Pre-harvest retting of flax with glyphosate: effects of novel methods of spray application on penetration of the canopy and uniformity of deposition

Four methods of spray application were compared in 1986 with the recommended method of glyphosate application for pre-harvest retting of flax (Linum usitatissimum). Sampling individual plants immediately after application of glyphosate and dye demonstrated that spray penetration through the crop canopy, measured as dye deposition on the lower stem as a percentage of total deposition per plant, was the same for all methods of application. None of the methods of application improved the uniformity of dye deposition.
The following year, effects of deseeding prior to spraying and of using an air-assisted sprayer were investigated. Deseeding increased dye deposition on the lower stem but did not improve the efficacy of glyphosate. Application with an air-assisted sprayer increased the proportion of spray deposited on the lower stem but not the absolute amount deposited or the efficacy of glyphosate. Dye deposition on individual plants varied greatly for all methods of application and, when glyphosate was applied using the same methods, undesiccated plants occurred in all plots.
Considerable improvements in spray penetration and uniformity of spray deposition appear necessary for successful pre-harvest retting of flax with glyphosate.     

Pre-harvest retting of flax: effects of water stress on uptake and efficacy of glyphosate

The effects of water stress on the uptake, translocation and efficacy of glyphosate in flax were investigated in relation to pre-harvest retting. Glyphosate (applied at a rate equivalent to 1.44 kg a.e. ha^-1 at 0, 10, 20 or 30 days after the start of flowering) caused little desiccation of flax grown in pots under restricted watering. Glyphosate application to well-watered plants caused the moisture content to decline from an initial value of 70 – 80% to approximately 40% at 3 wk after spraying. Glyphosate was applied 2 wk after the mid-point of flowering to flax grown in soil with moisture contents of 35, 31, 26, 22, 16 or 12%. Soil moisture levels (16% and 12%) which restricted evapotranspiration also reduced the efficacy of glyphosate but did not affect uptake of ¹⁴C-glyphosate. Translocation of ¹⁴C-glyphosate out of treated leaves was reduced only in the most severely stressed plants (12% soil moisture). Experiments with young plants (4 wk old) confirmed that water stress slightly reduced downward translocation of glyphosate. When the herbicide was applied to young plants under conditions which minimised differences in translocation, 10.8 μg glyphosate was sufficient to desiccate unstressed plants but 108 μg had little effect on stressed plants. This indicates that, in addition to any reduction in translocation which occurs during drought, water stress may reduce the susceptibility of flax to glyphosate. Thus only relief of plant water stress by irrigation is likely to improve response of the flax crop to glyphosate.     

Pre-harvest retting of flax with glyphosate: effect of growth stage at application on uptake, translocation and efficacy of glyphosate

Flax cv. Hera was grown in pots at a density of 2000 plants/m². At various stages of maturity plants were either sprayed with glyphosate (Roundup, Monsanto plc) at a rate equivalent to 1.4 kg a.e./ha or the uppermost leaves of the main stem were treated with ¹⁴C-glyphosate. Uniform desiccation to greater than 80% DM within 5 wk was obtained with sprays applied up to and at 2 wk after the mid-point of flowering (MP) and also at 10 wk after MP, when all seed had been shed. Sprays at 4–6 wk after MP caused little or no desiccation of the lower stem. Autoradiography demonstrated that ¹⁴C-glyphosate applied to the 10 uppermost leaves of the main stem at MP, or to the upper stem at 10 wk after MP, was distributed throughout the plant within 48 h. However, no downward translocation was detected within 48 h of applications during seed filling, i.e. at 2 or 5 wk after MP. Quantitative determinations of distribution of ¹⁴C-glyphosate were made at 48 h after application either to the five uppermost leaves on the main stem or, after leaf drop, to the base of the inflorescence. Translocation out of the treated zone was 58–67% of applied ¹⁴C-glyphosate with applications at 0, 2 or 6 wk after MP but only 13% with application to senescing leaves at 4 wk after MP. Little downward translocation occurred with any of these applications, ¹⁴C-glyphosate accumulating mainly in the upper stem and inflorescence. Thus failure of glyphosate to desiccate the lower stem may be related to poor downward translocation during seed development and to reduced uptake into, and export out of, senescing leaves.     

