The comparative phosphorus requirements of some temperate perennial legumes

The phosphorus (P) responses of seven temperate perennial pasture legumes and two species of lupins were compared in a field trial over a range of nine P rates, from 0 to 800 kg ha^-1. The two lupins produced more than 5 t ha^-1 of dry matter in the absence of added P and showed no response to the fertiliser. In contrast, the pasture legumes initially failed to grow without added P and responded to applications of between 200 and 800 kg ha^-1. At the higher rates of P, dry matter production of the pasture legumes was equivalent to that of the lupins.In the first 2 years of the trial; the most productive pasture legume species at the higher rates of added P were also the most productive at the lower rates. Phosphorus requirements for 90% of maximum yield varied greatly between species, but were closely related to maximum yield. Thus species with low P requirements for maximum yield were not necessarily P-efficient species. In the third and subsequent years of the trial Lotus corniculatus performed better than the other pasture legumes at the lower rates of added P. In contrast to other studies Lotus pedunculatus showed no ability to outyield Trifolium repens at low rates of P. Critical P concentrations of the pasture species for the late spring-early summer period declined in the order Trifolium repens (0.34%) > Lotus pedunculatus (0.30%) > Triofolium pratense (0.28%) > Trifolium hybridum (0.27%) > Trifolium ambiguum (0.26%) > Lotus corniculatus (0.23%).Mineralisable nitrogen (N) levels were determined in soils under three species in the 7th year of the trial. At the lowest rates of added P, mineralisable N levels were much higher under Lupinus polyphyllus than under Trifolium repens or Lotus corniculatus. With increasing P rate, levels under the latter species increased, and at 100 kg P ha^-1 were equivalent to those under the lupin with no added P.     

Elevated CO2 and the mineral content of herbaceous and woody plants

The CO₂ enrichment effects (300–650 µmol mol^-1) on mineral concentration (N, P, K, Ca, Mg, Mn, Fe, Zn), absolute total mineral contents per individual and of whole stands of four herbaceous (Trifolium repens L.,Trifolium pratense L.,Lolium perenne L.,Festuca pratensis HUDS.) and two woody species (Acer pseudo-platanus L.,Fagus sylvatica L.) were investigated.In general, the mineral concentration of the plant tissues decreased (all six species: N>Ca>K>Mg) with the exception of P. Mn and Fe were only determined for the tree species. Both decreased in concentration (Mn>Fe). Zn was only analysed forTrifolium pratense andFestuca pratensis and decreased significantly in the grass.Despite of decreases in concentrations of as much as 20% in some cases there were increases in absolute amounts per individual and, therefore, in the whole vegetation up to 25% because of the enhanced dry matter accumulation at elevated CO₂ supply.Dedicated to Prof. Dr. R. Bornkamm, TU-Berlin, on behalf of his 60th birthday     

Benefit and cost analysis and phosphorus efficiency of VA mycorrhizal fungi colonizations with sorghum (Sorghum bicolor) genotypes grown at varied phosphorus levels

Sorghum [Sorghum bicolor (L.) Moench] was grown in a greenhouse in a low P (3.6 mg kg^-1) soil (Typic Argiudolls) inoculated with the vesicular-arbuscular mycorrhizal fungi (VMAF) Glomus fasciculatum and P added at 0, 12.5, 25.0, and 37.5 mg kg^-1 soil to determine the effects of VAMF-root associations on plant growth, benefit and cost analysis, and P efficiency (dry matter produced/unit P absorbed). Root colonization with VAMF and shoot growth enhancements decreased with increased soil P applications. Mycorrhizal plants were less P efficient than nonmycorrhizal plants. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants were considered the benefit derived by plants from VAMF-root associations. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants with similar P concentrations were considered the costs paid by plants for VAMF-root associations. Values of benefit and cost analysis for VAMF-root associations were highest when soil P was lowest and decreased with increasing P applications. Genotypic differences for calculated costs were pronounced, but not benefits. Benefit and cost analysis.may be helpful to evaluate host plant genotypes and VAMF species to optimize efficiencies of VAMF symbiosis in different soil environments.     

Growth and root dry matter allocation by pasture legumes and a grass with contrasting external critical phosphorus requirements

Background and aims

This work aimed to quantify the critical external requirement for phosphorus (P) (i.e. extractable-P concentration required for 90 % of maximum yield) for a number of temperate legume species and understand differences in dry matter allocation, P distribution and P acquisition efficiency among these species.

