Total Plant Growth Research Articles

Cadmium‐induced alterations in nutrient composition and growth ofbetula pendulaseedlings: The significance of fine roots as a primary target for cadmium toxicity

Abstract Birch seedlings (Betula pendula) were cultivated in nutrient solution with 0–2 μM cadmium (Cd). The effects of 2–10 days of Cd exposure on root and shoot element composition [potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), sulfur (S), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), and Cd] and growth (as percentage dry weight increase) were investigated. The element composition of fine roots and remaining root parts were analysed separately to elucidate the significance of the fine roots as a primary target for Cd toxicity. The nutrient composition of the roots was considerably altered by the Cd exposures, whereas the nutrient composition of the shoot was less affected. After eight days, the whole root (fine roots + remaining roots) concentrations of K, Ca, Mg, and Mn were reduced, whereas the opposite was found for Cu and Mo. The element distribution between fine roots and remaining roots was altered by the Cd exposures. Cadmium was accumulated in the roots and in fine roots especially. Fine roots also exhibited a capacity for Cu accumulation and a retainment of Ca and S. Total plant growth was stimulated by 0.05 μM Cd but was reduced by the 0.5–2 μM Cd treatments. Root growth was increased by the Cd exposures and growth reductions were restricted only to the shoot. Accumulation of Cd and Cu and a retainment of Ca and S in the fine roots together with a preference for root growth, imply that the explanation for the Cd effects obtained may include mechanisms for Cd tolerance.

Solid matrix and liquid culture procedures for growth of potatoes

This report discusses the advantages and limitations of several different procedures for growth of potatoes for CELSS. Solution culture, in which roots and stolons are submerged, and aeroponic culture were not found useful for potatoes because stolons did not produce tubers unless a severe stress was applied to the plants. In detailed comparison studies, three selected culture systems were compared, nutrient film technique (NFT), NFT with shallow media, and pot culture with deep media. For the NFT and NFT plus shallow media, plants were grown in 0.3 m 2 trays and for the deep medium culture, in 20 liter pots. A 1 cm depth of arcillite, a baked montmorillonite clay, was used as shallow media (NFT-arc). Peatvermiculite mixture was used to fill the pots for the deep media. Nutrient solution, modified half-strength Hoagland's was recirculated among the tray culture plants with pH automatically controlled at 5.5, and conductivity maintained ≈ 1100 μS cm −1 by adding stock nutrients or renewing the solution. A separate nutrient solution was used to water the pot plants four times daily to excess and the excess was discarded. Plants of Norland cv. were utilized and transplanted from sterile-propagated stem cutting plantlets. The plants were grown for 66 days under 12 h photoperiod in a first study and grown for 54 days under 24 h photoperiod in a second study. Under both photoperiods, total plant growth was greater in NFT-arc than in either NFT or pot culture. Under 12 h photoperiod, tuber dry weight was 30% higher with NFT-arc, but 50% lower with NFT, than with pot culture. Under 24 h photoperiod, however, tuber dry weight in both NFT and NFT-arc was only 20% of that in pot culture. The NFT and NFT-arc produced a greater shoot growth and larger number of small tubers than pot culture, especially with 24 h photoperiod. It is concluded that there are serious limitations to the use of NFT alone for growth of potatoes in a CELSS system. These limitations can be minimized by using a modified NFT with a shallow layer of media, such as arcillite, yet additional work is needed to ensure high tuber production with this system under long photoperiods.

