Cellular basis for limitation of poplar leaf growth by water deficit

During the summers of 1986 and 1987, stem and leaf growth were measured on coppiced plants of Populus trichocarpa Torr. & A. Gray, P. deltoides Bartr. ex Marsh, and P. trichocarpa x deltoides growing in the field in Puyallup, WA. The trees were either irrigated periodically throughout the season, or grown without irrigation. In both treatments, stem volume at the end of the growing season was directly proportional to total leaf area in all three genotypes. The rate of individual leaf growth was reduced by lack of irrigation more in the parental species than in the hybrid. Only in the parental species did unirrigated trees have lower leaf water potentials (predawn and midday) than irrigated trees. However, stomatal conductances of all three genotypes were lower in unirrigated trees than in irrigated trees. Osmotic potentials of growing leaves of all three genotypes were also lower in unirrigated trees than in irrigated trees. As a consequence, turgor of growing leaves was as great in unirrigated trees as in irrigated trees, which indicates that turgor differences cannot explain the lower rates of leaf growth in the unirrigated trees. However, cell wall extensibility of leaves was lower in unirrigated trees than in irrigated trees, and the difference was greater in the parental species than in the hybrid. Unlike its effect on leaf area growth, irrigation increased stem volume growth of the hybrid and the parental species by a similar amount (12-16%).     

Influence of soil water on the physiological and morphological components of plant water balance in Populus trichocarpa, Populus deltoides and their F(1) hybrids

Patterns of leaf growth, transpiration and whole-plant water balance in Populus trichocarpa, P. deltoides and their F(1) hybrids were studied during a soil drying cycle. Plant responses were analyzed during three distinct stages of dehydration. In stage I, the transpiration rate of drought-stressed plants remained constant and equal to that of well-watered plants even though soil water content declined by more than 40%. Stage II began as soil and plant water deficits induced stomatal closure. When soil water was expressed as a fraction of transpirable soil water, the transition from stage I to stage II occurred at soil water fractions of 0.35, 0.45 and 0.60 for P. trichocarpa, P. deltoides and their F(1) hybrids, respectively. Reductions in leaf growth coincided with the shift from stage I to stage II. As soil water declined further, decreases in relative transpiration and whole-plant leaf area were significantly greater in parental species than in F(1) hybrids. Inherent feedbacks controlling stomatal water loss and the maintenance and growth of leaf tissue appeared to differ between F(1) and parental genotypes in a pattern characteristic of an overdominant mode of inheritance.Stage III began once the ability of stomata to compensate for water loss had been exhausted. Substantial differences were found in plant survival during stage III, with F(1) hybrids surviving longer than parental species. Survival was more strongly correlated with the hydraulic conductivity of xylem tissues than with the dehydration tolerance of leaf tissues. Collectively, these responses suggest that F(1) hybrids were more drought resistant than either parental species and highlight the importance of whole-plant studies of functional relationships between plant growth, water balance and hydraulic conductivity.     

Growth, leaf anatomy, and physiology of Populus clones in response to solar ultraviolet-B radiation

We compared the physiological and morphological responses of rooted cuttings of Populus trichocarpa Torr. & Gray and P. trichocarpa x P. deltoides Bartr. ex Marsh. grown in either near-ambient solar ultraviolet-B (UV-B; 280-320 nm) radiation (cellulose diacetate film) or subambient UV-B radiation (polyester film) for one growing season. Midday biologically effective UV-B radiation was 120.6 and 1.6 mJ m(-2) s(-1) under the cellulose diacetate and polyester films, respectively. Gas exchange, leaf chlorophyll, light harvesting efficiency of photosystem II, and foliar UV-B radiation-absorbing compounds (i.e., flavonoid derivatives) were measured in expanding (leaf plastochron index (LPI) 5), nearly expanded (LPI 10), and fully expanded mature (LPI 15) leaves of intact plants of plastochron index 30 to 35. Plants were then harvested and height, diameter, biomass allocation and leaf anatomical attributes determined. Net photosynthesis, transpiration, and stomatal conductance were significantly greater in mature leaves exposed to subambient UV-B radiation than in mature leaves exposed to near-ambient UV-B radiation. Concentrations of UV-B radiation-absorbing compounds (measured as absorbance of methanol-extracts at 300 nm) were significantly greater in mature leaves exposed to near-ambient UV-B radiation than in mature leaves exposed to subambient UV-B radiation. The UV-B radiation treatments had no effects on chlorophyll content or intrinsic light harvesting efficiency of photosystem II. Height, diameter, and biomass were not significantly affected by UV-B radiation regime in either clone. Leaf anatomical development was unaffected by UV-B radiation treatment in P. trichocarpa x P. deltoides. For P. trichocarpa, leaf anatomical development was complete by LPI 10 in the near-ambient UV-B radiation treatment, but continued through to LPI 15 in the subambient UV-B radiation treatment. Mature leaves of P. trichocarpa were thicker in the subambient UV-B radiation treatment than in the near-ambient UV-B radiation treament as a result of greater development of palisade parenchyma tissue. We conclude that exposure to near-ambient UV-B radiation for one growing season caused shifts in carbon allocation from leaf development to other pools, probably including but not limited to, UV-B absorbing compounds. This reallocation curtailed leaf development and reduced photosynthetic capacity of the plants compared with those in the subambient UV-B radiation treatment and may affect growth over longer periods of exposure.     

