Endogenous gibberellins in Lolium perenne and influence of defoliation on their contents in elongating leaf bases and in leaf sheaths

Gibberellin (GA) levels in leaf sheaths and in elongating leaf bases of perennial ryegrass ( Lolium perenne L. cv. Bravo) were monitored in undefoliated and defoliated plants. Nine C-13-hydroxylated GAs (GA8, GA97, GA29, GA1, GA20, GA44, GA19, GA17, GA53) and one C-13-non-hydroxylated GA (GA9) were identified by combined gas chromatography-mass spectrometry in leaf extracts. The total level of GA8, GA29, GA1, GA20, GA44, GA19 and GA53, determined by selected ion monitoring, was 7 times higher in elongating leaf bases than in mature leaf sheaths. In both leaf tissues, defoliation induced an increase in GA53 level, while GA20 and GA1 levels decreased, suggesting that the GA53→GA44, as well as GA19→GA20, conversions were slowed down. The roles of GA1 in the control of leaf elongation and fructan mobilization following defoliation are discussed.     

Effects of sheath tube length on leaf development in perennial ryegrass (Lolium perenne L.)

Position in and contribution of leaf laminae to the canopy of forage grasses are important both in determining herbage growth rates and intake rate by grazing animals. These canopy characteristics are controlled by the way dry matter is apportioned between sheath and lamina in growing leaves. The objective of this work was to determine how the development of individual leaves is affected by altering the effective length of the psuedostem tube, on the assumption that the light environment within the tube varied. The development of a leaf from initiation at the apex to maturity was followed by successive destructive dissections of tillers. Vertical incisions were made in the pseudostem of each tiller to three different depths. The three treatments imposed were — no incision (control), moderate and severe incision of the sheath length. Destructive harvests of tillers followed 3, 6, 12 and 24 days after imposition of treatments. Incision resulted in the length of the monitored leaf being reduced significantly at all harvests, and differentiation of the sheath beginning earlier. The length reduction reflected a reduction in both cell size and cell number and the effects were evident at the earliest harvest. The data support the theory that leaf size and timing of onset of sheath development are influenced by the environment of the developing leaf. The present results indicate that sheath tube length affects leaf development and suggests that the effects are substantially explained by a direct light effect on the location and depth of the elongation zone.     

Cell growth analysis during steady and non-steady growth in leaves of perennial ryegrass (Lolium perenne L.) subject to defoliation

The effect of defoliation on leaf elongation rate (LER) and on the spatial distribution of epidermal cell lengths in the leaf growth zone was studied in vegetative main tillers of perennial ryegrass (Lolium perenne L. cv Modus) grown in a controlled environment. A new material approach was used to analyse the responses of epidermal cell expansion and production during the initial, non‐steady growth phase following defoliation. The analysis involved assigning an identity to individual expanding cells, assessing the displacement and estimating the expansion of cells with assigned identity during day 1 and day 2 after defoliation. LER decreased by 34% during the first 2 d after defoliation and did not recover to the pre‐defoliation rate within the 14 day regrowth period. Decreased LER on day 1 and day 2 after defoliation was associated with (i) a decrease in the length of the leaf growth zone; (ii) a decrease in the length at which epidermal cells stopped expanding; (iii) a reduced expansion of cells at intermediate growth stages; and (iv) a reduction in cell production (i.e. division) and an associated decrease in the number of expanding cells in the growth zone. However, defoliation had no effect on the expansion of cells located in the proximal part of the growth zone. Reduced LER at 14 d after defoliation was associated with a reduced cell production rate (27% lower than the pre‐defoliation rate) and decreased final cell size ( ? 28%).     

Spatial distribution of growth rates and of epidermal cell lengths in the elongation zone during leaf development in Lolium perenne L.

Relative elemental growth rates (REGR) and lengths of epidermal cells along the elongation zone of Lolium perenne L. leaves were determined at four developmental stages ranging from shortly after emergence of the leaf tip to shortly before cessation of leaf growth. Plants were grown at constant light and temperature. At all developmental stages the length of epidermal cells in the elongation zone of both the blade and sheath increased from 12 m at the leaf base to about 550 m at the distal end of the elongation zone, whereas the length of epidermal cells within the joint region only increased from 12 to 40 m. Throughout the developmental stages elongation was confined to the basal 20 to 30 mm of the leaf with maximum REGR occurring near the center of the elongation zone. Leaf elongation rate (LER) and the spatial distributions of REGR and epidermal cell lengths were steady to a first approximation between emergence of the leaf tip and transition from blade to sheath growth. Elongation of epidermal cells in the sheath started immediately after the onset of elongation of the most proximal blade epidermal cells. During transition from blade to sheath growth the length of the blade and sheath portion of the elongation zone decreased and increased, respectively, with the total length of the elongation zone and the spatial distribution of REGR staying near constant, with exception of the joint region which elongated little during displacement through the elongation zone. Leaf elongation rate decreased rapidly during the phase when only the sheath was growing. This was associated with decreasing REGR and only a small decrease in the length of the elongation zone. Data on the spatial distributions of growth rates and of epidermal cell lengths during blade elongation were used to derive the temporal pattern of epidermal cell elongation. These data demonstrate that the elongation rate of an epidermal cell increased for days and that cessation of epidermal cell elongation was an abrupt event with cell elongation rate declining from maximum to zero within less than 10 h.Abbreviations LER leaf elongation rate - REGR relative elemental growth rates     

