Ernesto Gianoli

FEEDING BY THE APHID Sipha flava PRODUCES A REDDISH SPOT ON LEAVES OF Sorghum halepense: AN INDUCED DEFENSE?

Feeding by the aphid Sipha flava produces a reddish spot on mature leaves of Sorghum halepense. The present work is aimed at determining whether this plant response entails induced resistance against the aphid. Old and young leaves showed the same response to aphid feeding (reddish coloration). Water-stressed plants displayed a similar reddish coloration to aphid-infested plants. This was verified by evaluation of absorbance peaks of the respective leaf extracts. Aphid fecundity was reduced on previously infested (and hence reddish colored) leaves. However, aphid fecundity was not affected on water stressed plants. Furthermore, aphid survival was not different on artificial diets containing increasing concentrations of the reddish pigment. It is concluded that the reddish spot is correlated with, but is not itself responsible for, the observed induced resistance of S. halepense against S. flava.

Characteristics of hydroxamic acid induction in wheat triggered by aphid infestation

Hydroxamic acids (Hx) are natural products of Gramineae that are associated with cereal resistance to pests. We aimed at characterizing the induction of Hx accumulation in seedlings of wheat,Triticum aestivum, by short-term infestation of the cereal aphid,Rhopalosiphum padi. A load of 25 aphids increased significantly the Hx levels in the infested primary leaf in comparison with control levels. Lower loads did not increase Hx concentration. Aphid infestation lasting 16 hr did not elicit induction of Hx, even after a time-lag of 32 hr to allow the expression of any induced response. Forty-eight hours was the minimum duration of aphid infestation required to trigger Hx induction. The age of the infested tissue (the primary leaf) did not affect induction. Similar increases of Hx were found in unfolding, expanding, and totally expanded primary leaves. It was determined that the regime of nutrient supply (N-intensive nutritive solutions at low and high concentration) to wheat seedlings had no effect on the magnitude of the aphid-induced Hx (N-based secondary metabolites). Results obtained are discussed in the framework of general theories of plant defense allocation.

Phenotypic responses of the twining vine Ipomoea purpurea (Convolvulaceae) to physical support availability in sun and shade

Vines depend on external support to prevent shading by neighbouringplants. Hence, it is important to determine whether shading enhances thephenotypic responses of vines to support availability. I evaluated theconsequences of support availability (a vertical stake) on shoot and leaftraitsof the morning glory Ipomoea purpurea (Convolvulaceae)under full sunlight and extreme shade. It was hypothesised that phenotypicresponses of vines to support availability should be greater in the shade. Inaddition, to investigate possible constraints to such phenotypic responses, thecorrelations among phenotypic traits and the plasticity of such correlationswere evaluated. The phenotypic variation of the main stem length and of thenumber of branches was consistent with the hypothesis, i.e. greater responsestosupport availability in the shade. In contrast, both internode length and leafarea (two traits that showed a significant and positive correlation) decreasedin the sun and increased in the shade with support availability. Petiole lengthdecreased with support in the sun but had no response in the shade. On theotherhand, the number of significant trait correlations found in plants in the sunand supported plants was higher than those of shade and non-supported plants,respectively. Several of the correlations were significantly sensitive to theenvironment. Flowering only occurred in the sun treatment. Whereas no shoot orleaf trait was significantly correlated with flower number in supported plants,both petiole length and shoot biomass showed a significant correlation withsuchestimate of plant fitness in non-supported plants.

DEFOLIATION AFFECTS CHEMICAL DEFENSES IN ALL PLANT PARTS OF RYE SEEDLINGS

The effect of defoliation and consecutive defoliation (condefoliation) of rye seedlings on the allocation patterns of biomass and hydroxamic acids (Hx) was evaluated five days after treatment. Growth of condefoliated seedlings was lower than that of defoliated and nondefoliated ones. Concentration of Hx decreased in shoots of condefoliated seedlings compared to nondefoliated ones, while concentration of Hx in root exudates increased. Allocation of Hx to roots and root exudates increased at the expense of allocation to the shoots in condefoliated seedlings. The ratio of Hx-aglucone to Hx-glucoside was higher in shoots of defoliated and condefoliated seedlings. The decrease in quantity of defense in shoots was accompanied by an increase in its quality, given that aglucones are more toxic than glucosides. The increase in concentration of Hx—an allelopathic compound also involved in mineral uptake by roots—in root exudates of condefoliated seedlings was suggested to lead to an advantage in the acquisition of resources for the regeneration of lost biomass.

Environmental effects on the induction of wheat chemical defences by aphid infestation

The effects of temperature and photoperiod on the ability of wheat (Triticum aestivum L.) seedlings to show induced responses (increased accumulation of hydroxamic acids, Hx) upon infestation by the bird cherry-oat aphid, Rhopalosiphum padi L. were evaluated under laboratory conditions. Induction of Hx was significantly higher at lower temperatures. No such clear trend was found for the photoperiod effect. The significant effect of environmental conditions on growth rate of seedlings and the significant negative correlation between growth rate prior to infestation and induction of Hx suggested that environmental effects on induced responses were at least partially mediated through their effect on plant growth rate. After statistically uncoupling the effect of environmental conditions from the effect of plant growth rate, the effect of temperature on induction of Hx was no longer significant. Therefore, the temperature effect was mediated by plant growth rate.