Responses of the resurrection plant <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> to high irradiance

The effect of high irradiance (HI) during desiccation and subsequent rehydration of the homoiochlorophyllous desiccation-tolerant shade plant Haberlea rhodopensis was investigated. Plants were irradiated with a high quantum fluence rate (HI; 350 μmol m⁻² s⁻¹ compared to ca. 30 μmol m⁻² s⁻¹ at the natural rock habitat below trees) and subjected either to fast desiccation (tufts dehydrated with naturally occurring thin soil layers) or slow desiccation (tufts planted in pots in peat-soil dehydrated by withholding irrigation). Leaf water content was 5 % of the control after 4 d of fast and 19 d of slow desiccation. Haberlea was very sensitive to HI under all conditions. After 19 d at HI, even in well-watered plants there was a strong reduction of rates of net photosynthesis and transpiration, contents of chlorophyll (Chl) and carotenoids, as well as photosystem 2 activity (detected by the Chl fluorescence ratio R_Fd). Simultaneously, the blue/red and green/red fluorescence ratios increased considerably suggesting increased synthesis of polyphenolic compounds. Desiccation of plants in HI induced irreversible changes in the photosynthetic apparatus and leaves did not recover after rehydration regardless of fast or slow desiccation. Only young leaves survived desiccation.     

Abstracts of papers read at the winter meeting at the university of Newcastle Upon Tyne

Young plants of Laminaria hyperborea collected from the field were grown for 2·5–4 weeks in blue, green, red and white (simulated underwater) light fields at 5, 20 and 100 μmol m^-2s^-1. The absolute concentrations of all pigments showed little variation with irradiance in green and white light, but decreased in high irradiances of red and blue light. The ratio of fucoxanthin to chlorophyll a also increased in the latter treatments, as did the chlorophyll c:a ratio in bright red light. There was little difference in the action spectrum for photosynthesis between the different light qualities at any one irradiance, but the action spectra for plants grown at 100 μmol m^-2s^-1 showed deeper troughs and higher peaks than those for plants grown at lower irradiances. Gross photosynthesis per unit of thallus area at 10 μmol m^-2s^-1 decreased in plants with low total pigment concentrations, but the photosynthesis per unit of pigment concentration increased. This suggestion of self-shading of pigment molecules within the algal thalli was supported by a flattening of the action spectrum in plants with higher chlorophyll a contents. The variations observed between the action spectra for different plants could thus be attributed to the decrease in pigment content at high irradiances, and not to the light quality in which the plants were grown.     

Effects of Irradiance on Photosynthesis and Activity of Protease Inhibitors in Amaranthus hypochondriacus

Amaranthus hypochondriacus plants were grown under three photosynthetic photon flux densities (PPFD). Mature plants grown at full sunlight (38.8 mol m^–2 d^–1) had higher maximum net photosynthetic rate (P_N) and significantly higher leaf trypsin inhibitor activity than plants that developed under lower PPFD (19.4 and 12.8 mol m^–2 d^–1). In contrast, seeds collected from plants fully exposed to sunlight showed the lowest activity of trypsin inhibitor, higher rate of germination and susceptibility to infection by Aspergillus niger.     

A culture unit system for the study of responses of mycorrhizal and non-mycorrhizal seedling to treatments

