The patterns of growth, assimilation of 14CO2 and distribution of 14C-assimilate within vegetative plants of Loliumperenne at low and high density

The patterns of growth, assimilation of ¹⁴CO₂ and distribution of ¹⁴C-labelled assimilate were followed for 12 wk from sowing in individual plants of Lolium perenne grown in miniswards at either low (500 plants m^-2) or high (5000 plants m^-2) density. At the latter density, plants were characterised by a 50% reduction in RGR, by the production of fewer tillers, especially second- and third-order tillers, and by a reduction in mean tiller weight. All the green and senescing leaves of each tiller assimilated ¹⁴CO₂ and the overall assimilatory capacity of a tiller was directly related to its dry weight. At both densities the plant consisted of a main shoot and established tillers with comparable assimilatory activities and a range of developing tillers that assimilated relatively small amounts of ¹⁴CO₂. As each successive primary tiller developed it was supplied with assimilate from the main shoot and the degree of support was inversely proportional to the dry weight of the tiller. At both densities it was concluded that the first primary tiller could be regarded as an independent assimilatory unit when it reached a dry weight of about 25 mg even though some import of main shoot assimilate continued until the tiller was double this weight. The supply of assimilate to the root system was greatly reduced at both densities compared with previous observations on plants grown singly.     

Assimilation of 14CO2, Assimilate Translocation and Grain Yield in Three Primitive Nepalese Varieties and a European Cultivar (Senta) of Six-row Barley (Hordeum vulgare L.)

Three primitive varieties of six-row barley from Nepal withdifferent degrees of awn development were compared with a bredEuropean cultivar, Senta, under glasshouse conditions, fromanthesis onwards. Apart from the flag leaf the last three mainshoot leaves of the Nepalese varieties were smaller than thoseof Senta, and the lower leaves did not become senescent duringthe first 3 weeks of grain filling. In all varieties the penultimateleaf was the major assimilatory organ but in Senta the relativecontribution of the awns increased with time as the leaves senescedand fixed almost half of the total ¹⁴C assimilated by the shoot.Whereas the supply of assimilate to the ear increased with timein Senta, the reverse was found for the Nepalese varieties andoverall a smaller proportion of assimilate was supplied to theear in these varieties. The grain yield of the main shoot earof the two Nepalese varieties studied in detail was one-thirdof that of Senta; there were only approximately half the numberof grains in each median and lateral row of the ear and thegrains were smaller. There was no correlation between the degreeof awn development or the pattern of awn arrangement withinthe ear and the size of individual grains. The total biomassof the Nepalese varieties was much lower than that of Sentabut there were no differences in the harvest index or in tillerproduction. However, the Nepalese varieties, in contrast toSenta, had a mean grain yield per tiller comparable to thatof their main shoot and thus it appears that selection by plantbreeding may have increased the dominance of the main shoot. Hordeum vulgare L., barley, primitive barley varieties, carbon dioxide assimilation, assimilate translocation, grain filling     

Yellowing and photosynthetic decline of barley primary leaves in response to atmospheric CO2 enrichment

The photosynthetic response of barley (Hordeum vulgare L. cv. Brant) primary leaves was studied as a function of chlorosis induced by CO₂ enrichment. Leaf yellowing, measured as changes of chlorophyll a and b, was more extensive in controlled environments at elevated (680 ± 17 µl l^?1) than at ambient (380 ± 21 µl l^?1) CO₂. Stomatal conductance of primary leaves was decreased by growth in elevated CO₂ between 11 and 18 days after sowing (DAS) when measured at both 380 and 680 µl l^?1 CO₂. Internal leaf CO₂ concentration (C_i) was also lower for elevated- compared to ambient-CO₂-grown primary leaves between 11 and 14 DAS. Results suggest that non-stomatal factors were responsible for the decreased photosynthetic rates of elevated- compared to ambient-CO₂-grown primary leaves 18 DAS. Various photochemical measurements, including quantum absorptance (α), minimal (F₀), maximal (F_m), and variable (F_v) chlorophyll fluorescence, as well as the F_v/F_m ratio, were significantly decreased 18 DAS in the elevated- compared to ambient-CO₂ treatment. Photochemical (qP) and nonphotochemical (qN) chlorophyll fluorescence quenching coefficients of 18-day-old primary leaves did not differ between CO₂ treatments. Photosynthetic electron transport rates of photosystem II were slightly lower for elevated- compared to ambient-CO₂-grown primary leaves 18 DAS. Concentrations of α-amino N (i.e. free amino acids) in barley primary leaves were increased by CO₂ enrichment 10 DAS, but subsequently, α-amino N decreased in association with photosynthetic decline. Total acid protease activity was greater in elevated- than in ambient-CO₂-grown leaves 18 DAS. The above findings suggest that photoinhibition and premature senescence were factors in the CO₂-dependent yellowing of barley primary leaves.     

