Leaf growth in monocotyledons results from the flux of newly born cells out of the division zone and into the adjacent elongation-only zone, where cells reach their final length. We used a kinematic method to analyze the effect of phosphorus nutrition status on cell division and elongation parameters in the epidermis of Lolium perenne. Phosphorus deficiency reduced the leaf elongation rate by 39% due to decreases in the cell production rate (219%) and final cell length (220%). The former was solely due to a lower average cell division rate (0.028 versus 0.046 cell cell 21 h 21 ) and, thus, a lengthened average cell cycle duration (25 versus 15 h). The number of division cycles of the initial cell progeny (five to six) and, as a result, the number of meristematic cells (32-64) and division zone length were independent of phosphorus status. Accordingly, low-phosphorus cells maintained meristematic activity longer. Lack of effect of phosphorus deficiency on meristematic cell length implies that a lower division rate was matched to a lower elongation rate. Phosphorus deficiency did not affect the elongation-only zone length, thus leading to longer cell elongation duration (99 versus 75 h). However, the substantially reduced postmitotic average relative elongation rate (0.045 versus 0.064 mm mm 21 h 21) resulted in shorter mature cells. In summary, phosphorus deficiency did not affect the general controls of cell morphogenesis, but, by slowing down the rates of cell division and expansion, it slowed down its pace.

Lotus corniculatus L. and Lotus glaber Mill. are warm-season legume species adapted to many kinds of environmental stress, including flooding conditions, whereas other popular forage legumes, like alfalfa or white clover, cannot thrive. This study evaluates the relationship between root aerenchyma, water relations and leaf gas exchange and the differential tolerance to soil flooding of L. corniculatus and L. glaber. Adult plants of these species, established independently in grasslands mesocosms, were subjected to 40 days of early spring flooding at a water depth of 6 cm. Both species presented constitutive aerenchyma tissue in the roots. Under flooding conditions, this parameter was 26.2% in L. glaber and 15.3% in L. corniculatus. In addition, flooded plants of L. glaber presented a leaf biomass 47.5% higher above water while L. corniculatus showed a leaf biomass 59.6% lower in the same layer, in comparison to control plants. Flooded plants of L. glaber maintained the stomatal conductance (g s ) and transpiration rate (E) for 25 days, although these parameters reduce slightly to 40À60% in comparison to controls after 40 days of flooding. In this species, a reduction in photosynthesis (A) in flooding conditions was detected only on the last day of measurement. In L. corniculatus, the same parameters (g s , E and A) were affected by flooding since day 18 of treatment, and values reached 25À40% in comparison to control plants after 40 days of flooding. Flooding did not affect above-ground biomass in L. glaber; while in L. corniculatus, above-ground biomass was 35% lower than in control plants. Our results confirmed that L. glaber is more able to cope with flooding stress than L. corniculatus, even in the presence of natural competitors. On the whole, this experiment provides information that can aid in the identification of anatomical and physiological parameters associated with flood-tolerance in this forage legume species, with economic potential for the agricultural areas subject to periodic flooding.

The objective of this work was to study the existence of a trade-off between aerenchyma formation and root mechanical strength. To this end, relationships among root anatomical traits and mechanical properties were analysed in plant species with contrasting root structural types: Paspalidium geminatum (graminaceous type), Cyperus eragrostis (cyperaceous type), Rumex crispus (Rumex type) and Plantago lanceolata (Apium type). Variations in anatomical traits and mechanical strength were assessed as a function of root diameter by exposing plants to 0, 7, 15 and 30 d of control and flooded conditions. For each species, the proportion of root cortex was positively associated with the increment of root diameter, contributing to the increase in root porosity under both control and flooded conditions. Moreover, cell lysis produced an additional increase in root porosity in most species under flooded conditions (except R. crispus). Both structural types that presented a uniseriate layer (epidermis) to cope with compression (Rumex and Apium types) were progressively weakened as root porosity increased. This effect was significant even when the increment of root porosity was solely because of increased root diameter (R. crispus), as when both processes (root diameter and cell lysis) added porosity to the roots (P. lanceolata). Conversely, structural types that presented a multiseriate ring of cells in the outer cortex (graminaceous and cyperaceous types) maintained mechanical strength over the whole range of porosity, in spite of lysogenic processes registered in the inner cortex. In conclusion, our study demonstrates a strong trade-off between aerenchyma formation and mechanical strength in root structural types that lacked a multiseriate ring of tissue for mechanical protection in the outer cortex. The results suggest that this ring of tissue plays a significant role in maintaining the mechanical strength of roots when flooding induces the generation of additional aerenchyma tissue in the root cortex.