763 PB 307 ENVIRONMENTAL CONTROL ON FLOWERING AND GROWTH OF LYSIMACHIA CONGESTIFLORA

Lysimachia congestiflora Wils. (Primulaceae) is a new crop for American nurseries and may be used as an annual in the north and a half-hardy perennial in the south. The purpose of this study was to investigate the influence of photoperiod, temperature, and irradiance on its flowering and growth. Three experiments were conducted with photoperiod of 8, 12, 16 hrs day-1, temperature of 10, 18, 26C, and irradiance of 100, 200, 300 μmol m-2s-1, respectively. Plant.9 given long day photoperiod (16 hours) flowered 21 and 34 days earlier, respectively, than plants at 12 sad 8 hour photoperiods. Plants under long day treatment produced more flowers than those at 8 and 12 hours. Plant dry weight did not differ between treatments, but plants grown in the long day treatment produced fewer but larger leaves. Total plant growth increased as temperature increased, but lower temperature (10C) decreased flower initiation and prevented flower development, while high temperature (26C) reduced the longevity of the open flowers. Flowering was accelerated and dry weight increased as plants were subjected to high irradiance levels. The results suggest that Lysimachia congestiflora is a quantitative long day plant. It should be grown under a photoperiod of at least 12 hours at a temperature of approximately 20C. Low light areas should be avoided and supplemental lighting to provide the long days may improve the plant quality.

Growth of Eucalyptus marginata (Jarrah) seedlings in a greenhouse in response to shade and soil temperature.

The effects of shade and soil temperature on growth of Eucalyptus marginata Donn ex Sm (jarrah) seedlings were studied in greenhouse experiments. Plant dry weight and that of all plant parts declined in response to shade, as did root/shoot ratio. Plant leaf area was less in unshaded plants than in plants grown in shade, and specific leaf area increased with shade. Unshaded seedlings had a higher light-saturated rate of photosynthesis, a higher light compensation point and a higher light saturation point than seedlings grown in 70% shade. The relationship between plant dry weight and leaf dry weight was independent of shading, whereas the relationship between plant dry weight and plant leaf area was dependent on shading. Therefore, leaf dry weight may be a better predictor of biomass production than leaf area in forest stands where shade is likely to affect growth significantly. Soil temperature had a significant effect on the growth of all plant parts except cotyledons. Total plant growth and shoot growth were maximal at a soil temperature of 30 degrees C, but root growth had a slightly lower temperature optimum such that the root/shoot ratio was highest at 20 degrees C. Roots grown at 15 degrees C were about 30% shorter per unit of dry weight than roots grown at 20 to 35 degrees C. We conclude that increases in irradiance and soil temperature as a result of overstory removal in the forest will cause significant increases in growth of E. marginata seedlings, but these increases represent a relatively small component of the growth response to overstory removal.

Establishment of Vegetation on By‐Product Gypsum Materials

AbstractThe production of by‐product gypsum materials is likely to increase greatly in the future and will require storage in large stacks. Therefore, technology to cover these materials with vegetation will be required. The nutrient requirements for successful plant growth were investigated on three materials [phosphogypsum (PG) from Florida, and two flue gas desulfurization gypsums (FGDG) from Florida and Illinois] using weeping lovegrass (Eragrostic curvula) and lespedeza (Sericia lespedeza) as indicator plants. In the first greenhouse experiment, the requirements for all the essential nutrients were studied in a subtractive design using PG. Significant growth reductions were obtained with the singular elimination of all nutrients except B. Growth on the two FGDG materials was poor due to the presence of high levels of soluble salts. These materials were leached before being studied in the second experiment in which only N, P, and K were varied at constant basal rates of the other essential elements. The 100N‐50P‐150K (mg kg−1) treatment proved to be the best for total plant growth on all three gypsum materials. The results indicate that providing the proper nutrient balance is important in obtaining good growth on by‐product gypsum materials.

