Improving grasslands: the influence of soil moisture and nitrogen fertilization on the establishment of seedlings

1. In order to investigate the factors influencing the establishment of seedlings in permanent grassland, the influence of soil moisture and nitrogen fertilization on competition between established plants of Lolium perenne and seedlings of Phleum pratense or Trifolium pratense was studied in two experiments under greenhouse conditions using the 'split-box'-technique.
2. There was no difference in the production of plant dry matter of P. pratense or T. pratense between 30% volumetric soil water content (−0·005 MPa) and 22% (−0·04 MPa), but 15% soil moisture (−0·33 MPa) reduced plant growth. L. perenne yields were linearly reduced by reduced soil moisture content.
3. Shoot competition from L. perenne reduced the plant dry matter yield of P. pratense and T. pratense more than did root competition in these experiments. When shoot competition was present, differences between moisture contents were not detected, indicating that light was probably the limiting resource under such conditions. No significant interaction between root competition and soil moisture was observed for plant weight.
4. Root competition was not prevented even though sufficient water and nitrogen were supplied. This indicated either that some other growth factor was limiting or the plants competed for resources at the root hair level even though sufficient resources were supplied at the pot or field scale. Therefore, in the situation of direct drilling of species during grassland renovation, it may be difficult to alleviate competition by adequate provision of water and nitrogen.     

Lindera melissifolia responses to flood durations and light regimes suggest strategies for recovery and conservation

Passive management to preserve endangered plant species involves measures to avoid anthropogenic disturbance of natural populations, but this approach may not sustain plants that require disturbance-maintained habitats. Active management is often necessary to maintain existing habitats or provide new habitats for endangered species recovery. Our objective was to examine the effects of two disturbances in floodplain forests, soil flooding and light availability, on survival, stem length, stem diameter and ramet production of endangered Lindera melissifolia (Walt.) Blume. We used a water impoundment facility to control the timing and duration of flooding (0, 45 or 90 days) and shade houses to vary light availability (70, 63 or 5 % ambient light). Hydroperiod had little direct effect on steckling survival, stem length growth and stem diameter growth, supporting indications that soil flooding may be important for reduction of interspecific competition in L. melissifolia habitat. Greater ramet production by stecklings receiving no soil flooding likely resulted from longer periods of favorable soil conditions during each growing season. Positive stem length growth and stem diameter growth under all light levels demonstrates the plasticity of this species to acclimate to a range of light environments, though, greatest survival and stem length growth occurred when L. melissifolia received 37 % light, and stem diameter growth was greatest beneath 70 % light. Further, female clones produced more ramets as light availability increased. These results indicate that passive management absent natural disturbance could jeopardize sustainability of extant L. melissifolia populations, and this species would respond favorably to active management practices that create canopy openings to increase understory light availability.     

Effect of nutrient level on competition intensity in the field for three coexisting grass species

A field experiment encompassing both neighbour- and nutrient-manipulations was conducted in a nutrient-impoverished old-field habitat to investigate how the intensity of plant competition was affected by soil nutrient level. Three perennial grasses were used as target species: Agropyron repens, Poa pratensis and Phleum pratense. Neighbour manipulations involved the removal (through herbicide application) of all neighbouring vegetation within a 20 cm or 40 cm radius around target plants. Target performance was measured under five levels of added nutrients (N-P-K) in both the neighbour-removal plots and in non-removal (control) plots. Both neighbour and nutrient manipulations had a highly significant effect on both biomass and tiller production but the interaction between these treatments was generally insignificant. Below-ground/above-ground biomass quotient was affected only by neighbour manipulations and was greatest in the control plots (with no neighbours removed) for all three species. The suppressive effect of neighbours was not markedly affected by nutrient level. However, yield suppression showed a significant decreasing trend with increasing nutrient level for biomass production in Agropyron and an increasing trend for tiller production in Phleum. For Poa, there was no trend in the intensity of competition across nutrient level. The results suggest that the general intensity of competition within this community neither increases nor decreases with increasing nutrient level. Rather, coexisting species appear to respond individually in terms of the intensity of competition that they experience. These results conflict with predictions from the triangular C-S-R model of plant strategies. However, they are consistent with a recently modified ‘habitat templet’ model for vegetation.     

