf nitrogen content.     

Changes in photosynthesis caused by adaptation of maize seedlings to short-term drought

Detached leaf is in the state of increasing water deficit; it is a good experimental model for looking into the hardening effect of adaptation of eight-day-old maize (Zea mays L.) seedlings to short-term drought (five days without watering). The light stage of photosynthesis and photosynthetic CO₂/H₂O exchange in detached leaves were studied. Specific surface density of leaf tissue (SSDL), the content of chlorophylls a and b, proline, MDA as well as photosynthetic parameters: quantum yield of photosystem II fluorescence, assimilation of CO₂, and transpiration at room temperature and light saturation (density of PAR quantum flux of 2000 μmol/(m² s)) at normal and half atmospheric CO₂ concentration were determined. The leaves of seedlings exposed to short-term drought differed from control material by a greater SSDL and higher content of proline. The hardening effect of the stress agent on the dark stage of photosynthesis was detected; it was expressed in the maintenance of the higher photosynthetic CO₂ assimilation against control material due to the elevation of stomatal conductance for CO₂ diffusing into the leaf. Judging from the lack of differences in the MDA content, short-term drought did not injure photosynthetic membranes. In detached leaves of experimental maize seedlings, photosynthesis was maintained on a higher level than in control material.     

Traits of an invasive grass conferring an early growth advantage over native grasses

Aims Invasive species often have higher relative growth rates (RGR) than their native counterparts. Nutrient use efficiency, total leaf area and specific leaf area (SLA) are traits that may confer RGR differences between natives and invasives, but trait differences are less prominent when the invasive species belongs to the same plant functional type as the dominant native species. Here, we test if traits displayed soon after germination confer an early size advantage. Specifically, we predicted that invasive species seedlings grow faster than the natives because they lack trade-offs that more strongly constrain the growth of native species.Methods We quantified plant morphological and physiological traits and RGR during early seedling growth at high and low nutrient levels in three dominant perennial native C₄ grasses: Panicum virgatum L. (switchgrass), Schizachyrium scoparium (Michx.) Nash (little bluestem) and Andropogon gerardii Vitman (big bluestem); and a perennial C₄ exotic invasive grass, Sorghum halepense (L.) Pers. (Johnsongrass).Important findings After 2 weeks of growth, Johnsongrass seedlings had greater biomass, SLA and photosynthetic nitrogen use efficiency, but lower leaf N concentrations (% leaf N) and root:shoot ratio than natives. As growth continued, Johnsongrass more quickly produced larger and thicker leaves than the natives, which dampened the growth advantage past the first 2 to 3 weeks of growth. Investment in carbon gain appears to be the best explanation for the early growth advantage of Johnsongrass. In natives, growth was constrained by an apparent trade-off between allocation to root biomass, which reduced SLA, and production of leaves with high N content, which increased carbon gain. In Johnsongrass, root:shoot ratio did not interact with other traits, and % leaf N was decoupled from RGR as a result of a trade-off between the positive indirect association of % leaf N with RGR and the negative direct association of % leaf N with RGR.     

Hormonal changes in papaya seedlings subjected to progressive water stress and re-watering

Changes on abscisic acid (ABA), jasmonic acid (JA) and indole-3-acetic acid (IAA) levels were investigated in papaya seedlings (Carica papaya L.) cv. “Baixinho de Santa Amalia” under progressive water stress and subsequent rehydration. Also, the behaviour of leaf gas exchange and leaf growth was determined under stress condition. The results indicated that ABA and JA differ in their pattern of change under water stress. ABA continuously increased in leaves and roots during the whole period of stress whereas JA showed a sharp increase and a later decrease in both organs. Re-watering reduced rapidly (24 h) leaf and root ABA to control levels whereas the influence on JA levels could not be assessed. Drought and recovery did not alter IAA levels in leaf and root tissues of papaya seedlings. In addition, water stress reduced stomatal conductance, photosynthetic rate, transpiration rate, the percentage of attached leaves and leaf growth. Rehydration reverted in few days the effects of stress on leaf growth and gas exchange parameters. Overall, the data suggest that ABA could be involved in the induction of several progressive responses such as the induction of stomatal closure and leaf abscission to reduce papaya water loss. In addition, the pattern of accumulation of JA is compatible with a triggering signal upstream ABA. The unaltered levels of IAA could suggest a certain adaptive ability of papaya to maintain active physiological processes under progressive drought stress.     

