No abstract

Experimental infection of Prosopis species with the mistletoe Phoradendron tomeniosum subsp. tomentosum in a uniform garden demonstrated that differences in infection success between species, populations, and individuals can be caused by genetically‐based differences in both hosts and parasites. Prosopis glandulosa var. glandulosa was least resistant to infection; P. laevigata was more resistant; and P. torreyana was most resistant. A population of P. glandulosa var. glandulosa grown from seeds collected in the lower Rio Grande valley was significantly more resistant to infection than three populations of the same variety from central and northern Texas. The higher resistance of the Rio Grande valley population appears to be due to introgression with P. laevigata. The experimental population of Phoradendron tomentosum subsp. tomentosum from northeastern Mexico and the population of the same subspecies from southern Texas had greater infectivity than did those from central and northern Texas, corresponding to the greater resistance of the southern Prosopis populations.

MAY. 1977. Temperature relations ofgas exchange in altitudinal populations of Taroxoc~rtn officinrrle. Can. J. Bot. 55: 2496-2502. Trrraxncrrr?7 offieinale plants representative of three altitudinally diverse populations were grown under uniform conditions. Temperature responses of net photosynthesis, photorespiration, and transpiration were obtained from four plants of each population over the range of 10 to 40°C at saturating irradiances (1OOOpE . m-2 . S-I). Dark respiration rates were obtained from the same plants over the range of 10 to 30°C. All plants exhibited similar gas exchange responses to temperature regardless of population origin. Maximum rates of net photosynthesis occurred near 20°C in all plants and averaged 20.8 mg C 0 2 . dm-2 . h-I (mean of 12 plants). Dark respiration and photorespiration rates increased nearly linearly with temperature in all plants. These results are in contrast with previous studies of the same populations in which differences in Hill activity and succinate dehydrogenase activity were reported. However, the photosynthetic patterns and lack of genetic differentiation of photosynthesis are similar to the results obtained for another weedy species. Verbasc~~nl thnps~rs, along the same altitudinal transect.

No abstract

Investigation of the morphology and physiology of the mistletoe, Phoradendron tomentosum subsp. tomentosum, in Texas reveals that the subspecies is differentiated into at least two ecological races, one in north central Texas, the other in south central Texas. The races differ in leaf length/width ratio, fruit diameter, seed set, spike internode length, freeze tolerance, and flavonoid patterns. Clines occur in leaf length/width ratio and fruit diameter between the races, and these characteristics are negatively correlated. The northern race generally exhibits greater freeze tolerance than the southern race. Field observations, herbarium specimens, and a collection of fresh specimens from northeastern Mexico indicate that a third race in the subspecies may exist there. Racial variation within the subspecies appears to permit it to occupy a diversity of habitats. Morphological and physiological patterns within the two Texas races suggest that introgressive hybridization has occurred between them, allowing for the occupation of intermediate habitats.