Leaf and root respiration of Lolium perenne populations selected for contrasting leaf respiration rates are affected by intra- and interpopulation interactions

Autotoxicity and allelopathy affect the yield of GL66 and GL72, two populations of perennial ryegrass (Lolium perenne L. cv. S23) that were originally selected for contrasting rates of mature leaf dark respiration, but under conditions where allelopathic effects could not occur and autotoxic effects were minimal. To investigate if the respiration properties are also affected, the populations were subjected to two conditions (monoculture and mixed culture) and two treatments (grown in `renewed', i.e. solution completely replaced every week, and `replenished', i.e. nutrients added to the solution every week, to compensate for the nutrients that had been absorbed from the solution). When plants were grown in mixed culture with a `renewed' nutrient solution, the rate of mature leaf respiration in the absence of inhibitors was higher in GL66 than in GL72. In GL66, this rate was unaffected by condition or treatment, whereas in GL72 it increased in the `replenished' cultures, due to autotoxic and allelopathic effects. In contrast with GL66, for GL72 the effect of an inhibitor of the alternative path (salicylhydroxamic acid, SHAM) differed between conditions and treatments. Effects of an inhibitor of the cytochrome path (KCN) were affected by treatment in both populations. The increase in the rate of root respiration of GL66 due to autotoxic and allelopathic effects was accompanied by a higher rate of the SHAM-resistant component, i.e. maximum activity of the cytochrome path. Roots of GL72 showed a higher control respiration rate and a higher rate of the SHAM-resistant component when long-term accumulation of root exudates was allowed to occur in mixed cultures (allelopathy). We conclude that allelochemicals can modify both leaf and root respiration and that there is no consistent correlation between yield and respiration for GL66 and GL72 in the present study. This shows that selection for low rates of mature leaf respiration is not an appropriate method to select for high-yielding cultivars in perennial ryegrass.     

Metabolism and translocation of nitrogen in two Lolium perenne populations with contrasting rates of mature leaf respiration and yield

Barneix, A. J., Cooper, H. D., Stulen, I. and Lambers, H. 1988. Metabolism and translocation of nitrogen in two Lolium perenne populations with contrasting rates of mature leaf respiration and yield. - Physiol. Plant. 72: 631–636.
Several aspects of nitrogen metabolism and transport were investigated to determine whether these processes could account for the observed differences in the dark respiration rate of mature leaves between two populations of Lolium perenne L. cv. S23: GL72 - a slow respiring, high growth rate line, and cv. GL66 - a fast respiring, low growth rate line.
No differences were found in total nitrogen or soluble protein concentrations between the populations, but GL72 showed a higher concentration of soluble amino acids, accounted for mainly by increases in the amounts of asparagine and glutamine. There were no differences in the glutamine synthetase (EC 6.3.1.2) or nitrate reductase (EC 1.6.6.1) activities between populations, but the fast respiring GL66 line showed higher glutamate dehydrogenase (EC 1.4.1.3) and peroxidase (EC 1.11.1.7) activities than GL72. The protein turnover rate, determined from ³H disappearance from leaves labelled with [³H]-acetic anhydride, appeared to be larger in GL66, but the difference was not significant and could not account for the differences in respiration rate.
The apparent extent of ¹⁵N cycling between roots and shoots was low in Lolium compared to other grass species, and there were no differences between the two populations.
It is concluded that the differences in dark respiration rate are not due to differences in demand for ATP by nitrogen assimilatory processes, but may be related to faster leaf senescence in the GL66 population.     