Photochemical efficiency of Photosystem II and xanthophyll cycle components in Zea mays leaves exposed to water stress and high light

The effects of two light treatments (photosynthetically active photon flux density of either 650 or 1950 µmol m^–2 s^–1) on the photochemical efficiency of Photosystem II (PS II) (measured as variable to maximum fluorescence ratio) and on the xanthophyll cycle components was studied in wilted Zea mays leaves. For comparison, these parameters were followed under the same light conditions in well-hydrated leaves maintained either in normal or CO₂-free air. The net CO₂ assimilation of dehydrated leaves declined rapidly as their relative water content (RWC) decreased from 100 to 60% while the PS II efficiency measured after a prolonged dark period of 16 h declined only when RWC leaves was lower than 60%. Furthermore, drought caused an increase in the pool size of the xanthophyll cycle pigments and the presence of a sustained elevated level of zeaxanthin and antheraxanthin at the end of the long dark period. The leaf water deficit enhanced the sensitivity of PS II efficiency to light exposure. During illumination, strong inhibition of PS II efficiency and large violaxanthin deepoxidation was observed in wilted leaves even under moderate photon flux density compared to control leaves in the same conditions. After 2 h of darkness following the light treatment, the PS II efficiency that is dependent on the previous PPFD, decreased with leaf water deficit. Moreover, zeaxanthin epoxidation led to an accumulation of antheraxanthin in dehydrated leaves. All these drought effects on PS II efficiency and xanthophyll cycle components were also obtained in well-hydrated leaves by short-term CO₂ deprivation during illumination. We conclude that the increased susceptibility of PS II efficiency to light in wilted maize leaves is mainly explained by the decrease of CO₂ availability and the resulting low net CO₂ assimilation.     

Simulated sea level change alters anatomy, physiology, growth, and reproduction of red mangrove (Rhizophora mangle L.)

Tropical coastal forests – mangroves – will be one of the first ecosystems to be affected by altered sea levels accompanying global climate change. Responses of mangrove forests to changing sea levels depend on reactions of individual plants, yet such responses have not been addressed experimentally. We report data from a long-term greenhouse study that assessed physiological and individual growth responses of the dominant neotropical mangrove, Rhizophora mangle, to levels of inundation expected to occur in the Caribbean within 50–100 years. In this study, we grew potted plants in tanks with simulated semidiurnal (twice daily) high tides that approximated current conditions (MW plants), a 16-cm increase in sea level (LW plants), and a 16-cm decrease in sea level (HW plants). The experiment lasted 2½ years, beginning with mangrove seedlings and terminating after plants began to reproduce. Environmental (air temperature, relative humidity, photosynthetically active radiation) and edaphic conditions (pH, redox, soil sulfide) approximated field conditions in Belize, the source locale for the seedlings. HW plants were shorter and narrower, and produced fewer branches and leaves, responses correlated with the development of acid-sulfide soils in their pots. LW plants initially grew more rapidly than MW plants. However, the growth of LW plants slowed dramatically once they reached the sapling stage, and by the end of the experiment, MW plants were 10–20% larger in all measured growth parameters. Plants did not exhibit differences in allometric growth as a function of inundation. Anatomical characteristics of leaves did not differ among treatments. Both foliar C:N and root porosity decreased from LW through MW to HW. Relative to LW and HW plants, MW plants had 1–7% fewer stomata/mm², 6–21% greater maximum photosynthetic rates, 3–23% greater absolute relative growth rates (RGRs), and a 30% higher RGR for a given increase in net assimilation rate. Reduced growth of R. mangle under realistic conditions approximating future inundation depths likely will temper projected increased growth of this species under concomitant increases in the atmospheric concentration of CO₂.     