Changes in the amide and amino acid composition of xylem exudate from perennial ryegrass (Lolium perenne L.) during regrowth after defoliation

The paper presents the results of amino acid analyses in xylem sap during leaf regrowth of ryegrass plants defoliated firstly at the 8th and secondly at the 12th week of culture. The free amino acid composition of leaves, stubble and roots was also determined and some of the results are reported. Prior to defoliation, xylem sap contained a high proportion of amides, particularly glutamine. During regrowth after defoliation, the proportion of asparagine in the xylem sap increased until the third day when the highest ratios of asparagine/glutamine appeared. The results are compared with relative amounts of free amino acids in the different plant parts and discussed in relation to source-sink nitrogen transfer.     

Carbon and nitrogen deposition in expanding tissue elements of perennial ryegrass (Lolium perenne L.) leaves during non-steady growth after defoliation

The effect of defoliation on the deposition of carbon (C) and nitrogen (N) and the contribution of reserves and current assimilates to the use of C and N in expanding leaf tissue of severely defoliated perennial ryegrass (Lolium perenne L.) was assessed with a new material element approach. This included ¹³C/¹²C-and ¹⁵N/¹⁴N-steady-state labelling of all post-defoliation assimilated C and N, analysis of tissue expansion and displacement in the growth zone, and investigation of the spatial and temporal changes in substrate and label incorporation in the expanding elements prior to and after defoliation. The relationship between elemental expansion and C deposition was not altered by defoliation, but total C deposition in the growth zone was decreased due to decreased expansion of tissue at advanced developmental stages and a shortening of the growth zone. The N deposition per unit expansion was increased following defoliation, suggesting that N supply did not limit expansion. Transition from reserve- to current assimilation-derived growth was rapid (<1 d for carbohydrates and approximately 2 d for N), more rapid than suggested by label incorporation in growth zone biomass. The N deposition was highest near the leaf base, where cell division rates are greatest, whereas carbohydrate deposition was highest near the location of most active cell expansion. The contribution of reserve-derived relative to current assimilation-derived carbohydrates (or N) to deposition was very similar for elements at different stages of expansion     

Partitioning of reserve and newly assimilated carbon in roots and leaf tissues of Lolium perenne during regrowth after defoliation: assessment by 13C steady-state labelling and carbohydrate analysis

The relative significance of the use of stored or currently absorbed C for the growth of leaves or roots of Lolium perenne L. after defoliation was assessed by steady-state labelling of atmospheric CO₂. Leaf growth for the first two days after defoliation was to a large extent dependent on the use of C reserves. The basal part of the elongating leaves was mainly new tissue and 91% of the C in this part of the leaf was derived from reserves assimilated prior to defoliation. However, half of the sucrose in the growth zone was produced from photosynthesis by the emerged leaves. Fructans that were initially present in elongating leaf bases were hydrolysed (loss of 93 to 100%) and the resulting fructose was found in the new leaf bases, suggesting that this pool may be used to support cell division and elongation. Despite a negative C balance at the whole-plant level, fructans were synthesized from sucrose that was translocated to the new leaf bases. After a regrowth period of 28 d, 45% of the C fixed before defoliation was still present in the root and leaf tissue and only 1% was incorporated in entirely new tissue.     

Influence of initial levels of carbohydrates, fructans, nitrogen, and soluble proteins on regrowth of Lolium perenne L. cv. Bravo following defoliation

An experiment was designed to evaluate the role of N and C reserves on regrowth of Lolium perenne cv. Bravo following defoliation. By using two nitrogen fertilization levels together with three photoperiodic conditions, plants with variable contents of water-soluble carbohydrates (43-216 mg g^-1 DW in stubble) and contrasting amounts of nitrogen (7-49 mg g^-1 DW) were obtained. Plants were severely defoliated and regrowth was followed for 28 d under the same environmental conditions. The yield of leaf dry matter at the end of the regrowth period was not related to the initial level of carbohydrate reserves. However, levels of fructan in leaf sheaths and in elongating leaf bases strongly influenced the shoot yield during the first 2 d following defoliation. Fructan exohydrolase activity increased 2-3-fold in sheaths and 3.5-5-fold in elongation leaf bases, suggesting that not only fructans from sheaths but also fructans from immature cells may be used as substrates for growth. In contrast, no direct relationship was found between shoot production and nitrogen or soluble protein accumulation in source organs during early regrowth. A significant correlation existed with the initial amount of soluble proteins in sheaths and in elongating leaf bases after only 6 d of regrowth.     