Regulation of Respiration in the Leaves and Roots of Two Lolium perenne Populations with Contrasting Mature Leaf Respiration Rates and Crop Yields

Measurements of O₂ uptake were made on leaves and roots of two populations of Lolium perenne L. cv S23 (GL66 and GL72), previously shown to have contrasting rates of CO₂ evolution and yields of dry matter. O₂ uptake was faster in the mature leaves of GL66 than those of GL72, but no difference was observed in the respiratory rates of meristematic leaf bases or mature roots. The growth rate of GL72 was faster than that of GL66. Cyanide resistance was substantial in mature leaves but the alternative path did not contribute to O₂ uptake in the dark. In both populations, adding malate and glycine stimulated O₂ uptake, but exogenous sucrose only stimulated when uncoupler was also present. The difference between the respiratory rates of the two populations was maintained under all investigated conditions. We conclude that the rate of mature leaf respiration in the dark in L. perenne is limited by adenylate control of glycolysis. The difference between the fast (GL66) and slow (GL72) respiring populations reflected a greater respiratory capacity and higher turnover of ATP in GL66. Alternative path capacity was also high in the roots of both and contributed substantially to O₂ uptake, as indicated by inhibition by salicylhydroxamic acid in the absence of KCN. The alternative path capacity of meristematic leaf bases was considerably less than that in mature leaves.

Transverse and cross-sections were made of mature leaves of both populations to study anatomical features which might explain the differences in ATP turnover, suggested by the biochemical experiments. Leaves of GL72 were thicker but did not show a different anatomy when compared with GL66. The increased thickness was not due to more or larger cells but entirely to a larger intercellular volume.

     

Effect of Nitrogen Supply on the Growth and Senescence of Leaves of Lolium perenne with Contrasting Rates of Leaf Respiration

Respiration rates remain high during senescence which is anenergy demanding process. Different rates of senescence mayexplain the contrasting respiration rates of mature leaves oftwo populations of perennial ryegrass cv. S23. This hypothesiswas tested by measuring the lengths of elongating and senescingleaves of plants of two populations (GL72, a slow and GL66,a fast respiring population) for 76 d following the transplantationof the seedlings into soil-filled pots in a growth room. Nitrogenwas supplied at two different rates at the beginning of theexperiment and again at day 36. At high levels of nitrogen supply the slow respiring populationhad a faster elongation rate and so a greater leaf length thanthe fast respiring population. There was no difference betweenthem in the rate of leaf senescence or in the duration of leafgrowth. Consequently, the fast respiring population is consideredprofligate in its use of carbon. By contrast, under low nitrogensupply both elongation and senescence rates were higher in thefast respiring population, although there was no differencein final leaf length. Therefore the longevity of leaves of thefast respiring population was reduced. It is argued that theleaves of the fast respiring population may turn-over mineralsmore rapidly than those of the slow, which may be more advantageousin conditions of low nitrogen supply and offset the benefitsof a more conservative use of carbon normally seen under conditionsof high nitrogen supply. Lolium perenne L., perennial ryegrass cv. S23, nitrogen supply, respiration rate, leaf senescence rate, leaf elongation rate, leaf longevity, population     

Effect of Selection for Dark Respiration Rate of Mature Leaves on Crop Yields of Lolium perenne cv. S23

Growth and nutritive quality of two polycross progeny (GL 83and GL 112) from F₁ plants of Lolium perenne ex cv. S23 showingslow dark respiration rates of mature leaves were compared inthe field with S23 in 3 years. Plots were cut either nine (simulatedgrazing) or five (conservation) times annually. In the firsttrial, sown in 1977, dry matter yields from plots of GL 83 werecompared with S23 during 1978 and 1979. A similar trial sownin 1979 compared GL 83, S23 and GL 112 during 1980. The slowrespiration populations displayed consistently greater (6–13per cent) annual dry matter yields than S23, mostly during midto late summer. In the first trial the difference in yield wasgreater in 1979 than 1978 and more under conservation than simulatedgrazing. In the second trial GL 83 and GL 112 yielded 11 and13 per cent more dry matter than S23 under simulated grazingand 5–6 per cent under conservation. There were no consistentdifferences in dry matter digestibility, water soluble carbohydratesor protein between the populations. Cellulose content of GL83 and to a much lesser extent GL 112, tended to be greaterthan that of S23 from July to September. Analysis of one harvestfrom another trial revealed no significant difference in energyvalue, as measured by adiabatic bomb calorimetry, between GL83 and S23. Thus, it appears that the greater dry matter productionof the progeny with slower mature leaf dark respiration ratesreflected a true increase in total energy yield not apparentlyassociated with any agronomically undesirable characteristics.The general significance of the results for crop improvementare discussed. Lolium perenne L., perennial ryegrass, yield of dry matter, energy value, dry matter digestibility     

