Showing papers by "Bill Shipley published in 1999"

Interacting determinants of specific leaf area in 22 herbaceous species: effects of irradiance and nutrient availability

Abstract: We measured specific leaf area (SLA) and six of its determinants (the thickness of lamina, mesophyll, epidermis, mid-vein and mid-vein support tissues and leaf water content) in a collection of 22 herbaceous species grown in factorial combinations of high μ1100 (mol m -2 s -1 ) and low (200) irradiance crossed with high (1: 1) and low (1: 6 dilution) concentrations of a modified Hoagland hydroponic solution. SLA increased with both decreasing irradiance and with increasing nutrient availability but there was a strong interaction between the two. Lamina and mesophyll thickness both increased with increasing irradiance and nutrient availability without any interaction. The experimental treatments had complicated effects on mid-vein thickness and its support tissues. Leaf water content (a measure of leaf tissue density) decreased with increasing irradiance levels and with decreasing nutrient supply, but with an interaction between the two treatments. Changes in nutrient supply had no effect on SLA at high irradiance because leaf thickness and leaf tissue density changed in a compensatory way. A path analysis revealed that each of the components affected SLA when the others were statistically controlled but the strengths of the effects of mesophyll thickness, mid-vein thickness and water content differed between treatment groups. The effect of epidermal thickness on SLA was constant across environments and it showed no significant covariation with the other determinants. There was significant covariation between mesophyll thickness, mid-vein thickness and water content and this covariation was constant across the treatment groups.

Leaf structure and specific leaf mass: the alpine desert plants of the Eastern Pamirs, Tadjikistan

Abstract: This study examines interrelationships between eight leaf attributes (specific leaf mass, area, dry mass, lamina thickness, mesophyll cell number per cm2, mesophyll cell volume, chloroplast volume, and number of chloroplasts per mesophyll cell) in field-grown plants of 94 species from the Eastern Pamir Mountains, at elevations between 3800 and 4750 m. Unlike most other mountain areas, the Eastern Pamirs, Karakorum system, Tadjikistan provide localities where low temperatures and radiation combine with moisture stress at high altitudes. For all the attributes measured, significant differences were found between plants with different mesophyll types. Leaves with dorsiventral palisade structure (dorsal palisade, ventral spongy mesophyll cells) had thicker leaves with larger but fewer mesophyll cells, containing more and larger chloroplasts. These differences in mesophyll type are reflected in differences in the total surface of mesophyll cells per unit leaf area (Ames/A) or volume (Ames/V). Plants with isopalisade leaf structure (palisade cells under both dorsal and ventral surfaces) are more commonly xerophytes and their increased values of Ames/A and Ames/V decrease CO2 mesophyll resistance, which is an important adaptation to drought. Path analysis shows the critical importance of mesophyll cell volume in leading to the covariance between the different leaf attributes and hence to specific leaf mass (SLM), even though mesophyll cell volume is not itself strongly correlated with SLM. This is because mesophyll cell volume increases SLM through its effects on leaf thickness and chloroplast number per cell, but decreases SLM through its negative effect on mesophyll cell density.

Testing causal explanations in organismal biology : causation, correlation and structural equation modelling

Abstract: This paper describes some recent statistical developments that specify under what conditions multivariate causal explanations can be tested with observational data. The specific questions that motivate the paper involve testing hypotheses concerning the causal relationships between different traits of an organism. I argue that in such cases neither randomized experiments nor controlled experimental manipulations can test such hypotheses but that structural equation modelling (path analysis) can be used for such purposes. I conclude by describing some of the limitations of path analysis.

Interacting components of interspecific relative growth rate: constancy and change under differing conditions of light and nutrient supply

Abstract: 1. One thousand three hundred and twenty plants from 22 species of herbaceous angiosperms, typical of open sunny habitats, were grown from seed for 35 days under controlled conditions in four experimental environments: high (1100 μmol m–2 s–1 PAR) and low (200 μmol m–2 s–1 PAR) light, combined with high (full-strength) and low (1/6 dilution) levels of hydroponic nutrient solution. Plants of each species were harvested at 15, 20, 25, 30 and 35 days post-germination. 2. Relative growth rate (RGR), net assimilation rate (NAR), specific leaf area (SLA) and leaf mass ratio (LWR) were estimated for each species in each treatment combination. 3. Both light and nutrient levels affected RGR and each of the growth components. Average NAR increased, while SLA and LWR decreased, with increasing light. Increasing nutrient levels increased all three growth components. Light and nutrient levels interacted in their effects on NAR and SLA, but not in their effects on LWR. 4. Those species having the highest RGR in the most productive environment were most severely depressed in the less productive environments. This same pattern also occurred with NAR and SLA, but not with LWR. 5. The rank order of species with respect to SLA and LWR remained similar across the treatment environments; with respect to NAR the rank order remained similar in only four of the six treatment contrasts; with respect to RGR the rank order remained similar in only two of the six treatment contrasts and the rank order actually reversed in one treatment contrast. 6. RGR did not display strong bivariate correlations with any of the growth components in any of the treatment groups. It had positive, moderately strong correlations with NAR but only in the low-nutrient treatments. It had positive, moderately strong correlations with LWR only in the high-nutrient treatments. After controlling for the other growth components, each showed strong positive relationships with RGR. Furthermore, the effects of the treatments disappeared after controlling for the components, showing that the treatment effects were completely mediated through their effects on these underlying components. 7. There were also strong negative partial correlations between each of the growth components. These compensatory relationships buffered RGR from large changes in response to changing environments.

Do plant species with high relative growth rates have poorer chemical defences

Abstract: 1. Most theories of plant strategies assume the presence of certain ‘trade-offs’. One such evolutionary trade-off assumes a decrease in growth rate with increasing investment in chemical defences in species adapted to different levels of habitat fertility. 2. To test this hypothesis, we grew 31 herbaceous species of Asteraceae under controlled conditions of temperature (25 °C), humidity (80%), light (500 μmol m–2 s–1) and photoperiod (16 h day–1) in a modified Hoagland hydroponic solution. The plants grew from seed for 35 days post-germination and were harvested at 14, 21, 28 and 35 days. Relative growth rate (RGR) was calculated as well as a general measure of potential phytochemical toxicity (LC50) using an alcohol extraction of secondary compounds followed by Brine Shrimp bioassay and an assay of total phenolics. 3. The interspecific correlation between RGR and the potential phytochemical toxicity was weak and non-significant (rS = 0·12, P = 0·53). The correlation between RGR and total phenolics was weak, positive but significant (rS = 0·40, P = 0·03). 4. These results suggest that such an evolutionary trade-off does not exist in this group of Asteraceae.

Interacting determinants of interspecific relative growth: Empirical patterns and a theoretical explanation

Abstract: This paper discusses the ways in which physiology, morphology and biomass partitioning interact to determine interspecific patterns of relative growth rate (RGR) We measure the correlations between RGR and six growth components based on above-ground and below-ground plant attributes, and use these results to propose a general model of how these interacting growth components combine to determine RGR based on the assumption of balanced growth The data come from 28 wild herbaceous angiosperm species grown in hydroponic sand culture for 40 days under controlled standardised conditions in a growth chamber, with 16 hours daily of 550 µmol/m−2s −1 PAR Interspecific variation in RGR was largely a result of variation in unit leaf and unit root rates Variation in specific leaf areas, leaf weight ratios, specific root areas and root weight ratios were of secondary importance in explaining the differences in RGR These components of growth were not independent of each other Leaf attributes were negativel