Host water potentials may restrict development of snow mould fungi in low temperature-hardened grasses

The growth of the snow mould fungi Fusarium nivale (Fr.) Ces. and Typhula ishikariensis Imai at different water potentials was measured on potato dextrose broth supplemented with KCl or polyethylene glycol 6000. The dew point hygrometer method was used for determination of water potentials in both artificial growth media and plant material. Both fungi had a considerable decrease in dry weight production when water potentials of growth media were decreased from −0.7 to −3 MPa. Crown tissue from hardened plants of Phleum pratense L. (timothy, cvs Engmo, Grindstad) and Phalaris arundinacea L. (reed canary grass, cvs Hansvoll, American common, Vantage) showed a significant decrease in water potentials compared with unhardened plants. Thus, reduced availability of water may partly explain increased resistance to fungal pathogens in grasses after hardening.     

Nurse plants have the potential to accelerate vegetation recovery in Lapalala Wilderness old fields,South Africa

Nurse plants offer microclimates that are favourable to plant growth of understory native vegetation, thus facilitating ecological restoration in degraded old fields. This study examined the impact of three nurse plants on vegetation diversity and soil physical properties in old fields located at Lapalala Wilderness, South Africa. Vegetation surveys in plots measuring 5 m x 5 m under and outside the canopy of nurse plants in both old field and natural sites were conducted. Top soils under and outside the nurse plants canopy were collected in all plots and quantified for soil moisture, soil penetration resistance and soil water repellency. Results indicate that species diversity was high under plant canopy compared to outside plant canopy for all nurse plants. Soils under nurse plants canopy showed improved soil moisture and soil penetration resistance compared to soils outside plant canopy, but no differences were reported for soil water repellency. The study concludes that the presence of native plants under nurse plants canopy points to a positive vegetation recovery trajectory. For vegetation and soil restoration to be effective in Lapalala Wilderness old fields, nurse plants should be protected and active restoration, e.g. seeding or seedling sowing under nurse plants canopy should be considered.     

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.     

Effects of competitive stress on vegetative growth, storage, and regrowth after defoliation in Phleum pratense

The capacity for resource storage is important to population persistence in nonforaging plants with a consolidation strategy. In a competitive environment, selection may favor genetic variants with a well-developed ability to store resources for future vegetative growth or to regrow following herbivory. To determine the evolutionary potential for changes in vegetative growth, storage and regrowth after defoliation in response to competitive stress, half-sib families of the caespitose grass Phleum pratense were grown in pots in a glasshouse either alone or in competition with four individuals of Lolium perenne . They were defoliated after 16 wk and permitted to regrow for 8 wk. Production of new tillers, leaf area and leaf dry mass were recorded before and after defoliation; resource storage was estimated by the dry mass of stem bases. Tiller numbers, leaf area and mass, regrowth, and stem base mass were significantly reduced by competition; however, there was significant variation among families for many variables. A significant proportion of the variation in clipped and regrowth leaf mass, and in stem base mass, was due to a competition by family interaction. Storage in stem bases was correlated with total accumulated leaf mass in both control and competition groups. Genetic variation for competitive ability and the ability to regrow rapidly after defoliation exist in this P. pratense population. A well-developed ability to regrow following release from competitive or grazing pressures is likely to be a critical adaptation of caespitose grasses.     

Enhanced mineralization of [U-(14)C]2,4-dichlorophenoxyacetic acid in soil from the rhizosphere of Trifolium pratense

Enhanced biodegradation in the rhizosphere has been reported for many organic xenobiotic compounds, although the mechanisms are not fully understood. The purpose of this study was to discover whether rhizosphere-enhanced biodegradation is due to selective enrichment of degraders through growth on compounds produced by rhizodeposition. We monitored the mineralization of [U-(14)C]2,4-dichlorophenoxyacetic acid (2,4-D) in rhizosphere soil with no history of herbicide application collected over a period of 0 to 116 days after sowing of Lolium perenne and Trifolium pratense. The relationships between the mineralization kinetics, the number of 2,4-D degraders, and the diversity of genes encoding 2,4-D/alpha-ketoglutarate dioxygenase (tfdA) were investigated. The rhizosphere effect on [(14)C]2,4-D mineralization (50 microg g(-1)) was shown to be plant species and plant age specific. In comparison with nonplanted soil, there were significant (P < 0.05) reductions in the lag phase and enhancements of the maximum mineralization rate for 25- and 60-day T. pratense soil but not for 116-day T. pratense rhizosphere soil or for L. perenne rhizosphere soil of any age. Numbers of 2,4-D degraders in planted and nonplanted soil were low (most probable number, <100 g(-1)) and were not related to plant species or age. Single-strand conformational polymorphism analysis showed that plant species had no impact on the diversity of alpha-Proteobacteria tfdA-like genes, although an impact of 2,4-D application was recorded. Our results indicate that enhanced mineralization in T. pratense rhizosphere soil is not due to enrichment of 2,4-D-degrading microorganisms by rhizodeposits. We suggest an alternative mechanism in which one or more components of the rhizodeposits induce the 2,4-D pathway.     