The effect of handling on the yield of two populations of Lolium perenne selected for differences in mature leaf respiration rate

Two populations of Lolium perenne L. S23 (perennial ryegrass), selected for differences in mature leaf dark respiration, were used in a non-destructive indexing system for individual plants, to determine growth parameters. Population GL66, selected for high respiratory rates and low yield, responded strongly to the indexing treatment, when grown at low plant density. Dry weights of all plant parts decreased strongly, as did dry matter percentages of the leaf blades. At high density this population demonstrated the same trend, but additionally allocation to the shoot increased. In contrast, GL72, selected for low respiratory rates and a high yield, responded only at a high plant density. It is argued that there might be a relation between the dissimilar response of the two populations to mechanical influences and the presence of the genotypes of the low-yielding population in the parent variety. The results also emphasize that non-destructive growth analyses can only be used when their effects on the plants are known.     

红松和紫椴叶片暗呼吸及其光抑制性在幼、成树间的差异

叶片暗呼吸是森林碳循环的重要组分,深入分析幼、成树的叶片暗呼吸及其光抑制性的差异,对生态系统总生产力(GPP)的准确估算具有重要意义.本研究以长白山阔叶红松林主要树种(红松和紫椴)的幼树和成树为研究对象,分别测算不同光照下叶片暗呼吸与无光暗呼吸,比较叶片暗呼吸及其光抑制性在幼、成树间的差异,结合幼、成树叶片生理生态参数的对比,对幼、成树叶片暗呼吸及其光抑制性差异的原因进行探讨.结果表明: 两个树种幼树叶片光下暗呼吸的值高于成树,在生长季(6—9月),幼树的值比成树高6.8%～39.6%;两个树种幼树叶片暗呼吸光抑制程度低于成树,幼树叶片暗呼吸光抑制性的值比成树低2.5%～14.1%;红松幼、成树间叶片暗呼吸光抑制性的差异总体高于紫椴幼、成树间叶片暗呼吸光抑制性的差异,差值最高可达18.6%;幼树中较高的光下暗呼吸值和较低的光抑制程度可能与最大净光合速率、比叶面积、气孔导度的变化有关,与叶片氮含量的变化无关.     

The effect of handling on photosynthesis, transpiration, respiration, and nitrogen and carbohydrate content of populations of Lolium perenne

Two populations of perennial ryegrass ( Lolium perenne L.) S23, selected for contrasting rates of yield and mature leal dark respiration, were used in this study. Since previous work showed that yield decreased severely in population GL66 due to mechanical perturbation, possible effects on respiration rates were examined. Apart from a transient increase in population GL72, there was no effect on the respiration rate but mechanical perturbation did affect other processes. Handling caused transpiration ratesinthe light to increas for GL72, and in the dark to decrease for GL66. In the logger term, the nitrogen content decreased in handled plants grown at high density. These results emphasize that handling plants should be reduced to a minimum since it might have major effect on several processes. It is concluded that the original selection for the Lolium population was indeed on contrasting rates in yield and respiration. Furthermore it is argued that the existence of low- and high-yielding genotypes can be attributed to the dissimilar responses of different genotypes to mechanical influence.     

