Leaf area expansion and assimilate production in sunflower (Helianthus annuus L.) growing under low phosphorus conditions

Reductions in leaf area and plant growth as a consequence of phosphorus (P) limitations have been attributed both to direct effects of P shortage on leaf expansion rate and to a reduced production of assimilates required for growth. Canopy assimilation and leaf area expansion are closely interrelated processes. In this work we used experimental and simulation techniques to identify and study their importance in determining leaf area on sunflower (Helianthus annuus L.) growing under P-deficient conditions. Experiment 1 was done outdoors, in Buenos Aires, Argentina, and Experiment 2 in a glasshouse in Wageningen, The Netherlands. In both experiments we studied the effects of soil P addition on leaf appearance, leaf expansion, dry matter accumulation, and leaf photosynthesis of non-water stressed plants grown in pots containing a P-deficient soil. Before sowing the equivalent amounts of 0–600 kg of super phosphate ha^-1 were added to the pots. Phosphorus deficiency delayed leaf appearance increasing the value of the phyllochron (PHY) up to 76%, the rate of leaf area expansion during the quasi-linear phase of leaf expansion (LER) was reduced by up to 74%, with respect to high P plants. Phosphorus deficiency reduced by up to 50% the rate of light saturated photosynthesis per unit of leaf area (AMAX) in recently expanded leaves, while at low levels of leaf insertion in the canopy, AMAX was reduced by up to 85%, when compared to that in high P plants. Phosphorus deficiency also reduced the duration of the quasi-linear phase of leaf expansion by up to eight days. The values of LER were related (r = 0.56, P < 0.05) to the mean concentration of P in all the leaves (Leaves P%) and not to the concentration of P in the individual leaf where LER was determined (r = 0.22, P < 0.4) suggesting that under P deficiency individual leaf expansion was not likely to be regulated by the total P concentration at leaf level. The values of AMAX of individual leaves were related (r = 0.79, P < 0.01) to the concentration of total P in the corresponding leaf (Leaf P%). LER showed a hyperbolic relationship with Leaves P% (R² = 0.94, P < 0.01, n = 13) that saturate at 0.14%. AMAX showed a hyperbolic relationship with Leaf P% (R² = 0.73, P < 0.01, n = 53) that saturated with values of Leaf P% higher than 0.22. A morphogenetic model of leaf area development and growth was developed to quantify the effect of assimilate supply at canopy level on total leaf area expansion, and to study the effects of model parameters on the growth of sunflower plants under P-deficient conditions. With this model we identified the existence of direct effects of P deficiency on individual leaf area expansion. However, we calculated that under mild P stress conditions up to 83% of the reduction in the observed leaf area was explained by the particular effects of P% on the rate of leaf appearance, on the duration of the linear period of leaf expansion, and on the value of AMAX. We also calculated that the effects of P deficiency on the value of AMAX alone, explained up to 41% of the observed reductions in total leaf area between the highest and the intermediate P level in Experiment 2. Possible mechanisms of action of the direct effects of P on individual leaf expansion are discussed in this paper.     

From individual leaf elongation to whole shoot leaf area expansion: a comparison of three Aegilops and two Triticum species

BACKGROUND AND AIMS: Rapid leaf area expansion is a desirable trait in the early growth stages of cereal crops grown in low-rainfall areas. In this study, the traits associated with inherent variation in early leaf area expansion rates have been investigated in two wheat species (Triticum aestivum and T. durum) and three of its wild relatives (Aegilops umbellulata, A. caudata and A. tauschii) to find out whether the Aegilops species have a faster leaf area expansion in their early developmental stage than some of the current wheat species. METHODS: Growth of individual leaves, biomass allocation, and gas exchange were measured on hydroponically grown plants for 4 weeks. KEY RESULTS: Leaf elongation rate (LER) was strongly and positively correlated with leaf width but not with leaf elongation duration (LED). The species with more rapidly elongating leaves showed a faster increase with leaf position in LER, leaf width and leaf area, higher relative leaf area expansion rates, and more biomass allocation to leaf sheaths and less to roots. No differences in leaf appearance rate were found amongst the species. CONCLUSIONS: Aegilops tauschii was the only wild species with rapid leaf expansion rates similar to those of wheat, and it achieved the highest photosynthetic rates, making it an interesting species for further study.     

