Polyphenol oxidase (PPO) activity of Russet Burbank potato was inhibited by sense and antisense PPO RNAs expressed from a tomato PPO cDNA under the control of the 35S promoter from the cauliflower mosaic virus. Transgenic Russet Burbank potato plants from 37 different lines were grown in the field. PPO activity and the level of enzymatic browning were measured in the tubers harvested from the field. Of the tubers from 28 transgenic lines that were sampled, tubers from 5 lines exhibited reduced browning. The level of PPO activity correlated with the reduction in enzymatic browning in these lines. These results indicate that expression of tomato PPO RNA in sense or antisense orientation inhibits PPO activity and enzymatic browning in the major commercial potato cultivar. Expression of tomato PPO RNA in sense orientation led to the greatest decrease in PPO activity and enzymatic browning, possibly due to cosuppression. These results suggest that expression of closely related heterologous genes can be used to prevent enzymatic browning in a wide variety of food crops without the application of various food additives.

Ascorbic acid, better known as vitamin C, is a crucial nutrient in the human diet. It performs many physiological functions in its primary roles as an electron donor and antioxidant. Vitamin C has been directly linked to collagen formation, iron absorption, cancer prevention, immunomodulation, and maintenance of normal nerve function. It is suspected to decrease the likelihood of strokes, cataracts, hypertension, and lead toxicity. Vitamin C deficiency leads to a condition called scurvy, accompanied by a weakening of blood vessels, bones and connective tissues, hair and tooth loss, joint swelling, and eventually death. Intake of vitamin C is considered inadequate, even among some parts of the population in developed countries where diet is not restricted, but more especially for at-risk populations in developing countries. Potatoes are an important worldwide source of vitamin C, contributing about 20% of the dietary intake in Europe. They are a vital source of vitamin C not only because of relatively high content, but because they can be stored, leading to consistent availability. Any improvement in the vitamin C content of potato products will have a beneficial impact on human nutrition. A three-pronged approach can be used to increase the vitamin C content of potatoes involving breeding, improved crop management, and modification of cooking processes. Breeding has tremendous potential for increasing vitamin C content in tubers as evidenced by research results in studies documenting germplasm variability and inheritance patterns. Management research may define practices that will slow the natural decline that occurs near the end of field growth and storage, a response partially conditioned by plant stress. Research into cooking procedures may help reduce the oxidative and enzymatic degradation of vitamin C that results from exposure to moisture, heat, and air.Resumen El ácido ascórbico, mejor conocido como vitamina C, es un nutriente imprescindible en la dieta humana. Realiza muchas funciones cuyo principal rol es como donante de electrones y antioxidante. La vitamina C ha sido directamente ligada a la formación de colágeno, absorción de hierro, prevención de cáncer, modulación inmunológica y mantenimiento de la función nerviosa. Parece que disminuye la posibilidad de ataques de apoplejía, cataratas, hipertensión y toxicidad por plomo. La deficiencia de vitamina C produce escorbuto, acompañado por un debilitamiento de las venas, huesos y tejido conectivo, pérdida de pelo y dientes, hinchazón de las articulaciones y eventualmente la muerte. El consumo de vitamina C es considerado inadecuado, aún entre parte de la población en los países en desarrollo. La papa es una importante fuente de vitamina C a nivel mundial que contribuye con un 20% del consumo dietético en Europa. Además es una fuente vital de vitamina C no solo por su alto contenido, sino porque puede ser almacenado, contribuyendo a una permanente disponibilidad. Cualquier mejora en el contenido de vitamina C de los productos de papa tiene un impacto bené...

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A few potato lone, such as A6948-4, had higher rates ofphotosynthesis in the f than the Russet Burbank and were able to maintain higher rates not only dring miday but also in the early mornng and late evning For many species, more than 95% of the plant dry weight is derived from photosynthetically fixed CO2 (17). Many physiologists have studied rates of photosynthesis in various crops, and some have found differences in photosynthetic rates. For example, Bhagsari and Harmon (2) reported differences in rates of CO2 uptake among 20 sweet potato genotypes. Crosbie et al. (5) have demonstrated that leaf photosynthesis of maize can be improved by recurrent selection, and Mahon and Hobbs (12) have reported progress in the selection of peas for high photosynthetic CO2 exchange rate.Cieply (3) concluded that assimilation rates can be used as a physiological criterion for rapid selection in potato breeding. Moll and Henniger (13) measured rates of "4CO2 uptake by 18 clones of potato under standard conditions and obtained a correlation of 0.6 (P < 0.01) between tuber yield and 'whole photosynthetic capacity' (the product of photosynthetic rate and leaf area). Col-

The partitioning of photoassimilates was compared for four field‐grown potato clones. Russet Burbank was used as a standard for comparison, and the other three clones were selected because they generally produce higher yields and have higher photosynthetic rates than Russet Burbank. In this study we examined whether the patterns of assimilate partitioning change with the stage of plant development and with the time after labelling and also whether these partitioning patterns vary for different clones. The most recently‐expanded leaf on each plant was labelled with 14CO2 at three different growth stages: the beginning of tuberization, 3 and 5‐weeks later; and representative plants from each treatment were harvested at three time intervals: 2 h after labelling, 7 days after labelling, and the end of the growing season. The total quantity of 14C‐1abel exported from the exposed leaf increased beth with age of the plant and with time after labelling. The exported assimilates were partitioned in varying quantities to all plant organs, including trace amounts to the mother tubers (seed pieces). Some of the assimilates produced after tuber set and originally partitioned to other organs may have been remobilized to the tubers near the end of the season. At later growth stages, some clones delivered assimilates to the tubers more rapidly than others. The potential exists for plant breeders to combine late season maintenance of photosynthesis with rapid and/or greater delivery of assimilates to the tubers as an approach for increasing tuber yield.