Morphological variation in Plantago lanceolata L.: effects of light quality and growth regulators on sun and shade populations

The effects of different ratios of red to far-red light (R/FR-ratio) and of exogenously applied growth regulators on the morphology of plants from sun and shade populations were studied. Large differences in growth form were found between populations adapted to either sun or shaded habitats. Low R/FR-ratios, simulating vegetation shade, induced a growth form similar to that of plants from the shade population. High R/FR-ratios, simulating sunlight, had the opposite effect. Most morphological differences between shade and sun populations and effects of low R/FR-ratios on growth form could be mimicked by exogenously applied gibberellin (GA₃). In contrast, application of a gibberellin inhibitor (CCC) induced a growth form similar to that of the sun population and of plants grown under a high R/FR-ratio. Interactions between genetic background, the R/FR-ratio, and hormone treatment, were small and the factors exerted their action independently. The results are discussed in relation to the influence of developmental constraints on the evolvability of optimal phenotypes and the plastic responses therein.     

Phenotypic responses of the twining vine Ipomoea purpurea (Convolvulaceae) to physical support availability in sun and shade

Vines depend on external support to prevent shading by neighbouringplants. Hence, it is important to determine whether shading enhances thephenotypic responses of vines to support availability. I evaluated theconsequences of support availability (a vertical stake) on shoot and leaftraitsof the morning glory Ipomoea purpurea (Convolvulaceae)under full sunlight and extreme shade. It was hypothesised that phenotypicresponses of vines to support availability should be greater in the shade. Inaddition, to investigate possible constraints to such phenotypic responses, thecorrelations among phenotypic traits and the plasticity of such correlationswere evaluated. The phenotypic variation of the main stem length and of thenumber of branches was consistent with the hypothesis, i.e. greater responsestosupport availability in the shade. In contrast, both internode length and leafarea (two traits that showed a significant and positive correlation) decreasedin the sun and increased in the shade with support availability. Petiole lengthdecreased with support in the sun but had no response in the shade. On theotherhand, the number of significant trait correlations found in plants in the sunand supported plants was higher than those of shade and non-supported plants,respectively. Several of the correlations were significantly sensitive to theenvironment. Flowering only occurred in the sun treatment. Whereas no shoot orleaf trait was significantly correlated with flower number in supported plants,both petiole length and shoot biomass showed a significant correlation withsuchestimate of plant fitness in non-supported plants.     

Within-population variability in morphology and life history of Plantago major L. ssp. pleiosperma Pilger in relation to environmental heterogeneity

Summary An attempt was made to relate variation in life-history characteristics within a population of Plantago major ssp. pleiosperma to small-scale environmental variability. At a beach plain, embanked in 1966, a mosaic environment was distinguished with spatial variability in vegetation structure as well as in nutrient availability and water content of the soil. Differences between three subsites in comtemporary selection were demonstrated, e.g. in shoot morphology and allocation to reproductive tissue. The effects of nutrient supply and waterlogging on morphology and life history were studied on lines from the three subsites in a greenhouse. For most of the traits high levels of phenotypic plasticity were observed, covering almost entirely the observed phenotypic variability at the beach plain. In all treatments lines from the shrubs had, however, a higher leaf-area ratio as well as delayed flowering when compared to lines from more open subsites. In addition, in a reciprocal transplant experiment it was demonstrated that lines from the shrubs had larger shoots with e.g. broader leaves in the shady environment of the shrubs than other lines.From the experiments no indications were obtained that lines from any subsite were especially adapted to specific levels of nutrient supply or water content of the soil. With respect to these environmental factors P. major ssp. pleisoperma might occur and reproduce at all subsites by means of phenotypic plasticity, e.g. in plant form. However, it is suggested that spatial variability in vegetation structure caused a population subdivision in allocation patterns, leaf form and life history at the beach plain, over distances of about 15–25 m. This differentation occurred during primary succession over a period of twenty years.     