Effects of ambient rain chemistry on field-grown radish ? An exploratory approach by multiple linear regression

In a modeling approach multiple regression was used to describe plant growth of radish (Raphanus sativus L. cv. ‘Cherry Belle’) by a range of environmental variables including chemical characteristics of ambient rain. In a 2-yr field study, staggered radish crops were harvested in 2-d intervals. During the entire vegetative growth, the weights of the major organs (shoot, hypocotyl, root) were determined and mean relative growth rates (RGR) were calculated. Different climatic variables, which were modified by a range of time lag periods, were combined to describe total plant RGR and hypocotyl RGR for each of 2 or 3 developmental phases of the crop. In a second step, variables characterizing the ion concentration and deposition of ambient rain (mean pH 4.35 and 4.15 in 1989 and 1990, respectively) were introduced and tested for their significance of contribution to the models. Generally, the earlier development stages of radish were more responsive to changes in environmental variables and the growth models were improved significantly by introducing variables of rain chemistry during these early growth phases only. Hypocotyl growth was not more sensitive to the chemical composition of rain than was total plant growth. Disparities in frequency and amount of rain between the 2 yr may have caused differences in the relative contribution of rain chemistry variables towards the models. The limitations and advantages of this modeling approach are discussed in respect to the importance of different growth stages on the sensitivity to acid rain stress.

Field Study of the Interaction between Solar Ultraviolet-B Radiation and Drought on Photosynthesis and Growth in Soybean

Soybean, Glycine max (L.) Merr. cv Essex, plants were grown in the field in a 2 x 2 factorial design, under ambient and supplemental levels of ultraviolet-B (UV-B) radiation (supplemental daily dose of 5.1 effective kilojoules per square meter) and were either well-watered or subjected to drought. Soil water potentials were reduced to -2.0 megapascals by the exclusion of natural precipitation in the drought plots and were maintained at approximately -0.5 megapascal by supplemental irrigation in well-watered plots. Plant growth and gas exchange characteristics were affected under both drought and supplemental UV-B radiation. Whole-leaf gas exchange analysis indicated that stomatal limitations on photosynthesis were only significantly affected by the combination of UV-B radiation and drought but substrate (ribulose bisphosphate) regeneration limitations were observed under either stress. The combined effect of both drought and UV-B radiation on photosynthetic gas exchange was a reduction in apparent quantum efficiency and the rapid appearance of biochemical limitations to photosynthesis concomitant with reduced diffusional limitations. However, the combination of stresses did not result in additive effects on total plant growth or seed yield compared to reductions under either stress independently.

Leaf Color Retention, Dark Respiration, and Growth of Red-leafed Japanese Maples Under High Night Temperatures

Red- and purple-leafed seedlings and clonal material selected for superior color and growth under northern climatic conditions may exhibit progressive color loss and reduced growth rates when exposed to the hot summers and high night temperatures of more southern climates. Studies were conducted to characterize the color loss associated with red-leafed seedlings of Acer palmatum Thunb. (Japanese maple), and to determine to what extent night temperatures affect the dark respiration, growth, and anthocyanin expression of A. palmatum `Bloodgood'. The percentage of seedlings within each of five color classes was determined for five dates from spring to early fall. Significant shifts in class distribution occurred on every evaluation date tested. The class changes contributing the most to these shifts varied with age of leaf material and date. Dark respiration rates increased by 0.09 mg CO2/g leaf dry weight per hour for every 1C rise in temperature, regardless of exposure duration. Dark respiration rates of 0.69 and 1.73 mg CO2/g per hour were found at 14 and 26C, respectively. The greatest amount of growth occurred during weeks 6 through 8 at a night temperature of 14C. Plant growth during this period increased by an average 51%, compared to that at warmer night temperatures. Ultimately, total plant growth at 14C decreased 7%, 19%, and 32% as night temperatures increased from 18 to 22 to 26C. Leaf redness index values at 14 or 18C were from two to seven times greater than those at warmer night temperatures.