Stomatal responses to water stress and to abscisic Acid in phosphorus-deficient cotton plants

Cotton (Gossypium hirsutum L.) plants were grown in sand culture on nutrient solution containing adequate or growth-limiting levels of P. When water was withheld from the pots, stomata of the most recently expanded leaf closed at leaf water potentials of approximately −16 and −12 bars in the normal and P-deficient plants, respectively. Pressure-volume curves showed that the stomata of P-deficient plants closed when there was still significant turgor in the leaf mesophyll. Leaves of P-deficient plants accumulated more abscisic acid (ABA) in response to water stress, but the difference was evident only at low water potentials, after initiation of stomatal closure. In leaves excised from unstressed plants, P deficiency greatly increased stomatal response to ABA applied through the transpiration stream. Kinetin blocked most of this increase in apparent sensitivity to ABA. The effect of P nutrition on stomatal behavior may be related to alterations of the balance between ABA and cytokinins.     

Effects of phosphorus nutrition on tiller emergence in wheat

Phosphorus (P) deficiency limits the yield of wheat, particularly by reducing the number of ears per unit of area because of a poor tiller emergence. The objectives of this work were to (i) determine whether tiller emergence under low phosphorus availability is a function of the availability of assimilates for growth or a direct result of low P availability, (ii) attempt to establish a quantitative relation between an index of the availability of P in the plant and the effects of P deficiency on tiller emergence, and (iii) to provide a better understanding of the mechanisms involved in tiller emergence in field-grown wheat. Wheat (Triticum aestivum L., cv. INTA Oasis), was grown in the field under drip irrigation on a typic Argiudol, low in P (5.5 μg P g^-1 soil Bray & Kurtz I) in Balcarce, Argentina. Treatments consisted of the combination of three levels of P fertilization 0, 60 and 200 kg P₂O₅ ha^-1, and two levels of assimilate availability, a control (non-shaded) and 65% of reduction in incident irradiance from seedling emergence until the end of tillering (shaded). Phosphorus treatments significantly modified the pattern of growth and development of the plants. Shading reduced the growth and concentration of water-soluble carbohydrates in leaves and stems. Leaf photosynthetic rate at saturating irradiance was reduced by P deficiency, but was not affected by shading. At shoot P concentrations less than 4.2 g P kg^-1 the heterogeneity in the plant population increased with respect to the number of plants bearing a certain tiller. At a shoot P concentration of 1.7 g P kg^-1 tillering ceased completely. Phosphorus deficiency directly altered the normal pattern of tiller emergence by slowing the emergence of leaves on the main stem (i.e. increasing the phyllochron), and by reducing the maximum rate of tiller emergence for each tiller. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of leaf position and nitrogen supply on the expansion of leaves of field grown sunflower (Helianthus annuus L.)

Leaf growth responses to N supply and leaf position were studied using widely-spaced sunflower plants growing under field conditions. Both N supply (range 0.25 to 11.25 g added N per plant) and leaf position significantly (p=0.001) affected maximum leaf area (LA_max) of target leaves through variations in leaf expansion rate (LER); effects on duration of expansion were small. Specific leaf nitrogen (SLN, g N m^-2) fell quite rapidly during the initial leaf expansion phase (LA < 35% LA_max) but leveled off during the final 65% increase of leaf area. This pattern held across leaf positions and N supply levels. Leaf nitrogen accumulation after 35% LA_max continued up to achievement of LA_max; reductions in the higher SLN characteristic of the initial phase were insufficient to cover the nitrogen requirements for expansion during the final phase. LER in the quasi-linear expansion phase (35 to 100% of LA_max) was strongly associated with SLN above a threshold that varied with leaf position (mean 1.79±0.225 g N m^-2). This contrasts with the response of photosynthesis at high irradiance to SLN, which has previously been shown to have a threshold of 0.3 g N m^-2; in the present work saturation of photosynthetic rate was evident when SLN reached 1.97 g N m^-2. Thus, once the area of a leaf exceeds 35% of LA_max, expansion proceeds provided SLN values are close to the levels required for maximum photosynthesis. However, growth of leaves during the initial expansion phase ensures a minimum production of leaf area even at low N supply levels.     