Methods

Shoot and root growth of five legume and one grass species was assessed in response to six rates of P mixed into the top 45 mm of soil in a pot experiment. Dactylis glomerata and Trifolium subterraneum were used as benchmark species; they are commonly grown together in mixed temperate pastures and have low and high critical external requirements for P, respectively. Growth was compared with four alternative legume species: Ornithopus compressus, Ornithopus sativus, Biserrula pelecinus and Trifolium hirtum, that have root morphologies better suited to soil exploration and nutrient acquisition than that of Trifolium subterraneum.

Results

Dactylis glomerata, Ornithopus compressus and Ornithopus sativus had maximum yields equal to or greater than Trifolium subterraneum but achieved this at rates of P less than half that of Trifolium subterraneum. Biserrula pelecinus and Trifolium hirtum had critical P requirements between that of Trifolium subterraneum and the Ornithopus species, but also had lower yields. Root dry matter of Dactylis glomerata and the Ornithopus species in the fertilised soil layer was only marginally changed in response to low P supply. In contrast, Trifolium subterraneum, Trifolium hirtum and to a lesser extent Biserrula pelecinus markedly increased root dry matter allocation to this soil layer. Species with lower critical P requirements were able to take up more P per unit root dry mass than those with higher critical P requirements, particularly at lower levels of P addition.

Conclusions

The high P acquisition efficiencies of the Ornithopus species and Dactylis glomerata were likely to have contributed to their low critical external P requirements. It was surmised that differences in root morphology traits underpin the differences in acclimation to low P stress and P acquisition efficiency among the species.

     

Interspecific and intraspecific differences in shoot and leaf lifespan of four Carex species which differ in maximum dry matter production

The effect of N supply on shoot and leaf lifespan was investigated in established stands of four herbaceous Carex species which differed in maximum dry matter production. These species were, in rank order of increasing maximum dry matter production (per unit ground area): Carex diandraC. rostrata. The observed patterns of shoot and leaf turnover were compared with data on leaf characteristics and nitrogen use efficiency indices of these species. There was no consistent difference in shoot production (number of shoots produced per unit ground area) between species with low production and those with high production: Carex diandra (low production) and C. lasiocarpa (high production) had high shoot production, while shoot production in c. rostrata (low production) and C. acutiformis (high production) was much lower. The rank order of the mean lifespan of shoots was: C. diandra. Thus, the lifespan of shoots increased with increasing maximum dry matter production of these Carex species. In all species, increased N supply led to a significant reduction in shoot lifespan. The reduction of shoot lifespans in response to enhanced N supply will result in increased nutrient turnover rates in these species. There was no consistent difference in the number of leaves produced per shoot between low-production and high-production species. C. diandra and C. lasiocarpa had relatively low leaf production, while C. rostrata and C. acutiformis had relatively high leaf production per shoot. Thus, this pattern is opposite to the pattern in shoot production. The rank order of the mean lifespan of leaves was: C. diandra. This implies that the high-production species had longer mean leaf lifespans than the low-production species. Mean leaf lifespan was not significantly affected by enhanced N supply, except in C. diandra, where leaf lifespan decreased in response to enhanced N supply. Shoot lifespans did not show any significant relation with the specific leaf area (SLA, leaf area per unit leaf mass) or the leaf area ratio (LAR, leaf area per unit plant mass) of the species under study. There was, however, a negative relation (r ²=0.71) with the nitrogen concentration in the leaves. Shoot lifespans were positively related (r ²=0.79) with whole-plant nitrogen use efficiency (NUE, dry matter production per unit N-loss) and with the mean residence time of nitrogen (MRT, the average time-span during which a unit of nitrogen is present in the plant) (r ²=0.78), but not with the nitrogen productivity (A, annual dry matter production per unit N in the plant). Leaf lifespan was positively related with the mean residence time of nitrogen in the plants (r ²–0.70). For all the other parameters, there were no significant relations with leaf lifespan. From these results we conclude that: (1) at the stand level, shoot and leaf lifespans are positively related with maximum dry matter production; and (2) shoot and leaf lifespan are important determinants of whole-plant nitrogen economy.     