Comparative phosphate requirements of yellow serradella (Ornithopus compressus) burr medic (Medicago polymorpha var. brevispina) and subterranean clover (Trifolium subterraneum)

The phosphate (P) requirements of burr medic (Medicago polymorpha) on marginally acidic, medium-textured soils and yellow serradella (Ornithopus compressus) on acidic, light-textured soils in the low rainfall (<400 mm) wheatbelt of Western Australia are not known. The hypothesis that yellow serradella, subterranean clover (Trifolium suhrerraneum) and burr medic have the same external requirement for applied P was tested in a glasshouse trial on 2 P-deficient soils. Species were compared on an equal total seed weight and similar maturity length basis. It was found that yellow serradella required less soil applied P to achieve 90% maximum shoot production or total plant (shoots + roots) growth and had a greater curvature co-efficient from fitted Mitscherlich functions than either subterranean clover or burr medic. Burr medic was the most responsive to applied P and had the largest external requirement for applied P. These differences in external P requirements were related to differences in ability of the 3 legumes to absorb P and to transport it to their shoots and were not due to differences in internal efficiency of the shoots. Yellow serradella was able to take up more P (total plant P content) per g roots than subterranean clover, and subterranean clover more than burr medic. The same relationship applied to the translocation of absorbed P from roots to shoots.

Effects of Plant Density on Growth and Biomass Partitioning in Pickling Cucumbers

Abstract Pickling cucumbers (Cucumis sativus L. cvs. Tamor and Castlepik) were direct-seeded at six plant densities (in thousands, 44, 77, 97, 121, 152, 194) using two between-row spacings (71 and 36 cm) and three within-row spacings (29, 14, and 11 cm between plants). Compared with the 29 cm within-row spacing, the 11- and 14-cm spacings resulted in significantly lower total above-ground plant dry weights, growth rates, and total leaf areas for both cultivars as early as 21 to 27 days after planting. The between-row spacing effects on plant growth were similar, but were of a lower magnitude and appeared later in plant development than for the within-row spacing effects. Leaf lamina and fruit tissue exhibited the largest reduction in tissue dry weights per plant compared to stem and petiole tissue when plant density was increased from ≈4.5 to 20 plants/m−2 (45,000 to 200,000 plants/ha). Lower fruit productivity per plant at higher plant densities resulted from fewer fruit set per plant and lower fruit : shoot ratios. Unit leaf rate (g dry weight/day per g of lamina dry weight) was not affected by plant spacing during the fruit development period. Increased densities resulted in significantly higher leaf area indexes, and vegetative and total above-ground dry weights/m−2. Total fruit yield with a single harvest did not increase above ≈77,000 plants/ha for both cultivars. A high correlation (r = 0.877) between leaf lamina dry weight and fruit growth rate indicates that net photosynthetic capacity might be limiting fruit productive potential in pickling cucumbers.

Patterns of water use and the tissue water relations in the dioecious shrub, Salix arctica: the physiological basis for habitat partitioning between the sexes

Within the high arctic of Canada, Salix arctica, a dioecious, dwarf willow exhibits significant spatial segregation of the sexes. The overall sex ratio is female-biased and female plants are especially common in wet, higher nutrient, but lower soil temperature habitats. In contrast, male plants predominate in more xeric and lower nutrient habitats with higher soil temperatures that can be drought prone. Associated with the sex-specific habitat differences were differences in the seasonal and diurnal patterns of water use as measured by stomatal conductance to water vapor and the bulk tissue water relations of each gender. Within the wet habitats, female plants maintained higher rates of stomatal conductance (g) than males when soil and root temperatures were low (<4° C). In contrast, within the xeric habitats, male plants maintained higher g and had lower leaf water potentials Ψleaf at low soil water potentials and a high leaf-to-air vapor pressure gradient (Δw) when compared to females. Female plants had more positive carbon isotope ratios than males indicating a lower internal leaf carbon dioxide concentration and possibly higher water use efficiency relative to males. Tissue osmotic and elastic properties also differed between the sexes. Male plants demonstrated lower tissue osmotic potentials near full tissue hydration and at the turgor loss point and a lower bulk tissue elastic modulus (higher tissue elasticity) than female plants. Males also demonstrated a greater ability to osmotically adjust on a diurnal basis than females. These properties allowed male plants to maintain higher tissue turgor pressures at lower tissue water contents and Ψsoil over the course of the day. The sex-specific distributional and ecophysiological characteristics were also correlated with greater total plant growth and higher fecundity of females in wet habitats, and males in xeric habitats respectively. The intersexual differences in physiology persisted in all habitats. These results and those obtained from growth chamber studies suggest that sex-specific differences have an underlying genetic basis. From these data we conjecture that selection maintaining the intersexual differences may be related to different costs associated with reproduction that can be most easily met through physiological specialization and spatial segregation of the sexes among habitats of differing conditions.