The Effects of Copper Supply and Shading on Retranslocation of Copper from Mature Wheat Leaves

Plants of Gamenya wheat (Triticum aestivum L.) were grown inpots of a Cu-deficient sand at two levels of Cu (deficient andsufficient), and harvested on days 13, 22, 28 and 38. In 50per cent of the pots in each Cu treatment, the oldest leaf andleaf 2 of the main stem were shaded when they reached full expansion. The Cu content of the oldest leaf of Cu-sufficient, unshadedplants was high at day 13 and declined rapidly to day 38. Thatof Cu-deficient, unshaded plants was initially relatively lowand declined much more slowly, so that at day 38 it resembledthat of Cu-sufficient plants. Shading the oldest leaf acceleratedthe loss of its Cu in both Cu-deficient and Cu-sufficient plants.The effects of shading and of Cu supply on the loss of Cu fromthe oldest leaf paralleled their effects on the loss of N andchlorophyll. The results suggest that most of the Cu in theoldest leaf does not move out until the leaf senesces. In Cu-deficient plants retention of Cu by old green leaves accentuatedCu deficiency. The release of Cu, resulting from shading theold leaves of Cu-deficient plants, stimulated the growth ofnew leaves. In Cu-sufficient plants, shading depressed growth. copper, shading, retranslocation, wheat, Triticum aestivum L.     

Accumulation of damaging concentrations of phosphorus by leaves of selkirk wheat

Summary Leaves of P-deficient wheat plants were damaged after 1 mM P was included in the previously zero P nutrient solution. Wheat plants continuously grown in 1 mM P in modified Hoagland's solution were not damaged. Maximum damage and P accumulation occurred in the apical regions of the youngest leaves. We attributed this leaf damage to the accumulation of abnormally high (in excess of 3 per cent dry weight) amounts of P in the leaves of P-deficient plants from nutrient solution that was nontoxic to plants containing adequate P. re]19720721     

Leaf area establishment of a maize (Zea Mays L.) field crop under potassium deficiency

The effect of K deficiency on leaf area index (LAI) establishment of a maize field crop (Zea Mays L.) was studied. The experimental work was carried out in 2000 and 2001 on a long-term K fertilization trial. Three K fertilization regimes (K0, K1 and K4) have been applied since 1995, thus leading to contrasted levels of available K in soils (14, 23 and 44 µg exchangeable K per g of dry soil for the three fertilization regimes, respectively). The rate of leaf appearance, the leaf elongation rate (LER), the leaf elongation duration (LED), their final length and width and the number of senescent leaves were investigated. K concentrations in shoot tissue water were lower in K0 plants, whereas concentrations of Ca and Mg were higher. The LAI was reduced in the K0 treatment, mainly because of a slower rate of leaf appearance and a reduced final size of individual leaves. The reduced final length of individual leaves was almost entirely accounted for by a reduced LER during the quasi linear elongation phase. The LED was only slightly affected. A rough parallelism was observed between the relative reduction of leaf length and the relative reduction of plant water content during leaf elongation. Conversely, there was no evidence that leaf elongation was limited by carbohydrate availability in leaf growing zones. This suggests that K deficiency reduced LER probably because of altered plant-water relationships. On the whole, these results strengthen the idea that leaf growth is a key variable for analyzing, and later on modeling, crop growth under K deficiency.     

De novo arginine biosynthesis in leaves of phosphorus-deficient citrus and poncirus species

Young, fully expanded leaves from 7-month-old P-deficient citrus rootstock seedlings had levels of nonprotein arginine that were 10- to 50-fold greater than those from P-sufficient control plants. Arginine content of the protein fraction increased 2- to 4-fold in P-deficient leaves. Total arginine content, which averaged 72 ± 6 micromoles per gram dry weight of P-sufficient leaf tissue (mean ± se, n = the four rootstocks) was 207, 308, 241, and 178 micromoles in P-deficient leaves from Citrus limon cv rough lemon, Poncirus trifoliata × C. sinensis cv Carrizo citrange and cv Troyer citrange, and P. trifoliata cv Australian trifoliate orange, respectively. For each rootstock, the accumulation of arginine paralleled an increase in the activity of the pathway for the de novo biosynthesis of arginine. The ratio of the nanomoles NaH¹⁴CO₃ incorporated into the combined pool of arginine plus urea per gram fresh weight intact leaf tissue during a 3-hour labeling period for P-deficient to P-sufficient plants was 91:34, 49:11, 35:11, and 52:41, respectively. When P-deficient plants were supplied with P, incorporation of NaH¹⁴CO₃ into arginine plus urea was reduced to the level observed for the P-sufficient control plants of the same age and arginine ceased to accumulate. Arginase and arginine decarboxylase activity were either unaffected or slightly increased during phosphorus deficiency. Taken together, these results provide strong evidence that arginine accumulation during phosphorus deficiency is due to increased activity of the de novo arginine biosynthetic pathway.     

