Daniel O. Giménez

The effects of submergence on anatomical, morphological and biomass allocation responses of tropical grasses Chloris gayana and Panicum coloratum at seedling stage

Abstract. Submergence is a major factor affecting seedling recruitment in lowland grassland ecosystems. Our aim was to evaluate the tolerance to increasing flooding intensity of the seedlings of tropical grasses Chloris gayana K. and Panicum coloratum L., whose use as a forage species is increasing in humid grasslands. For this purpose, 2-week-old seedlings of C. gayana and P. coloratum were subjected to control, partial submergence (PS) and complete submergence (CS) in clear water for 14 days and allowed to grow for a subsequent 12-day period to assess their recovery. The following responses were assessed: generation of root aerenchyma, morphological changes and emergence from water, biomass allocation in relation to plant size, and biomass accumulation. Results showed that constitutive root aerenchyma was high in both species. Under PS and CS, root aerenchyma increased by up to 50–55% in C. gayana and up to 40–48% in P. coloratum. Under PS, the increase in seedling height for both species was the same as for controls. Under CS, C. gayana further increased its height and emerged more quickly from water; P. coloratum was not able to increase its height, and therefore the seedlings always remained underwater. The escape-from-water response of C. gayana was associated with preferential biomass allocation towards shoots and with a marked lengthening of leaf blades. By contrast, there was no change in allocation in P. coloratum, and its leaves were shorter under CS. The final biomass of C. gayana under CS was similar to that under PS, and equivalent to 54% of its controls. In P. coloratum, biomass under PS and CS were 64 and 21% of its controls (respectively), which indicates that injury caused by CS persisted during the post-submergence period. In conclusion, both species are tolerant to PS at the seedling stage. However, when flood depth increases by submerging the seedlings, C. gayana is able to escape from water while P. coloratum is not, thus strongly affecting its recovery. Therefore, C. gayana appears to be a more promising species for cultivation in lowland grasslands prone to flooding of unpredictable intensity.

Greenbug systemic effect on barley phosphate influx

Abstract Greenbugs ( Schizaphis graminum Rond.) cause considerable yield loss in cereals. Local feeding damage of greenbug-infested leaves includes collapsed mesophyll cells, chlorosis, alterations in photosynthesis and respiration. However, this damage cannot explain rapid changes taking place in plant metabolism (inhibition of new leaf primordia and new root differentiation, within a few hours after attack), or the early death of such plants. This study was aimed at determining whether greenbug feeding induces systemic damage to barley. The phosphate influx by roots of susceptible and tolerant barley ( Hordeum vulgare L.) plants was evaluated as an estimate of aphid systemic damage. Phosphate (P)-influx was determined at two plant growth stages, with two levels of greenbug infestation, at two different greenbug life stages. Plants grown in hydroponics in a glasshouse were infested for 0 (control), 3, 6, 12, 24, 48, and 72 h with the Argentinean biotype C greenbug. The P-influx was not significantly affected in tolerant barley plants by greenbug infestation. In contrast P-influx was significantly reduced 6 h post-infestation in the susceptible cultivar. Plants with one expanded leaf suffered a significantly greater reduction in P-influx than plants with two expanded leaves. By 48 h after infestation, the P-influx of the two-expanded-leaf treatment was similar to that of the controls, whereas P-influx in plants with one expanded leaf remained significantly less than on the controls 72 h after infestation. A larger greenbug population resulted in greater reduction in P-influx. Adult greenbugs, but not third stage nymphs, affected P-influx. In summary, the intensity of greenbug-induced systemic damage was greater when young plant stages were infested by the aphid. Reductions of P-influx may become critical under increasing natural infestation levels.

Ability to recover overrides the negative effects of flooding on growth of tropical grasses Chloris gayana and Panicum coloratum

Abstract. This study assessed the flooding tolerance of the tropical grasses Chloris gayana Kunth and Panicum coloratum L. at different times of the year: (i) late winter flooding for 50 days (WF), (ii) early spring flooding (SF) for 20 days, and (iii) long-term flooding covering both periods (WF + SF, 70 days). A growth period under well-watered conditions was allowed after each flooding event to assess recovery of plant species. Plants were harvested after each flooding event and at the end of the recovery period. Panicum coloratum had higher tolerance to WF than C. gayana. Treatment WF did not affect biomass in P. coloratum, whereas it reduced biomass of flooded plants by 38% in C. gayana. Treatment SF did not differentiate the species for tolerance; both registered moderate reduction in their growth (20–30%). Under WF + SF, C. gayana showed additional reduction in its growth over that observed when subjected separately to either WF or SF, whereas P. coloratum did not. Both species displayed remarkably fast recovery from flooding when temperatures rose during early summer, attaining biomass equivalent to that of non-flooded plants 1 month after water subsided. Therefore, although P. coloratum appears slightly more tolerant during flooding than C. gayana, both species are promising for introduction in temperate lowland grasslands.