Defining phosphorus efficiency in plants

The many different definitions for "nutrient efficiency" make the use of the term ambiguous. We evaluated nutrient efficiency using data from a study of response to phosphorus (P) supply in white clover (Trifolium repens L.) and lucerne (Medicago sativa L.). Application of various criteria identified in the literature as measures of nutrient efficiency did not clarify differences between purportedly P efficient and inefficient germplasm. Germplasm differed in maximum shoot and total dry mass and in solution P concentration ([P]_s) required to achieve 80% maximum yield, but not in P concentration of tissue ([P]_t), internal P utilization, or P uptake per unit of fine root dry mass. Differences in yield may have resulted from factors other than efficient use of P. To reduce the confounding effects that other factors have on nutrient efficiency, it is essential that equivalent yields of germplasm be demonstrated where nutrients are not limiting. Mechanisms that enable enhanced nutrient efficiency can be identified less ambiguously using this approach.Joint contribution of the Minn. Agric. Exp. Stn. and the USDA-ARS     

Mycorrhizal responses of barley cultivars differing in P efficiency

The purpose of this study was to investigate how barley cultivars which are different in dry matter yield at low phosphorus (P) supply (i.e. they differ in agronomic P efficiency) respond to mycorrhizal infection. In a preliminary experiment, six mycorrhizal fungi were tested for their ability to colonize barley (Hordeum vulgare L.) roots at a soil temperature of 15°C.Glomus etunicatum was the most effective species and was used in the main experiment. The main experiment was conducted under glasshouse conditions in which soil temperature was maintained at 15°C. Treatments consisted of a factorial arrangement of 8 barley cultivars, 2 mycorrhiza (inoculated and non-inoculated), and 3 rates of P (0, 10 and 20 mg kg^-1). P utilization efficiency (dry matter yield per unit of P taken up) and agronomic P efficiency among the barley cultivars was significantly negatively correlated with mycorrhizal responses. However, the response to mycorrhizal infection was positively correlated with response to P application. Poor correlation was observed between P concentration when neither mycorrhiza nor P were supplied and the percentage of root length infected. The extent of mycorrhizal infection among the barley cultivars in soil without P amendment varied from 8.6 to 28.6%. Significant interactions between cultivar and P addition, and between mycorrhiza and P addition were observed for shoot dry weight but not root dry weight.     

High altitude acclimatization in fourArtemisia species: Changes in free amino acids and nitrogen contents in leaves

During high altitude acclimatization the highest number of amino acids were found in temperateArtemisia species (A. vestita) and the lowest one in tropical species (A. scoparia). The amount of free amino acids in temperateArtemisia species was higher when this was grown at 3600 m altitude. InA. scoparia, A. vulnaris andA. parviflora, the higher amount of individual amino acids was ascertained in plants grown at lower altitudes. InA. vestita, the nitrogen contents per unit dry matter was lower in plants grown at high altitude. In the other three species, the contents were significantly higher in the leaves of plants grown at a higher altitude. The nitrogen contents per unit leaf dry matter determined during active growth of plants were minimum inA. parviflora and maximum inA. vestita. With progressing plant senescence the concentration of nitrogen decreased in all species studied.     

Dry matter response of ryegrass to ammonium and nitrate sources of nitrogen as a function of soil texture and moisture

Summary InLolium perenne cv. S23 grown in pots of soil to which nitrogen was applied at rates of 0, 60, 120 and 180 mg N/kg, ammonium significantly increased root mass compared with nitrate, whereas both nitrogen sources produced similar yields of herbage. Total dry matter production in the herbage and roots increased within the textural sequence: loamy sand, coarse sandy loam and loam. The increases in the latter two soils over the loamy sand were 35% and 50%, respectively. Depletions of available water of 50% and 80% reduced yields at the first harvest by 19% and 50% compared with the field capacity. The corresponding reductions in the second harvest were only 16% and 10%, because residual effects were then limiting. The amount of dry matter produced per unit of nitrogen absorbed indicated that textural class was less restrictive than moisture supply to efficient use of nitrogen within the plant. Furthermore, its efficient physiological use was limited at the 50% depletion, although its absorption from the soil was not affected. The results illustrated the inefficiency of the substitution of nitrogen for soil moisture: even at low yield levels three times more nitrogen was absorbed per unit increase in dry matter at 80% depletion of available water compared with the field capacity. The significant first-order interactions that affected yield were moisture by soils, and moisture by nitrogen rate.     