Sheep Grazing as a Silvicultural Tool to Suppress Brush

The possibility of using livestock as a biological agent to control unwanted ground vegetation in Pacific Northwest coniferous forests has been discussed for over 50 years. However, little quantitative information has yet been published documenting the efficacy of livestock in suppressing brush and other ground vegetation in commercial Douglas-fir (Pseudotsuga menziesii) plantations. Therefore, a study was conducted in 1981 and 1982 to evaluate the potential for using herded sheep to control competing vegetation in Douglas-fir plantations in Pacfic Northwest coniferous forests. Three 4to 6-year-old plantations were grazed once each year during the May to September grazing season. Estimates of current year's growth present in October, both inside and outside a livestock exclosure on each study plantation, were used to evaluate the effects of grazing. In general, utilization of brush by sheep was moderate to heavy, except in the spring of 1982, when brush was lightly utilized. Sheep grazing effectively reduced (p<0.01) both total understory plant growth and brush net current year's growth on all plantations. Reduced brush biomass on grazed areas was associated with greater Douglas-fir diameter growth in 1981-82 and 1982-83. By 1985, trees in grazed areas were 5% taller (p<0.0S) and 7% greater in diameter (p<0.01) compared to ungrazed controls. Our data and observations suggest that sheep may be effectively used as a biological control agent for brush control in coastal Douglas-fir forests.

Soil acidity, and the growth, biomass partitioning and leaf mineral composition of honeylocust (Gleditsia triacanthos L.) seedlings.

Honeylocust seedlings (Gleditsia triacanthos L.) were grown in cylinders containing soil adjusted to pH 4, 5 or 6, and harvested every 10 days for 40 days for dry weight and leaf mineral analysis. Total weight of plants grown at pH 4 was less than that of plants grown at pH 5 or 6. Root weight accounted for a greater proportion of total weight in plants grown at pH 4 than in plants grown at pH 5 or 6. Root growth as a function of total plant growth was higher in plants grown at pH 4 than in plants grown at pH 5 or 6, whereas leaf growth as a function of total plant growth was less in plants grown at pH 4 than in plants grown at pH 5 or 6. However, the relationships between root biomass and root length and between leaf biomass and leaf area were the same in all treatments. An analysis of total leaf concentrations of Ca, P, K, Mg, Mn and Al indicated that Al accumulation in leaves was significantly related to a decrease in plant growth at pH 5. A leaf tissue aluminum concentration of 35 microg g(-1) was associated with toxicity symptoms and a 25% reduction in total plant weight.

