EFFECT OF SHADING AT DIFFERENT TIMES ON GROWTH AND YIELD OF POTATO

The effect of shading during the periods of new tuber initiation and subsequent bulking were studied with 3 crops of field-grown potatoes, Solanum tuberosum ''Sebago''. Plots were shaded (34% reduction on full sunlight) up to or after the stage when the young tubers had formed (3 wk after the onset of initiation), receiving full sunlight for the other half of growth. In other treatments plots were shaded throughout growth or not shaded at all. In 2 experiments plots were split for 2 planting densities (7.14 and 14.28 tubers planted m-2), but density had no effect on the response to shade. Shade during the 1st part of growth did not affect the numbers of tubers initiated but, compared with unshaded plots, did reduce the numbers which reached a size of 1 cm in diameter or more, which was the same as the final numbers which reached a size of 1 cm in diameter or more, which was the same as the final numbers harvested at maturity. Shade also delayed by a few days the onset of the rapid tuber-bulking period. Shade during later growth had some additional effect in reducing the maximum bulking rate achieved, so that the final yields from the differentially shaded plots were intermediate between those from the plots either unshaded or shaded throughout. Plants receiving late shade had a higher percentage of their total dry weight in the haulm than did unshaded plants.