We studied individual responses to flooding in the perennial grass Paspalum dilatatum , a widespread species in the Flooding Pampa of Argentina, using plants established in species-diverse grassland microcosms.• Flooding effects were evaluated on root and leaf sheath anatomy and shoot morphological traits. Leaf water status and CO 2 exchange rates were monitored in flooded and unflooded plants under changing, natural and controlled atmospheric conditions. • Root porosity and leaf sheath aerenchyma increased with flooding. Leaf extension rates and tiller height were also higher in flooded plants, which resulted in a large fraction of the shoot architecture emerging above the water surface. Flooding enhanced stomatal conductance, leaf water potential and net photosynthesis, especially under conditions leading to high air-vapour pressure deficits. Therefore, flooded plants experienced fewer water deficits during periods of high atmospheric evaporative demand. • P. dilatatum showed tight regulation of water and carbon relations under severe soil-oxygen deficiency, even in the presence of natural competitors. The suite of adaptive responses documented here might help to explain the observed increase in abundance of this species during extensive floods.

The forage legume L. tenuis has the flexibility either to escape from partial submergence by elongating its shoot more vigorously to avoid becoming totally submerged or to adopt a non-elongating quiescent strategy when completely immersed that is based on utilizing stored reserves. The possession of these alternative survival strategies helps to explain the success of L. tenuis in environments subjected to unpredictable flooding depths.

This study tested whether leaf elongation rate ( LER , mm h) and its components -average relative elemental growth rate ( REGR avg , mm mm) and leaf growth zone length ( L LGZ , mm) -are related to phosphorus (P) concentration in the growth zone ( P LGZ , mg P g− 1 tissue water) of Lolium perenne L. cv. Condesa and whether such relationships are modified by the arbuscular mycorrhizal fungus (AMF) Glomus hoi . Mycorrhizal and non-mycorrhizal plants were grown at a range of P supply rates and analysed at either the same plant age or the same tiller size (defined by the length of the sheath of the youngest fully expanded leaf). Both improved P supply (up to 95%) and AMF (up to 21%) strongly increased LER . In tillers of even-aged plants, this was due to increased REGR avg and L LGZ . In even-sized tillers, it was exclusively due to increased REGR avg . REGR avg was strictly related to P LGZ ( r 2 = = = = 0.95) and independent of tiller size. Conversely, L LGZ strictly depended on tiller size ( r 2 = = = = 0.88) and not on P LGZ . Hence, P status affected leaf growth directly only through effects on relative tissue expansion rates. Symbiosis with AMF did not modify these relationships. Thus, no evidence for P status-independent effects of AMF on LER was found.Key-words : arbuscular mycorrhizal fungi; Glomus hoi ; leaf elongation rate; leaf growth zone; Lolium perenne L.; relative elemental growth rate Abbreviations : AMF, arbuscular mycorrhizal fungus/fungi; LER , leaf elongation rate; L LGZ , length of the leaf growth zone; P LGZ , concentration of P in leaf growth zone; REGR avg , average relative elemental growth rate; WSC, total water soluble carbohydrates.

Summary 1.We studied the differences in root strength of species with contrasting root structural types (the grass Paspalum dilatatum and the dicot Lotus glaber ), and their relationship with tolerance to simulated cattle trampling under flooding conditions. 2. Root strength was analysed through measurement of the pressure required to cause root collapse. The responses of aerenchyma and plant mass to flooding and trampling were studied. 3. Root aerenchyma increased from 28·0 to 40·2% in P. dilatatum and from 12·9 to 19·7% in L. glaber under flooding conditions. The increase in aerenchyma did not affect root strength in the relatively trampling-resistant roots of P. dilatatum : roots cracked at >380 kPa in all treatments. In contrast, roots of L. glaber were weaker, cracking at 260 kPa; flooded roots with air spaces irregularly dispersed in the cortex cracked at 115 kPa. 4. Trampling, flooding or their combination did not affect the biomass of P. dilatatum . Conversely, the isolated effects of either trampling or flooding both decreased biomass accumulation in L. glaber . The combination of both treatments killed all Lotus plants . 5. In conclusion, root strength was positively associated with soil trampling tolerance. The effect of aerenchyma tissue generation on root strength varies among root structural types. Aerenchyma tissue increases root weakness in the less stable structural type of the dicot species, but had no effect on the strength of the grass.

Natural flooding is one of the major factors affecting vegetation dynamics in many regions of the world. The Flooding Pampa Grasslands (Argentina) are frequently exposed to flooding events of diverse intensity and duration, some of which Leontodon taraxacoides, an exotic dicot. frequent in these grasslands, seems to survive. Its responses to four different water depths (0, 1, 7 and 13 cm) were studied. The results indicate that plants in conditions of total submergence (depth of 13 cm) did not survive. In less severe flood conditions, increases in the leaf insertion angle resulted in the maintenance of a large proportion of the total leaf area above the water. Differences in leaf length and a decrease in the width and the proportion of lobes per leaf were also found under partial submergence conditions (depth of 7 cm). Root and leaf aerenchyma, present in unflooded plants, showed a significant increase in flood conditions. In spite of the anatomical and morphological responses, total biomass and leaf area were severely affected by water depth. Control plants allocated more biomass to reproductive organs, while partly submerged plants allocated more to leaves and less to reproductive organs. Mature L. taraxacoides plants presented a wide range of plastic adjustment as a survival strategy in soil anaerobiosis, and appear to be able to survive short spring floods in a vegetative state ; in contrast, they might not tolerate total submergence conditions imposed by more intense and long-lasting floods.