Efecto del glifosato sobre el crecimiento y acumulación de azúcares libres en dos biotipos de lolium perenne de distinta sensibilidad al herbicida

El movimiento sistemico del glifosato esta determinado por el transporte de fotoasimilados. A su vez, la capacidad de un destino de consumir los asimilados esta condicionada por su actividad metabolica. Pese a su importancia, la relacion entre el glifosato y la sintesis de azucares en hojas fuente ha sido poco abordada. El objetivo del presente trabajo fue evaluar los efectos del glifosato sobre el crecimiento y la acumulacion de azucares libres en dos biotipos de Lolium perenne de baja y alta sensibilidad al herbicida. Se trabajo con clones de ambos tipos de plantas, en macollaje, tratados con 1.440 g e.a. ha-1 de glifosato y sin tratamiento herbicida como controles. Se evaluo periodicamente el efecto del glifosato sobre el rebrote de hojas hasta las 50 horas post-aplicacion y sobre los niveles de azucares libres totales, reductores y no reductores en hojas a 1, 2, 3 y 5 dias post-aplicacion. A partir de las 25 horas post-aplicacion, el glifosato provoco una disminucion del crecimiento del 58% en el biotipo susceptible, con una acumulacion de azucares libres superior al 90% con relacion al control, desde el primer dia post-aplicacion en adelante. La inhibicion del crecimiento, inducida por el glifosato en plantas susceptibles, no depende de la limitacion del traslado de fotoasimilados desde la parte aerea. Por tanto, la acumulacion de azucares libres en hojas podria explicarse por la caida en la tasa de crecimiento. En el biotipo de baja sensibilidad, en el que no se detecto inhibicion del crecimiento, estos efectos fueron limitados.

Increase in forage yield in narrowleaf birdsfoot trefoil (Lotus tenuis Waldst & Kit ex Willd) in a permanent pasture with foliar applied gibberellic acid (GA3), and phosphorus

Lotus tenuis is a perennial legume with a good adaptation to infertile, heavy and waterlogging soils. It can replace alfalfa in these sites with a similar feeding value. An important constraint is its weak competitive capacity with other graminae and weed species in permanent pastures, having consequently a poor forage yield. The objective of the present research is to overcome this disadvantage, enhancing its competitive ability with foliar applications of GA3 (GA) and phosphorus (P), increasing L. tenuis forage yield. Field experiments were conducted during 1994 with foliar application of GA (50 mg.l-1) and during 1995 with foliar application of GA (25 and 50 mg.l-1), phosphorus (8 kg.ha-1, as P2O5) and their combinations, in permanent pastures with L. tenuis and other companion grasses. In 1994 GA 50 increased significantly L. tenuis dry matter (DML) in 64.6% but not the dry matter of graminae fraction (DMG) and in consequence the total dry matter of the pasture (TDMP) was increased. In 1995 all GA treatments and their combinations with phosphorus enhanced DML but not DMG. In this sense GA 25 + P was the most effective treatment with a 151% increment of DML. Consequently TDMP was significantly increased due to a larger participation of L. tenuis in the forage yield. This increase was achieved due to a greater length and diameter of L. tenuis branches, with a logical modification in leaf:stem ratio. Moreover GA treatments reduced L. tenuis flower number. Phosphorus treatment, applied alone, showed an increase in the DML. GA treatments did not modify the feeding value of the forage in L. tenuis and graminae fractions, except GA 50 and GA 50 + P in acid detergent fiber (ADF), neutral detergent fiber (NDF) and crude protein (CP), respect to the control. The total crude protein (CP.m-2) was enhanced in all GA and GA + P treatments. Foliar GA3 and phosphorus spray applications increased the competitiveness of trefoil for light, on account of morphological changes in the spatial disposition of L. tenuis stems reaching faster the top of the pasture canopy. This practice can be an adequate alternative to increase the forage yield and total crude protein in permanent and cultivated pastures with a low cost-benefit ratio.