The time-dependence of Mn accumulation was confirmed in potato foliage (Solanum tuberosum. L.cv. Norland) grown in solution culture. Older leaves grown at 0.61 mM Mn had substantially higher Mn concentrations than younger leaves and stem samples. Levels of Mn in older leaves increased steadily from 4000 µg g^–1 at one week to 8–10,000 µg g^–1 at 6 weeks, but were relatively constant in the emerging leaves. Even foliage grown at low Mn levels (0.01 mM Mn) had 4 fold gradients in Mn concentration from younger (40 µg g^–1) to older leaves (180 µg g^–1).At 0.61 mM Mn, concentrations of 3–4000 µg g^–1 in the youngest fully-developed leaves did not bring about any decline in yield, and levels of up to 5000 µg g^–1 occurred in individual potato leaves before Mn toxicity symptoms were observed. Potato foliage grown at the high Mn had similar leaf numbers, but showed an increased stem length and smaller leaves than foliage grown at 0.01 mM Mn. In particular, the leaf area of the middle and lower leaf fractions were affected by the high Mn level.The ability of rapidly growing plants to withstand high concentrations of Mn is discussed in relation to the pattern of dry matter and Mn accumulation shown by potato foliage.     

Stable isotope analysis of water extraction from subsoil in upland rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) as affected by drought and soil compaction

Water extraction from subsoil in upland rice (Oryza sativa L.) was examined as related to topsoil desiccation and subsoil compaction. The water extraction was observed by measurements of heavy water concentrations in transpiring plants. The plants were grown in pots that were filled with sandy soil and vertically compartmented into two columns. Heavy water was applied to the subsoil. Plants exposed to mild topsoil desiccation (–120 kPa in water potential) eventually increased water extraction from the subsoil and maintained photosynthetic rate and stomatal conductance at the wet condition level. The rates of the plants subjected to severely droughted topsoil (–190 kPa) were significantly lowered due to less water uptake from the subsoil. Subsoil compaction at bulk densities of 1.45 and 1.50 Mg m^–3 inhibited increase of root length densities. Limited water extraction from the subsoil was insufficient to maintain plant productivity under drought conditions. Daily water uptake per unit of root length in the lower tube did not apparently increase even if water demand on the unit root length increased. When water to topsoil was completely withheld, water extraction from the subsoil gradually increased as the topsoil dried out. Plants that were watered and rewatered took up very little water from the subsoil. The extraction from the subsoil occurred only when water potential of the topsoil was below about –190 kPa.     

Enhanced systemic resistance to bacterial speck disease caused by Pseudomonas syringae pv. tomato by dl-β-aminobutyric acid under salt stress

Plants have evolved different but interconnected strategies to defend themselves against microbial pathogens and stress conditions. The defense responses of tomato (Lycopersicon esculentum Mill) seedlings treated with dl ‐β‐aminobutyric acid (BABA) were investigated with and without abiotic stress (100 mM NaCl) against bacterial speck disease caused by Pseudomonas syringae pv. tomato. The plants were sprayed with 50, 125, 250 or 500 μg ml⁻¹ BABA and were inoculated with 10⁸ colony‐forming units ml⁻¹ bacterial suspension 1 day after treatment. Abiotic stress led to an increase in plant resistance. When BABA was additionally applied as a foliar spray at 125 μg ml⁻¹, the effect on plants was almost identical to that on plants that were sprayed with BABA at 500 μg ml⁻¹ alone. The bacterial multiplication in the plants was 250‐fold lower than in the water‐treated (control) plants and in plants that were sprayed with 500 μg ml⁻¹ BABA alone within 48 h postinoculation (hpi). Physiological studies were carried out in the plants treated with BABA in order to investigate the reason for this synergistic effect. Abiotic stress with BABA spray resulted in high H₂O₂ generation and guaiacol peroxidase activity in the plants. The activity of the enzymatic antioxidative protective system of the plants, superoxide dismutase, ascorbate peroxidase and catalase (CAT), also showed a significant delayed increase in BABA‐treated plants under abiotic stress conditions. These increases in enzyme activity coincided with the initiation of the most suppressive effect of BABA on bacterial growth by 24 hpi, which were significantly higher than the control. Salt stress alone did not lead to any significant increase in CAT activity, but salt stress with BABA did. These findings indicate a synergistic effect between salt stress and BABA at low concentrations, resulting in induced plant resistance. Furthermore, a stress regulation effect of BABA under abiotic stress can be associated with plant resistance.     