Responses of nitrogen metabolism in N-sufficient barley primary leaves to plant growth in elevated atmospheric carbon dioxide

Effects of atmospheric carbon dioxide enrichment on nitrogen metabolism were studied in barley primary leaves (Hordeum vulgare L. cv. Brant). Seedlings were grown in chambers under ambient (36 Pa) and elevated (100 Pa) carbon dioxide and were fertilized daily with complete nutrient solution providing 12 millimolar nitrate and 2.5 millimolar ammonium. Foliar nitrate and ammonium were 27% and 42% lower (P ≤ 0.01) in the elevated compared to ambient carbon dioxide treatments, respectively. Enhanced carbon dioxide affected leaf ammonium levels by inhibiting photorespiration. Diurnal variations of total nitrate were not observed in either treatment. Total and Mg²⁺inhibited nitrate reductase activities per gram fresh weight were slightly lower (P ≤ 0.01) in enhanced compared to ambient carbon dioxide between 8 and 15 DAS. Diurnal variations of total nitrate reductase activity in barley primary leaves were similar in either treatment except between 7 and 10 h of the photoperiod when enzyme activities were decreased (P ≤ 0.05) by carbon dioxide enrichment. Glutamate was similar and glutamine levels were increased by carbon dioxide enrichment between 8 and 13 DAS. However, both glutamate and glutamine were negatively impacted by elevated carbon dioxide when leaf yellowing was observed 15 and 17 DAS. The above findings showed that carbon dioxide enrichment produced only slight modifications in leaf nitrogen metabolism and that the chlorosis of barley primary leaves observed under enhanced carbon dioxide was probably not attributable to a nutritionally induced nitrogen limitation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Estimation of N2-fixation by sugarcane,Saccharum sp., and soybean,Glycine max,grown in soil with15N-labelled organic matter

Summary Two varieties of sugarcane, and nodulated and non-nodulated soybean isolines, were planted in a soil previously mixed with¹⁵N-labelled plant material. 45 days was allowed to elapse before planting, to permit initiation of organic matter mineralization. Plants were grown for 60 days, then harvested, dried, weighed and analysed for total N. Analysis of soil samples pre-incubated in the laboratory was carried out to evaluate ammonium and nitrate from added organic matter. Dry weights of the soybean isolines were similar, but total N was higher for the nodulated line. Both sugarcane varieties showed similar weight and total N. Nitrogen derived from applied organic matter (NdfOM) was higher in non-nodulated soybean than in all other plants. Although there is the possibility of different¹⁵N availabilities between species, nitrogen derived from fixation (Nfix) was calculated based on the¹⁵N enrichment of the non-nodulating soybean. Nfix was 72% for nodulating soybean and ranged from 19 to 39% for different parts of sugarcane plants, despite high levels of available-N. Nitrogen derived from soil was calculated by difference. NdfOM was lower in roots than in upper parts (leaves+stalks) of plants. Use of¹⁵N labelled organic matter seems a useful approach to the longer term measurement of N₂-fixation.IAEA Project BRA/5/009-CENA.     