Variation in petiole and internode length affects plant performance in <Emphasis Type="Italic">Trifolium repens</Emphasis> under opposing selection regimes

We studied the effects of genotypic and plastic variation in vertical and horizontal spacer lengths on plant performance in a stoloniferous herb subjected to opposing selection regimes. We hypothesized that longer vertical structures are beneficial if plants are subjected to competition, but they should negatively affect plant performance if plants are exposed to aboveground disturbance. To test these hypotheses we subjected 34 genotypes of Trifolium repens to competition and disturbance treatments. Competition was imposed by a grass canopy consisting of Lolium perenne, and disturbance was simulated by regularly clipping the target plants and all the surrounding vegetation at 1 cm above soil level. Conform to our hypothesis, genotypes with longer vertical structures (petioles) produced fewer ramets than genotypes with shorter petioles in the disturbance treatment. However, genotypes with longer petioles did not perform better under competition than genotypes with shorter petioles. Genotypes with highly plastic vertical structures tended to produce more shoot mass under competition, and they produced fewer ramets if subjected to disturbance. Unexpectedly, horizontal structures (stolon internodes) expanded in response to competition which, furthermore, was associated with enhanced plant performance. However, producing longer internodes is inherently associated with costs in terms of increased resource allocation to the longer structures, but not to benefits in terms of increased resource capture. Positive correlations among the length and plasticity of vertical and horizontal structures may explain the apparent positive effect of producing longer internodes on plant performance. Our data thus support the notion that trait correlations may weaken selective forces acting on a focal trait in a specific environment if opposing selection pressures act on genetically correlated traits.     

Predation induced changes in behavior and growth rate in three populations of the intertidal snail, Littorina sitkana (Philippi)

We investigated the sublethal effects of a predatory crab, Cancer productus (Randall), on the behavior and growth of its snail prey, Littorina sitkana, by setting up controlled rearing and prey-size selection experiments. L. sitkana were collected from three sites on San Juan Island, WA, USA. These sites varied in snail size, abundance, and vertical distribution, and in the abundance of the crab predator C. productus. Snails from all three populations were raised for 34 days under the following treatments: no-crab control, a non-feeding C. productus encased in mesh box, and an encased C. productus feeding on L. sitkana. The non-feeding crab treatment did not affect snail foraging behavior or growth rate in comparison with the no-crab control. In contrast, the presence of a feeding crab elicited escape behavior in the snails, halted grazing, and consequently reduced growth rates. A population difference in escape behavior was observed: upward migration in snails from rocky shores and hiding in crevices in snails from a mud flat. It thus appears that chemicals leaching from crushed conspecific snails, rather than the presence of the crab predator, act as the “alarm substance” to which L. sitkana react. The magnitude of the growth depression in the presence of feeding crabs was 85%, with no difference among the three populations. Once the feeding crab stimulus was removed, snails in all populations resumed normal growth, suggesting that this response to feeding predators is reversible with changing environmental conditions. Laboratory experiments were set up to determine if all size classes of L. sitkana are equally susceptible to C. productus predation. C. productus consistently selected the largest of three size classes of L. sitkana. These results suggest that slow growth rate and small size in L. sitkana may actually be an adaptation for coexisting with high C. productus abundance, rather than simply a cost of escape behavior.     

Cessation of tillering in spring wheat in relation to light interception and red : far-red ratio

BACKGROUND AND AIMS: The production of axillary shoots (tillering) in spring wheat (Triticum aestivum) depends on intraspecific competition. The mechanisms that underlie this competition are complex, but light within the wheat canopy plays a key role. The main objectives of this paper are to analyse the effects of plant population density and shade on tillering dynamics of spring wheat, to assess the canopy conditions quantitatively at the time of tillering cessation, and to analyse the relationship between the tiller bud and the leaf on the same phytomer. METHODS: Spring wheat plants were grown at three plant population densities and under two light regimes (25 % and 100 % light). Tiller appearance, fraction of the light intercepted, and red : far-red ratio at soil level were recorded. On six sampling dates the growth status of axillary buds was analysed. KEY RESULTS: Tillering ceased earlier at high population densities and ceased earlier in the shade than in full sunlight. At cessation of tillering, both the fraction of light intercepted and the red : far-red ratio at soil level were similar in all treatments. Leaves on the same phytomer of buds that grew out showed more leaf mass per unit area than those on the same phytomer of buds that remained dormant. CONCLUSIONS: Tillering ceases at specific light conditions within the wheat canopy, independent of population density, and to a lesser extent independent of light intensity. It is suggested that cessation of tillering is induced when the fraction of PAR intercepted by the canopy exceeds a specific threshold (0.40-0.45) and red : far-red ratio drops below 0.35-0.40.     