DIBOA in wild Poaceae: Sources of resistance to the Russian wheat aphid (Diuraphis noxia) and the greenbug (Schizaphis graminum)

Hydroxamic acids are secondary metabolites typical of Poaceae that play a role in cereal resistance against pests and pathogens. The content of the hydroxamic acid aglucones DIBOA (2,4-dihydroxy-1,4-benzoxazin-3-one) and DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) was evaluated in wild Poaceae belonging to the tribes Triticeae (genera Hordeum and Elymus) and Aveneae (genera Deschampsia and Phalaris). The concentration of DIBOA in seedling extracts of the wild barleys Hordeum chilense, H. brevisubulatum subsp. violaceum and H. bulbosum was negatively correlated with parameters related to performance of the cereal aphids Schizaphis graminum and Diuraphis noxia. The relevance of the results obtained for breeding programs aimed at increasing cereal resistance against aphids is discussed.

Environmental Effects on the Accumulation of Hydroxamic Acids in Wheat Seedlings: The Importance of Plant Growth Rate

The effects of temperature and photoperiod on accumulation of hydroxamic acids (Hx) in wheat (Triticum aestivum L.) seedlings were evaluated under laboratory conditions. Hx concentrations were significantly higher at higher temperatures. No such clear trend was found for the photoperiod effect. The significant effect of temperature and photoperiod on growth rate of seedlings and the significant positive correlation between growth rate prior to analysis and levels of Hx, suggested that environmental effects on Hx accumulation were at least partially mediated through their effect on plant growth rate. After uncoupling the effect of environmental conditions from the effect of plant growth rate by statistical means the effect of temperature on Hx was no longer significant. Therefore, temperature effect was fully mediated by plant growth rate. Implications of the patterns found are discussed in issues of plant-defense general theories.

A phenotypic trade-off between constitutive defenses and induced responses in wheat seedlings

Abstract In plant defense theory, a trade-off between constitutive and induced defenses in plants is expected on the basis of cost-saving arguments. Thus, assuming that investment in defense is costly to the plant in terms of reduced growth and/or reproduction, induced responses would be a less expensive alternative when constitutive defenses are low. Conversely, plants with high levels of constitutive defenses would gain little from inducible defense systems. Despite this rationale, such a trade-off has rarely been detected. The present study reports the occurrence of a phenotypic trade-off between constitutive and aphid-induced levels of secondary metabolites in wheat seedlings. The trade-off was detected after examining the variation in induced responses as a function of the variation in constitutive defenses generated by plant ontogeny (8 ontogenetic stages, from 5 to 26 days after germination) and the environment (different regimes of temperature, light, and nutrient availability). Plant growth rate and the rate of change of constitutive levels of defense were identified as correlated traits potentially mediating the negative association between induced responses and constitutive defenses. The possible generalization of this result is discussed. Results are also discussed invoking efficacy issues as well as a hypothesis that variability in plant quality may function per se as a resistant factor against herbivores. It is suggested that both induced responses and increased rates of change of constitutive defenses may provide such beneficial variability.

Effect of defoliation on the patterns of allocation of a hydroxamic acid in rye (Secale cereale)

Abstract Hydroamic acids are a class of secondary metabolites typical of grasses which play a role in resistance to herbivores. The effects of defoliation (simulated herbivory) on the allocation patterns of a hydroxamic acid (DIBOA) in rye seedlings were evaluated. Defoliated plants were compared with undefoliated plants 5, 7, 9 and 11 days after defoliation and germination, respectively. Shoot DIBOA of defoliated and undefoliated seedlings decreased with increasing age, but DIBOA concentration in defoliated seedlings was generally higher than in undefoliated ones. On the other hand, DIBOA concentration of the primary leaf was not affected by defoliation. The contribution of the primary leaf to the DIBOA content of the whole shoot as well as to the biomass of the whole shoot decreased with time for both defoliation treatments. As age increased the primary leaf of undefoliated seedlings decreased in relative DIBOA content at a greater rate than in relative biomass. The primary leaf of defoliated seedlings showed a similar rate of decrease for both variables. This pattern as well as that for DIBOA concentration in defoliated vs. undefoliated seedlings was interpreted as consistent with predictions of the Optimal Defense theory. In addition, the increased level of DIBOA in the shoot and, to a lesser extent, the unchanged DIBOA level in primary leaf of defoliated seedlings supported an extension of the ‘reversion to juvenile phase’ phenomenon described for woody plant-mammal interactions.

Changes in growth and chemical defences upon defoliation in maize

The effect of defoliation on growth and on levels and allocation patterns of hydroxamic acids (Hx) in maize seedlings was evaluated 6 days after treatment. No significant differences were found between defoliated and nondefoliated plants for the Hx concentration, relative Hx content and Hx-aglucone to Hx-glucoside ratio in shoots, roots and root exudates, with the exception of Hx concentration in shoots, which decreased upon defoliation. However, growth of defoliated seedlings was considerably higher than that of nondefoliated ones. These results indicate that maize responds to defoliation by allocating its resources mainly to growth rather than to defence. Since previous work described the opposite strategy in rye, responses to defoliation in both species are discussed in relation to current theories of plant defence.