Changes in the condition of photosynthetic apparatus of a diatom alga <Emphasis Type="Italic">Thallassiosira weisflogii</Emphasis> during photoadaptation and photodamage

Two populations of a diatom alga Thallassiosira weisflogii were grown at photon flux densities (PFD) of 0.8 and 8 μmol/(m² s). For both diatom populations, the recovery of chlorophyll fluorescence parameters (F ₀, F _m, F _v/F _m, and NPQ) was monitored after nondestructive irradiation by visible light at PFD of 40 μmol/(m² s) and after high-intensity irradiation by visible light (1000–4000 μmol/(m² s)). The exposure of diatoms to PFD of 40 μmol/(m² s)—higher than PFD used for algal growth but still nondamaging to photosynthetic apparatus—induced nonphotochemical quenching (NPQ), which was stronger in algae grown at higher PFD (8 μmol/(m² s)) than in algae grown at low light. After irradiation with high-intensity light, the recovery of chlorophyll fluorescence parameters was more pronounced in algae grown at elevated PFD level. During short-term irradiation of diatoms with high-intensity visible light (1000 μmol/(m² s)), a stronger NPQ was observed in the culture adapted to high irradiance. After the treatment of algae with dithiothreitol (an inhibitor of carotenoid deepoxidase in the diadinoxanthin cycle) or NH₄Cl (an agent abolishing the proton gradient at thylakoid membranes), a short exposure of algae to PFD of 40 μmol/(m² s) induced hardly any nonphotochemical quenching. The results indicate the dominant contribution of xanthophyll cycle carotenoids to energy-dependent quenching.     

Is photosynthetic acclimation to free-air CO<Subscript>2</Subscript> enrichment (FACE) related to a strong competition for the assimilatory power between carbon assimilation and nitrogen assimilation in rice leaf?

Net photosynthetic rate (P _N) of leaves grown under free-air CO₂ enriched condition (FACE, about 200 μmol mol⁻¹ above ambient air) was significantly lower than P _N of leaves grown at ambient CO₂ concentration (AC) when measured at CO₂ concentration of 580 μmol mol⁻¹. This difference was found in rice plants grown at normal nitrogen supply (25 g m⁻²; NN-plants) but not in plants grown at low nitrogen supply (15 g m⁻²; LN-plants). Namely, photosynthetic acclimation to FACE was observed in NN-plants but not in LN-plants. Different from the above results measured in a period of continuous sunny days, such photosynthetic acclimation occurred in NN-plants, however, it was also observed in LN-plants when P _N was measured before noon of the first sunny day after rain. Hence strong competition for the assimilatory power between nitrogen (N) and carbon (C) assimilations induced by an excessive N supply may lead to the photosynthetic acclimation to FACE in NN-plants. The hypothesis is supported by the following facts: FACE induced significant decrease in both apparent photosynthetic quantum yield (Φ_c) and ribulose-1,5-bisphosphate (RuBP) content in NN-plants but not in LN-plants.     

Reduced photoinhibition with stem curling in the resurrection plant Selaginella lepidophylla

Summary Selaginella lepidophylla, the resurrection plant, curls dramatically during desiccation and the hypothesis that curling may help limit bright light-induced damage during desiccation and rehydration was tested under laboratory conditions. Restraint of curling during desiccation at 25° C and a constant irradiance of 2000 mol m^–2 s^]t-1 significantly decreased PSII and whole-chain electron transport and the F_v/F_m fluorescence yield ratio following rehydration relative to unrestrained plants. Normal curling during desiccation at 37.5°C and 200 mol m^–2 s^–1 irradiance did not fully protect against photoinhibition or chlorophyll photooxidation indicating that some light-induced damage occurred early in the desiccation process before substantial curling. Photosystem I electron transport was less inhibited by high-temperature, high-irradiance desiccation than either PSII or whole-chain electron transport and PSI was not significantly affected by restraint of curling during desiccation at 25°C and high irradiance. Previous curling also helped prevent photoinhibition of PSII electron transport and loss of whole-plant photosynthetic capacity as the plants uncurled during rehydration at high light. These results demonstrate that high-temperature desiccation exacerbated photoinhibition, PSI was less photoinhibited than PSII or whole-chain electron transport, and stem curling ameliorated bright light-induced damage helping to make rapid recovery of photosynthetic competence possible when the plants are next wetted.     