Effect of light quality (red: far-red ratio) and defoliation treatments applied at a single phytomer on axillary bud outgrowth in Trifolium repens L.

We studied the effects of light quality and defoliation on the rate of phytomer appearance and axillary bud outgrowth in white clover. The treatments were applied to one phytomer, a phytomer being defined as the structural unit comprising a node, internode, axillary bud, subtending leaf and two nodal root primordia. Light of a low red:far-red (R:FR) ratio (0.27) was applied to a target phytomer either (i) within the apical bud and then to the axillary bud after emergence of the phytomer from the apical bud, or (ii) to the axillary bud only after emergence. The light conditions were directed to these specific parts of the plant by collimating light from small FR light-emitting diodes; with this technique we were able to change the light quality without any change in the level of photosynthetically active radiation. The subtending leaf of the target phytomer was retained or defoliated when it had emerged from the apical bud. FR light applied from the time the phytomer was within the apical bud caused a delay in branch appearance at the target phytomer. In contrast, direct treatment of the axillary bud with FR light after it had emerged from the apical bud did not result in any delay in branch appearance. As the light treatment of the apical bud may have changed the light environment of any of the organs contained in the bud we were unable to ascribe the delay in branch appearance to light perception by any particular organ. However, indirect evidence leads to the conclusion that the likely site of light perception was the developing leaf subtending the axillary bud while it was the outermost phytomer within the apical bud. These results do not support the hypothesis that the R:FR ratio of light incident at an axillary bud site is the environmental factor that controls bud development. Defoliation of the unfolding leaf reduced the rate of phytomer appearance on the main stolon but had no immediate effect on branch appearance. As a consequence there was a reduction in the number of phytomers between the stolon apical meristem and the first phytomer with a branch. This is frequently taken to indicate a relaxation of apical dominance, but in this case was found not to involve a direct effect on bud activity. A current model of white clover growth suggests that there is integration of activity between apical meristems but independence of activity and response to the local micro-environment by axillary buds. In contrast, we found that (i) defoliation reduced phytomer appearance only at the main stolon apical meristem and not at all the meristems in the plant and (ii) that a change in the local light environment of an axillary bud had no discernible effect on bud activity once the bud had emerged from the apical bud but could delay branching if applied before emergence. These results are at variance with the predictions of the model.     

Photosynthetic and genetic differentiation in sun and shade populations of Holcus mollis L.

Three Holcus mollis L. populations, one with 2n = 28 chromosomes living in a forest and two with 2n = 35 chromosomes, the first living in a forest, the second in open land, are compared for photosynthesis.Simultaneous measurements of oxygen and carbon dioxide, either in high light, low light, or dark experiments indicate that the 2n = 28 chromosomes population is photosynthetically well adapted to shade, while 2n = 35 chromosomes forest population, is not.The 2n = 35 chromosomes plants growing in the forest does not automatically acquire the photosynthetic character of a shade plant, the genome must show an evolution for this. In our study, only the plants with 2n = 28 chromosomes demonstrated the shade adaptation.     

Effects of red, far-red and blue light in maintaining growth in latitudinal populations of Norway spruce (Picea abies)

Seedlings of trees with a free growth pattern cease growth when night-lengths become shorter than a critical value, and this critical night-length (CNL) decreases with increasing latitude of origin. In northern populations, the light quality also appears to play an important role and a clinal variation in requirement for far-red (FR) light has been documented. In this study we dissected the light quality requirements for maintaining growth in different latitudinal populations of Norway spruce (Picea abies (L.) H. Karst.) using light emitting diodes for red (R), FR and blue (B) light, as 12 h day extension to provide 24 h photoperiod. At equal spectral photon flux, FR light was more effective than R light in maintaining growth, and the requirement of both R and FR increased with northern latitude of origin. One-to-one mixtures of R and FR light were more effective in maintaining growth than either FR or R light alone, indicating a possible interaction between R and FR light maintaining growth. Using the blue light as day extension could not prevent growth cessation in any of the populations, but delayed the bud set slightly in all populations. Our results suggest that phytochrome(s) are the primary photoreceptors in high irradiance responses maintaining growth in Norway spruce seedlings.     