A comparative study on growth and morphology of wasabi plantlets under the influence of the micro-environment in shoot and root zones during photoautotrophic and photomixotrophic micropropagation

The growth of wasabi (Wasabia japonica Matsumura) plantlets under different micro-environments inside culture vessels in photoautotrophic micropropagation (PA) and photomixotrophic micropropagation (PM) conditions were compared. After 28 days of culture, dry weight, relative growth rate, leaf area, and leaf chlorophyll contents of plantlets in PA were greater than those in PM. The number of leaves did not differ significantly between PA and PM conditions. PA promoted root growth and development with a greater number of roots, root length, root diameter, root fresh weight, root dry weight, and root xylem vessel system. Dissolved oxygen concentration in PA culture medium sharply decreased after 7 days of culture and then recovered. In PM culture medium, no significant fluctuation of dissolved oxygen concentration was apparent. The net photosynthetic rates of plantlets in PA were much higher than those in PM and increased with culture time. In contrast, the net photosynthetic rates of wasabi plantlets in PM kept a low and constant value during the culture period. With the presence of gas exchange membranes attached to the vessel lids, the detected vapor pressure deficit was higher in PA than in PM conditions. Higher stomatal density and larger stomatal aperture on the abaxial and adaxial surfaces of the leaves in PM medium promoted leaf water loss following ex vitro conditions. Thus, PA is applicable for producing healthy wasabi transplants.     

Seasonal changes in photosynthetic characteristics of Pachysandra terminalis (Buxaceae), an evergreen woodland chamaephyte,in the cool temperate regions of Japan

Summary Seasonal changes in photosynthetic capacity, and photosynthetic responses to intercellular CO₂ concentration and irradiance were investigated under laboratory conditions on intact leaves of Pachysandra terminalis. Photosynthetic capacity and stomatal conductance under saturating light intensity and constant water vapor pressure deficit showed almost the same seasonal trend. They increased from early June just after the expansion of leaves, reached the maximum in late-Septemer, and then decreased to winter. In over-wintering leaves they recovered and increased immediately after snow-melting, reached a first maximum in late April, and then decreased to early July in response to the reduction of light intensity on the forest floor. There-after, they increased from mid August, reached a second maximum in late September, and then decreased to winter. The parallel changes of photosynthesis and stomatal conductane indicate a more or less constant intercellular CO₂ concentration throughout the year. The calculated values of relative stomatal limitation of photosynthesis were nearly constant throughout the year, irrespective of leaf age. The results indicate that the seasonal changes in light-saturated photosynthetic capacity are not due to a change of stomatal conductance, but to a change in the photosynthetic capacity of mesophyll. Indeed, carboxylation efficiency assessed by the inital slope of the Ci-photosynthesis curve changed in proportion to seasonal changes of the photosynthetic capacity in both current-year and over-wintered leaves. High photosynthetic capacity in current-year leaves as compared with one-year-old leaves was also due to the high photosynthetic capacity of mesophyll. Nevertheless, stomatal conductance changed in proportion to photosynthetic capacity, indicating that stomatal conductance is regulated by the mesophyll photosynthetic capacity such that the intercellular CO₂ concentrations are maintained constant. The quantum yield also changed seasonally parallel with that in the photosynthetic capacity.Contribution No. 2893 from the Institute of Low Temperature Science     

Differential responses of three grasses to defoliation,water and light availability