Phytohormones and shoot growth in a three-generation hybrid poplar family

Hybrid vigor for secondary growth in poplar has been linked with increased gibberellin (GA) concentration in cambial tissue, but the relationship between concentrations of GAs and hybrid vigor of primary growth has not been investigated. We explored associations between concentrations of GAs, abscisic acid (ABA) and indoleacetic acid (IAA) and shoot extension in a hybrid family of Populus deltoides Bartr. ex Marsh. (Clone ILL-129) and P. trichocarpa Torr. & Gray (Clone 93-968) and two F1 and 67 F2 genotypes. Rapidly elongating subapical internodes from shoots of 4-year-old clonal saplings were selected for hormone analysis. The F1 hybrids displayed heterosis (hybrid vigor) for primary growth as a result of the complementation of dominance for increased internode length from the P. trichocarpa parent and dominance for increased diameter from the P. deltoides parent. Internodes from the faster-elongating shoots of the P. trichocarpa parent had a fourfold higher concentration of bioactive GA1 and higher concentrations of GA20, GA44, GA29 and GA8 than the P. deltoides parent. However, the two fast-growing F1 hybrids had low concentrations of all five GAs, with concentrations similar to those of the slower-elongating P. deltoides parent. Concentrations of ABA and IAA were correlated with GA concentrations and there was thus no evidence of a consistent promoting or inhibiting effect of ABA or IAA within the F1 family. These results indicate that heterosis for internode growth was not primarily regulated by hormone concentration. The segregating population of F2 hybrids was analyzed to assess the possible utility of hormone screening as a selection tool. The internodes of most of the F2 hybrids were smaller than those of their F1 parents and the larger P. trichocarpa parent. Among the F2 hybrids, mature internode length varied fourfold, and was correlated with lengths of young, subapical internodes from which hormones were analyzed. In these internodes, GA1 concentrations were negatively correlated with length (r = 0.41), diameter (0.33) and mass (0.50). Indoleacetic acid concentration was negatively correlated only with diameter (r = 0.37) and positively correlated with GA1 (r = 0.46), whereas ABA concentration was not correlated with any parameter. Thus, in the F2 population, variation in hormone concentration was not primarily responsible for the variation in shoot growth, indicating that selection for individuals with high GA concentration may not be an effective strategy for identifying vigorous hybrid genotypes.     

Abscisic acid relations and the response of Populus trichocarpa stomata to leaf water potential

The role of abscisic acid (ABA) in the mediation of stomatal responses to low leaf water potential was examined with intact plants and epidermal strips of Populus trichocarpa Torr. & A. Gray. Clones of this species grown under well-watered conditions maintain a high leaf conductance when the foliage wilts. However, foliar ABA concentration in P. trichocarpa increased manyfold in response to water stress as it did also in P. deltoides Bartr. ex Marsh. and P. trichocarpa x deltoides hybrids. Application of ABA to epidermal strips appeared to cause solute leakage, however stomata of P. trichocarpa remained partially open even when the guard cells were plasmolyzed. Foliar application of ABA induced closure of stomata in young expanding leaves, but not in fully expanded foliage. Ten days after ABA application, stomata on young leaves were open at high water potential but closed at low water potential. These characteristics are discussed with respect to wilty mutants of tomato and potato, which also have stomata unresponsive to leaf wilting.     

