THE INFLUENCE OF THE PARASITIC ANGIOSPERM STRIGA GESNERIOIDES ON THE GROWTH AND PHOTOSYNTHESIS OF ITS HOST, VIGNA-UNGUICULATA

Cowpea plants (Vigna unguiculata) infected with the root hemiparasitic angiosperm Striga gesnerioides accumulated less biomass than uninfect ed plants over a growth period of 60 d, The allometric relationship be tween shoot and root dry weight was similar in parasitized plants rela tive to control plants, as was the proportion of dry matter partitione d into leaf, stem and root tissue, However, infected plants failed to make any significant investment of dry matter in pods, The rate of pho tosynthesis of the youngest fully expanded leaf of parasitized plants was significantly lower than for control plants, The lower rates of ph otosynthesis were not attributable to stomatal limitation, a loss of c hlorophyll or to an accumulation of carbohydrate, The depression of ph otosynthesis in the young leaves was transient, As control leaves aged , photosynthesis declined, This also occurred in Striga-infected plant s, but to a lesser extent resulting in higher rates of photosynthesis in mature leaves when compared to those of uninfected plants, The foli ar nitrogen content of parasitized plants was higher than control plan ts consistent with the slower rate of photosynthetic decline of older leaves, The data are discussed with respect to the influence of parasi tic weeds on host growth and photosynthesis.