Response of ‘Tifblue’ Rabbiteye Blueberry to Soil-applied Paclobutrazol

Abstract Bud development in ‘Tifblue’ rabbiteye blueberry (Vaccinium ashei Reade) was reduced with increasing levels of soil-applied paclobutrazol (PB). Flowering was delayed from 10 to 15 days on plants receiving ≥3 g a.i. PB. This effect still existed 2 years after treatment. Rates of ≤2 g per plant did not influence leaf area. Floral or vegetative bud development, photosynthesis, transpiration, or fruit size when measured 1 year after treatment, but fruit yields were inversely correlated to PB levels. Two years after application, ≤2 g rates had not influenced total plant growth. Stems arising from mature canes were reduced by ≥ 0.5 g PB, but length of juvenile canes was increased by rates up to 2 g PB. Paclobutrazol applications had no influence on leaf content of N, P, Ca, Mg, Fe, or Cu. Only the highest level resulted in increased leaf levels of K and Zn, but Mn leaf content was increased by all levels of PB. In general, rates of soil-applied PB which reduced vegetative growth also reduced fruit yield. Chemical name used: β [(4-chlorophenyl)methyl]-α-(1,1 dimethylethyl)-1H-1,2,4-triazole-l-ethanol (paclobutrazol).

Increased Stress Tolerance of Nodule Activity in the Medicago-Rhizobium-Glomus Symbiosis Under Drought

Summary A comparison was made between Medicago sativa L. cv. Tierra de Campos plants in tripartite symbiosis with Rhizobium meliloti strain 102F51 and the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe, irrigated with N- and P-free nutrient solution, and nodulated plants plus soluble added phosphorus. When 45 days old, both groups of plants were stressed by cyclic withholding of water and rewatering at different soil water potentials of −0.1, −0.3, −0.5 and −0.7MPa. After a subsequent 1 month drought-treatment period, the following parameters were evaluated: leaf water potential, intensity of VAM infection, nodule dry weight, total plant growth, phosphorus content and acetylene reduction activity. Drought stress significantly reduced the dry weight of total plants and nodules as well as P content of non-VAM plants when compared with the tripartite symbiosis. The intensity of VAM infection was not affected by drought. During drought, nodule activity was significantly higher in VAM than in non-VAM plants; this higher activity cannot be explained by improved P uptake caused by the fungus since P-concentration in mycorrhizal plants was always lower than in phosphorus fertilized plants.

Carbon and Mineral Element Accumulation and Allocation in Two Annual Plant Species in Response to Timing of Nutrient Addition

(1) The effects of the timing of a large 'pulse' addition of mineral nutrients on accumulation and allocation of C, N, P, K and Ca, and on seed production and element status, were examined in the annual plants Abutilon theophrasti and Datura stramonium. (2) Nutrient addition increased total plant weight and element content, but the increase occurred over a period of several weeks rather than within the week after the addition. (3) The various mineral elements were allocated independently of carbon and of each other in both species. A. theophrasti started reproduction later than did D. stramonium and had lower proportional allocations of weight, N and P to reproduction at the final harvest. (4) Although nutrient pulse addition produced a temporary increase in proportional allocation of weight, N and P to leaves shortly after the addition, pulse timing had little effect on final proportional allocations. Plant age had a greater effect on allocation patterns than did the nutrient treatments. (5) Differences in seed production observed among nutrient treatments appeared to result primarily from differences in total accumulation of carbon and other elements and hence total plant growth, rather than from differences in proportional allocations. (6) Element concentrations of seeds and capsules, and mineral element contents per seed, did not differ among the treatments, and variation in mean seed weight among treatments was relatively small.

Growth, Yield, and Nutrient Uptake of Transplanted Fresh-market Tomatoes as Affected by Plastic Mulch and Initial Nitrogen Rate

Abstract Two field experiments were conducted with two cultivars of transplanted tomatoes (Lycopersicon esculentum Mill.) with and without plastic mulch, varying the initial rate of N fertilizer, but maintaining the total N rate at 168 kg·ha–1 by sidedressing. In 1982, 0 and 112 kg·ha–1 initial N rates, and bare ground, black mulch, and clear plastic mulch were compared on a gravelly loam soil. In 1983, initial N rates used were 34, 67, 101, or 134 kg·ha–1, with bare ground and clear mulch on a silt loam soil. Effects of the plastic mulch dominated both experiments. Mulching increased rate of basal branch appearance and led to early flowering on branches. Total plant growth, as measured by vine weights at final harvest, was increased by mulch in both years. Mulching increased early yield only in 1983, but increased total yields by 13% and 79% in 1982 and 1983, respectively. Initial N fertilizer rates did not influence total yields significantly in either experiment, although high initial N rate, combined with clear plastic mulch, led to a significant decrease in percent marketable fruit in 1982. In 1983, mulching increased shoot concentrations of N, NO3-N, P, K, Ca, Mg, Cu, and Β (P = &lt; 0.01) in spite of the fact that mulched plants were larger than unmulched plants at sampling time, 24 days after transplanting. Nitrogen fertilizer increased only the N and P concentrations and to a lesser extent than did the mulch.