Light use efficiency of dry matter gain in five macro-lichens: relative impact of microclimate conditions and species-specific traits

Relations between irradiance (I) and lichen growth were investigated for five macro‐lichens growing at two sites in Sweden. The lichens represented different mycobiont–photobiont associations, two morphologies (foliose, fruticose) and two life forms (epiphytic, terricolous). The lichens were transplanted at two geographically distant sites in Sweden (1000 km apart) from Sept 1995 to Sept 1996 in their typical microhabitats, where microclimate and growth were followed. Between April/May and Sept 96, the terricolous species had a dry matter gain of 0·2 to 0·4 g (g DW)^–1 and the epiphytes 0·01 to 0·02 g (g DW)^–1. When related to area, growth amounted to 30 to 70 g m^?2 for the terricolous species and to 1 to 4 g m^?2 for the epiphytes. There was a strong correlation between growth and intercepted irradiance when the lichens were wet (I_wet), with 0·2 to 1·1 g lichen dry matter being produced per MJ solar energy. Across the 10 sets of transplants, light use efficiencies of dry matter yield (e) ranged between 0·5 and 2%, using an energy equivalent of 17·5 kJ g^?1 of lichen dry matter. The higher productivity of the terricolous species was due to longer periods with thallus water contents sufficient for metabolic activity and because of the higher mean photon flux densities of their microhabitat. A four‐fold difference in photosynthetic capacity among the species was also important. It is concluded that lichen dry matter gain was primarily related to net carbon gain during metabolically active periods, which was determined by light duration, photon flux density and photosynthetic capacity.     

Nutrient Supply, Dry-matter Production and Nutrient Uptake of Forest Tree Seedlings

The dry-matter production and nutrient uptake of forest-treeseedlings grown under controlled levels of nutrition in sub-irrigatedsand culture were investigated. Hardwood seedlings attainedgreater dry weight than conifer seedlings, and Pinus radiataa greater dry weight than P. contorta and P. nigra, but thesegreater dry weights were not related to greater leaf/weightratios, although seed size may have been an important influenceon final seedling dry weight. P. radiata was found to have alower requirement for P than P. contorta and P. nigra in oneexperiment. The demand (total nutrient uptake per unit of time) made bythree species for nutrients upon a site was shown to dependmore upon growth-rate per annum than on efficiency (quantityof nutrients required to produce a unit of dry matter). It isconcluded that the ability of tree species to grow satisfactorilyon soils of low nutrient status depends largely on their abilityfor relatively slow growth. On the other hand, species showinggood response to favourable sites may also make satisfactorygrowth on poor sites.     

The influence of water stress preconditioning on dark respiration

The respiration rate of individual leaves of cotton (Gossypium hirsutum L. cv. Stoneville 213), beans (Phaseolus vulgaris L. cv. Bush Blue Lake), and sorghum (Sorghum vulgare Pers.) which had been fully expanded prior to a series of severe water stresses was compared with those of unstressed leaves of similar age. Measurements were made over a range of leaf temperatures. The respiration rate per unit area of the leaves of all rewatered plants were significantly lower than those of the plants which had not undergone water stress. During the stress periods, the leaves of all species suffered dry matter loss. The respiration rates per unit dry matter for cotton and beans were the same for the plants which had undergone stress as they were for the plants which had not undergone stress, thus for these two C3 plants the decrease in dark respiration due to water stress may be explained entirely by the loss of dry matter. Respiration rates of the water stressed sorghum leaves expressed on a per unit weight basis were significantly lower than those which had not undergone water stress preconditioning. The lower respiration rates of the stressed leaves when expressed on both a per unit area basis and a per unit weight basis for the C4 species indicate that water stress adaptation of C4 plants may include alterations in the respiratory mechanism or on the amount of residual respirable substrate. The light compensation points of all the species were not altered by the water stress preconditioning. This indicates that the mechanisms controlling the net photosynthetic exchange per unit leaf area underwent adaptations as a result of repeated water stress which decreased its efficiency.     

Evaluation of tuber bearing Solanum species belonging to different ploidy levels for its yielding potential at low soil fertility

Virus free tubers from thirty-six clones of tuber bearing Solanum species belonging to different ploidy levels, from the world gene bank at the International Potato Center, were cultivated under field conditions at low and high fertilizer rates. Nine of the clones were chosen for their high yielding potential (advanced group) and twenty-seven for their extensiveness (native group). The tuber yields of the thirty-six clones at the two fertilizer levels were positively correlated and the advanced group yielded more than the native group, even under conditions of low soil fertility.A broad genetic variation in root size at both fertilizer levels was apparent. Although some native clones showed rather large root systems as a group, the advanced clones had a similar root dry weight to the native clones.Tuber dry weight was significantly correlated with root dry weight but the advanced clones had a higher tuber yield per unit of root dry matter produced.     