Influence of Species of Vesicular-Arbuscular Mycorrhizal Fungi and Phosphorus Nutrition on Growth,Development, and Mineral Nutrition of Potato (Solanum tuberosum L.)

Growth, development, and mineral physiology of potato (Solanum tuberosum L.) plants in response to infection by three species of vesicular-arbuscular mycorrhizal (VAM) fungi and different levels of P nutrition were characterized. P deficiency in no-P and low-P (0.5 mM) nonmycorrhizal plants developed between 28 and 84 d after planting. By 84 d after planting, P deficiency decreased plant relative growth rate such that no-P and low-P plants had, respectively, 65 and 45% less dry mass and 76 and 55% less total P than plants grown with high P (2.5 mM). A severe reduction in leaf area was also evident, because P deficiency induced a restriction of lateral bud growth and leaf expansion and, also, decreased the relative plant allocation of dry matter to leaf growth. Root growth was less influenced by P deficiency than either leaf or stem growth. Moreover, P-deficient plants accumulated a higher proportion of total available P than high-P plants, indicating that P stress had enhanced root efficiency of P acquisition. Plant P deficiency did not alter the shoot concentration of N, K, Mg, or Fe; however, the total accumulation of these mineral nutrients in shoots of P-stressed plants was substantially less than that of high-P plants. P uptake by roots was enhanced by each of the VAM symbionts by 56 d after planting and at all levels of abiotic P supply. Species differed in their ability to colonize roots and similarly to produce a plant growth response. In this regard, Glomus intraradices (Schenck and Smith) enhanced plant growth the most, whereas Glomus dimorphicum (Boyetchko and Tewari) was least effective, and Glomus mosseae ([Nicol. and Gerd.] Gerd. and Trappe) produced an intermediate growth response. The partial alleviation of P deficiency in no-P and low-P plants by VAM fungi stimulated uptake of N, K, Mg, Fe, and Zn. VAM fungi enhanced shoot concentrations of P, N, and Mg by 28 d after planting and, through a general improvement of overall plant mineral nutrition, promoted plant growth and development.     

Leaf primordia initiation,leaf emergence and tillering in wheat (Triticum aestivum L.) grown under low-phosphorus conditions

In two simultaneous experiments we examined the effects of phosphorus (P) supply on leaf area development in wheat (Triticum aestivum L.) grown in sand with nutrient solutions. In Experiment 1 we studied leaf emergence, leaf elongation, tiller emergence, shoot growth, and P uptake under four levels of P supply (mM) 0.025 (P1), 0.05 (P2), 0.1 (P3), and 0.5 (P4), and. In Experiment 2 there were two levels of P supply, P1 and P4, and we examined the effects of P on leaf primordia differentiation and leaf emergence. The phyllochron was calculated as the inverse of the rate of leaf emergence calculated from the regression of number of leaf tips (PHY-Ltip), Haun index (PHY-Haun), and as the cumulated thermal time between the emergence of two consecutive leaves (PHYtt). The plastochron was calculated from the inverse of the rate of leaf primordia initiation in the apex. P deficiency delayed the emergence of leaves on the main stem and on the tiller 1. Phosphorus deficiency increased the time from emergence to double ridge and anthesis. The final number of leaves was not affected by P. The effects of P on the value of the phyllochron were attributed to both a reduced rate of leaf primordia initiation, and to a reduced leaf elongation rate. P deficiency delayed or even suppressed the emergence of certain tillers. In this work a phosphorus deficiency that reduced shoot growth by 25% at 44 days after emergence significantly modified the structure of the plants by increasing the value of the phyllochron and delaying tillering. These results suggest that any attempt to simulate leaf area development and growth of wheat plants for P-limited conditions should include the effects of the deficiency on leaf emergence.     