Grazing not only modifies the structure and functioning of grasslands, it also changes micro-environmental conditions that alter the availability of resources. The aim of this study was to analyze the response of grasses with different photosynthetic pathways (C₃/C₄), growth forms (prostrate/erect), and grazing responses (increaser/decreaser) to defoliation and resource availability. In a greenhouse, we performed a factorial experiment with three factors: defoliation, light, and water and three species: Axonopus affinis (C₄ prostrate, increaser), Coelorachis selloana (C₄ erect, decreaser), and Bromus auleticus (C₃ erect, decreaser). We measured the relative growth rate (RGR), biomass assignment, and specific leaf area. The RGR of both C₄ species was affected by light availability, while the decreaser C₃ did not respond to any factor. Biomass allocation to leaves and stolons changed with the interaction between light and water in the C₄ prostrate species (increaser). In the C₄ erect grass (decreaser), biomass allocation was more affected by defoliation under low levels of light and water. Low light availability and defoliation reduced the assignment to leaves, while the allocation to rhizomes increased. Species-specific responses to resources availability that are modified by grazing were related to photosynthetic pathway, growth form, and grazing responses. Biomass allocation was related to strategies to avoid and/or tolerate grazing. The investment to leaves was limited by light and water availability in prostrate species, while in erect grasses it was controlled by defoliation and water availability. Our results highlight the importance of species responses to changes in resource availability associated to grazing regimes.     

Compensatory growth and photosynthetic responses of <Emphasis Type="Italic">Pharbitis purpurea</Emphasis> seedlings to clipped cotyledon and second leaf

Leaf tissue damaging to seedlings can limit their subsequent growth, and the effects may be more extensive. Compensatory photosynthesis responses of the remnant cotyledon and primary leaf of Pharbitis purpurea to clipping and the effect of clipping on seedling growth were evaluated in a pot-cultivated experiment. Three treatments were conducted in the experiment, which were clipped cotyledon (CC), clipped second leaf (CL), and control group (CG). The area, thickness, mass, and longevity of the remaining cotyledon of CC exhibited over-compensatory growth. In contrast, seedlings of CC had under-compensatory growth in seedling height, root length, seedling mass, and root to shoot ratio. However, the traits of remnant cotyledon and seedling in CL treatment exhibited equal-compensatory growth. Net photosynthetic rate of the cotyledon of CC was significantly higher than those of CL and CG treatments, and the diurnal changes in photosynthetic rates showed significantly different patterns which were unimodal curve (CC) and bimodal curve (CL and CG), respectively. There was no significant difference between CL and CG treatment. Net photosynthetic rate of the primary leaf of CL was significantly higher than that of CG treatment. However, the photosynthetic rates of primary leaves of CL and CG treatments showed similar photosynthetic patterns characterized by a bimodal curve. P. purpurea seedlings used a compensatory growth strategy in the remaining cotyledon or the primary leaf to resist leaf loss and minimize any adverse effects.     

Effects of light and soil water availability on leaf photosynthesis and growth of Arisaema heterophyllum,a riparian forest understorey plant

The effects of soil-water availability on leaf light acclimation and whole-plant carbon gain were examined in Arisaema heterophyllum Blume, a riparian deciduous forest understorey plant. Photosynthesis, above-ground morphology and ramet biomass accumulation (relative growth rate: RGR of a corm for a full leaf life-span) were measured on plants raised under three light treatments combined with two soil water conditions. The two higher light treatments during growth (high: max. 550 μmol photons m^–2 s^–1; medium: 150 μmol photons m^–2 s^–1) resulted in a twofold increase in RGRs, 30% higher photosynthetic capacities and 20% less photosynthetic low-light use efficiency than those under a low light condition (50 μmol photons m^–2 s^–1). Leaf area was the smallest and leaf mass area ratio was the largest under the high light treatment. Water stress decreased both photosynthetic rate and leaf area and, hence, RGR in all the light regimes. However, water stress did not alter the general patterns of physiological and morphological responses to different light regimes. We estimated that higher photosynthetic low-light use efficiency and larger leaf area in the low light leaf would lead to a threefold carbon gain as compared with the high light leaf under simulated low light conditions. Both experimental and simulation results suggest that the physiological and morphological acclimations tend to be beneficial to carbon gain when light availability is low, whereas they favor increased water use efficiency when light availability is sufficiently high. Electronic Publication     

Can CO2 enrichment modify the effect of water and high light stress on biomass allocation and relative growth rate of cork oak seedlings?