An investigation into the relations between some growth parameters and yield of barley

Twenty-five field experiments on barley were done at four sites from 1970 to 1975, with an additional site in 1975, comparing five rates of nitrogen application. The crops were sampled at c. 14-day intervals from about the four-leaf stage to maturity to follow changes with time in the dry weights of the constituent plant parts and to measure fertile tiller number, grain number per ear and grain weight. Some data have been selected from this body of information and regression analysis has been used to assess the possible importance of pre- and post-anthesis growth in determining the yield of barley. Ear emergence was taken as an approximate indicator of anthesis. Yield was closely related (r=+0–96) and almost equal to the amount of dry matter accumulated before ear emergence for twenty-one of the experiments, with a regression coefficient of 0–95. In two of the remaining experiments the regression coefficient was greater, 1–14; and smaller in the other two, o-8i. Plant population is also shown to affect the regression coefficient. Despite these anomalies grain yield was more closely related to plant dry weight at ear emergence than to the increase in weight after ear emergence (r =+0–79). The increase in plant dry weight after ear emergence was frequently less than the yield of grain, suggesting that the plant can compensate for inadequate photosynthesis during grain filling. The result of this compensation is a reduction in straw dry weight. This implies that previously assimilated dry matter is transported to the grain, or that respiratory losses from the straw are not replaced by current photosynthesis. These observations suggest that yield may be limited by sink capacity rather than by photosynthesis after ear emergence. The main component of sink capacity, grain number per m² was closely related to yield (r =+0–95) and is known to be determined at or before anthesis. Grain size was shown to be related to grain number per ear (r= 4- 0–99), suggesting that grain size is also, at least partially, determined at ear emergence. Therefore, all the yield components are determined, wholly or partly before ear emergence.     

Intra-specific variation in growth and P accumulation of Leucaena leucocephala and Gliricidia sepium as influenced by soil phosphate status

Twenty-three provenances of Gliricidia sepium and eleven isolines of Leucaena leucocephala were examined at a low and at high phosphate levels (20 and 80 mg P kg^-1 soil) for growth, phosphate (P) uptake and use efficiency. Large differences in growth at the low P level, and in growth response to the higher P rate occurred among L. leucocephala isolines and G. sepium provenances. Shoot dry weight at low P varied from 1.30 to 3.01 g plant^-1 for L. leucocephala and from 1.44 to 3.07 g plant^-1 for G. sepium.Leucaena isolines had only half the root weight of G. sepium provenances yet produced approximately 90% of the shoot weight of the corresponding G. sepium treatments, i.e. more than a 2-fold difference in root/shoot ratios. Total P in shoots of G. sepium was some 85% greater than of the respective L. leucocephala isolines in corresponding treatments. Physiological phosphate use efficiency (g shoot/mg P in shoots) (PPUE) was not a simple reciprocal relation, being markedly lower at higher shoot % P and content. However, for the same shoot P both species produced the same shoot weight. Nevertheless, there were apparent genotypic differences within species in the root development, shoot P and PPUE.In another study, the numbers of rhizobia in the rhizosphere of L. leucocephala, nodulation, N₂ fixation at five different levels of P were determined. The numbers of rhizobia in the rhizosphere of inoculated L. leucocephala during the first two weeks were lower when P was added but later became similar to those without added P. Nodules formed earlier than inoculated plants fertilized with P and in greater numbers (4- to 5-fold) and dry weights than in those without P. However, the percentage of N₂ derived from fixation did not change with increasing levels of P application. These results suggest that the observed P effect did not operate via stimulated growth of rhizobia in the rhizosphere, nor through increased N₂ fixation rate. The major effect appeared to be due to effects via plant growth.     