Growth and carbon economy of a fast-growing and a slow-growing grass species as dependent on nitrate supply

In previous experiments systematic differences have been found in the morphology, carbon economy and chemical composition of seedlings of inherently fast- and slow-growing plant species, grown at a non-limiting nutrient supply. In the present experiment it was investigated whether these differences persist when plants are grown at suboptimal nutrient supply rates. To this end, plants of the inherently fast-growing Holcus lanatus L. and the inherently slow-growing Deschampsia flexuosa (L.) Trin. were grown in sand at two levels of nitrate supply. Growth, photosynthesis, respiration and carbon and nitrogen content were studied over a period of 4 to 7 weeks. At low N-supply, the potentially fast-growing species still grew faster than the potentially slow-growing one. Similarly, differences in leaf area ratio (leaf area:total dry weight), specific leaf area (leaf area:leaf dry weight) and leaf weight ratio (leaf dry weight:total dry weight), as observed at high N-supply persisted at low N-availability. The only growth parameter for which a substantial Species × N-supply interaction was found was the net assimilation rate (increase in dry weight per unit leaf area and time). Rates of photosynthesis, shoot respiration and root respiration, expressed per unit leaf, shoot and root weight, respectively, were lower for the plants at low N-availability and higher for the fast-growing species. Species-specific variation in the daily carbon budget was mainly due to variation in carbon fixation. Lower values at low N were largely determined by both a lower C-gain of the leaves and a higher proportion of the daily gain spent in root respiration. Interspecific variation in C-content and dry weight:fresh weight ratio were similar at low and high N-supply. Total plant organic N decreased with decreasing N-supply, without differences between species. It is concluded that most of the parameters related to growth, C-economy and chemical composition differ between species and/or are affected by N-supply, but that differences between the two species at high N-availability persist at low N-supply.     

The Alternative Respiration Pathway in Leaves ofRhodiola roseaand Ajuga reptans: Presumable Physiological Role

The effect of growth conditions and plant age on the relationships between respiratory pathways was investigated in Rhodiola roseaand Ajuga reptans.The alternative pathway (AP) contributed 0–50% to the leaf respiration; however, this pathway was absent from the overwintered leaves of A. reptans.In both plant species, AP contributed 15–20% to the respiration of mature leaves, and in the young rapidly expanding leaves the contribution was twice higher. The highest AP contribution (40–50%) was found in the leaves of A. reptansplants grown in an experimental plot in full light. As compared to the plot-grown plants, A. reptansplants grown in their natural habitats were characterized by a lower AP contribution to the respiration of leaves; they contained two times less nonstructural carbohydrates and accumulated less biomass. We conclude that a high AP contribution to the respiration of leaves correlates with their rapid growth and that a high supply of respiratory substrates is one of prerequisite for the AP activation.     

Effects of Water and Nutrient Availability on Water Relations, Gas Exchange and Growth Rate of Mature Plants and Resprouts of Arbutus unedo L.

This study examines the effects of water supply and nutritionon the water status, gas exchange and growth of mature plantsand resprouts of Arbutus unedo, a Mediterranean evergreen shrubadapted to drought and poor nutrition. Mature plants of A. unedorespond to irrigation with increased leaf water potential duringsummer drought, but they show a very conservative use of waterand they do not increase leaf conductance. There is also a verysmall increase in net photosynthesis and growth, which doesnot significantly increase productivity. Resprouts of A. unedo increase water potential, leaf conductance,transpiration rate, net photosynthesis and growth rate in responseto watering, showing a less conservative use of water than matureplants. Increased growth rates, both in mature plants and resprouts,are likely to be due to the higher cell turgor caused by improvedleaf water potential, rather than to increased photosynthesis. The only effect of nutrient addition on mature plants is anincrease in leaf nutrient content, and other aspects of thephysiology and growth of resprouts were unaffected. We thereforeconclude that water is a more limiting factor than nutrientsfor mature plants and resprouts of A. unedo growing in the studyarea. These results support previous data which indicate thathigher growth rates in resprouts than in mature plants of A.unedo are mainly the result of a higher water availability.Copyright1994, 1999 Academic Press Arbutus unedo L., strawberry tree, resprouts, water stress, nutrient availability, water relations, gas exchange, growth rate, regeneration     