The functional ecology of shoot architecture in sun and shade plants of Heteromeles arbutifolia M. Roem., a Californian chaparral shrub

The functional roles of the contrasting morphologies of sun and shade shoots of the evergreen shrub Heteromeles arbutifolia were investigated in chaparral and understory habitats by applying a three-dimensional plant architecture simulation model, YPLANT. The simulations were shown to accurately predict the measured frequency distribution of photosynthetic photon flux density (PFD) on both the leaves and a horizontal surface in the open, and gave reasonably good agreement for the more complex light environment in the shade. The sun shoot architecture was orthotropic and characterized by steeply inclined (mean = 71^o) leaves in a spiral phyllotaxy with short internodes. This architecture resulted in relatively low light absorption efficiencies (E _A) for both diffuse and direct PFD, especially during the summer when solar elevation angles were high. Shade shoots were more plagiotropic with longer internodes and a pseudo-distichous phyllotaxis caused by bending of the petioles that positioned the leaves in a nearly horizontal plane (mean = 5^o). This shade-shoot architecture resulted in higher E _A values for both direct and diffuse PFD as compared to those of the sun shoots. Differences in E _A between sun and shade shoots and between summer and winter were related to differences in projection efficiencies as determined by leaf and solar angles, and by differences in self shading resulting from leaf overlap. The leaves exhibited photosynthetic acclimation to the sun and the shade, with the sun leaves having higher photosynthetic capacities per unit area, higher leaf mass per unit area and lower respiration rates per unit area than shade leaves. Despite having 7 times greater available PFD, sun shoots absorbed only 3 times more and had daily carbon gains only double of those of shade shoots. Simulations showed that sun and shade plants performed similarly in the open light environment, but that shade shoots substantially outperformed sun shoots in the shade light environment. The shoot architecture observed in sun plants appears to achieve an efficient compromise between maximizing carbon gain while minimizing the time that the leaf surfaces are exposed to PFDs in excess of those required for light saturation of photosynthesis and therefore potentially photoinhibitory. Received: 8 June 1997 / Accepted: 2 November 1997     

Genetic differentiation between sun and shade habitats in populations of Lindera benzoin L.

Differences in selection patterns among habitats can alter the distribution of genetic diversity even when this is estimated with neutral markers. For plants, light is an essential resource that can influence both abiotic and biotic components of habitat. We examined genetic differentiation between sun and shade habitats in Lindera benzoin L. (Spicebush), a perennial understory shrub. Genetic diversity of 127 plants from sun and shade habitats in two populations of L. benzoin was determined using 12 polymorphic microsatellite markers. We analyzed patterns of genetic diversity using analysis of molecular variance (AMOVA), and we assessed correlation between genetic and geographic distance using Mantel tests. We found (1) low levels of differentiation among populations (F _ST = 0.028), (2) little evidence of genetic structure within populations due to isolation-by-distance, and (3) some evidence of habitat-based genetic differentiation. Specifically, the AMOVA showed a small (0.5%) but significant portion of overall variation could be explained by differences between habitats. The overall low levels of differentiation we saw were likely a result of extensive gene flow in this dioecious, bird-dispersed species.     

Tillering responses to red:far-red light ratio during different phenological stages in Eragrostis curvula

Tillering responses to light quality in different phenological stages of a perennial warm-season grass Eragrostis curvula were investigated in controlled environments. In vegetative plants, the tillering rate was greater (P<0.01) in the high (1.1–1.3) than in the low (0.59–0.70) red:far-red ratio (R/FR) light regime. Tillering rates were higher in the low R/FR treatment when the plants in the high R/FR regime reached the reproductive stage, while the plants in the low R/FR regime remained vegetative. After the reproductive tillers were removed by defoliation, more tillers were produced in the defoliated plants grown in the high R/FR regime. When the plants in both light treatments entered the reproductive stage, the tillering rate under the two light regimes became similar, suggesting a significant interaction between tillering and inflorescence development. The more advanced inflorescence development in the high R/FR regime may have reduced assimilate availability to tiller growth and overshadowed the effect of high R/FR on tillering. Both tillering and inflorescence development appeared to be controlled by R/FR ratio. The higher rate of aerial tiller production on the reproductive culms during the post-anthesis period in the high R/FR regime suggested that high R/FR ratio stimulated not only basal tillering, but also aerial tillering.     