Lack of C4 photosynthetic metabolism in ears of C3 cereals

There is continuing controversy over whether a degree of C₄ photosynthetic metabolism exists in ears of C₃ cereals. In this context, CO₂ exchange and the initial products of photosynthesis were examined in flag leaf blades and various ear parts of two durum wheat (Triticum durum Desf.) and two six-rowed barley (Hordeum vulgare L.) cultivars. Three weeks after anthesis, the CO₂ compensation concentration at 210 mmol mol^?1 O₂ in durum wheat and barley ear parts was similar to or greater than that in flag leaves. The O₂ dependence of the CO₂ compensation concentration in durum wheat ear parts, as well as in the flag leaf blade, was linear, as expected for C₃ photosynthesis. In a complementary experiment, intact and attached ears and flag leaf blades of barley and durum wheat were radio-labelled with ¹⁴CO₂ during a 10s pulse, and the initial products of fixation were studied in various parts of the ears (awns, glumes, inner bracts and grains) and in the flag leaf blade. All tissues assimilated CO₂ mainly by the Calvin (C₃) cycle, with little fixation of ¹⁴CO₂ into the C₄ acids malate and aspartate (about 10% or less). These collective data support the conclusion that in the ear parts of these C₃ cereals C₄ photosynthetic metabolism is nil.     

Effect of irradiance during growth of Glycine max on photosynthetic capacity and percent activation of ribulose 1,5-bisphosphate carboxylase

The initial (in vivo) and total (activity present after preincubation with CO₂ and Mg²⁺) activities of ribulose bisphosphate carboxylase were both assayed in extracts of leaves of soybean (Glycine max) plants which had been grown under 4 different irradiance levels. The total carboxylase activity per unit leaf area decreased with decreased irradiance during growth but was not different on a dry weight basis. The initial activity as a percentage of the total activity was unchanged (approximately 95%) except in leaves of plants grown at the lowest irradiance (74%). When the plants grown at the lowest irradiance were exposed to high irradiance, the initial activity was increased to 93% of the total. Light saturated rates of photosynthesis per unit leaf area were lower and saturated at lower irradiance for plants grown at lower irradiances. Initial carboxylase activity was correlated closely (r²=0.84) with leaf photosynthesis rate on a dry weight basis.     

Trends in Biomass Fractionation in Wheat and Barley from Wild Ancestors to Modern Cultivars

Abstract: We tested the hypothesis that cultivar selection during the process of domestication in cereal plants led to a change in dry mass allocation, e.g., less root mass and more leaf mass or more leaf area per unit leaf mass. We divided 24 varieties of diploid, tetraploid and hexaploid winter wheat and two-rowed winter barley into three categories of domestication levels (wild species, old landraces and modern cultivars) and compared the patterns of dry matter fractionation at the time of anthesis under standardized outdoor growth conditions. In both cereals, total biomass per individual increased significantly with domestication level but, to our surprise, we found no significant change in dry matter investment between domestication levels: neither the dry mass fraction of leaves increased, nor was there a trend of reduced investment in stems and roots, contrary to what we expected. Specific leaf area (SLA) and leaf area ratio (LAR) of modern wheat and barley cultivars were significantly lower compared to wild varieties. Major differences in both cereals were of a purely morphological nature, namely a decrease in the number of stems and ears from wild species to domesticated varieties, along with more synchronous tiller development and therefore similar tiller size. Fertilizer increased total biomass in all domestication levels in both cereals, but influenced the dry matter fractionation only in barley. Tissue nitrogen concentration was unresponsive to both domestication and fertilization. The expected shift in functional traits, conventionally considered to determine plant growth, was not found. Indeed, dry matter fractionation among the major plant biomass components seems to be very conservative.     

Relationships between early vigour, grain yield, leaf structure and stable isotope composition in field grown barley