Interactive Effects of Soil Nutrients, Moisture and Sand Burial on the Development, Physiology, Biomass and Fitness of Cakile edentula

The relative importance and interactive effects of nutrientsupply, soil moisture content and sand burial on the development,physiology, biomass allocation and fitness ofCakile edentulawere examined under controlled greenhouse conditions. Planttraits were more frequently affected by nutrient supply thanby soil moisture content or sand burial. Measurements on mostplant traits also varied depending on the two or three way interactionsamong the three environmental factors. Plants partially buriedby sand had higher leaf chlorophyll concentration than thoseunburied at the early stages of development, especially underlow soil moisture content. High nutrient supply tended to lowerthe leaf chlorophyll concentration of mature plants, and thiseffect was more pronounced under high as compared to low soilmoisture content. High nutrient supply enhanced the photosyntheticcapacity of plants when they were water stressed. With adequatesoil moisture, high nutrient supply increased/decreased thephotosynthetic capacity of plants with/without previous experienceof water stress. High nutrient supply increased the biomassallocation to the root system of plants, especially at low soilmoisture content. Partial sand burial also promoted biomassallocation to the root system of plants grown at low soil moisturecontent. Soil nutrition; water supply; sand accretion; multiple stresses; biomass allocation; Cakile edentula     

Growth response to step-decrease in nutrient availability in small birch (Betula pendula Roth)

Abstract Changes in the uptake and allocation of carbon and nitrogen, after a step-decrease in nutrient availability, were investigated in small birch (Betula pendula Roth). By demonstrating stable nutrition, before and after the decrease in nutrient supply, it was possible to eliminate the effects of plant size and age. Immediately following the step-decrease in nutrient availability, net nitrogen uptake to leaves and the relative rate of increase in shoot area tended to zero. Although photosynthetic rate per shoot area decreased, carbon uptake remained in excess of that used in structural growth and respiration. More of the excess carbon was accumulated as starch in leaves than in roots. After a lag phase, the relative rates of increase in plant dry matter, starch amount, net nitrogen uptake to leaves and shoot area development equalled that of the reduced rate of nutrient supply. It is concluded that the reduction in plant relative growth rate was much more attributable to the reduced allocation of photosynthate to leaf area growth than to the reduction in photosynthesis per shoot area.     

Carbon and nitrogen economy of 24 wild species differing in relative growth rate

The relation between interspecific variation in relative growth rate and carbon and nitrogen economy was investigated. Twentyfour wild species were grown in a growth chamber with a nonlimiting nutrient supply and growth, whole plant photosynthesis, shoot respiration, and root respiration were determined. No correlation was found between the relative growth rate of these species and their rate of photosynthesis expressed on a leaf area basis. There was a positive correlation, however, with the rate of photosynthesis expressed per unit leaf dry weight. Also the rates of shoot and root respiration per unit dry weight correlated positively with relative growth rate. Due to a higher ratio between leaf area and plant weight (leaf area ratio) fast growing species were able to fix relatively more carbon per unit plant weight and used proportionally less of the total amount of assimilates in respiration. Fast growing species had a higher total organic nitrogen concentration per unit plant weight, allocated more nitrogen to the leaves and had a higher photosynthetic nitrogen-use efficiency, i.e. a higher rate of photosynthesis per unit organic nitrogen in the leaves. Consequently, their nitrogen productivity, the growth rate per unit organic nitrogen in the plant and per day, was higher compared with that of slow growing species.     