To test whether the impact of an enriched-CO₂ environment on the growth and biomass allocation of first-season Quercus suber L. seedlings can modify the drought response under shade or sun conditions, seedlings were grown in pots at two CO₂ concentrations × two watering regimes × two irradiances. Compared to CO₂, light and water treatment had greater effects on all morphological traits measured (height, stem diameter, number of leaves, leaf area, biomass fractions). Cork oak showed particularly large increases in biomass in response to elevated CO₂ under low-watered (W−) and high-illuminated conditions (L+). Allocation shifted from shoot to root under increasing irradiance (L+), but was not affected by CO₂. Changes in allocation related to water limitation were only modest, and changed over time. Relative growth rate (RGR) and net assimilation rate (NAR) were significantly greatest in the L+/W+ treatment for both CO₂ concentrations. Changes in RGR were mainly due to NAR. Growth responses to increased light, water or CO₂ were strongest with light, medium with water availability and smallest for CO₂, in terms of RGR. The rise in NAR for light and water treatments was counterbalanced by a decrease in SLA (specific leaf area) and LMF (leaf mass fraction). Results suggest that elevated CO₂ caused cork oak seedlings to improve their performance in dry and high light environments to a greater extent than in well-irrigated and low light ones, thus ameliorating the effects of soil water stress and high light loads on growth.     

Salicylic acid may be involved in the regulation of drought-induced leaf senescence in perennials: A case study in field-grown Salvia officinalis L. plants

Leaf senescence is a highly regulated physiological process that contributes to nutrient remobilization during stress, thus allowing the rest of the plant to benefit from the nutrients accumulated during the life span of the leaf. Here we studied drought-induced leaf senescence in a perennial plant, common sage (Salvia officinalis L.) grown under Mediterranean field conditions, with an emphasis on the possible involvement of the phytohormones, salicylic acid and jasmonic acid in the process. The initial stages of leaf senescence (0–27 days of water deficit) were characterized by salicylic acid accumulation (by 80%) and decrease of jasmonic acid levels (by 40%), which occurred in parallel with a severe loss of photosynthetic pigments (up to 65%) and increases in the de-epoxidation state (DPS) of the xanthophyll cycle (by 55%), while the maximum efficiency of photosystem II (F_v/F_m ratio) was maintained above 0.80, thus indicating the absence of damage to the photosynthetic apparatus. The latest stages of leaf senescence (until 42 days of water deficit) were instead characterized by maintenance of the levels of jasmonic acid and salicylic acid, while β-carotene and the F_v/F_m ratio decreased significantly, which was followed by cell death. Exogenous applications of methyl salicylic acid in leaves of water-stressed plants led to reductions in chlorophyll levels, thus confirming the promoting effects of salicylic acid on leaf senescence. It is therefore concluded that salicylic acid may be involved, together with other phytohormones, in the regulation of drought-induced leaf senescence in perennials.     

Contrasting traits,contrasting environments,and considerations on population dynamics under a changing climate: an ecophysiological field study of two co‐dominant tree species

The increase in light availability resulting from canopy changes or opening is not always beneficial and can inhibit photosynthesis of tree seedlings already under other environmental stress. Tree seedlings' responses to compounded abiotic stress depend on their life‐history traits, and understanding the variations of such responses is important for understanding population dynamics under a changing climate. In this study we investigate how the photosynthesis of juveniles of two canopy tree species with different life‐history traits, Abies sachalinensis and Betula ermanii, differs in two contrasting sites at a sub‐boreal forest in northern Japan—one under a deciduous canopy (Closed site) and the other at a wide canopy opening (Open site). Seedlings at the Open site had low F_v/F_m (quantum yield of photosystem II) for a longer period than those at the Closed site. Abies sachalinensis at the Closed site showed lower F_v/F_m in spring than those at the Open site, but recovered after the canopy's new leaves flushed, indicating its acclimation to the shaded condition. Mean P_max (light‐saturated photosynthetic rate at ambient CO₂ levels) of A. sachalinensis seedlings was affected by site and air temperature, while B. ermanii seedlings were also affected by precipitation. Only B. ermanii's seedlings presented growth in the period studied, in spite of observed mid‐day drops to F_v/F_m attributed to water‐deficit‐related photoprotection. Results suggest that the climate change predicted for the Hokkaido area may increase the competitive advantage of broad‐leaved deciduous species, such as B. ermanii, in relation to evergreen conifers like A. sachalinensis.     