Diversity of leaf traits related to productivity in 31 Populus deltoides x Populus nigra clones

To test if some leaf parameters are predictors of productivity in a range of Populus deltoides (Bartr.) Marsh. x P. nigra L. clones, we assessed leaf traits and productivity in 2-month-old rooted cuttings from 31 clones growing in 4-l pots in a greenhouse, under conditions of controlled temperature and optimal irrigation. We evaluated four groups of variables describing (1) productivity (total biomass), (2) leaf growth (total leaf number increment and total leaf area increment rate), (3) leaf structure (specific leaf area and nitrogen and carbon contents) and (4) carbon isotope discrimination (delta), which is negatively correlated with time-integrated water-use efficiency. High-yielding clones did not necessarily display high leaf growth rates, but they displayed a larger total leaf area, lower specific leaf area and lower leaf nitrogen concentration than clones with low productivity. Total leaf area was mainly controlled by maximal individual leaf area and total leaf area increment rate (r = 0.51 and 0.56, respectively). Carbon isotope discrimination did not correlate with total biomass, but it was associated with total number of leaves and total leaf area increment rate (r = 0.39 and 0.45, respectively). Therefore, leaf area and specific leaf area were better indicators of productivity than leaf growth traits. The observed independence of delta from biomass production provides opportunities for selecting poplar clones combining high productivity and high water-use efficiency.     

Gas exchange characteristics of Populus trichocarpa, Populus deltoides and Populus trichocarpa x P. deltoides clones

Responses of net photosynthesis, dark respiration, photorespiration, transpiration, and stomatal conductance to irradiance, temperature, leaf-to-air vapor density difference (VDD), and plant water stress were examined in two Populus trichocarpa clones (one from a moist, coastal climate in western Washington and one from a dry, continental climate in eastern Washington), one P. deltoides clone, and two P. trichocarpa x P. deltoides clones. Light saturation of photosynthesis in greenhouse-grown trees occurred at about 800 micromol m(-2) s(-1) for P. deltoides, P. trichocarpa x P. deltoides, and the eastern Washington ecotype of P. trichocarpa, but at about 600 micromol m(-2) s(-1) for the western Washington ecotype of P. trichocarpa. Average net photosynthesis (at saturating irradiance and the optimum temperature of 25 degrees C) was 20.7, 18.8, 18.2 and 13.4 micromol CO(2) m(-2) s(-1) for P. deltoides, P. trichocarpa x P. deltoides, and the eastern and western Washington clones of P. trichocarpa, respectively. In all clones, net photosynthesis decreased about 14% as VDD increased from 3 to 18 g H(2)O m(-3). Stomatal conductance decreased sharply with decreasing xylem pressure potential (XPP) in all clones except the western Washington clone of P. trichocarpa. Stomata in this clone were insensitive to changes in XPP and did not control water loss. Complete stomatal closure (stomatal conductance < 0.05 cm s(-1)) occurred at about -2.0 MPa in the eastern Washington clone of P. trichocarpa and around -1.25 MPa in the P. deltoides and P. trichocarpa x P. deltoides clones. Transpiration rates were highest in the P. trichocarpa x P. deltoides clone and lowest in the western Washington clone of P. trichocarpa. The P. deltoides clone and eastern Washington clone of P. trichocarpa had the highest water use efficiency (WUE) and the western Washington clone of P. trichocarpa had the lowest WUE. The hybrids were intermediate. It was concluded that: (1) gas exchange characteristics of eastern and western Washington clones of P. trichocarpa reflected adaptation to their native environment; (2) crossing the western Washington clone of P. trichocarpa with the more drought resistant P. deltoides clone produced plants better adapted to the interior Pacific Northwest climate, although the stomatal response to soil water deficits in the hybrid was conservative compared with that of the eastern Washington clone of P. trichocarpa; and (3) introducing eastern Washington clones of black cottonwood into breeding programs is likely to yield lines with favorable growth characteristics combined with enhanced WUE and adaptation to soil water deficits.     

