Components of variation in seedling potential relative growth rate: phylogenetically independent contrasts

Variation between species in seedling potential relative growth rate (RGR) is among the most important spectra of plant adaptation. Investigations are reported into the components responsible for this variation, using phylogenetically independent contrasts (PICs). The two species for each PIC were selected to diverge in seed mass at least four-fold, seed mass being a known correlate of RGR. Consistent with previous reports, the main influence on RGR differences between species was leaf area per unit leaf mass (SLA), rather than net assimilation rate per leaf area (NAR_a). The PIC design showed that SLA differences both underpinned old RGR divergences between orders and families, and also were repeatedly responsible for more recent RGR divergences between genera and species.     

Relative growth rate and biomass allocation in ten woody species with different leaf longevity using phylogenetic independent contrasts (PICs)

In this study, we compare the relative growth rate (RGR) and biomass allocation of 10 woody species (5 deciduous and 5 evergreen) from the Mediterranean region using phylogenetic independent contrasts (PICs) to test if these two functional groups differ in these traits. In general, the results were similar when using PICs or without taking into account phylogenetic relations. Deciduous species had a higher RGR than evergreen species, due to the higher net assimilation rate (NAR). Deciduous species had a higher specific leaf area (SLA) but a lower leaf mass ratio (LMR), resulting in a similar LAR for deciduous and evergreen species (LAR = SLA x LMR). In some cases, the use of PICs revealed patterns that would not have appeared if phylogeny had been overlooked. For example, there was no significant correlation between RGR and final dry mass (after 4 months of growth) but PICs revealed that there was a positive relation between these two variables in all deciduous-evergreen pairs. In general, RGR decreased with time and this temporal variation was due primarily to NAR variations (r = 0.79, p < 0.01), and also to variations in LAR (r = 0.69, p < 0.05). Considering the phylogeny, the only variable constantly different for all deciduous-evergreen pairs was SLA. This result, and the fact that SLA was the best correlated variable with RGR (r = 0.81, p < 0.01), reinforce the value of SLA as a variable closely associated to growth and to the functional groups (deciduous vs. evergreen).     

High seedling relative growth rate and specific leaf area are traits of invasive species: phylogenetically independent contrasts of woody angiosperms

Understanding causal factors of exotic species invasions is important not only for prevention and prioritizing control efforts, but also for providing valuable insights into the underlying biology of contrasting life-history strategies. In seedling growth analyses, invasive woody species were compared with less-invasive woody species commonly cultivated in California using phylogenetically corrected procedures (12 phylogenetically independent contrasts). Invasive species were hypothesized to have higher seedling relative growth rates (RGRs) and specific leaf areas (SLAs) than did related less-invasive species. In phylogenetically independent contrasts conducted among taxa within families, high seedling RGRs and SLAs have significant positive associations with woody plant invasiveness. For contrasts containing species invasive in mediterranean regions, invasive species had significantly larger root biomass allocation than did less-invasive species. Optimization of fast seedling growth (high RGR) associated with opportunistic resource acquisition (high SLA) and increased root allocation to survive summer drought may be critical for the success of plant invaders in regions with mediterranean climates.     

Toward a causal explanation of plant invasiveness: seedling growth and life-history strategies of 29 pine (Pinus) species

We studied 29 pine (Pinus) species to test the hypothesis that invasive species in disturbed habitats have distinct attributes. Seedling relative growth rate (RGR) and measures of invasiveness were positively associated across species as well as within phylogenetically independent contrasts. High RGR, small seed masses, and short generation times characterize pine species that are successful invaders in disturbed habitats. Discriminant analysis and logistic regression revealed that RGR was the most significant factor among these life-history traits separating invasive and noninvasive species. We also explored the causes of differences in RGR among invasive and noninvasive species. While net assimilation rate, leaf mass ratio, and specific leaf area (SLA) were all found to be contributing positively to RGR, SLA was found to be the main component responsible for differences in RGR between invasive and noninvasive pines. We investigated differences in SLA further by studying leaf anatomy, leaf density, and leaf thickness. We also evaluated relative leaf production rate as an important aspect of SLA. We proposed a hypothetical causal network of all relevant variables.     