Responses of Papaya Seedlings (Carica papaya L.) to Water Stress and Re-Hydration: Growth, Photosynthesis and Mineral Nutrient Imbalance

The effects of water stress and subsequent re-hydration on growth, leaf abscission, photosynthetic activity, leaf water potential and ion content were investigated in papaya seedlings (Carica papaya L.) cv. “Baixinho de Santa Amalia”. Water stress was imposed by suspending irrigation during 34 days. Thereafter, plants were regularly re-watered. Drought arrested plant growth, induced leaf abscission and drastically decreased photosynthetic rate. However, leaf water potential was hardly reduced. Water deficit also induced sodium, potassium and chloride accumulation in leaves and roots, and did not modify nitrogen levels in both organs. Re-hydration stimulated growth, promoted emergence of new leaves, reactivated photosynthetic machinery function and reduced ion content to control levels. The results indicated that the ability of papaya plants to improve drought tolerance is not mediated through the reduction of leaf abscission, the detention of growth or the decrease of net CO₂ assimilation. In contrast, the data suggested that under water stress conditions these plants appear to posses a certain capacity to increase ion content, which might contribute to osmotic adjustment.     

Water relations of rough lemon (Citrus jambhiri Lush.) citrus seedlings infected withFusarium solani

Summary Rough lemon citrus seedlings were inoculated withFusarium solani and evaluated for changes in water relations of leaves, stems, and roots. Inoculated seedlings had decreased leaf stomatal conductance, lower leaf water potential, lower water content, and higher leaf osmotic values compared to healthy plants. Visible wilt symptoms occurred as early as 24 h after inoculation. Transpiration and root conductivity were lower in diseased plants but stem conductivity in diseased plants did not differ from the control. Thus, wilting appears to be due to the inability of roots to supply water to the leaves.     

Effects of drought-rewatering-drought on photosynthesis and growth of maize

Aims Our objective was to investigate the responses of maize photosynthesis and growth to repeated drought.Methods Maize seedlings were exposed to different soil water deficit for three weeks, then rewatering for one week, and again to different water deficit for three weeks, to examine the effects of repeated drought on photosynthesis and growth.Important findings After the first water deficit treatments, under severe drought, plant height, total leaf area of individual plant, shoot and root biomass declined significantly, also transpiration rate (T_r), stomatal conductance (G_s), intercellular CO₂ concentration (C_i), net photosynthetic rate (P_n), maximum net photosynthetic rate (A_max), but light compensation point and dark respiration rate increased significantly. Under medium drought, plant height, leaf area, and shoot biomass decreased significantly, but root biomass did not vary, hence, the ratio of roots to shoots (R/S) increased. Moreover, plants did not show significant differences in photosynthetic parameters. After rewatering, photosynthesis and growth rate of plants previously exposed to water deficit could recover to the levels of well-watered plants, but plant height and leaf area did not recover to the levels of the control. When maize were subjected to recurrent drought, plants pre-exposed to medium drought showed no significant difference in plant height, biomass, and photosynthetic parameters, but a significant decrease in leaf area, compared to plants only exposed to second medium drought. Plants pre-exposed to severe drought had significantly higher T_r, G_s, C_i, P_n, A_max, and, apparent quantum yield but significantly lower plant height, leaf area, and biomass than plants without previous exposure. These results indicated that the first severe drought significantly reduced photosynthetic capacity and maize growth, rewatering could recover photosynthesis and growth rate to the levels of well-watered plants, but could not eliminate the adverse influence of the first drought on growth. The first medium drought could stimulate the growth of maize root system and significantly increased R/S, which can enhance maize drought resistance to subsequent repeated drought, and maintain the total biomass in the control level; the first severe drought could enhance maize drought resistance to subsequent repeated drought in the aspect of photosynthesis, but could not compensate for the adverse effect of early drought on plant growth. Hence, in practice, drought hardening should be limited in the level of medium drought, and avoiding severe drought.     