Mechanical perturbation affects conductivity, mechanical properties and aboveground biomass of hybrid poplars

Xylem development in trees is affected by dynamic mechanical stresses imposed on stems by wind. To assess clonal differences in response to mechanical perturbation (MP), we subjected seven greenhouse-grown F1 hybrids of Populus trichocarpa Torr. and A. Gray. x P. deltoides Bartr. ex Marsh. to a standard MP treatment consisting of 20 manually imposed stem flexures per day for 70-90 days. Effects of MP on aboveground biomass, hydraulic conductivity (k(h)), specific conductivity (k(s)), flexural stiffness (EI), modulus of elasticity (MOE) and modulus of rupture (MOR) were determined. Treatment increased stem radial growth and decreased height growth, leaf area and total aboveground biomass. It also significantly decreased k(s), MOE and MOR, but significantly increased EI and wood specific gravity in most clones. Mechanical perturbation caused greater stem rigidity, without having a significant effect on whole-stem k(h) or percent loss of conductivity due to embolism. Maximum k(h) was positively correlated with EI in both control (r(2) = 0.54, P < 0.0001) and MP-treated (r(2) = 0.61, P < 0.0001) plants, and k(s) and MOE were positively correlated with percent vessel lumen area (r(2) = 0.45, P < 0.0001 and r(2) = 0.28, P = 0.002, respectively). Thus, contrary to our expectation of a trade-off between conductivity and wood strength, there may be an opportunity to select clones for woody biomass production that are superior in both mechanical strength and hydraulic conductivity, as is the triploid Clone 19-61.     

Water stress, photosynthesis and early growth patterns of cuttings of three Populus clones

Photosynthetic attributes, leaf area and early root growth patterns were studied in three Populus clones to identify traits associated with superior growth potential on sites where water could be a limiting factor. It was found that early root growth and superior leaf area production were more closely related to growth potential than were photosynthetic capacity or carboxylation efficiency. A hybrid clone of Populus nigra var. charkowiensis (syn. P. nigra var. plantierensis) x P. nigra cv. 'Incrassata' (NE308) had more leaf area production and greater root system development in both wet and dry soil than did a P. trichocarpa clone (T6) and a P. balsamifera clone (B3). Despite greater above- and below-ground productivity, plants of clone NE308 had significantly lower photosynthetic capacity and carboxylation efficiency and a slightly higher CO(2) compensation point than plants of clones T6 and B3. Rapid early leaf and root growth appear to be key attributes associated with productivity in these clones regardless of soil water availability.     

Petiole length and biomass investment in support modify light interception efficiency in dense poplar plantations

Leaf architecture, stand leaf area index and canopy light interception were studied in 13 poplar clones growing in a second rotation of a coppice plantation, to determine the role of leaf architectural attributes on canopy light-harvesting efficiency and to assess biomass investment in leaf support tissue. Stand leaf area index (L) varied from 2.89 to 6.99, but L was only weakly associated with canopy transmittance (TC). The weak relationship between TC and L was a result of a higher degree of foliage aggregation at larger values of L, leading to lower light-interception efficiency in stands with greater total leaf area. We observed a strong increase in leaf aggregation and a decrease in light-harvesting efficiency with decreasing mean leaf petiole length (PL) but not with leaf size, possibly because, in cordate or deltoid poplar leaves, most of the leaf area is located close to the petiole attachment to the lamina. Although PL was the key leaf characteristic of light-harvesting efficiency, clones with longer petioles had larger biomass investments in petioles, and there was a negative relationship between PL and L, demonstrating that enhanced light harvesting may lead to an overall decline in photosynthesizing leaf surface. Upper-canopy leaves were generally larger and had greater dry mass (MA) and nitrogen per unit area (NA) than lower-canopy leaves. Canopy plasticity in MA and NA was higher in clones with higher foliar biomass investment in midrib, and lower in clones with relatively longer petioles. These relationships suggest that there is a trade-off between photosynthetic plasticity and biomass investment in support, and also that high light-harvesting efficiency may be associated with lower photosynthetic plasticity. Our results demonstrate important clonal differences in leaf aggregation that are linked to leaf structure and biomass allocation patterns within the leaf.     

Transpiration by two poplar varieties grown as coppice for biomass production

Fast-growing tree clones selected for biomass plantations are highly productive and therefore likely to use more water than the agricultural crops they replace. We report field measurements of transpiration through the summer of 1994 from two poplar clones, Beaupré (Populus trichocarpa Torr. & A. Gray x P. deltoides Bartr. ex Marsh.) and Dorschkamp (P. deltoides x P. nigra L.), grown as unirrigated short-rotation coppice in southern England. Stand transpiration was quantified by scaling up from sap flow measurements made with the heat balance method in a sample of stems. Leaf conductances, leaf area development, meteorological variables and soil water deficit were also measured to investigate the response of the trees to the environment. High rates of transpiration were found for Beaupré. In June, when soil water was plentiful, the mean (+/- SD) transpiration rate over an 18-day period was 5.0 +/- 1.8 mm day(-1), reaching a maximum of 7.9 mm day(-1). Transpiration rates from Dorschkamp were lower, as a result of its lower leaf area index. High total leaf conductances were measured for both Beaupré (0.34 +/- 0.17 mol m(-2) s(-1)) and Dorschkamp (0.39 +/- 0.16 mol m(-2) s(-1)). Leaf conductance declined slightly with increasing atmospheric vapor pressure deficit in both clones, but only in Beaupré did leaf conductance decrease as soil water deficit increased.     