Interaction of night temperature and photoperiod on Bromus unioloides H.B.K. aerial biomass components

Abstract The interaction between night temperature and photoperiod with aerial biomass components in two Bromus unioloides H.B.K. populations during the first months of their growth cycle was studied. Plants were cultivated at three temperatures (17, 21 or 27°C) under two photoperiods (SD: 8 hr sunlight and LD: 8 hr daylight+8 hr low intensity light). As temperature decreased tiller number increased; it was higher at 17°C for SD and for LD at 27°C. No differences due to photoperiod were registered at 21°C. At 27°C leaf emergence was minor in the main tiller but increased during stem extension at all three temperatures under LD. Leaf number per plant was determined by the number of tillers (r2 = 0.88). As a result of the interaction, foliar area was high under LD at 17°C and under SD at 27°C, and increased as temperature decreased, with wider differences between means under LD. Sheath length showed a similar morphogenic response, with no differences between temperatures under SD. The variations in leaf number per plant and foliar area per leaf were inversely related. Consequently, total leaf area was not affected by daylength and increased as temperature decreased. Aerial biomass depended upon foliar area (r2 = 0.95). It is concluded that high relative night temperature limited Bromus productivity and that photoperiod interacted with night temperature to adapt tillering and foliar area to natural conditions.

Mechanism of Resistance to Glyphosate in Lolium perenne from Argentina

In Argentina, glyphosate resistance was reported in a Lolium perenne population after 12 years of successful herbicide use. The aim of the current paper was to put in evidence for the mechanism of glyphosate resistance of this weed. Susceptible leaves treated with different doses of glyphosate and incubated in vitro showed an accumulation of shikimic acid of around three to five times the basal level, while no changes were detected in leaves of glyphosate-resistant plants. The resistance mechanism prevents shikimate accumulation in leaves, even under such tissue-isolation conditions. The activity of the glyphosate target enzyme (EPSPS: 5-enolpyruvylshikimate-3-phosphate synthase) was quantified at different herbicide concentrations. EPSPS from resistant plants showed no difference in glyphosate-sensitivity compared to EPSPS from susceptible plants, and, accordingly, no amino acid substitution causing mutations associated with resistance were found. While the glyphosate target enzymes were equally sensitive, the basal EPSPS activity in glyphosate resistant plants was approximately three-fold higher than the EPSPS activity in susceptible plants. This increased EPSPS activity in glyphosate resistant plants was associated with a 15-fold higher expression of EPSPS compared with susceptible plants. Therefore, the over-expression of EPSPS appears to be the main mechanism responsible for resistance to glyphosate. This mechanism has a constitutive character and has important effects on plant fitness, as recently reported.

Effects of deferred forage as winter cover on spring growth of the tropical grasses Chloris gayana and Panicum coloratum

This pot study assessed the effects of deferring forage during autumn and leaving as winter cover on reducing cold damage to plants of 2 tropical (C4) grasses ( Chloris gayana and Panicum coloratum ) in a temperate environment in La Plata, Buenos Aires, Argentina. Mature plants were subjected to the following treatments: (i) autumn-deferred forage retained (DF) as control; and (ii) autumn-deferred forage removed (DFR) cutting at 15 cm from soil level at beginning of winter. This experiment had 10 replicates per treatment and 1 plant per pot (experimental unit). Plants of both species were extracted from a commercial beef farm and transplanted into an experimental garden in pots where they grew outdoors from 2 February to 23 May (111 days) when treatments were applied. After winter, both grasses were cut to 15 cm in early spring (27 September) and spring growth was measured in November. Shoot biomass was harvested at ground level and separated into lower and upper layers (above and below 15 cm), leaf blades, sheaths and stolons. Daily air temperature, relative humidity and frost events were registered. Allometric analysis of shoot biomass was performed to determine the stress incidence by cold. DF plants achieved 55‒80% higher shoot biomass than DFR plants during spring in both species. The allometric analysis revealed for P. coloratum significant relationships between shoot biomass from plant compartments (lower and upper layers, leaf blade and sheath) and total shoot biomass in both treatments, indicating good cold tolerance. However, for C. gayana, unlike DF plants, DFR plants were strongly stressed, showing a lack of shoot biomass fit. These results suggest that deferring autumn forage growth and retaining as winter cover may improve survival during winter and productivity during spring of these two tropical grasses in temperate pastoral systems. However, the study needs to be repeated under field conditions and under grazing or cutting over a number of years in different situations to verify these preliminary results. Keywords : Winter cover, cold damage, frost protection, pasture management.