Interactive Effects of Intraspecific Competition and Drought on Stomatal Conductance and Hormone Concentrations in Different Tomato Genotypes

Plant physiological responses to various stresses are characterized by interaction and coupling, while the intrinsic mechanism remains unclear. The effects of intraspecific competition on plant growth, stomatal opening, and hormone concentrations were investigated with three tomato genotypes (WT-wild type, Ailsa Craig; FL-a abscisic acid (ABA) deficient mutant, flacca; NR-a partially ethylene-insensitive genotype) under two water regimes (full irrigation, irrigation amount = daily transpiration; deficit irrigation, 60% of irrigation amount in full irrigation) in this study. Three kinds of competitions were designed, i.e., root and canopy competition, non-root competition, and non-canopy competition, respectively. Intraspecific competition reduced plant leaf area and stomatal conductance (gs) of wild-type tomato, accompanied by ABA accumulation and ethylene evolution. Intraspecific competition-induced decrease in gs was absent in FL and NR, indicating ABA and ethylene involved in plant response to intraspecific competition. As soil water becomes dry, the competition decreased gs by elevating ABA and ethylene accumulations. Under severe drought, the competition-induced decline in gs was covered by the severe drought-induced decrease in gs, as hydraulic signals most probably dominate. The absence of canopy competition insignificantly influenced plant stomatal opening of well-watered tomato, as canopy separation minimized the plant neighbor sensing by ethylene and other signals. Whereas under water deficit condition, the absence of canopy competition significantly reduced ABA accumulation in roots and then stomatal conductance, indicating the belowground neighbor detection signals maybe enhanced by soil drought. The absence of root competition increased ethylene evolution, confirming the importance of ethylene in neighbor detection and plant response to environmental stress.

Partitioning above and below ground interactions and their effects on juvenile Dacrycarpus dacrydioides (kahikatea) and Podocarpus totara (totara)

<p>Competitive and facilitative interactions play an important role in determining plant community structure and development. Historically, competitive interactions have been considered to be more prevalent in nature. However, in the past few decades strong facilitative interactions have been identified as being more important than competition in certain environments. Recent evidence has also suggested that interactions occurring in the above and below ground environments may be unevenly contributing to the net interaction effects between a target plant and nurses species. This study partitions the above and below ground interactions and determines their strength and directions in order to help better understand their relative importance to plant community dynamics. In Chapter 2 I develop species specific allometric models which aim to accurately estimate the total above- and below- ground biomass of individual D. dacrydioides and P. totara juveniles using measurements which are easily and non-destructively obtained in the field. The best model for each species is then used to construct total above and below ground biomass estimates for use in Chapter 3. Eight models using stem height, diameter, and volume either alone or in combination are examined for their predictive power and tested for their goodness of fit. Models using diameter alone are found to be less powerful in predicting total tree biomass, while models containing height either alone or in combination with diameter are more powerful. The absolute best model for predicting D. dacrydioides total biomass was BTOTAL = 0.0099(Height²)⁰˙⁸⁷⁴⁹, whereas the absolute best model for P. totara was BTOTAL = 0.2635((Height*Diameter)²)⁰˙⁵⁶⁹⁵. In Chapter 3 I use the Relative Interaction Index (RII) to determine the strength and direction of the net interactions affecting D. dacrydioides and P. totara juveniles. To partition the above ground interactions, I examined the effects of a conspecific or interspecific neighbour. I found that my two study species D. dacrydioides and P. totara showed different responses to the treatments that they received. D. dacrydioides showed net facilitation and gained biomass when it had access to the mycorrhizal network and a neighbour. Whereas, P. totara showed net neutral interactions and did not gain biomass. P. totara also showed net competition when it did not have access to the mycorrhizal network and was grown next to neighbours. The role of above ground interactions was found to be less important than below ground interactions, overall. In general, these results mean that D. dacrydioides juveniles should be expected to have higher growth, reproductive, and survival rates when grown next to nurse species in comparison to P. totara. Chapter 4 details the significance of this study for the restoration of Wairio wetland, and wetlands in general. Given the result in chapter 3 and the current restoration method at Wairio wetland, this study suggests that it may be worth exploring the benefit of planting new P. totara juveniles farther away from older woody species in order to avoid root competition.</p>

Partitioning above and below ground interactions and their effects on juvenile Dacrycarpus dacrydioides (kahikatea) and Podocarpus totara (totara)