Leaf anatomy during leaf development of photoautotrophically <Emphasis Type="Italic">in vitro</Emphasis>-grown tobacco plants as affected by growth irradiance

Tobacco (Nicotiana tabacum L.) plants were cultured in vitro photoautotrophically at three levels of irradiance (PAR 400–700 nm): low (LI, 60 μmol m⁻² s⁻¹), middle (MI, 180 μmol m⁻² s⁻¹) and high (HI, 270 μmol m⁻² s⁻¹). Anatomy of the fourth leaf from bottom was followed during leaf development. In HI and MI plants, leaf area expansion started earlier as compared to LI plants, and both HI and MI plants developed some adaptations of sun species: leaves were thicker with higher proportion of palisade parenchyma to spongy parenchyma tissue. Furthermore, in HI and MI plants palisade and spongy parenchyma cells were larger and relative abundance of chloroplasts in parenchyma cells measured as chloroplasts cross-sectional area in the cell was lower than in LI plants. During leaf growth, chloroplasts crosssectional area in both palisade and spongy parenchyma cells in all treatments considerably decreased and finally it occupied only about 5 to 8 % of the cell cross-sectional area. Thus, leaf anatomy of photoautotrophically in vitro cultured plants showed a similar response to growth irradiance as in vivo grown plants, however, the formation of chloroplasts and therefore of photosynthetic apparatus was strongly impaired.     

Influence of Cotyledon Excision and Sucrose on Root Formation in Pea Cuttings

Sucrose was supplied to stock plants of Pisum sativum L. cv. Alaska grown at different levels of irradiance. There was no significant effect on the rooting of the cuttings by sucrose supply to intact plants regardless of the irradiance. However, an increase in the number of roots per cutting was obtained at increasing concentrations of sucrose when the stock plants had been grown at 4 W m^?2 and their cotyledons had been removed two days before the cuttings were excised. Cotyledons were removed from stock plants at different times before the excision of cuttings with the intent to regulate the endogenous supply of carbohydrate. The number of roots per cutting was reduced by removal of the cotyledons and this reduction was correlated to the number of days the stock plants had grown without cotyledons as well as to the irradiance pre-treatment. A greater reduction occurred in cuttings from plants grown under 4 W m^?2 than from those grown under 38 W m^?2. The growth of the stock plants and the subsequent stem growth of the cuttings was determined by the irradiance to the stock plants and by the time of removal of the cotyledons. Exogenous supply of sucrose had no effect on the stem growth of the cuttings.     

RESPONSE OF PROROCENTRUM MARIAE-LEBOURIAE (DINOPHYCEAE) TO LIGHT OF DIFFERENT SPECTRAL QUALITIES AND IRRADIANCES: GROWTH AND PIGMENTATION1

Growth and pigment concentrations of the, estuarine dinoflagellate, Prorocentrum mariae-lebouriae (Parke and Ballantine) comb. nov., were measured in cultures grown in white, blue, green and red radiation at three different irradiances. White irradiances (400–800 nm) were 13.4, 4.0 and 1.8 W · m^?2 with photon flux densities of 58.7 ± 3.5, 17.4 ± 0.6 and 7.8 ± 0.3 μM quanta · m^?2· s^?1, respectively. All other spectral qualities had the same photon flux densities. Concentrations of chlorophyll a and chlorophyll c were inversely related to irradiance. A decrease of 7- to 8-fold in photon flux density resulted in a 2-fold increase in chlorophyll a and c and a 1.6- to 2.4-fold increase in both peridinin and total carotenoid concentrations. Cells grown in green light contained 22 to 32% more peridinin per cell and exhibited 10 to 16% higher peridinin to chlorophyll a ratios than cells grown in white light. Growth decreased as a function of irradiance in white, green and red light grown cells but was the same at all blue light irradiances. Maximum growth rates occurred at 8 μM quanta · m^?2· s^?1 in blue light, while in red and white light maximum growth rates occurred at considerably higher photon flux densities (24 to 32 μM quanta · m^?2· s^?1). The fastest growth rates occurred in blue and red radiation. White radiation producing maximum growth was only as effective as red and blue light when the photon flux density in either the red or blue portion of the white light spectrum was equivalent to that of a red or of blue light treatment which produced maximum growth rates. These differences in growth and pigmentation indicate that P. mariae-lebouriae responds to the spectral quality under which it is grown.     