南方型美洲黑杨基因库物候期及生长比较

以美国不同区域引种的美洲黑杨为材料建立基因库,通过对基因库资源物候期和生长调查发现,基因库资源内存在普遍的遗传变异,遗传多样性丰富。落叶晚的系号比落叶早的系号最长可多60 d左右的生长期;相关分析发现,叶片长、宽、叶宽基距等决定叶片大小的指标与树高和胸径呈负相关,甚至达到显著或极显著水平,说明单个叶片越大越不利于植株材积量增加。正态分布检验结果为树高、叶片长、叶柄长呈显著正态分布(P0.05),而其他特征虽然不呈显著正态分布(P0.05),但峰度和偏度值也较低,接近正态分布;基因库资源中不同性状变异系数从11%到29%不等。胸径超出群体均值25%的系号有32个,占到总数的7.13%。总体上,引进的美洲黑杨资源变异丰富,选育潜力巨大。     

Shoot culture dynamics of six Populus clones

Shoot tips of five genotypically diverse Populus clones, P. alba x P. grandidentata 'Crandon,' P. nigra 'Betulifolia' x P. trichocarpa, P. nigra x P. laurifolia 'Strathglass,' P. maximowiczii x P. trichocarpa 'Androscoggin' and P. deltoides x P. nigra 'Eugenei,' were collected from hardwood cuttings, sterilized,and established in vitro. Stable shoot cultures were obtained from all clones except P. deltoides x P. nigra 'Eugenei'. The four poplar clones that formed stable shoot cultures together with a previously established P. tremula 'Erecta' clone were placed as two-node explants on either Murashige and Skoog medium or Woody Plant Medium containing benzyladenine to determine the rate of shoot multiplication, shoot growth and other responses of the clones. All five poplar clones showed rapid shoot multiplication when cultured in the presence of 0.4-1.0 microM benzyladenine on Murashige and Skoog medium, although P. tremula 'Erecta' produced a greater number of healthy shoots when grown on Woody Plant Medium. Individual shoot growth of all clones was more vigorous when the medium contained 0-0.1 microM benzyladenine, and 100% of such shoots rooted ex vitro.     

Drought resistance of two hybrid Populus clones grown in a large-scale plantation

Poplar hybrids were grown with irrigation in a large-scale plantation to investigate the mechanisms underlying clonal differences in drought resistance. Beginning in spring 1992, Populus trichocarpa x P. deltoides (TD) and P. deltoides x P. nigra (DN) cuttings received 46, 76, or 137 cm year(-1) of irrigation to supplement the 18-20 cm of annual precipitation, and all trees received the same fertilization regime. Stem volume, assessed as the square of stem diameter at breast height times tree height (D(2)H), and water relations of the trees were studied from the end of their second growing season until the end of their fifth growing season. By the end of the second growing season, stem volume of Clone TD was 40-146% larger than that of Clone DN, but stem volume growth was independent of irrigation in excess of 46 cm year(-1) in both clones. During the third growing season, stem volume growth of both clones was limited by both the 46- and 76-cm irrigation treatments, so that by the end of the third growing season trees in the 46-cm irrigation treatment were only half the size of trees in the 137-cm irrigation treatment. These treatment differences were maintained through the fifth growing season. Although stem volumes of Clone TD trees in the 76- and 137-cm irrigation treatments were larger than the corresponding values for Clone DN trees at the end of the third growing season (1994), these clonal differences gradually decreased in subsequent years and were not detectable after 5 years, because stem volume relative growth rate of Clone DN was greater than that of Clone TD in all treatments. Although both clones exhibited similar predawn leaf water potentials, Clone DN typically maintained higher midday leaf water potentials, suggesting better stomatal control of water loss. Clonal and treatment differences in osmotic potential at full turgor were minimal and could not explain the clonal differences in drought resistance. Root density and root density to stem volume ratio increased more in response to moderate drought in Clone DN than in Clone TD, resulting in enhanced drought resistance (high stem volume growth rate under moderate drought conditions) and an increased capacity to withdraw water from the soil. We conclude that the greater drought resistance of Clone DN compared with Clone TD was the result of the maintenance of a more favorable water balance by stomatal regulation and greater carbon allocation to roots during the early stages of drought. However, the low root density to stem volume ratio in Clone DN growing in the 46-cm irrigation treatment suggests that severe water limitation restricted the preferential allocation of carbon to belowground tissues, so that both root and shoot growth were constrained by severe drought.     

