Evergreen Foliage Contributions to the Spring Growth of Taxus

Taxus media cv. Hicksii plants were grown one season under a low and high level of nitrogen fertilization. Before growth in the spring the plants were divided into two groups, one of which was defoliated and the other left intact. The growth and spring utilization of the nitrogen and carbohydrate reserves of defoliated plants were compared to the intact plants 0, 2, 4 and 6 weeks after growth started in the spring. The plants were separated into buds (all new growth), roots and stems and analyzed for changes in total nitrogen, basic and non-basic amino acids, hemicelluloses, soluble sugars, organic acids and chlorophyll. The older evergreen needles from plants grown under low nitrogen levels contain 20 % of the carbohydrate and 24% of the nitrogen used in spring growth. The needles from plants grown under high nitrogen levels contained 56% of the carbohydrate and 49% of the nitrogen used in spring growth. Removal of the old needles before spring growth removed this nitrogen and carbohydrate reserve and reduced the total plant chlorophyll content after 6 weeks of growth to 50% of that found in intact plants, with the result that defoliated plants did not show a growth response to nitrogen. Amino acids accumulated in the stems and buds of defoliated plants as carbohydrates became limiting. The defoliated plants removed 25% more available carbohydrates from the roots and stems than intact plants and their buds contained 50% less available carbohydrates. Plants without old needles showed similar growth rates under low and high nitrogen regimes and produced 33% of the dry weight of intact plants grown under high nitrogen levels and 66% of the dry weight of intact plants grown under low nitrogen levels. The old needles of taxus plants contain substantial amounts of reserve nitrogen and carbohydrate and these needles greatly influence the extent and rapidness of growth in the spring. When the needles are removed, the other tissues can supply an adequate amount of nitrogen but the carbohydrate supply becomes limiting for spring growth.