<p>Competitive and facilitative interactions play an important role in determining plant community structure and development. Historically, competitive interactions have been considered to be more prevalent in nature. However, in the past few decades strong facilitative interactions have been identified as being more important than competition in certain environments. Recent evidence has also suggested that interactions occurring in the above and below ground environments may be unevenly contributing to the net interaction effects between a target plant and nurses species. This study partitions the above and below ground interactions and determines their strength and directions in order to help better understand their relative importance to plant community dynamics. In Chapter 2 I develop species specific allometric models which aim to accurately estimate the total above- and below- ground biomass of individual D. dacrydioides and P. totara juveniles using measurements which are easily and non-destructively obtained in the field. The best model for each species is then used to construct total above and below ground biomass estimates for use in Chapter 3. Eight models using stem height, diameter, and volume either alone or in combination are examined for their predictive power and tested for their goodness of fit. Models using diameter alone are found to be less powerful in predicting total tree biomass, while models containing height either alone or in combination with diameter are more powerful. The absolute best model for predicting D. dacrydioides total biomass was BTOTAL = 0.0099(Height²)⁰˙⁸⁷⁴⁹, whereas the absolute best model for P. totara was BTOTAL = 0.2635((Height*Diameter)²)⁰˙⁵⁶⁹⁵. In Chapter 3 I use the Relative Interaction Index (RII) to determine the strength and direction of the net interactions affecting D. dacrydioides and P. totara juveniles. To partition the above ground interactions, I examined the effects of a conspecific or interspecific neighbour. I found that my two study species D. dacrydioides and P. totara showed different responses to the treatments that they received. D. dacrydioides showed net facilitation and gained biomass when it had access to the mycorrhizal network and a neighbour. Whereas, P. totara showed net neutral interactions and did not gain biomass. P. totara also showed net competition when it did not have access to the mycorrhizal network and was grown next to neighbours. The role of above ground interactions was found to be less important than below ground interactions, overall. In general, these results mean that D. dacrydioides juveniles should be expected to have higher growth, reproductive, and survival rates when grown next to nurse species in comparison to P. totara. Chapter 4 details the significance of this study for the restoration of Wairio wetland, and wetlands in general. Given the result in chapter 3 and the current restoration method at Wairio wetland, this study suggests that it may be worth exploring the benefit of planting new P. totara juveniles farther away from older woody species in order to avoid root competition.</p>

Interactive Effects of Intraspecific Competition and Drought on Stomatal Conductance and Hormone Concentrations in Different Tomato Genotypes

We elucidated the effects of intraspecific competition on plant growth, stomatal opening and hormone concentrations in different tomato genotypes under different water regimes. Intraspecific competition reduced plant leaf area and stomatal conductance (gs) of wild-type tomato (Ailsa Craig), which was accompanied by abscisic acid (ABA) accumulation and ethylene evolution. Intraspecific competition-induced decrease in gs was absent in flacca, an ABA-deficient mutant, and in never-ripe, a partially ethylene-insensitive genotype, indicating ABA and ethylene involved in plant response to intraspecific competition. As soil water becomes dry, the competition decreased gs by elevating ABA and ethylene accumulations. Under severe drought, the competition-induced decline in gs was covered by the severe drought-induced decrease in gs, as hydraulic signals most probably dominate. Absence of canopy competition had no significant influence on plant stomatal opening of well-watered tomato, due to canopy separation minimized the plant neighbor sensing by ethylene and other signals. Whereas under water deficit condition, absence of canopy competition significantly reduced ABA accumulation in roots and then stomatal conductance, indicating the belowground neighbour detection signals maybe enhanced by soil drought. Absence of root competition increased ethylene evolution, confirming the importance of ethylene in neighbor detection and plant response to environmental stress.