Fast growth and early development in barley are used in breeding programmes to improve the water use efficiency and transpiration efficiency of this crop in Mediterranean conditions. Here, we examine the use of several simple traits based on the structure and stable isotope composition of seedling leaves to assess differences in early vigour, phenology and grain yield, and also the interaction with low temperatures in barley. A set of 260 F₈ lines of two-row barley (Hordeum vulgare L.) derived from the cross of Tadmor and WI 2291 were cultivated in two locations in northwest Syria. Total chlorophyll content on an area basis (SPAD) and specific leaf dry weight (SLDW) were measured in recently fully expanded intact leaves of seedlings. Total leaf area and total dry weight per seedling were evaluated in the same seedlings. The stable isotope compositions of carbon and nitrogen (δ¹³C and δ¹⁵N, respectively) were analyzed in the same leaves on a subset of 75 genotypes. Number of days from planting to heading and grain yield were recorded at both sites. The grain yield measured at both locations was positively correlated with the SPAD value of seedlings, but showed no relationship with SLDW. Days to heading was negatively correlated with SPAD values. Regarding early vigour, a negative relationship between the SLDW and the total leaf area of seedlings was observed. However, no relationship between the δ¹³C of seedlings and early vigour was observed, except when only the genotypes most resistant to low temperatures (i.e. showing the highest SPAD values) were considered. This subset of genotypes showed negative relationships between δ¹³C and either total leaf area or total dry weight. In addition, δ¹⁵N was negatively correlated with SPAD only within the high-SPAD genotypes. This suggests that within the genotypes resistant to low temperatures, those with higher chlorophyll content assimilate more nitrogen from nitrate.     

Efficacy of Exserohilum longirostratum on barnyard grass (Echinochloa crus-galli spp. crusgalli) under field conditions

An isolate of Exserohilum longirostratum obtained from Rottboelia cochinchinensis in Malaysia was highly pathogenic to barnyardgrass (Echinocloa crus-galli,) a serious weed in rice fields in Malaysia. In glasshouse trials, high levels of barnyardgrass control were achieved when E. longirostratum was applied as a conidial concentration at 10⁵ conidia/mL. This conidial concentration may not be sufficient for field use; therefore, a mini plot trial was carried out to assess the bioherbicidal potential of the fungus and the efficacy of different inoculum types under field conditions. Excellent barnyard grass disease as indicated by Area Under Disease Progress Curve (AUDPC) was achieved with mycelium (AUDPC = 583.8 unit²); mycelium + pretichlaclor (AUDPC = 610.4 unit²) and conidia alone (AUDPC = 468.3 unit²) compared to conidia + pretichlaclor (AUDPC = 395.8 unit²). Although conidia caused lower disease severity on barnyard grass compared to mycelium, this inoculum equally reduced the competition from barnyard grass as indicated by lower mean dry weight and fewer barnyard grass tiller numbers. Positive correlations between AUDPC of rice and its tiller number and AUDPC of rice and its dry weight were recorded while negative correlations were observed for the AUDPC of barnyard grass and its tiller number and AUDPC of barnyard grass and its dry weight. These findings confirmed that E. longirostratum applied either in the form of conidia or mycelium was effective against barnyard grass under field conditions.     

甜叶菊同源四倍体离体诱导及鉴定

用不同浓度秋水仙素溶液处理甜叶菊不定芽,诱导同源四倍体,并进行解剖学、染色体鉴定和流式细胞仪鉴定倍性。结果表明:(1)用0.20%的秋水仙素溶液浸泡甜叶菊不定芽12h,同源四倍体诱导率最高,可达32.14%。(2)同源四倍体植株与二倍体(对照)相比,其气孔、叶片等均表现巨大性,且叶片变厚、叶色浓绿、叶片皱缩。(3)对照植株染色体2n=2x=22,四倍体植株染色体2n=4x=44;流式细胞仪倍性鉴定结果显示,对照DNA相对含量为100,四倍体DNA相对含量为200。(4)该研究共鉴定出48株甜叶菊同源四倍体植株,为进行倍性植株的诱导奠定了技术基础,为进一步开展甜叶菊同源四倍体新品种的选育提供了实验材料。     

Productivity of Agave deserti: measurement by dry weight and monthly prediction using physiological responses to environmental parameters