Measuring the RGR of Individual Grass Plants

Vegetative growth of grasses was analysed by dry mass increaseof growing leaves.Holcus lanatuswas grown in a controlled environmentand leaf extension rates of leaf numbers 5–10 of the maintiller were monitored daily. Leaf appearance and leaf extensionrates (LER) of leaves 5–7 enabled the prediction of thefinal length and dry mass of leaf 8 during its growth. A linearincrease of leaf mass per unit leaf length (LML) of leaf 8 wasobserved during growth. After harvest the daily increase indry mass of growing leaves was calculated from the LER and correspondingincrease in LML. The relative growth rate (RGR) of the maintiller showed day-to-day fluctuations and was gradually reducedby 50% over a 16-d period. The RGR of the shoot was maintainedby tillering. The RGR of a single (grass) plant can be calculatedfrom four parameters only: LER, LML, leaf appearance and tillering.Variation of RGR over a period can be reconstructed after harvestand the impact of these four parameters on RGR can be established.Copyright1998 Annals of Botany Company. Relative growth rate, grass, leaf growth,Holcus lanatus.     

Transport, synthesis and catabolism of abscisic acid (ABA) in intact plants of castor bean (Ricinus communis L.) under phosphate deficiency and moderate salinity

Flows of abscisic acid (ABA) were investigated in whole plantsof castor bean (Ricinus communis) grown in sand culture undereither phosphate deficiency or moderate salinity. Xylem transportof ABA in P-deficient plants was stimulated by a factor of 6whereas phloem transport was affected only very slightly. ABAdeposition into leaves of P-deficient plants was not appreciablydifferent from the controls because of strong net degradationin leaves. Since conjugation of ABA was strongly reduced inall organs of P-deficient plants ABA was presumably metabolizedmainly to phaseic acid and dihydrophaseic acid. The increasedimport of ABA occurred predominantly into fully differentiatedbut not senescent leaves and showed a good correlation withthe inhibition of leaf conductance under P deficiency. As with low-P-plants salt stress increased ABA synthesis inroots and associated transport in the xylem. However, salinitycaused a distinctly greater accumulation of ABA in the leaves,stem segments and the apex than in P-deficient plants. As opposedto P deficiency, ABA export in the phloem from the leaves wasstimulated by salinity. Modelling of ABA flows within an individualleaf over its life cycle showed that young growing leaves importedABA from both phloem and xylem, whereas the adult non-senescentleaves were a source of ABA and thus provided a potential shoot-to-rootstress signal as well as an acceptor for reciprocal signalsfrom root to shoot. In senescing leaves ABA flows and accumulationwere somewhat retarded and ABA was lost in net terms by exportfrom the leaf. Key words: Abscisic acid, phosphorus deficiency, salt stress, phloem and xylem transport     

Does water and phosphorus uptake limit leaf growth of Rhizoctonia-infected wheat seedlings?

Wheat seedlings infected with a pure inoculum of the root-rotting fungus Rhizoctonia solani were grown in pots designed to fit in pressure chambers, to allow the effects of the Rhizoctonia infection on leaf growth to be studied while maintaining the leaves at elevated water status. Wheat was grown to the third leaf stage in soil inoculated with three different levels of Rhizoctonia, and the pots were then pressurised for seven days to maintain the leaf xylem at the point of bleeding (ie. the leaves were at full turgor). The reduction in leaf expansion caused by Rhizoctonia was not overcome by pressurisation, indicating that a reduced supply of water to the leaves was not responsible for reduced leaf growth. The addition of phosphorus to pots marginally deficient in P did not increase the leaf growth of Rhizoctonia-infected plants, despite increased P uptake to the leaves. These results indicate that a reduced supply of water to the leaves and a supply of phosphorus that was bordering on deficient was not the cause of the growth reduction in seedlings with Rhizoctonia infection. The nature of this reduced growth remains uncertain but may involve growth regulators produced by the fungus, or by the plant as a result of the infection process. The mechanism of these growth reductions is of interest as it may provide a key to the development of plant resistance mechanisms. This revised version was published online in June 2006 with corrections to the Cover Date.     

Specific leaf area in barley: individual leaves versus whole plants

We have explored the relationships between specific leaf area calculated for a whole plant and its individual leaves. Barley was grown in hydroponics in controlled environment cabinets. Plants were harvested on the basis of physiological age (defined as the number of days after full expansion of leaves on the main stem) and the area and weight of whole, fully expanded, leaves measured and specific leaf area (SLA) of individual leaves or whole plants calculated. Specific leaf area calculated for individual leaves (SLA_L) varied with leaf position and with leaf age after full expansion whereas SLA calculated for whole plants (SLA_P) varied with plant age. The same conclusions were reached whether the results were based on total dry weight or dry weight minus soluble carbohydrates ('structural weight'). Transferring plants to shade on the day of full expansion of the third leaf on the main stem increased the SLA_P, and also SLA_L of leaves 3 and 4 on the main stem (leaf 4 being the younger leaf of the two), because of a decrease in the 'structural weight' of these leaves. However SLA_L of leaf 2 (which was older than leaf 3) was not affected by shading; the effect was confined to leaves developing in the new conditions.     