The effect of an experimental growth retardant (WL83801) on leaf growth and development in Lolium perenne L.

The experimental growth retardant WL83801, applied as a root drench, had a rapid and persistent effect in retarding the growth and development of leaves in L. perenne. Leaves of the main shoot were greatly reduced in length, were broader, and appeared faster than in control plants. The rate of extension of individual leaves was greatly reduced in retarded plants but still followed a diurnal pattern that closely corresponded with temperature. There was evidence that leaf extension was far less responsive to temperature in treated plants. At the cellular level WL83801 had no significant effect on leaf blade cell number, thus reductions in leaf length were associated with the retardation of cell elongation. Changes in leaf structure were also observed. These changes in the pattern of leaf growth and development are discussed in relation to the primary mode of action of the growth retardant in interfering with gibberellin biosynthesis.     

Species-specific trade-offs between regrowth and mycorrhizas in the face of defoliation and phosphorus addition

Arbuscular mycorrhizal fungi (AMF) enhance nutrient provision in exchange for carbon. However, a shift from nutrient to carbon limitation, induced by repeated or intense defoliation, can represent a trade-off between plant regrowth and the maintenance of mycorrhiza. The combined effects of AMF, phosphorus and defoliation on growth of Agropyron elongatum (C₃ grass, low mycorrhizal responsiveness) and Brachiaria brizantha (C₄ grass, high mycorrhizal responsiveness) were explored. Each species was subjected to a manipulative experiment with AMF inoculation (non-inoculated, inoculated), soluble P supply (low, high) and defoliation (non-defoliated, 60% defoliated). In A. elongatum, at low P supply, mycorrhizal plants showed increased growth rates following defoliation without substantial changes in AMF colonization. At high P supply instead, we found a clear trade-off between regrowth and the maintenance of mycorrhiza evidenced by growth depression (biomass and tillers) and lower AMF activity (reduction of arbuscules). In contrast, in B. brizantha, defoliation effects on plant regrowth were independent from AMF at any P supply (no trade-off). This indicates that cost-benefit relationship in defoliated plants is highly context-dependent and may vary with mycorrhizal responsiveness of species. This variation of responses can play a decisive role on plant recovery in pastures and natural grasslands subjected to herbivory.     

Effects of a stay-green mutation on plant nitrogen relations in Lolium perenne during N starvation and after defoliation

The stay-green mutation of the nuclear gene sid results in inhibition of chlorophyll degradation during leaf senescence in grasses, reducing N remobilization from senescing leaves. Effects on growth of Lolium perenne L. were investigated during N starvation (over 18 d) and after severe defoliation, when leaf growth depends on the remobilization of internal N. Rates of dry mater production, partitioning between shoots and roots, and re-partitioning of N from shoots to roots were very similar in stay-green and normal plants under N starvation. Km and Vmax for net uptake of NH4+ were also similar for both genotypes, and Vmax increased with the duration of N deprivation. The mutation had little effect on recovery of leaf growth following severe defoliation, but stay-green plants recommenced NO3- and K+ uptake 1 d later than normal plants. Import of remobilized N into new leaves was generally similar in both lines. However, stay-green plants remobilized less N from stubble compared with normal plants. It was concluded that the sid locus stay-green mutation has no significant adverse effect on the growth of L perenne during N starvation, or recovery from severe defoliation when plants are grown under an optimal regime of NO3- supply both before and after defoliation. The absence of any effect on leaf dry matter production implies that the difference in foliar N availability attributable to this mutation has little bearing on productivity, at least in the short to medium term.     

黑麦草内生真菌感染状况的检测及定量分析

选取内生真菌特异性引物,成功建立了利用PCR技术对黑麦草中内生真菌感染状况的检测和定量分析方法。此检测方法的准确性高于常规乳酸-苯胺蓝染色法。利用实时荧光PCR定量分析的结果表明：不同植株之间内生真菌含量差异较大,而同株植物相同龄级分蘖之间内生真菌含量无显著差异。由此可见内生真菌的含量不仅与植物种以及品种有关,也与植物的基因型密切相关。