不同光照处理下青藏高原克隆植物黄帚橐吾(Ligulariavirgaurea)种子大小对其幼苗生长的影响

以高寒草甸克隆植物黄帚橐吾为实验材料,通过遮荫网模拟植被遮荫,研究种子大小与萌发及幼苗生长能力的关系和幼苗对光照条件的反应。结果表明：（1）在自然光照下,黄帚橐吾种子大小对种子萌发的影响显著,大种子的萌发率高于小种子。遮荫生境下,大、小种子萌发率有所降低,但遮荫对小种子萌发的影响比大种子显著。小种子的萌发率下降了近1/8,而大种子的萌发率仅下降了1/11。（2）黄帚橐吾种子大小对幼苗生物量积累影响显著,大种子幼苗总生物量（TB）大于小种子幼苗的。但生物量的分配与播种时间相关,播种后60 d,在自然光照条件下,大种子幼苗对根生物量的分配大于小种子幼苗,而对叶生物量的分配则正好相反。在遮荫环境中,大、小种子幼苗普遍对根的生物量分配增加,大种子幼苗根冠比（R/S）大于小种子幼苗。（3）黄帚橐吾种子大小对幼苗的生长也有明显影响。在自然光照下,小种子幼苗的相对生长速率（RGR）较大于大种子幼苗,但叶面积比率（LAR）、叶面积干质量比（SLA）、叶干质量（LWR）差别不明显。在遮荫条件下,幼苗的LAR、SLA、LWR显著增加,但大、小种子幼苗间差异不显著,幼苗的RGR减小,小种子幼苗的减小趋势大于大种子幼苗。     

Relative growth rate in phylogenetically related deciduous and evergreen woody species

Relative growth rate (RGR) and other growth parameters were studied in eight pairs of closely related deciduous and evergreen species (within the same genus or family). The main objective of this study was to test the association between leaf turnover rate and RGR, specific leaf area (SLA, leaf area/leaf dry weight) and other growth variables. Plants were grown for 6 months in a greenhouse under favourable water and nutrient conditions. Variation in RGR among the 16 woody species was due mainly to differences in morphological parameters such as leaf area ratio (LAR, whole plant area/whole plant dry weight) and SLA). However, temporal variation in RGR within species was due mainly to variation in net assimilation rate. When phylogeny was not taken into account, analyses showed that deciduous species grew faster than evergreens. In contrast, when phylogeny was taken into account, the data analysis showed that a faster RGR is not consistently associated with the deciduous habit (in five pairs it was, but in the other three it was not). The faster growth of the deciduous trees (in the five positive contrasts) could be explained by their higher LAR and higher SLA relative to evergreens. The lack of differences in RGR between deciduous and evergreens (in three pairs) was due to the higher leaf mass ratio (LMR, leaf dry biomass/total dry biomass) for the evergreens, which offset the higher SLA of the deciduous species, resulting in a similar LAR in both functional groups (LAR=LMR2SLA). Deciduous species had consistently higher SLA than evergreens. We suggest that SLA, more than RGR, could be an important parameter in determining adaptive advantages of deciduous and evergreen species.     

Seed size effect on seedling growth under different light conditions in the clonal herb Ligularia virgaurea in Qinghai-Tibet Plateau

We studied the influence of seed size on germination, seedling growth and seedling responses to light in Ligularia virgaurea, a clonal herb native to the Qinghai-Tibet Plateau. (1) Under unshaded conditions, large seeds had significantly (P < 0.001) higher rates of germination than did small seeds. Both large and small seeds showed significantly reduced levels of germination under shaded conditions. The magnitude of this effect was greater for small seeds than for large seed. (2) Seedlings from large seeds had significantly higher rates of biomass accumulation (g · day⁻¹) than did seedlings from small seeds. The total biomass of seedlings from larger seeds is larger than that from smaller ones. And seedlings from large and small seeds also differed in biomass allocation. (3) Seedlings from small seeds have higher relative growth rates (RGR; g · g⁻¹ · day⁻¹) than do seedlings from large seeds under both shaded and unshaded conditions. In contrast, there was no significant difference in leaf area ratio (LAR), specific leaf area (SLA) or leaf weight ratio (LWR) between seedlings from small and large seeds. RGR, LAR, SLA and LWR were all significantly higher in seedlings grown under shaded conditions than under unshaded conditions.     