Morphology and response of roots of pasture species to phosphorus and nitrogen nutrition

The root morphology of ten temperate pasture species (four annual grasses, four perennial grasses and two annual dicots) was compared and their responses to P and N deficiency were characterised. Root morphologies differed markedly; some species had relatively fine and extensive root systems (Vulpia spp., Holcus lanatus L. and Lolium rigidum Gaudin), whilst others had relatively thick and small root systems (Trifolium subterraneum L. and Phalaris aquatica L.). Most species increased the proportion of dry matter allocated to the root system at low P and N, compared with that at optimal nutrient supply. Most species also decreased root diameter and increased specific root length in response to P deficiency. Only some of the species responded to N deficiency in this way. Root morphology was important for the acquisition of P, a nutrient for which supply to the plant depends on root exploration of soil and on diffusion to the root surface. Species with fine, extensive root systems had low external P requirements for maximum growth and those with thick, small root systems generally had high external P requirements. These intrinsic root characteristics were more important determinants of P requirement than changes in root morphology in response to P deficiency. Species with different N requirements could not be distinguished clearly by their root morphological attributes or their response to N deficiency, presumably because mass flow is relatively more important for N supply to roots in soil.Section editor: H. Lambers     

Shoot growth, radiation interception and dry matter production and partitioning during the establishment phase of Miscanthus sinensis'Giganteus' grown at two densities in the UK

Photosynthetic area index (PAI), radiation interception (I) and dry matter partitioning between shoots and roots were measured for Miscanthus sinensis‘Giganteus' grown from micro-propagated transplants on a fertile peaty loam soil in eastern England. In the establishment year, Miscanthus plants produced 35 and 70 shoots plant^-1 at densities of 4.0 and 1.8 plants m^-2 respectively. At the higher density, there were 140 shoots m^-2 with the largest reaching a height of 1.8 m; these canopies attained a maximum PAI of 5.45, intercepting 94% of incident radiation. Leaf lamina contributed c. 90% of total photosynthetic area with stems contributing the remainder. At the lower density, maximum PAI and I values were 2.88 and 86% respectively. PAI was related to I by calculating attenuation coefficients (k); these indicated that Miscanthus canopies were more effective at intercepting radiation per unit PAI at the lower density (k= -0.31) compared with the higher density (k= -0.20). Radiation interception was related to dry matter accumulated by calculating conversion efficiencies (e). At 4 plants m^-2, × for shoot dry matter production was 1.17g MJ^-1. Miscanthus partitioned a relatively large amount of total dry matter into below-ground biomass. By plant senescence, c. 30% of total dry matter had been partitioned into root and rhizome; rhizome biomass contributed 80% of below-ground dry matter, × increased to 1.62 g MJ^-1 when calculated on a total dry matter basis (shoot + root + rhizome). Total dry matter production was increased 68% by a 2.2-fold increase in plant density.     

Effect of soil moisture and sowing density on population growth ofEupatorium adenophorum andE. riparium

Summary Efect of sowing density on germination, establishment and growth of two perennial weeds,Eupatorium adenophorum spreng. andE. riparium Regel, was studied by sowing varying number of seeds of each species in pots. At high sowing density, seedling emergence did not proportionately increase with seed input. The yield and seed output per unit area was independent of sowing density. Survival of established plants was independent of soil moisture stress. The dry matter yield of both species declined at low moisture regime, the reduction being more inE. riparium. E. adenophorum produced more seeds at low moisture level, whileE. riparium at high moisture level. The resource allocation to roots was reduced due to moisture stress, especially inE. andenophorum.     

The influence of simulated shadelight and daylight on growth, development and photosynthesis of Pinus radiata, Agathis australis and Dacrydium cupressinum