Summary An environmental productivity index based on physiological responses to three environmental variables was used to predict the net productivity of a common succulent perennial of the Sonoran Desert, Agave deserti, on a monthly basis. Productivity was also independently measured in the field from dry weight changes. The index was based on soil water availability, day/night air temperatures, and photosynthetically active radiation (PAR), which were individually varied in the laboratory and the effect on net CO₂ uptake by the leaves determined. From monthly precipitation, temperature, and PAR at the field site together with the responses measured in the laboratory, an index (maximum value of unity) was assigned to each of these three environmental variables and their product was termed the environmental productivity index. This index indicates the fraction of maximal CO₂ uptake expected in the field for each month (well-watered A. deserti assimilated 285 mmol CO₂ m^-2 leaf area day^-1 at PAR saturation and optimal day/night temperatures of 25° C/15° C). The dry weight analysis was based on the monthly unfolding of new leaves from the central spike of the rosette and their seasonal increase in dry weight, which were determined in the field. The production of new leaves was highly correlated with the environmental productivity index (r²=0.93), which in turn was highly correlated with the water status index (r²=0.97). After correction for respiration by folded leaves, stem, and roots, plant productivity predicted by the average environmental productivity index (0.36) over a wet June-to-October period agreed within 4% with the productivity based on the conventional dry weight analysis. The net productivity of A. deserti over this 5-month period was 0.57 kg m^-2 ground area (5.7 Mg ha^-1), a large value for a desert CAM plant. The environmental productivity index proposed here may provide a reliable means for predicting net productivity on a monthly basis, which may be particularly useful for species in relatively variable environments such as deserts.     

Variation in monoculture and in mixture for grain yield and other characters in spring barley

Fourteen north-west European spring barley cultivars were grown alone or in binary mixtures sampled according to a partial diallel scheme. On the basis of the association between cultivars in mixture and monoculture, three groups of characters were distinguished. Group A characters, plant height, ear weight/tiller, grain yield/tiller, number of grains/tiller and 1000-grain weight, showed strong positive associations between performance in monoculture and mixture. Group B characters, number of tillers/plant and harvest index showed incomplete positive associations, while for group C characters, dry matter/plant, ear weight/plant, grain yield/plant and number of grains/plant, associations were weak or non-existent. Compound characters in group C showed less genetic variation in monoculture and lower general competitive effects in mixture than component characters in groups A or B. These results clearly indicate that while selection for grain yield and other characters on a per plant basis (group C characters) is confounded by intergenotypic competition, characters such as the yield components number of grains/tiller and 1000-grain weight (group A characters) are hardly affected in this range of cultivars. Selection for opposing group A characters may start in the F₂ generation, while any selection for group B and C characters should be delayed until later generations. The merits of indirect selection for yield using visual assessment of yield components are discussed. Separate analyses obtained by the inclusion of spring wheat cv. Timmo in monoculture and in the set of mixtures indicated that the use of spring wheat plants to minimise intergenotypic competition ranges from superfluous (group A characters) to useless (group C characters). A large degree of mixture advantage and the lack of complementary dominance and suppression between competitor and associate was attributed to the relatively low density of plants in the experiment which, though suitable for single plant selection, is not typical of normal seed rates for cropping.     

Effects of irradiance on growth,photosynthesis, and water use efficiency of seedlings of the chaparral shrub,Ceanothus megacarpus