INORGANIC CARBON LIMITATION OF PHOTOSYNTHESIS IN ULVA ROTUNDATA (CHLOROPHYTA)1

A computerized oxygen electrode Astern was used to make rapid and accurate measurements of photosynthetic light and dissolved inorganic carbon (DIC) response cures with a macroalga. Ulva rotundata Blid. was grown in an outdoor, continuous flow system in seawater under sunlight or 9% of sunlight at Beaufort, North Carolina. The light compensation points in the shade- and sun-grown plants, measured in seawater, were at photon flux densities (PFDs) of 16 and 27 μmol. Photons·m^?2·s^?1, respectively but the quantum yield of O₂ evolution was not significantly different. Rates of photosynthesis in seawater per unit area of thallus under saturating light and rates of dark respiration were about 1.5-fold higher in sun- than in shade-grown plants. The concentration of DIC in seawater (approximately 2 mM) limited photosynthesis at absorbed PFDs above 60–70 μmol photons·m^?2·s^?1 Addition of 20 mM inorganic carbon had no effect on quantum yield but caused about a 1.5-fold increase in the light-saturated photosynthetic rate in both shade- and sun-grown Ulva. The effect of DIC supplementation was greatest in plants grown in October and least in plants grown in June. The light- and DIC-saturated rate of photosynthesis in seawater was similar to the maximum rate obtained by exposing Ulva to 10% CO₂, in the gas phase. The carbon isotope values (δ¹³C, reflecting the ¹³C/¹²C ratio compared to a standard) of Ulva grown in the same seawater supply were dependent on light and agitation. Samples from Beaufort Inlet were more negative (δ¹³C value, ?20.03‰) than those grown in bright light with agitation (δ¹³C value, ?17.78‰ outdoors; ?17.23‰ indoors), which may indicate DIC supply limited carbon uptake in seawater.     

NMR imaging of root water distribution in intact Vicia faba L plants in elevated atmospheric CO2

The effect of elevated atmospheric CO₂ on water distribution in the intact roots of Vicia faba L. bean seedlings grown in natural soil was studied noninvasively with proton (¹H) nuclear magnetic resonance (NMR) imaging. Exposure of 24-d-old plants to atmospheric CO₂-enriched air at 650 cm³ m^?3 produced significant increases in water imaged in upper roots, hypogeal cotyledons and lower stems in response to a short-term drying-stress cycle. Above ground, drying produced negligible stem shrinkage and stomatal resistance was unchanged. In contrast, the same drying cycle caused significant depletion of water imaged in the same upper root structures in control plants subject to ambient CO₂ (350 m³ m^?3), and stem shrinkage and increased stomatal resistance. The results suggest that inhibition of transpiration caused by elevated CO₂ does not necessarily result in attenuation of water transport from lower root structures. Inhibition of water loss from upper roots and lower stem in elevated CO₂ environments may be a mitigating factor in assessing deleterious effects of greenhouse changes on crops during periods of dry climate.     

Physiological ecology of photosynthesis in Prasiola stipitata (Trebouxiophyceae) from the Bay of Fundy,Canada