Growth and photosynthesis of soybean (Glycine max (L.) Merr.) in simulated vegetation shade: influence of the ratio of red to far-red radiation*

Abstract. Glycine max (L.) Merr. was grown under several light conditions to determine the role of red and far-red radiation in plant adaptation to vegetation shade. Neutral density,‘neutral’ density with elevated far-red radiation, and green shade treatments were used in a greenhouse, producing calculated phytochrome photostationary state (P_fr/P_r+P_fr) values of 0.68, 0.63 and 0.51, respectively. Cool-white fluorescent lamps either alone or in conjunction with far-red fluorescent lamps were used in a growth chamber, providing P_fr/P_r+P_fr of 0.79 and 0.61, respectively. Daily photo-synthetically active radiation was about 25% of daylight and was approximately equal for both greenhouse (2.15MJ m^?2) and growth chamber (2.57MJ m^?2). Developmental stage 4 weeks after sowing was similar for all treatments, but axillary growth and rates of leaf area and dry matter accretion differed between plants from greenhouse and growth chamber. Light conditions simulating vegetation shade (i.e. a low ratio of red to far-red radiation) significantly promoted petiole elongation and retarded the rate of stem elongation in both greenhouse and growth chamber experiments. Other aspects of growth either were not significantly altered by spectral quality or were not modified consistently in both greenhouse and growth chamber environments. Net photosynthetic rates measured under growth conditions for unifoliate and first trifoliolate (TF₁) leaves of growth chamber plants between 9 and 21 d after sowing were generally unaffected by spectral quality, but supplemental FR enhanced TF₁ leaf area expansion. The latter effect was not correlated with increased dry matter accumulation. The significance of spectral quality for adaptation of soybeans to canopy closure and intercropping is discussed.     

Predation induced changes in behavior and growth rate in three populations of the intertidal snail, Littorina sitkana (Philippi)

We investigated the sublethal effects of a predatory crab, Cancer productus (Randall), on the behavior and growth of its snail prey, Littorina sitkana, by setting up controlled rearing and prey-size selection experiments. L. sitkana were collected from three sites on San Juan Island, WA, USA. These sites varied in snail size, abundance, and vertical distribution, and in the abundance of the crab predator C. productus. Snails from all three populations were raised for 34 days under the following treatments: no-crab control, a non-feeding C. productus encased in mesh box, and an encased C. productus feeding on L. sitkana. The non-feeding crab treatment did not affect snail foraging behavior or growth rate in comparison with the no-crab control. In contrast, the presence of a feeding crab elicited escape behavior in the snails, halted grazing, and consequently reduced growth rates. A population difference in escape behavior was observed: upward migration in snails from rocky shores and hiding in crevices in snails from a mud flat. It thus appears that chemicals leaching from crushed conspecific snails, rather than the presence of the crab predator, act as the “alarm substance” to which L. sitkana react. The magnitude of the growth depression in the presence of feeding crabs was 85%, with no difference among the three populations. Once the feeding crab stimulus was removed, snails in all populations resumed normal growth, suggesting that this response to feeding predators is reversible with changing environmental conditions. Laboratory experiments were set up to determine if all size classes of L. sitkana are equally susceptible to C. productus predation. C. productus consistently selected the largest of three size classes of L. sitkana. These results suggest that slow growth rate and small size in L. sitkana may actually be an adaptation for coexisting with high C. productus abundance, rather than simply a cost of escape behavior.     

藤本植物Potentilla reptans叶对风和遮阴的响应

生长在一个密集植物群中的植株由于相互遮蔽而不可能对风的影响做出反应,因为这样的环境条件(有限的光资源)对由风导致的矮小表型植株的生长是不利的。为弄清在密集植物群体中生长的植株对风的响应,利用藤本植物Potentilla reptans的10种基因型做实验材料,在温室条件下(光照强度为日光照的50%,红光/远红光=1.2)模拟冠层遮阴(相当于15%的日光照,红光/远红光=0.3),研究了藤本植物叶对风的响应。结果表明,Potentilla reptans的10种基因型植株在冠层遮阴下(低的红光/远红光)都表现出典型的避阴生长响应:较少的叶(叶生物量少),长而细但硬度系数高(higher Young's modulus)的叶柄;而受风影响的植株,无论遮阴或不遮阴,其植株的叶相对较多,叶柄短、粗且柔韧性强(lower Young's modulus),说明Potentilla reptans叶对风的响应并未因遮阴而被压抑,其可塑性变化不过是对复杂生境做出的一种生长权衡:尽可能增强抗风能力(矮壮)和获取最大光能(足够高而避免被遮光),即保证在存活下去的前提下获取最大的生长效率。     