No neighbour-induced increase in root growth of soybean and sunflower in mesh divider experiments after controlling for nutrient concentration and soil volume

Root competition is a key factor determining plant performance, community structure and ecosystem productivity. To adequately estimate the extent of root proliferation of plants in response to neighbours independently of nutrient availability, one should use a setup that can simultaneously control for both nutrient concentration and soil volume at plant individual level. With a mesh-divider design, which was suggested as a promising solution for this problem, we conducted two intraspecific root competition experiments one with soybean (Glycine max) and the other with sunflower (Helianthus annuus). We found no response of root growth or biomass allocation to intraspecific neighbours, i.e. an ‘ideal free distribution’ (IDF) norm, in soybean; and even a reduced growth as a negative response in sunflower. These responses are all inconsistent with the hypothesis that plants should produce more roots even at the expense of reduced fitness in response to neighbours, i.e. root over-proliferation. Our results suggest that neighbour-induced root over-proliferation is not a ubiquitous feature in plants. By integrating the findings with results from other soybean studies, we conclude that for some species this response could be a genotype-dependent response as a result of natural or artificial selection, or a context-dependent response so that plants can switch from root over-proliferation to IDF depending on the environment of competition. We also critically discuss whether the mesh-driver design is the ideal solution for root competition experiments.

Comparative analysis of the competition of stands of different species composition in relation to the Siberian stone pine undergrowth

Under the canopy of the parent native berry-green-moss Siberian stone pine forest and derivatives berry-green-moss pine and birch forests on the Urals, using the previously proposed complex of light, root, and integral competition indices of the stand, a comparative analysis of the parameters of the Siberian stone pine undergrowth is carried out. Under the canopy of Siberian stone pine forest two main determining factors have a negative complex effect on the growth of the Siberian stone pine undergrowth: the light competition of the stand (the level of photosynthetically active radiation interception by its canopy) and, almost equal to it, the root competition for soil nutrition. Under the pine forest canopy with the combined negative effect of factors of light and root competition of the stand on the development of Siberian stone pine undergrowth, light competition is decisive. The effect of the root competition is less than almost two times. Under the birch forest canopy the main factor determining the development of Siberian stone pine undergrowth is the light competition of the stand. The connection with the root competition of stand is not expressed.

Including tree spatial extension in the evaluation of neighbourhood competition effects in Bornean rain forest

Classical tree neighbourhood models use size variables acting at point distances. In a new approach here, trees were spatially extended as a function of their crown sizes, represented impressionistically as points within crown areas. Extension was accompanied by plasticity in the form of crown removal or relocation under the overlap of taller trees. Root systems were supposedly extended in a similar manner. For the 38 most abundant species in the focal size class (10 - <100 cm stem girth) in two 4-ha plots at Danum (Sabah), for periods P1 (1986-1996) and P2 (1996-2007), stem growth rate and tree survival were individually regressed against stem size, and neighbourhood conspecific (CON) and heterospecific (HET) basal areas within incremented steps in radius. Model parameters were critically assessed, and statistical robustness in the modelling set by randomization testing. Classical and extended models differed importantly in their outcomes. Crown extension weakened the relationship of CON effect on growth versus plot species’ abundance, showing that models without plasticity over-estimated negative density dependence. A significant negative trend of difference in CON effects on growth (P2 − P1) versus CON or HET effect on survival in P1 was strongest with crown extension. Model outcomes did not then support an explanation of CON and HET effects being due to (asymmetric) competition for light alone. An alternative hypothesis is that changes in CON effects on small trees, largely incurred by a drought phase (relaxing light limitation) in P2, and following the more shaded (suppressing) conditions in P1, were likely due to species-specific (symmetric) root competition and mycorrhizal processes. The very high variation in neighbourhood composition and abundances led to a strong ‘neighbourhood stochasticity’, and hence to largely idiosyncratic species’ responses. A need to much better understand the roles of rooting structure and processes at the individual tree level was highlighted.

The level of relatedness affects self/nonself discrimination in Eucalyptus urophylla seedlings

Plants can develop differentially because of their ability of self/nonself discrimination and the degree of kinship among them. Here, we evaluate the ability of self/nonself discrimination of Eucalyptus urophylla S.T. Blake roots in plant groups with different levels of relatedness. We used three plant groups: clonal, half-siblings, and population. Split-root plants were grown in pots containing either two roots of the same plant (SD) or of two different plants (NSD). The growth of root and leaves of the half-siblings and population plants was decreased in NSD in relation to SD, whereas this response was not observed in the clonal group. The multivariate analysis indicated that there was a progressive increase in plant responses likely caused by competitive interaction of roots, as the level of relatedness between individuals was lower. Our results suggest that the group of clonal plants minimized the competitive interaction among them, indicating low ability to discriminate from each other. However, half-sibling and populational plants reduced growth as a result of root competition, showing high capacity of self discrimination. Thus, a minimum degree of genetic variation between plants seems necessary for kin recognition to be expressed.