Nitrogen and potassium effects on xylem cavitation and water-use efficiency in poplars

Effects of N and K nutrition on drought and cavitation resistance were examined in six greenhouse-grown poplar clones: Populus trichocarpa (Torr. & Gray) and its hybrids with P. deltoides Bartr. and P. euramericana (Dole) Guinier, before and after preconditioning to water stress. Both tendency to cavitate and water-use efficiency (WUE) increased when N supply was increased, whereas K supply had little impact on cavitation. Mean xylem vessel diameters increased from 36.6 &mgr;m at low-N supply to 45.2 &mgr;m at high-N supply. Drought-hardy clones, which were relatively resistant to cavitation, had the smallest mean vessel diameters. Vulnerability to cavitation had a weakly positive relationship with vessel diameter, and a negative correlation with transpiration. Drought hardening offered no protection against cavitation in a subsequent drought. Under drought conditions, increasing N supply increased leaf loss and decreased water potentials, whereas increasing K supply decreased leaf loss. Drought-resistant clones exhibited similar WUE to drought-susceptible clones, but had smaller, more numerous stomata and greater leaf retention under drought conditions.     

Interspecific variation of photosynthesis and leaf characteristics in canopy trees of five species of Dipterocarpaceae in a tropical rain forest

Photosynthetic rate, nitrogen concentration and morphological properties of canopy leaves were studied in 18 trees, comprising five dipterocarp species, in a tropical rain forest in Sarawak, Malaysia. Photosynthetic rate at light saturation (Pmax) differed significantly across species, varying from 7 to 18 micro mol m(-2) s(-1). Leaf nitrogen concentration and morphological properties, such as leaf blade and palisade layer thickness, leaf mass per area (LMA) and surface area of mesophyll cells per unit leaf area (Ames/A), also varied significantly across species. Among the relationships with leaf characteristics, Pmax had the strongest correlation with leaf mesophyll parameters, such as palisade cell layer thickness (r2 = 0.76, P < 0.001) and Ames/A (r2 = 0.73, P < 0.001). Leaf nitrogen concentration and Pmax per unit area also had a significant but weaker correlation (r2 = 0.46, P < 0.01), whereas Pmax had no correlation, or only weakly significant correlations, with leaf blade thickness and LMA. Shorea beccariana Burck, which had the highest P(max) of the species studied, also had the thickest palisade layer, with up to five or more layers. We conclude that interspecific variation in photosynthetic capacity in tropical rain forest canopies is influenced more by leaf mesophyll structure than by leaf thickness, LMA or leaf nitrogen concentration.     

Growth and dry-matter partitioning of young Populus trichocarpa in response to carbon dioxide concentration and mineral nutrient availability

Young individuals of a single black cottonwood (Populus trichocarpa Torr. & Gray) clone were raised for three growing seasons in whole-tree chambers and exposed to either ambient or elevated atmospheric carbon dioxide concentration ([CO2]), with either a high or a low mineral nutrient supply, in a factorial experimental design. Nutrient availability had a larger effect on growth and dry matter partitioning than did [CO2]. Total biomass did not differ significantly with CO2 treatment when nutrient availability was low. However, elevated [CO2] increased whole-plant biomass by 47% in the high nutrient availability treatment. Carbon dioxide enrichment reduced leaf area ratio and specific leaf area significantly, but had no significant effect on mean leaf size or leaf mass ratio. Root mass ratio was significantly increased by elevated [CO2] at low, but not at high nutrient availability. A modified "demographic harvesting approach" made possible the retrospective estimation of stem and branch dry masses for different years. The relative growth rates of stem and branch were significantly enhanced by elevated [CO2] with high, but not with low nutrient availability. Canopy productivity index (CPI), i.e., the amount of stem and branch wood produced annually per unit leaf area, was raised 12% by elevated [CO2] when nutrient availability was high, but was reduced when nutrient availability was low, because of increased below ground allocation.     