Photosynthetic rates and vegetative production of Sorghastrum nutans in response to competition at two strip mines and a railroad prairie

The effect of differing environmental conditions on competition for resources was investigated by a comparison of net photosynthetic rate (PN) and vegetative production of Indiangrass [Sorghastrum nutans (L.) Nash.] at two strip mine sites with differing reclamation histories, and a railroad prairie site where this species occurs naturally. The treatment for a competition experiment consisted of tying back all species of neighboring plants around a target plant, and measuring its PN and vegetative performance during the growing season. Environmental variables at each site were also measured during the growing season. Soil bulk density and pH were higher at the two mine sites than at the prairie site, and soil texture, nutrients, and water potential were different at each of the three sites. PN of target plants compared closely among the three sites, and were lowest for plants at the railroad prairie. The competition experiment indicated that lower canopy leaves were most affected by competition for photosynthetically active radiation (PAR) at all sites. Significant differences in PN of upper canopy leaves were found between treatment and control plants at one of the mine sites. This site had higher soil water potentials and higher soil levels of P and K than the other mine site or the railroad prairie. Target plants at the other mine site experienced a low competition for PAR, likely due to lower soil moisture availability and therefore lower aboveground productivity. The largest differences in PN and irradiances between upper and lower canopy leaves occurred in target plants with neighbors at the railroad prairie, likely due to inter-specific competition. Vegetative production of the target plants also reflected the environment at each site, but did not reflect PN differences between treatments. S. nutans is well adapted to the varying environment at these three sites, and aboveground competition for radiant energy was probably not as limiting for this C4 grass as belowground competition.     

Analysis of competition effects in mono- and mixed cultures of juvenile beech and spruce by means of the plant growth simulation model PLATHO

Inter- and intra-specific competition between plants for external resources is a critical process for plant growth in natural and managed ecosystems. We present a new approach to simulate competition for the resources light, water, and nitrogen between individual plants within a canopy. This approach was incorporated in a process-oriented plant growth simulation model. The concept of modelling competition is based on competition coefficients calculated from the overlap of occupied crown and soil volumes of each plant individual with the occupied volumes of its four nearest neighbours. The model was parameterised with data from a two-year phytotron experiment with juvenile beech and spruce trees growing in mono- and mixed cultures. For testing the model, an independent data set from this experiment and data from a second phytotron experiment with mixed cultures were used. The model was applied to analyse the consequences of start conditions and plant density on plant-plant competition. In both experiments, spruce dominated beech in mixed cultures. Based on model simulations, we postulate a large influence of start conditions and stand density on the outcome of the competition between the species. When both species have similar heights at the time of canopy closure, the model suggests a greater morphological plasticity of beech compared with spruce to be the crucial mechanism for competitiveness in mixed canopies. Similar to the experiment, in the model greater plasticity was a disadvantage for beech leading to it being outcompeted by the more persistent spruce.     

Root growth dynamics linked to above-ground growth in walnut (Juglans regia)

Background and Aims Examination of plant growth below ground is relatively scant compared with that above ground, and is needed to understand whole-plant responses to the environment. This study examines whether the seasonal timing of fine root growth and the spatial distribution of this growth through the soil profile varies in response to canopy manipulation and soil temperature.Methods Plasticity in the seasonal timing and vertical distribution of root production in response to canopy and soil water manipulation was analysed in field-grown walnut (Juglans regia ‘Chandler’) using minirhizotron techniques.Key Results Root production in walnuts followed a unimodal curve, with one marked flush of root growth starting in mid-May, with a peak in mid-June. Root production declined later in the season, corresponding to increased soil temperature, as well as to the period of major carbohydrate allocation to reproduction. Canopy and soil moisture manipulation did not influence the timing of root production, but did influence the vertical distribution of roots through the soil profile. Water deficit appeared to promote root production in deeper soil layers for mining soil water. Canopy removal appeared to promote shallow root production.Conclusions The findings of this study add to growing evidence that root growth in many ecosystems follows a unimodal curve with one marked flush of root growth in coordination with the initial leaf flush of the season. Root vertical distribution appeared to have greater plasticity than timing of root production in this system, with temperature and/or carbohydrate competition constraining the timing of root growth. Effects on root distribution can have serious impacts on trees, with shallow rooting having negative impacts in years with limited soil water or positive impacts in years with wet springs, and deep rooting having positive impacts on soil water mining from deeper soil layers but negative impacts in years with wet springs.     