INTERACTION OF TEMPERATURE AND CO2 ENRICHMENT ON SOYBEAN: PHOTOSYNTHESIS AND SEED YIELD

Seed yield and photosynthetic responses of soybean (Glycine max L. Merr. ’Ransom’) were studied in growth chambers at day/night temperatures of 18/12, 22/16, and 26/20 °C and atmospheric CO2 concentrations of 350, 675 and 1000 μL L−1. No seeds were produced at 18/12 °C within any of the CO2 concentrations. Numbers of pods and seeds increased with increasing temperature and CO2 levels. Carbon dioxide enrichment increased seed yield of soybean grown at moderately cool temperatures. This increase was associated with an increase in net photosynthetic rate. Leaf photosynthesis in response to CO2 enrichment increased more at 22/16 °C than at 26/20 °C. Increases in temperature and CO2 levels enhanced total growth of plants but hastened senescence of leaves. The extended photosynthetic capacity at cool temperatures did not result in allocating more dry matter to developing pods. CO2 enrichment at 26/20 °C resulted in greater seed yield increases than CO2 enrichment at lower temperatures.Key words: Soybean yield, low temperature, CO2 × temperature

Influence of Pruning at Transplant Time on Growth and Establishment of Liquidambar styraciflua L., Sweet Gum

Pruning of container-grown sweet gum trees by heading 20, 30, and 50% of the top at transplant time produced no significant positive influence on growth and establishment when compared to nonpruned trees. Total growth of plants pruned 30 and 50% was restricted. Heading impaired the natural excurrent (conical) form of sweet gum. However the lost leader of some trees was replaced by a clearly dominant, upright lateral branch. Results of fall and spring transplanting were similar. Supplemental irrigation during prolonged dry periods produced significantly larger, more desirable landscape trees.

EFFECTS OF PHOTON IRRADIANCE ON THE GROWTH OF SHOOTS AND ROOTS, ON THE RATE OF INITIATION OF MYCORRHIZAL INFECTION AND ON THE GROWTH OF INFECTION UNITS IN TRIFOLIUM SUBTERRANEUM L.

SUMMARYThe effects of photon irradiance on the growth of young plants (up to three weeks old) of Trifolium subterraneum L. and on the development of mycorrhizal root systems were studied with plants grown in a soil/sand mixture inoculated with Glomus mosseae (Nicol. &amp; Gerd.) Gerdemann &amp; Trappe. Total plant growth was lower when photon irradiance was 100 μmol m−2 s−1, compared with 450 μmol m−2 s−1. Fresh weight of shoots was unaffected, but the fresh weight/dry weight ratio was increased at the lower irradiance in shoots, but not in roots. The main effect of decreased irradiance on the growth of the root system was a reduction in the numbers of first‐ and second‐order lateral roots initiated. The average rate of extension of axial and lateral roots was only slightly reduced. The fraction of the root length infected was lower at the lower irradiance, particularly in the first two weeks. Development of infection was analyzed in terms of the role of formation of mycorrhizal entry‐points (A) and the average rate of growth of infection units (B). Reduction in the fraction of the root length infected was found to be due to a reduction in A. B was remarkably constant at the two irradiances and in the different root subsystems. Thus both roots and infection units respond by making fewer units of unaltered rate of growth.