The physiological ecology of Prasiola stipitata was examined in situ from two supralittoral sites in the Bay of Fundy (Nova Scotian, Canada) during November 2011, when the population was undergoing major expansion. Photosynthetic parameters (effective quantum yield, Φ_PSII, maximum quantum yield, F_v/F_m, and relative electron transport rate, rETR) were evaluated using chlorophyll fluorescence of PSII. A largely shaded and continuously moist population showed no change in Φ_PSII from one hour after sunrise to sunset in which natural irradiance varied between 3 and 300 μmol photons m^?2 s^?1. High irradiance (up to 1800 μmol photons m^?2 s^?1) had no apparent negative impacts on either quantum yield or rETR, but high desiccation in the field reduced quantum yield to almost zero. When thalli were brought into the laboratory, no change in F_v/F_m was observed up to 60% dehydration; however, there was a steep decline in F_v/F_m between 60% and 85% dehydration. Thalli showed complete recovery of F_v/F_m within one hour of reimmersion in seawater after 2 days of desiccation. After 15 days of desiccation full recovery required 24 h and after 30 days of desiccation thalli showed only partial recovery. These observations confirm the adaptation to photosynthesis in high irradiances and the rapid recovery following extreme desiccation observed in other Prasiola species.     

Optimum plant densities for three semi-leafless combining pea (Pisum sativum) cultivars under contrasting field conditions

Yield and yield components of three semi-leafless pea (Pisum sativum) cultivars, of contrasting seed type/growth habit, were assessed at target planting densities of 40–140 plants/m² on nine sites over three years. Flat-topped parabolic/asymptotic yield/density relationships were obtained. The plant density required to maximise (p max) and optimise (p opt) yield differed between cultivars: Helka, small blue, p max 126 plants/m², p opt 101 plants/m²; Solara, large blue, p max 124 plants/m², p opt 94 plants/m²; and Countess, white-seeded, p max 104 plants/m², p opt 71 plants/m². Near-maximum yields were maintained between 70 and 140 plants/m² due to the ability of the pea crop to make compensatory increases in the number of pods per plant as density declined. Yield/density responses were influenced by site (e.g. soil type) more than by seasonal factors. The risk of yield reductions occurring at densities below 70 plants/m² was greater on a mineral soil than on a fertile organic soil. On the basis of agronomic and economic considerations, there was no evidence that target plant densities required to optimise yield should necessarily be higher for semi-leafless cultivars studied than for conventional leafed peas.     

THE IMPACT OF FROND CROWDING ON FROND BLEACHING IN THE CLONAL INTERTIDAL ALGA MAZZAELLA CORNUCOPIAE (RHODOPHYTA, GIGARTINACEAE) FROM BRITISH COLUMBIA, CANADA

The importance that frond crowding represents for the survival of fronds of the clonal intertidal alga Mazzaella cornucopiae (Postels et Ruprecht) Hommersand (Rhodophyta, Gigartinaceae) was investigated in Barkley Sound, British Columbia, Canada. Frond density is high for this species, up to 20 fronds·cm^?2 in the most crowded stands. Frond crowding imposes a cost in the form of reduced net photosynthetic rates when fronds are fully hydrated as a result of reduced irradiance compared with experimental (not found naturally) low-density stands. However, the interaction between desiccation and irradiance alters this relationship between net photosynthetic rates and frond density. During a typical daytime low tide in spring, irradiance is 10–30 μmol·m^?2·s^?1 below the canopy of fronds, and frond desiccation (relative to total water content) can reach 43% at the end of the low tide. In contrast to natural stands, fronds from experimentally thinned stands are subjected to irradiances up to 2000 μmol·m^?2·s^?1 because of the spatial separation among fronds and can desiccate up to 81% at the end of the same low tide. Laboratory experiments showed that negative net photosynthetic rates occur between 40% and 80% desiccation at an irradiance of 515 μmol·m^?2·s^?1, and the literature suggests that strong bleaching could occur as a result. At 20 μmol·m^?2·s^?1 of irradiance and desiccation levels up to 40%, simulating understory conditions of natural stands, net photosynthetic rates are never negative. Experimental thinning of stands of M. cornucopiae done during spring effectively resulted in a stronger extent of frond bleaching compared with natural stands. Therefore, the cost of reduced net photosynthetic rates at high frond densities when fronds are fully hydrated is counterbalanced by the protective effects of frond crowding against extensive bleaching, essential for survival at the intertidal zone. Future research will have to demonstrate the possible relationship between the frequency and duration of negative net photosynthetic rates and the extent of frond bleaching.