Light partitioning in experimental grass communities

Through complementary use of canopy space in mixtures, aboveground niche separation has the potential to promote species coexistence and increase productivity of mixtures as compared to monocultures. We set up an experiment with five perennial grass species which differed in height and their ability to compete for light to test whether plants partition light under conditions where it is a limiting resource, and if this resource partitioning leads to increased biomass production in mixtures (using relative yield-based methods). Further, we present the first application of a new model of light competition in plant communities. We show that under conditions where biomass production was high and light a limiting resource, only a minority of mixtures outperformed monocultures and overyielding was slight. The observed overyielding could not be explained by species differences in canopy structure and height in monoculture and was also not related to changes in the canopy traits of species when grown in mixture rather than monoculture. However, where overyielding occurred, it was associated with higher biomass density and light interception. In the new model of competition for light, greater light use complementarity was related to increased total energy absorption. Future work should address whether greater canopy space-filling is a cause or consequence of overyielding.     

Influence of bacterial strains isolated from lead-polluted soil and their interactions with arbuscular mycorrhizae on the growth of Trifolium pratense L. under lead toxicity

We isolated two bacterial strains from an experimentally lead (Pb)-polluted soil in Hungary, 10 years after soil contamination. These strains represented the two most abundant cultivable bacterial groups in such soil, and we tested their influence on Trifolium pratense L. growth and on the functioning of native mycorrhizal fungi under Pb toxicity in a second Pb-spiked soil. Our results showed that bacterial strain A enhanced plant growth, nitrogen and phosphorus accumulations, nodule formation, and mycorrhizal infection, demonstrating its plant-growth-promoting activity. In addition, strain A decreased the amount of Pb absorbed by plants, when expressed on a root weight basis, because of increased root biomass due to the production of indoleacetic acid. The positive effect of strain A was not only evident after a single inoculation but also in dual inoculation with arbuscular mycorrhizal fungi. Strain A also exhibited higher tolerance than strain B when cultivated under increasing Pb levels in the spiked soil. Molecular identification unambiguously placed strain A within the genus Brevibacillus. We showed that it is important to select the most tolerant and efficient bacterial strain for co-inoculation with arbuscular mycorrhizal fungi to promote effective symbiosis and thus stimulate plant growth under adverse environmental conditions, such as heavy-metal contamination.     

Tree seedling performance and below-ground properties in stands of invasive and native tree species

The establishment and subsequent impacts of invasive plant species often involve interactions or feedbacks with the below-ground subsystem. We compared the performance of planted tree seedlings and soil communities in three ectomycorrhizal tree species at Craigieburn, Canterbury, New Zealand – two invasive species (Pseudotsuga menziesii, Douglas-fir; Pinus contorta, lodgepole pine) and one native (Nothofagus solandri var. cliffortioides, mountain beech) – in monodominant stands. We studied mechanisms likely to affect growth and survival, i.e. nutrient competition, facilitation of carbon and nutrient transfer through mycorrhizal networks, and modification of light and soil conditions by canopy trees. Seedlings were planted in plastic tubes filled with local soil, and placed in monospecific stands. Effects of root competition from trees and mycorrhizal connections on seedling performance were tested by root trenching and use of tubes with or without a fine mesh (20 μm), allowing mycorrhizal hyphae (but not roots) to pass through. Survival and growth were highest in stands of Nothofagus and lowest under Pseudotsuga. Surprisingly, root trenching and mesh treatments had no effect on seedling performance, indicating canopy tree species affected seedling performance through reduced light availability and altered soil conditions rather than below-ground suppression from root competition or mycorrhizal facilitation. Seedlings in Pseudotsuga stands had lower mycorrhizal colonisation, likely as a result of the lower light levels. Soil organic matter levels, microbial biomass, and abundance and diversity of microbe-consuming nematodes were all highest under Nothofagus, and nematode community assemblages differed strongly between native and non-native stand types. The negative effects of non-native trees on nematodes relative to Nothofagus are likely due to the lower availability of soil organic matter and microbial biomass in these stands, and therefore lower availability of resources for nematodes. This study shows that established stands of non-native invasive tree species may adversely affect tree seedlings and soil communities through modifications of the microenvironment both above and below ground. As such, invasion and domination of new landscapes by these species is likely to result in fundamental shifts in community- and ecosystem-level properties relative to those under native forest cover.     