Summary The effects of irradiance during growth on biomass allocation, growth rates, leaf chlorophyll and protein contents, and on gas exchange responses to irradiance and CO₂ partial pressures of the evergreen, sclerophyllous, chaparral shrub, Ceanothus megacarpus were determined. Plants were grown at 4 irradiances for the growth experiments, 8, 17, 25, 41 nE cm^-2 sec^-1, and at 2 irradiances, 9 and 50 nE cm^-2 sec^-1, for the other comparisons.At higher irradiances root/shoot ratios were somewhat greater and specific leaf weights were much greater, while leaf area ratios were much lower and leaf weight ratios were slightly lower than at lower irradiances. Relative growth rates increased with increasing irradiance up to 25 nE cm^-2 sec^-1 and then leveled off, while unit leaf area rates increased steeply and unit leaf weight rates increased more gradually up to the highest growth irradiance.Leaves grown at 9 nE cm^-2 sec^-1 had less total chlorophyll per unit leaf area and more per unit leaf weight than those grown at 50 nE cm^-2 sec^-1. In a reverse of what is commonly found, low irradiance grown leaves had significantly higher chlorophyll a/b than high irradiance grown leaves. High irradiance grown leaves had much more total soluble protein per unit leaf area and per unit dry weight, and they had much higher soluble protein/chlorophyll than low irradiance grown leaves.High irradiance grown leaves had higher rates of respiration in very dim light, required higher irradiances for photosynthetic saturation and had higher irradiance saturated rates of photosynthesis than low irradiance grown leaves. CO₂ compensation irradiances for leaves of both treatments were very low, <5 nE cm^-2 sec^-1. Leaves grown under low and those grown under high irradiances reached 95% of their saturated photosynthetic rates at 65 and 85 nE cm^-2 sec^-1, respectively. Irradiance saturated rates of photosynthesis were high compared to other chaparral shrubs, 1.3 for low and 1.9 nmol CO₂ cm^-2 sec^-1 for high irradiance grown leaves. A very unusual finding was that leaf conductances to H₂O were significantly lower in the high irradiance grown leaves than in the low irradiance grown leaves. This, plus the differences in photosynthetic rates, resulted in higher water use efficiencies by the high irradiance grown leaves. High irradiance grown leaves had higher rates of photosynthesis at any particular intercellular CO₂ partial pressure and also responded more steeply to increasing CO₂ partial pressure than did low irradiance grown leaves. Leaves from both treatments showed reduced photosynthetic capability after being subjected to low CO₂ partial pressures (100 bars) under high irradiances. This treatment was more detrimental to leaves grown under low irradiances.The ecological implications of these findings are discussed in terms of chaparral shrub community structure. We suggest that light availability may be an important determinant of chaparral community structure through its effects on water use efficiencies rather than on net carbon gain.     

Utilization of Ammonium as a Nitrogen Source : Effects of Ambient Acidity on Growth and Nitrogen Accumulation by Soybean

Dry matter accumulation of plants utilizing NH₄⁺ as the sole nitrogen source generally is less than that of plants receiving NO₃⁻ unless acidity of the root-zone is controlled at a pH of about 6.0. To test the hypothesis that the reduction in growth is a consequence of nitrogen stress within the plant in response to effects of increased acidity during uptake of NH₄⁺ by roots, nonnodulated soybean plants (Glycine max [L.] Merr. cv Ransom) were grown for 24 days in flowing nutrient culture containing 1.0 millimolar NH₄⁺ as the nitrogen source. Acidities of the culture solutions were controlled at pH 6.1, 5.1, and 4.1 ± 0.1 by automatic additions of 0.01 n H₂SO₄ or Ca(OH)₂. Plants were sampled at intervals of 3 to 4 days for determination of dry matter and nitrogen accumulation. Rates of NH₄⁺ uptake per gram root dry weight were calculated from these data. Net CO₂ exchange rates per unit leaf area were measured on attached leaves by infrared gas analysis. When acidity of the culture solution was increased from pH 6.1 to 5.1, dry matter and nitrogen accumulation were reduced by about 40% within 14 days. Net CO₂ exchange rates per unit leaf area, however, were not affected, and the decreased growth was associated with a reduction in rates of appearance and expansion of new leaves. The uptake rates of NH₄⁺ per gram root were about 25% lower throughout the 24 days at pH 5.1 than at 6.1. A further increase in solution acidity from pH 5.1 to 4.1 resulted in cessation of net dry matter production and appearance of new leaves within 10 days. Net CO₂ exchange rates per unit leaf area declined rapidly until all viable leaves had abscised by 18 days. Uptake rates of NH₄⁺, which were initially about 50% lower at pH 4.1 than at 6.1, continued to decline with time of exposure until net uptake ceased at 10 days. Since these responses also are characteristic of the sequence of responses that occur during onset and progression of a nitrogen stress, they corroborate our hypothesis.     