Effect of cadmium on growth and respiration of barley plants grown under two temperature regimes

We studied cadmium effect on the respiratory pathways ratio in the organs of barley (Hordeum distichum L., cv. Novichok) plants grown in water culture at two temperature regimes. Mineral nutrients were supplied daily in exponentially increasing amounts in order to provide for steady-state growth. CdSO₄ (30, 60, or 100 μmol/l) was added to nutrient solution at a single time in the beginning of the exponential growth period (19 days after germination). In further 6 days, the relative growth rate and biomass accumulation declined stronger with the increase in the cadmium concentration in plants grown at 13/8°C (day/night) than at 21/17°C (day/night). Cadmium suppressed root respiration (down to 60% of control) stronger than leaf respiration, and the roots manifested a higher sensitivity to the inhibitor of alternative oxidase, salicylhydroxamic acid. The respiratory pathways ratio in the roots occurred against the background of activated lipid peroxidation (POL). The highest POL activity and the highest proportion of alternative respiration pathway (AP) (up to 46% of total respiration) were observed in the roots in the presence of the highest cadmium concentration (100 μM) under lower temperature (13/8°C). Thus, high cadmium concentrations affected strongly the total rate of respiration and respiratory pathways ratio. Growth temperature modulated Cd effects on respiration. AP activation is one of the mechanisms for maintenance of root cell homeostasis under cadmium-induced stress.     

Energy metabolism of Plantago lanceolata as dependent on the supply of mineral nutrients

Plantago lanceolata L., a grassland species from a relatively nutrient-poor habitat, was grown in nutrient-rich and in nutrient-poor culture solutions. Half of the plants were trensferred from high to low or from low to high nutrient conditions. Shoot growth was immediately reduced upon transfer to low nutrient conditions, whilst it reacted more slowly upon transfer of plants to high nutrient conditions. Root growth was less dependent on the supply of nutrients, but it was slightly reduced upon transfer of plants to high nutrient conditions.
Photosynthesis was largely independent of the nutrient supply, apart from an initial increase upon transfer of plants to low nutrient conditions. Photosynthesis decreased with age in all treatments, and this decrease was not due to mutual shading. The decrease of photosynthetic rate was not accompanied by a decreased relative growth rate: it was compensated by a more efficient root respiration, since the activity of the alternative nonphosphorylating pathway continuously decreased in plants grown in a high nutrient environment.
It is concluded that the alternative pathway was of significance in removal of carbohydrates, which could not be utilized for growth, energy production, etc. , due to a temporary or structural imbalance between assimilate production and requirement. The alternative pathway also appeared to allow P. lanceolata plants to adapt to a changed environment as regards mineral nutrition.
The experimental value for root growth respiration of P. lanceolata grown under high nutrient conditions was compared with a theoretical value, calculated from the biochemical composition of plant dry matter and the known energy costings for biosynthetic and transport processes. A good correlation between the experimental and theoretical value of root growth respiration was found if it was assumed that ion uptake required c . 1.0 molecule of ATP per ion per membrane passage.     

Diurnal patterns of respiration in the leaves of four forest tree species

Respiration along the cyanide-sensitive, cytochrome pathway and the cyanide-in-sensitive, alternative pathway was monitored over a 24-h period in the leaves of Acerrubrum, Betula papyrifera, Populus tremuloides and Quercus rubra excised in the field. Populus displayed the greatest rate of total respiration, followed by Betula, Quercus and Acer . All species displayed the capacity for the alternative pathway as well as some engagement of the pathway, although the level of engagement in Acer was very low. In all species investigated there were no marked diurnal trends in any respiration variable measured. Leaf soluble sugar levels remained at a relatively constant level throughout the sampling period. It is concluded that respiration did not show diurnal fluctuations because substrate supply in the leaf was maintained at a constant level.     