The Sensitivity of Evapotranspiration to Inter-Specific Plant Neighbor Interactions: Implications for Models

Evapotranspiration (ET) is an important water loss flux in ecosystem water cycles, and quantifying the spatial and temporal variation of ET can improve ecohydrological models in arid ecosystems. Plant neighbor interactions may be a source of spatial and temporal variation in ET due to their effects on the above- and belowground microclimate and increased water demand for transpiration. Over longer timescales (annual to multiple years), adjustments in plant physiological traits may occur in response to neighbor environments, potentially affecting the transpiration (T) component of ET. We used a dynamic soil water model to assess the sensitivity of ET and T estimates to neighbor effects on soil moisture via competition for water, aboveground microclimate effects via canopy shading, and physiological adjustments (specifically, root distribution, stomatal behavior, and canopy leaf area). We focus on a common desert shrub (Larrea tridentata) under different inter-specific neighbor environments and precipitation regimes. Neighbors impacted T of Larrea by as much as 75% at the patch scale (plant and surrounding soil) and 30% at the stand scale. Annual T estimates were highly sensitive to changes in soil moisture associated with competition for water, and the inclusion of physiological adjustments to neighbor environments significantly impacted seasonal T. Plant neighbor interactions can significantly influence ET and soil moisture, and their inclusion in models can help explain spatial and temporal variation in water fluxes in arid ecosystems. Furthermore, physiological adjustments to neighbor environments may be an important source of variation to include in models that operate over seasonal timescales or in studies focused on plant responses to precipitation under climate change.     

两种沉水植物黑藻和伊乐藻的种间竞争

采用取代系列实验方法,主要从竞争期的长短出发,研究了黑藻(Hydrilla verticillata)和伊乐藻(Elodea nuttallii)的种间竞争关系,并考查了在不同底质(土壤)肥力下两者种间竞争能力的变化情况。实验发现,伊乐藻由于具有较强的耐寒能力,在冬春时空竞争方面占有明显的优势,从而在周年实验中表现出较强的竞争优势,取代黑藻生长。而在短期实验中,黑藻由于可在水面生长形成较上位的冠层的特性,与伊乐藻相比在水体上层空间占领和阳光获取方面具有一定的优势,因此造成两种间竞争的不平衡,竞争偏利于黑藻,且这种优势随底质(土壤)肥力的增加而有所增强,但并没有明显取代现象的发生,两物种可以在混合种群中共存。     

A Study of the Nitrogen and Water Factors in Competition with Agropyron repens (L.) Beauv.

From a consideration of the possible relationships between thelevel of a nutrient in the soil, the depletion due to a competitor,and the response of a test plant, it is argued that a validcriterion for the implication of a nutrient factor in root competitionis that some growth response of the test plant to competitionshould be less at higher levels of supply of that nutrient thanat lower levels. Young test plants of Impatiens parviflora were transplantedinto pans of soil in which stands of Agropyron repens were growing.Two levels of nitgrogen and gtwo rates of watering were appliedin a multifactorial design, and from half of the test plantsroot competition was excluded, so that their responses to rootcompetition could be measured under the various nitrogen andwatering treatments. A preliminary experiment hadf shown, and the competition experimentconfirmed, that the optimum nitrogen level for root growth andNet Assimilation Rate was less than the higher level employed,whereas this level was not sufficient for maximum leaf-growth.In agreement with this, the depressions of relative root growth-rateand Net Assimilation Rate caused by competition at the lowernitrogen level were significantly reduced at the higher level,but for relative leaf-growth this reduction did not reach significance.The effects of competition were reduced by the higher rate ofwstering, and for relative leaf growth-rate this interactionwas highly significant. It is deduced that competition from A. repensfinvolves bothnitrogen and water, and that water is probably the more importantfactor under normal conditions. There is no need to postulkateany toxic root product to account for the experimentalo results.Compared with competition above ground, it appeared that rootcompetition could have an effect on the relative growth-rateof I. parviflora about as great as a reduction of light intensityfrom full to 5–10 per cent. of summer daylight.     