CO2 transfer conductance, leaf structure and carbon isotope composition of Polygonum cuspidatum leaves from low and high altitudes

Anatomy and some physiological characteristics of the leaves in Polygonum cuspidatum Sieb. et Zucc., a dioecious clonal herb, were compared between two populations, one from a lowland in Shizuoka City (10 m above sea level), and another from a highland on Mt. Fuji (2500 m above sea level). Leaf mass per area (LMA) of the highland plants was about twice that of the lowland plants. The greater leaf thickness, thicker mesophyll cell walls and higher mesophyll cell density in the highland leaves contributed to the larger LMA. Although mesophyll area exposed to intercellular airspaces was greater in the highland leaves than in the lowland leaves by 30%, the surface area of chloroplasts facing intercellular airspaces was similar between these leaves. CO₂ transfer conductance inside the leaf (g_i) of the highland leaves (0·75 μmol m^?2 s^?1 Pa^?1) is the lowest recorded for herbaceous plants and was only 40% of that in the lowland leaves. On the other hand, the difference in stomatal conductance was small. δ¹³C values in the leaf dry matter were greater in the highland leaves by 4‰. These data and the estimation of CO₂ partial pressures in the intercellular air spaces and in the chloroplast suggested that the greater dry matter δ¹³C in the highland leaves, indicative of lower long‐term ratio of the chloroplast stroma to the ambient CO₂ partial pressures, would be mainly attributed to their lower g_i.     

Photoautotrophic and photomixotrophic growth of strawberry plantlets in vitro and changes in nutrient composition of the medium

Explants excised from strawberry (Fragaria x ananassa Duch.) plantlets were cultured in vitro for 21 days on half-strength MS (Murashige & Skoog 1962) basal liquid medium with 20 g l^-1 sucrose and without sugar in the vessels capped with gas permeable microporous polypropylene film. The experiments were conducted under CO₂ nonenriched (350–450 mol mol^-1 in the culture room) and CO₂ enriched (2,000 mol mol^-1 during the photoperiod in the culture room) conditions with a PPF (photosynthetic photon flux) of 200 mol m^-2 s^-1. The CO₂ concentration in the vessels decreased to approximately 200 mol mol^-1 during the photoperiod on day 21 under CO₂ nonenriched conditions. The fresh and dry weight, net photosynthetic rate (NPR) per plantlet, NPR per g leaf fresh weight, NPR per g leaf dry weight, the number of unfolded leaves, and ion uptake of PO₄ ^3-, NO₃ ^-, Ca²⁺, Mg²⁺ and K⁺ on day 21 were the greatest under photoautotrophic (no sugar in the medium) and CO₂ enriched conditions. The residual percent of PO₄ ^3- was 3% on day 21 under photoautotrophic and CO₂ enriched conditions.Abbreviations MS Murashige & Skoog (1962) basal medium composition - NPR net photosynthetic rate - PPF photosynthetic photon flux     

Physiological characterizations of three barley genotypes in response to low potassium stress

Plants adopt several strategies for fighting against low potassium (LK) stress. Our previous study identified some Tibetan wild barley accessions which show the higher LK tolerance than cultivated barley. However, the physiological mechanisms underlying the wild barley are not well understood. In this study, growth performance, elements content, SPAD value, photosynthetic parameters, and ATPase activities were measured to investigate the effect of LK stress on the two wild barley genotypes (XZ153 and XZ141) and one barley cultivar (ZD9) differing in LK tolerance. The results revealed that LK stress inhibited barley growth and induced reduction in dry weight, with XZ153 being least inhibited. Moreover, XZ153 had less reduction in photosynthetic rate, SPAD value, and K concentrations in the younger leaves under LK stress compared to the other two genotypes. Although the activities of H⁺/K⁺-ATPase and Ca²⁺/Mg²⁺-ATPase were increased significantly in all three genotypes in response to LK, the highest H⁺/K⁺-ATPase activity was observed in XZ153. The current results indicate that higher LK tolerance of XZ153 is partly attributed to its high capacity of transferring K from the old leaves to younger ones.