Plasticity in seedling morphology,biomass allocation and physiology among ten temperate tree species in response to shade is related to shade tolerance and not leaf habit

• Mechanisms of shade tolerance in tree seedlings, and thus growth in shade, may differ by leaf habit and vary with ontogeny following seed germination. To examine early responses of seedlings to shade in relation to morphological, physiological and biomass allocation traits, we compared seedlings of 10 temperate species, varying in their leaf habit (broadleaved versus needle‐leaved) and observed tolerance to shade, when growing in two contrasting light treatments – open (about 20% of full sunlight) and shade (about 5% of full sunlight).
• We analyzed biomass allocation and its response to shade using allometric relationships. We also measured leaf gas exchange rates and leaf N in the two light treatments.
• Compared to the open treatment, shading significantly increased traits typically associated with high relative growth rate (RGR) – leaf area ratio (LAR), specific leaf area (SLA), and allocation of biomass into leaves, and reduced seedling mass and allocation to roots, and net assimilation rate (NAR). Interestingly, RGR was not affected by light treatment, likely because of morphological and physiological adjustments in shaded plants that offset reductions of in situ net assimilation of carbon in shade. Leaf area‐based rates of light‐saturated leaf gas exchange differed among species groups, but not between light treatments, as leaf N concentration increased in concert with increased SLA in shade.
• We found little evidence to support the hypothesis of a increased plasticity of broadleaved species compared to needle‐leaved conifers in response to shade. However, an expectation of higher plasticity in shade‐intolerant species than in shade‐tolerant ones, and in leaf and plant morphology than in biomass allocation was supported across species of contrasting leaf habit.

     

Consequences of early chilling stress in two Triticum species: plastic responses and adaptive significance

Phenotypic plasticity of two primitive wheat species (Triticum monococcum L. and Triticum dicoccum S.) was studied in response to early chilling stress. Selection pressure differentials, gradients and plasticity costs on plant morphogenesis, growth and reserve carbohydrate consumption were estimated. Regression analysis was applied to investigate differential developmental changes and patterns between treatments. Four‐day‐old seedlings of T. monococcum and T. dicoccum, differing in plant stature and reserve carbohydrates, were given an early chilling temperature (4 °C for 42 day) and compared with control plants grown at 23 °C. Early chilling stress resulted in a significant increase in leaf mass ratio (LMR) and relative growth rate (RGR), a reduction in flag leaf size, total biomass, specific leaf area (SLA) and reserve carbohydrate storage at flowering, together with advanced onset of flowering. Selection pressure within the early chilling environment favoured early flowering, smaller SLA, higher LMR and lower reserve carbohydrates, suggesting the observed responses were adaptive. Furthermore, a regression of daily cumulative plant biomass derived from a crop growth simulation model (CERES‐Wheat) on crop vegetation period revealed a divergent developmental pattern in early‐chilled plants. Using selection pressure gradient analysis, we found similar responses among these traits, except for SLA and sucrose, indicating that these two traits have indirect effects on fitness. Thus, the total effects of SLA and reserve sucrose on relative fitness seem to be buffered via the rapid growth rate in chilled plants. While lower SLA may reduce early chilling stress effects at an individual leaf level, a higher LMR and use of reserve carbohydrates indicated that compensatory growth of chilled plants during the recovery period relied on the concerted action of altered resource allocation and reserve carbohydrate consumption. However, a significant cost of plasticity was evident only for flowering time, LMR and fructan levels in the early chilling environment. Our results demonstrate that morphological and intrinsic developmental (ontogenetic) patterns in two Triticum species respond to early chilling stress.     

Modulation of relative growth rate and its components by water stress in Mediterranean species with different growth forms

Effects of water availability on seedling growth were analysed in eight Mediterranean species naturally occurring in the Balearic Islands. Seedlings were grown outdoors during summer under two irrigation treatments: field capacity and 35% of field capacity. The relative growth rate (RGR) strongly depended on the growth form, from highest values in herbs to lowest in woody perennials. The main component associated with interspecific variation in RGR was the specific leaf area (SLA), and a quantitative grouping of the different growth forms appeared along the regression line between both parameters. The slow-growing species, i.e. woody perennial shrubs, had the lowest SLA and the fast-growing perennial herbs, the highest, while woody semi-deciduous shrubs appeared intermediate. Decreases in RGR due to water stress were analysed in terms of the relative contribution of the leaf mass ratio (LMR), SLA and the net assimilation rate (NAR). Pooling all species, the decrease in RGR caused by water deficit was mainly explained by decreases in SLA. However, this general pattern was strongly dependent of growth form. Thus, in the woody perennial plants, the decrease in RGR was accompanied by a three-fold decrease in NAR which, however, increased in perennial herbs. SLA increased with decreasing water supply in woody perennial plants, and decreased in woody semi-deciduous shrubs and perennial herbs. Finally, decreases in LMR partly explained decreases in RGR in perennial herbs and woody perennial shrubs. This different response of the different growth forms may reflect differences in seedling adaptation and surviving strategies to drought periods.