GREENBUG SYSTEMIC EFFECT ON BARLEY PHOSPHATE INFLUX

Greenbugs (Sckizaphis graminum Rond.) cause considerable yield loss in cereals. Local feeding damage of greenbug-infested leaves includes co llapsed 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 ro ot differentiation, within a few hours after attack), or the early dea th of such plants. This study was aimed at determining whether greenbu g feeding induces systemic damage to barley. The phosphate influx by r oots of susceptible and tolerant barley (Hordeum vulgare L.) plants wa s evaluated as an estimate of aphid systemic damage. Phosphate (P)-inf lux was determined at two plant growth stages, with two levels of gree nbug 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-infl ux was not significantly affected in tolerant barley plants by greenbu g infestation. In contrast P-influx was significantly reduced 6 h post -infestation in the susceptible cultivar. Plants with one expanded lea f suffered a significantly greater reduction in P-influx than plants w ith two expanded leaves. By 48 h after infestation, the P-influx of th e two-expanded-leaf treatment was similar to that of the controls, whe reas P-influx in plants with one expanded leaf remained significantly less than on the controls 72 h after infestation. A larger greenbug po pulation resulted in greater reduction in P-influx. Adult greenbugs, b ut not third stage nymphs, affected P-influx. In summary, the intensit y of greenbug-induced systemic damage was greater when young plant sta ges were infested by the aphid. Reductions of P-influx may become crit ical under increasing natural infestation levels. (C) 1997 Elsevier Sc ience B.V.