Variation in phosphorus efficiency among 73 bread and durum wheat genotypes grown in a phosphorus-deficient calcareous soil

A greenhouse experiment was carried out to study the severity of phosphorus (P) deficiency symptoms on leaves, shoot dry matter production, and shoot concentration and content (the total amount per shoot) of P in 39 bread wheat (Triticum aestivum L.) and 34 durum wheat (Triticum durum L.) genotypes grown in a severely P-deficient calcareous soil with low (20mgPkg⁻¹ soil) and adequate (80mgPkg⁻¹ soil) P supply for 39 days. As the seed P concentration or content can affect plant performance under P-deficient conditions, the seeds of the genotypes used in the present study were also analyzed for P concentration. Phosphorus efficiency (relative shoot growth) of genotypes, calculated by the ratio of shoot dry matter production under low P to that under adequate P supply, significantly differed among the genotypes, and varied between 46.7% and 78.6%. Phosphorus efficiency ranged from 51% to 71% with an average of 61% for bread and from 47% to 79% with an average of 66% for durum wheat genotypes. There was no correlation between P efficiency ratio and P concentration of plants (R ²=0.0001), but P efficiency of all bread and durum wheat genotypes showed a very significant correlation with the P content (the total amount of P per shoot) (R ²=0.333^***). The relationship between the P efficiency and total amount of P per shoot was much more significant in bread (R ²=0.341^***) than in durum wheat (R ²=0.135^*). Like shoot P concentrations, also severity of visible leaf symptoms of P deficiency on older leaves, including leaf chlorosis and necrosis, did not correlate with P efficiency. In most cases, genotypes showing higher P efficiency had higher absolute shoot dry weight under P deficient conditions. Under P deficient conditions, the absolute shoot dry weight very significantly correlated with shoot P content (R ²=0.665^***), but the correlation between the absolute shoot dry weight and shoot P concentration tended to be negative. There was also variation in native seed P reserve of the genotypes, but this variation had no influence on the P efficiency. The results indicate that the total amount of P per shoot and shoot dry matter production at low P supply are most reliable parameters in ranking genotypes for P efficiency at early growth stage. In wheat germplasm tested in the present study, several wheat genotypes are available showing both very high P efficiency and very high shoot content and concentration of P suggesting that P acquisition ability should be most important mechanism for high P efficiency in such genotypes. On the other hand, there are also genotypes in the germplasm having more or less same P concentration or P content in shoot but differing substantially in P efficiency, indicating importance of P utilization at cellular level in P efficiency. All these results suggest that P efficiency mechanisms can be different from one genotype to other within a given plant species.     

Growth analysis of maize field crops under phosphorus deficiency

Biomass accumulation by crops depends both on light interception by leaves and on the efficiency with which the intercepted light is used to produce dry matter. Our aim was to identify which of these processes were affected for maize (Zea Mays L., cv Volga) field crops grown under phosphorus (P) deficiency, and assess their relative importance. In this paper, the effects of P deficiency on leaf appearance, leaf elongation rate, final individual leaf area and leaf senescence were studied. The experimental work was carried out in 1995–1977 on a long-term P fertilisation trial located on a sandy soil in the south-west of France. Three P fertilisation regimes have been applied since 1972: no-P (P0 treatment) and different rates of P fertiliser (P1.5:1.5 times the grain P export and P3:3 times the grain P export). These fertilisation regimes have led to contrasted levels of soil P supply, with the P0 treatment being limiting for growth. Very few differences were observed about leaf growth between the P1.5 and P3 treatments. Conversely, the leaf area index (LAI) was significantly reduced in the P0 treatment, especially during the first phases of the crop cycle (up to −60% between the 7- and 14-visible leaves). This effect gradually decreased over time. The lower LAI in P0 treatment was due to two main processes affecting the leaf growth. The final number of leaves per plant and leaf senescence were only slightly modified by P deficiency. Conversely, leaf appearance was delayed during the period between leaf 4 and leaf 9. The value of the phyllochron increased from 47 °C days in the P1.5 treatment to 65 °C days in the P0 treatment. Leaf elongation rates during the quasi-linear phase of leaf expansion were significantly reduced for lower leaves of P0 plants. The final size of leaves L2–L12 was reduced. On the opposite, leaf elongation duration was not greatly affected by P treatments. Before the emergence of leaf 9, the reduction of individual leaf size was the main factor responsible for the reduced LAI in the P0 treatment. After this stage, the delayed leaf appearance accounted for a great part of the reduced LAI in the P0 treatment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Alleviation of phosphorus deficiency stress by moderate salinity in the halophyte <Emphasis Type="Italic">Hordeum maritimum</Emphasis> L.