Control of growth of juvenile leaves of Eucalyptus globulus: effects of leaf age

Biophysical variables influencing the expansion of plant cells (yield threshold, cell wall extensibility and turgor) were measured in individual Eucalyptus globulus leaves from the time of emergence until cessation of growth. Leaf water relations variables and growth rates were determined as relative humidity was changed on an hourly basis. Yield threshold and cell wall extensibility were estimated from plots of leaf growth rate versus turgor. Cell wall extensibility was also measured by the Instron technique, and yield threshold was determined experimentally both by stress relaxation in a psychrometer chamber and by incubation in a range of polyethylene glycol solutions. Once emerging leaves reached approximately 5 cm(2) in size, increases in leaf area were rapid throughout the expansive phase and varied little between light and dark periods. Both leaf growth rate and turgor were sensitive to changes in humidity, and in the longer term, both yield threshold and cell wall extensibility changed as the leaf aged. Rapidly expanding leaves had a very low yield threshold and high cell wall extensibility, whereas mature leaves had low cell wall extensibility. Yield threshold increased with leaf age.     

Morphological and physiological adjustment to N and P fertilization in nutrient-limited Metrosideros polymorpha canopy trees in Hawaii

Leaf-level studies of Metrosideros polymorpha Gaud. (Myrtaceae) canopy trees at both ends of a substrate age gradient in the Hawaiian Islands pointed to differential patterns of adjustment to both nutrient limitation and removal of this limitation by long-term (8-14 years) nitrogen (N), phosphorus (P) and N + P fertilizations. The two study sites were located at the same elevation, had similar annual precipitation, and supported forests dominated by M. polymorpha, but differed in the age of the underlying volcanic substrate, and in soil nutrient availability, with relatively low N at the young site (300 years, Thurston, Hawaii) and relatively low P at the oldest site (4,100,000 years, Kokee, Kauai). Within each site, responses to N and P fertilization were similar, regardless of the difference in soil N and P availability between sites. At the young substrate site, nutrient addition led to a larger mean leaf size (about 7.4 versus 4.8 cm2), resulting in a larger canopy leaf surface area. Differences in foliar N and P content, chlorophyll concentrations and carboxylation capacity between the fertilized and control plots were small. At the old substrate site, nutrient addition led to an increase in photosynthetic rate per unit leaf surface area from 4.5 to 7.6 micromol m(-2) s(-1), without a concomitant change in leaf size. At this site, leaves had substantially greater nutrient concentrations, chlorophyll content and carboxylation capacity in the fertilized plots than in the control plots. These contrasting acclimation responses to fertilization at the young and old sites led to significant increases in total carbon gain of M. polymorpha canopy trees at both sites. At the young substrate site, acclimation to fertilization was morphological, resulting in larger leaves, whereas at the old substrate site, physiological acclimation resulted in higher leaf carboxylation capacity and chlorophyll content.     

Actual and potential transpiration and carbon assimilation in an irrigated poplar plantation

We examined the tradeoffs between stand-level water use and carbon uptake that result when biomass production of trees in plantations is maximized by removing nutrient and water limitations. A Populus trichocarpa Torr. x P. deltoides Bartr. & Marsh. plantation was irrigated and received frequent additions of nutrients to optimize biomass production. Sap flux density was measured continuously over four of the six growing-season months, supplemented with periodic measurements of leaf gas exchange and water potential. Measurements of tree diameter and height were used to estimate leaf area and biomass production based on allometric relationships. Sap flux was converted to canopy conductance and analyzed with an empirical model to isolate the effects of water limitation. Actual and soil-water-unlimited potential CO(2) uptakes were estimated with a canopy conductance constrained carbon assimilation (4C-A) scheme, which couples actual or potential canopy conductance with vertical gradients of light distribution, leaf-level conductance, maximum Rubisco capacity and maximum electron transport. Net primary production (NPP) was about 43% of gross primary production (GPP); when estimated for individual trees, this ratio was independent of tree size. Based on the NPP/GPP ratio, we found that current irrigation reduced growth by about 18% compared with growth with no water limitation. To achieve maximum growth, however, would require 70% more water for transpiration, and would reduce water-use efficiency by 27%, from 1.57 to 1.15 g stem wood C kg(-1) water. Given the economic and social values of water, plantation managers appear to have optimized water use.