The influence of savanna trees on nutrient,water and light availability and the understorey vegetation

In an East African savanna herbaceous layer productivity and species composition were studied around Acacia tortilis trees of three different age classes, as well as around dead trees and in open grassland patches. The effects of trees on nutrient, light and water availability were measured to obtain an insight into which resources determine changes in productivity and composition of the herbaceous layer. Soil nutrient availability increased with tree age and size and was lowest in open grassland and highest under dead trees. The lower N:P ratios of grasses from open grassland compared to grasses from under trees suggested that productivity in open grassland was limited by nitrogen, while under trees the limiting nutrient was probably P. N:P ratios of grasses growing under bushes and small trees were intermediate between large trees and open grassland indicating that the understorey of Acacia trees seemed to change gradually from a N-limited to a P-limited vegetation. Soil moisture contents were lower under than those outside of canopies of large Acacia trees suggesting that water competition between trees and grasses was important. Species composition of the herbaceous layer under Acacia trees was completely different from the vegetation in open grassland. Also the vegetation under bushes of Acacia tortilis was different from both open grassland and the understorey of large trees. The main factor causing differences in species composition was probably nutrient availability because species compositions were similar for stands of similar soil nutrient concentrations even when light and water availability was different. Changes in species composition did not result in differences in above-ground biomass, which was remarkably similar under different sized trees and in open grassland. The only exception was around dead trees where herbaceous plant production was 60% higher than under living trees. The results suggest that herbaceous layer productivity did not increase under trees by a higher soil nutrient availability, probably because grass production was limited by competition for water. This was consistent with the high plant production around dead trees because when trees die, water competition disappears but the high soil nutrient availability remains. Hence, in addition to tree soil nutrient enrichment, below-ground competition for water appears to be an important process regulating tree-grass interactions in semi-arid savanna.     

Iron toxicity to rice in an acid sulfate soil as influenced by water regimes

Summary The effects of two water regimes: Continuous flooding and flooding with soil drying on iron toxicity to rice in an acid sulfate soil was studied by continuously growing 7 crops of IR-32 rice in pots under the two water treatments. There was no plant growth upto the second crop under both water treatments due to iron toxicity. But there was good growth of rice under the continuous water regime from third cropping onwards, however, there was no growth of rice in the flooding with soil drying treatment even upto the seventh crop due to iron toxicity.The results of the study bring out that keeping an acid sulfate soil flooded for a few weeks and then planting rice when iron in soil solution has dropped below toxicity level may be a possible management practice for lowland rice culture on such soils. Drying and reflooding an acid sulfate soil on the other hand aggravates soil acidity and keeps iron in solution in high amounts to be toxic to rice plant.     

Photosynthetic acclimation to simultaneous and interacting environmental stresses along natural light gradients: optimality and constraints

There is a strong natural light gradient from the top to the bottom in plant canopies and along gap-understorey continua. Leaf structure and photosynthetic capacities change close to proportionally along these gradients, leading to maximisation of whole canopy photosynthesis. However, other environmental factors also vary within the light gradients in a correlative manner. Specifically, the leaves exposed to higher irradiance suffer from more severe heat, water, and photoinhibition stresses. Research in tree canopies and across gap-understorey gradients demonstrates that plants have a large potential to acclimate to interacting environmental limitations. The optimum temperature for photosynthetic electron transport increases with increasing growth irradiance in the canopy, improving the resistance of photosynthetic apparatus to heat stress. Stomatal constraints on photosynthesis are also larger at higher irradiance because the leaves at greater evaporative demands regulate water use more efficiently. Furthermore, upper canopy leaves are more rigid and have lower leaf osmotic potentials to improve water extraction from drying soil. The current review highlights that such an array of complex interactions significantly modifies the potential and realized whole canopy photosynthetic productivity, but also that the interactive effects cannot be simply predicted as composites of additive partial environmental stresses. We hypothesize that plant photosynthetic capacities deviate from the theoretical optimum values because of the interacting stresses in plant canopies and evolutionary trade-offs between leaf- and canopy-level plastic adjustments in light capture and use.