Hordeum maritimum (Poacea) is a facultative halophyte potentially useful for forage production in saline zones. Here, we assessed whether moderate NaCl-salinity can modify the plant response to phosphorus (P) shortage. Plants were cultivated for 55 days under low or sufficient P supply (5 or 60 μmol plant⁻¹ week⁻¹ KH₂PO₄, respectively), with or without 100 mM NaCl. When individually applied, salinity and P deficiency significantly restricted whole-plant growth, with a more marked effect of the latter stress. Plants subjected to P deficiency showed a significant increase in root growth (as length and dry weight) and root/shoot DW ratio. Enhanced root growth and elongation presumably correspond to the well-known root adaptive response to mineral deficiency. However, leaf relative water content, leaf P concentration, and leaf gas exchange parameters were significantly restricted. The interactive effects of salinity and P deficiency were not added one to another neither on whole plant biomass nor on plant nutrient uptake. Indeed, 100 mM NaCl-addition to P-deficient plants significantly restored the plant growth and improved CO₂ assimilation rate, root growth, K⁺/Na⁺ ratio and leaf proline and soluble sugar concentrations. It also significantly enhanced leaf total antioxidant capacity and leaf anthocyanin concentration. This was associated with significantly lower leaf osmotic potential, leaf Na⁺ and malondialdehyde (MDA) concentration. Taken together, these results suggest that mild salinity may mitigate the adverse effects of phosphorus deficiency on H. maritimum by notably improving the plant photosynthetic activity, the osmotic adjustment capacity, the selective absorption of K⁺ over Na⁺ and antioxidant defence.     

Activity of ascorbate-dependent H2O2-scavenging enzymes and leaf chlorosis are enhanced in magnesium- and potassium-deficient leaves, but not in phosphorus-deficient leaves

The effect of phosphorus (P), potassium (K), and magnesium (Mg)deficiency on the development of leaf symptoms (chlorosis andnecrosis) and activities of ascorbate-dependent H₂O₂ scavengingenzymes (ascorbate peroxidase, monodehydroascorbate reductase,dehydroascorbate reductase, and glutathione reductase) was studiedin bean (Phaseolus vulgans) plants over a 12 d period of growthin nutrient solution. With increasing plant age Mg- and K-deficientleaves developed severe interveinal chlorosis and, accordingly,chlorophyll concentrations were reduced. However, in P-deficientleaves neither chlorosis nor necrosis appeared; the leaves remaineddark green and even at an advanced stage of P deficiency, chlorophyllconcentrations were still higher than those of control plants.In K- and, particularly, Mg-deficient leaves with an increasein severity of leaf chlorosis, activity of ascorbate-dependentH₂O₂- scavenging enzymes was progressively increased. In contrast,in P-deficient leaves, as in leaves of the control plants, activityof H₂O₂-scavenging enzymes remained at a low level over the12 d period. Accordingly, compared with P-deficient and controlplants, Mg- and K-deficient leaves with elevated anti-oxidativepotential showed much higher resistance to chlorophyll destructionby the herbicide paraquat. Elevated levels of H₂O₂-scavengingenging enzymes in Mg- and K-deficient leaves indicate a higherproduction of H₂O₂ and related toxic O₂ species. It Is suggestedthat in Mg- and K-deficient leaves, utilization of photoreductantsin CO₂ fixation is restricted because of impaired export andthus accumulation of photosynthates. This disturbance mightlead to enhanced photoreduction of molecular O₂ to toxic O₂species causing chlorophyll destruction (chlorosis), a processwhich is not important in P-deficient leaves where export ofsucrose is not affected. Key words: Bean, hydrogen peroxide detoxification, leaf chlorosis, magnesium nutrition, oxygen activation, phosphorus nutrition, potassium nutrition