PRODUCTION OF GLANDS IN LEAVES OF POROPHYLLUM SPP. (ASTERACEAE) - ECOLOGICAL AND GENETIC-DETERMINANTS, AND IMPLICATIONS FOR INSECT HERBIVORES

1 The mean number of translucent glands containing potent insecticidal allelochemicals in leaves of Porophyllum gracile was observed to vary up to nearly twofold under field conditions in the area of Tucson, Ar izona (USA). 2 The production of glands appeared to be a beneficial tr ait to reduce herbivory in the field since individuals of P. gracile b earing 2.5 glands per leaf were submitted to an approximately fivefold higher herbivory pressure than those having 4.8 glands per leaf. 3 To investigate if the production of glands in leaves of P. gracile is un der genetic control, seeds were collected from individuals for which t his trait varied under field conditions. There was no correlation for the mean number of glands per leaf between individuals of P. gracile s ampled in the field and their progeny seedlings grown under controlled conditions. This suggested that the production of glands in leaves of P. gracile is not strongly hereditary. 4 The resource allocated to th e formation of foliar glands, which was determined as the ratio of tot al volume of glands per unit area of leaf, in seedlings of P. ruderale and P. gracile was enhanced by up to one order of magnitude by both n itrogen fertilization, 15 vs. 0 mM of NO3- in the watering solution, a nd high light regime, 380 vs. 50 mu mole photon m(-2) s(-1). These res ults suggest that the production of glands in leaves of Porophyllum sp p. depends greatly on resource availability. 5 The natural plasticity in the production of glands in leaves of P. ruderale was in turn emplo yed in controlled laboratory experiments to confirm the potential bene fit of the glands to repress insect herbivory as observed in the field . It was demonstrated that adults of the red-legged grasshopper are re pelled by the volatiles emitted from the glands and that they feed les s on leaves bearing more glands.