Growth,biomass distribution and CO2 exchange of northern hardwood seedlings in high and low light: relationships with successional status and shade tolerance

The physiology, morphology and growth of first-year Betula papyrifera Marsh., Betula alleghaniensis Britton, Ostrya virginiana (Mill.) K. Koch, Acer saccharum Marsh., and Quercus rubra L. seedlings, which differ widely in reported successional affinity and shade tolerance, were compared in a controlled high-resource environment. Relative to late-successional, shade-tolerant Acer and Ostrya species, early-successional, shade-intolerant Betula species had high relative growth rates (RGR) and high rates of photosynthesis, nitrogen uptake and respiration when grown in high light. Fire-adapted Quercus rubra had intermediate photosynthetic rates, but had the lowest RGR and leaf area ratio and the highest root weight ratio of any species. Interspecific variation in RGR in high light was positively correlated with allocation to leaves and rates of photosynthesis and respiration, and negatively related to seed mass and leaf mass per unit area. Despite higher respiration rates, early-successional Betula papyrifera lost a lower percentage of daily photosynthetic CO₂ gain to respiration than other species in high light. A subset comprised of the three Betulaceae family members was also grown in low light. As in high light, low-light grown Betula species had higher growth rates than tolerant Ostrya virainiana. The rapid growth habit of sarly-successional species in low light was associated with a higher proportion of biomass distributed to leaves, lower leaf mass per unit area, a lower proportion of biomass in roots, and a greater height per unit stem mass. Variation in these traits is discussed in terms of reported species ecologies in a resource availability context.     

Field assessment of sub-epidermal leaf anthocyanin, PSII photochemistry,and the xanthophyll-cycle as photoprotective mechanisms in two morphs of Agave striata

We tested the hypothesis that leaf epidermal pigments screen light of particular wavelengths from reaching the photosynthetic machinery, reducing dependence on the xanthophyll-cycle as an energy dissipation process. Under field conditions, photosynthesis and water relations were studied in two morphs of Agave striata that differ in leaf coloration (green vs. reddish-purple). Titratable acidity, chlorophyll fluorescence, and internal and surface leaf temperatures were measured under low irradiance by shading (30%) and full sunlight (100%) for six days. We also measured the reflectance ratio (R_RED: R_GREEN), an index of anthocyanin content and the change in photochemical reflectance index (ΔPRI), an index of xanthophyll-cycle de-epoxidation state (xanthophyll conversion). Our results showed that both morphs expressed typical CAM-activity with no significant differences under sun vs. shade. However, shading did reduce titratable acids in both morphs. Both morphs were well hydrated, with the relative water content (RWC) being greater than 93%. Leaf surface temperature was found to be significantly higher during the day in the green morph compared to the red morph under sun and shade. Dark level fluorescence (Fo), photochemical efficiency of PSII (Fv/Fm), and the quantum yield of PSII electron transport (Φ_PSII) were higher in the red morph under sun compared to the green morph. The value of qN (non-photochemical quenching) was significantly higher during the day for the green morph compared to the red morph and this higher qN value was associated with a greater xanthophyll conversion and surface leaf temperature. However, sunlight did not predispose either of the morphs to photoinhibition. It is clear that the sub-epidermal anthocyanins serve as a photoprotective mechanism in the red morph, screening light energy from reaching the photosynthetic machinery and reducing dependence on the xanthophyll-cycle. We concluded that under natural light conditions the leaves of two morphs tested utilized differential photoprotective mechanisms.