Ecophysiological responses to light availability in three Blechnum species (Pteridophyta, Blechnaceae) of different ecological breadth

In Chilean evergreen temperate forest, fern species of the genus Blechnum occur in diverse microhabitats ranging from large gaps to heavily shaded understoreys. We hypothesised that differences in the ecological breadth of three co-occurring Blechnum species would be associated with differences in magnitude of ecophysiological responses to light availability. We quantified the field distribution of each species in relation to diffuse light availability (% canopy openness), and measured in situ variation in photosynthetic capacity (A), dark respiration (R _d) and specific leaf area (SLA) across the light gradient. The response of SLA of each species was also evaluated in a common garden in two light conditions (understorey and forest edge). The three Blechnum species differed significantly in the range of light environments occupied (breadth: B. chilense > B. hastatum > B. mochaenum). Despite significant interspecific differences in average A and R _d, the response of these traits to light availability did not differ among species. However, there was significant interspecific variation in both the mean value and the plasticity of SLA to light availability, the species with least ecological breadth (B. mochaenum) showing a flatter reaction norm (lower response) than its two congeners. This pattern was also found in the common garden experiment. The adjustment of leaf morphology (SLA) to light availability appears to be an important mechanism of acclimation in these Blechnum species. The narrow range of light environments occupied by B. mochaenum may be at least partly attributable to its inability to display phenotypic plasticity in SLA to changes in light availability.     

Direct and correlated responses to selection on iridoid glycosides in Plantago lanceolata L.

Plantago lanceolata L. (ribwort plantain) produces two costly terpenoid secondary plant compounds, the iridoid glycosides aucubin and catalpol. We performed an artificial selection experiment to investigate direct and correlated responses to selection on the constitutive level of iridoid glycosides in the leaves for four generations. Estimated realized heritabilities (±SE) were 0.23 ± 0.07 and 0.23 ± 0.04 for upward and downward selection, respectively. The response to upward selection was caused by selection for a developmental pattern characterized by the production of fewer leaves that on average contain more iridoids, and by selection for a development‐independent increase in the level of these compounds. Significant correlated responses were observed for plant growth form. Upward selection resulted in plants with larger sized, but fewer leaves, fewer side rosettes, and fewer spikes, corresponding to a previously distinguished ‘hayfield’ ecotype, whereas downward selection produced the opposite pattern, corresponding to a ‘pasture’ ecotype. This indicates that the level of iridoid glycosides is genetically correlated with morphological traits in P. lanceolata, and is part of the complex of genetically correlated traits underlying the two ecotypes. The genetic association between iridoid level and growth forms suggests that there may be constraints to the simultaneous evolution of resistance to generalist insects (by iridoid glycosides) and to larger grazers (by a high production rate of prostrate leaves and inflorescences) in open grazed habitats where the ‘pasture’ ecotype is found.     

Shade light interaction with salicylic acid in regulating growth of sun (alpine) and shade (prairie) ecotypes of Stellaria longipes

Magnesium (Mg) deficiency in plants is a widespread problem, affecting productivity and quality in agriculture. The mechanism of Mg deficiency inducing antioxidant enzyme activities has not been elucidated in rice. We examined the relationship among abscisic acid (ABA), H₂O₂, and antioxidant enzymes in the leaves of rice seedlings grown under conditions of Mg deficiency. The expression of OsRab16A, an ABA responsive gene, was used to determine the content of ABA. Mg deficiency resulted in increased ABA content in leaves of rice seedlings. The production of H₂O₂ was examined by 3,3-diaminobenzidine staining and a colorimetric method. Mg deficiency also induced H₂O₂ production in leaves, which was blocked by dipehnyleneiodonium chloride (DPI), an NADPH oxidase inhibitor. Tungstate (Tu), an ABA biosynthesis inhibitor, was effective in reducing Mg deficiency-increased ABA content, as well as Mg deficiency-induced H₂O₂ production. Both Tu and DPI were effective in reducing Mg deficiency-induced activities of superoxide dismutase, ascorbate peroxidase, glutathione reductase, and catalase in the leaves. Mg deficiency-induced ABA accumulation may trigger increased production of H₂O₂, which may involve plasma-membrane NADPH oxidase, and, in turn, up-regulates the activities of antioxidant enzymes in leaves of rice seedlings.