Participation of the cyanide-resistant pathway in respiration of winter rape leaves as affected by plant cold acclimation

Leaf slices sampled from winter rape plants ( Brassica napus L., var. oleifera L., cv. GórczaánAski), grown in cold (5°C), showed an increase in the dark respiration rate (measured at 25°C) as compared to slices cut from control plants (grown at 20/15°C). The effect of low temperature was most pronounced after 4 days of plant growth in the cold. Oxygen uptake by control slices was 60% inhibited by 1 m M KCN and was insensitive to 2.5 m M salicylhydroxamic acid (SHAM). On the contrary, respiration of leaf slices from cold-pretreated plants was more resistant to cyanide (35% inhibition after 4 days of cold treatment) and was 30% inhibited by SHAM. The patterns of cold-induced changes in total respiratory activity and in the estimated activity of alternative pathway were similar. It seems that in leaf slices from plants grown in the cold, the cyanide-resistant, alternative pathway participates in oxygen uptake. Cold treatment of plants also brought about a 4-fold increase in the level of soluble sugars, which reached a maximum on day 4 of exposure to cold. Addition of sucrose to the incubation medium resulted in an immediate increase in oxygen uptake by slices with low endogenous sugar level. The respiration stimulated by sucrose addition was more resistant to cyanide than the basal respiration and it was inhibited by SHAM. It is concluded that the operation of the alternative pathway is responsible for the increased oxygen uptake by the cold-grown winter rape leaves and it may be induced by an increased sugar supply for respiratory processes.     

The modular character of growth in Nicotiana tabacum plants under steady-state nutrition

The impact of different plant growth rates on biomass allocation and growth distribution in tobacco was studied on the whole plant, total leaf area and single leaf level. On the whole plant level, constant relationships were found between the total leaf area and the biomass allocation to leaves and the nonphotosynthetic organs (roots and stem) independent from the overall growth rate and the nutrient addition rate to the plants. On the level of total leaf area, plants grown at lower nutrient supply reached a distinct distribution of leaf area later than those grown at higher nutrient supply, but the normalized distribution of leaf area along the stem at a certain plant size did not differ between plants growing at different nutrient supply and growth rates. On the leaf blade level, growth rates declined, initially linearly, from the leaf base to the leaf tip. Distinct gradients within the side veins were not observed, but the growth rates of the side veins were closely correlated to the adjacent mid-vein segments. These gradients flattened with increasing size of the leaf. The modular character of growth in tobacco is discussed in the context of basic growth analysis and as a framework for physiological, cytological, biochemical, and molecular studies in growing plants.Key words: Nicotiana tabacum, whole plant, total leaf area, leaf growth, growth rate, biomass.      

Studies on the Expansion of the Leaf Surface: III. THE INFLUENCE OF RADIATION ON CELL DIVISION AND LEAF EXPANSION

The numbers of leaves and the areas and cell numbers of leavesfrom the first five nodes of the cucumber were determined throughouttheir development with three different amounts of daily radiationand two conditions of nutrient supply. The rate of leaf production was found to be constant with timefor any one amount of radiation and to increase with increasedradiation. The transition from the rate in darkness to thatin light was sharp and occurred more quickly the higher theradiation. Provided the nutrient supply was high the ultimate areas ofindividual leaves were greater the higher the radiation; ifmineral nutrients were depleted the maximum area of a leaf occurredwith an intermediate amount of light. This arose because thesefactors exercised a differential effect on the various phasesof growth. Each leaf commenced its life as a mass of dividing cells andthe mean rate of division remained constant until it unfoldedfrom the terminal bud. The mean rate of division was much greaterat high than at low levels of radiation and was interpretedas being regulated by carbohydrate supply. Although expansionof some cells was likely before unfolding, after this stagethere was a marked decrease in the proportion of cells proceedingto division. The duration of division in the leaf after unfoldingwas independent of radiation; although 70–98 per cent,of the final number of cells were formed after unfolding, theultimate number was effectively determined by the rate of divisionprior to unfolding. Low nutrient supply restricted the duration of division in theleaf to a significant extent and the expansion of cells to avery considerable degree. The smaller leaves on plants receivinghigh rather than medium radiation were due to their cells beingmuch smaller.