Abstract. Juvenile seedlings, micropropagated plantlets, and adult rooted cuttings of Pinus radiata, together with seedlings of Agathis australis and Dacrydium cupressinum, were grown under either high (670/μmol m^?2 s^?1) or low (200 μmol m^?2 s^?1) photosynthetic photon flux density (PPFD) and, at low PPFD, under three red:far-red (R/FR) conditions, approximately simulating canopy shadelight, daylight and one intermediate value. In all types of P. radiata, a shade-intolerant species, stem height and diameter, stem and needle dry weight, and apical dominance were markedly increased by a reduction in R:FR ratio while fascicle density was decreased. In contrast, these increases were considerably less for the shade-tolerant D. cupressinum and moderately tolerant A. australis. With the exception of the P. radiata seedlings, height growth was greatest in all species under high compared to low PPFD for daylight R:FR ratio conditions. Total shoot (or plant) dry weight was increased approximately two-fold in all species and types by the higher PPFD. Shoot extension rates were negatively correlated with calculated phytochrome photoequilibrium with P. radiata, the most shade-intolerant species, having the highest change in rate per unit change in photoequilibrium (i.e. very responsive), and D. cupressinum, the most shade tolerant species evaluated, having the smallest change (i.e. largely unresponsive). Within the spectral quality treatments at low PPFD, it is suggested that higher rates of dry matter accumulation under the low R:FR ratio were the result of reduced mutual shading of adjacent leaves as a consequence of photomorphogenically-controlled internode lengths rather than of enhanced photosynthesis per se. The significance of the results is discussed in relation to planting stock management in nurseries, the management of forest canopies for understorey seedling growth, and to the construction of representative growth simulation models. Consequences for controlled environment lighting are also considered.     

Growth response and phosphorus uptake of rye with long and short root hairs: Interactions with mycorrhizal infection

Plant growth and phosphorus (P) uptake of two selections of rye (Secale cereale L.) differing in length of root hairs, in response to mycorrhizal infection were investigated. Rye plants with short root hairs (SRH) had a greater length of root infected by Glomus intraradices (up to 32 m pot^–1) than those with long root hairs (LRH) (up to 10 m pot^–1). Application of P decreased the percentage of root length infected in both selections. In low-P soil, mycorrhizal infection increased shoot and root P concentration, especially in LRH plants. Generally, LRH had higher shoot dry weight than SRH plants. P uptake was increased both by LRH and by mycorrhizal infection. Differences in specific P uptake and P utilization efficiency between SRH and LRH plants were observed in non-mycorrhizal plants. With low P supply, P utilization efficiency (dry matter yield per unit of P taken up) of LRH plants increased with time. However, mycorrhizal infection reduced P utilization efficiency, particularly of SRH plants. SRH plants, which were agronomically less efficient (i.e. low dry matter yield at low P supply) were more responsive to either mycorrhizal infection or P addition than the LRH plants. No interaction was observed between mycorrhizal infection and root hair length.     

The Relationship between the critical Leaf Area, Total Chlorophyll Content, and Maximum Growth-rate of some Pasture and Crop Planst

The amounts of leaf and of chlorophyll per unit area of landexposed to light above 400–500 f.c. on sunny days at localnoon were determined for pure stands of red clover, white clover,perennial ryegrass, short-rotation ryegrass, maize, kale, cocksfoot,and watercress, growing under near-optimum conditions. The valuesobtained for the grass species and maize were significantlyhigher than those of the dicotyledon species There was a highly significant correlation (r = + 0?912) betweenthe maximum growth-rates¹ of the different species and the amountsof chlorophyll above the measurement level per unit area ofland. However, the index was higher for species in which the leaves were disposed horizontally or whereflagging occurred (clovers, kale, and maize) than for the grassspecies, suggesting either that the chlorophyll of the firstgroup of species was more active or efficient in photosynthesisthan the chlorophyll in the species with erect leaves, or thata greater proportion of dry matter produced was accumulatedin underground organs of these erect-leaved species. Growth-ratedata were not available for cocksfoot or watercress. There was a significant correlation (r = + 0?815) between theleaf-area index absorbing 95 per cent. of the light and themaximum growth-rates of the different species, but this relationshipdid not take account of the photosynthetic activity of otherparts of the plant or of species differences in chlorophyllcontent. The results are discussed in relation to the availability andutilization of light energy and carbon dioxide by the speciesexamined.     

Alfalfa water status and cold hardiness as influenced by cold acclimation and water stress

Abstract Water stress at a nonacclimating temperature (18–20°C) increased the cold hardiness of Medicagosativa L. (alfalfa) plants. This increased cold hardiness was retained when the previously water-stressed plants were cold acclimated (2–9°C) in the absence of water stress. Water stress during cold acclimation also increased cold hardiness. Alfalfa was demonstrated to suffer injury, measured as decreased growth following freezing, at sub-lethal temperatures. During cold acclimation the turgor potential (ψ) of watered plants increased, whereas the solute potential and the water content per unit dry weight decreased. The large positive psgrdap of acclimated plants indicates that the decreased water content per unit dry weight is related to an increased proportion of tissue dry matter rather than to tissue dehydration.