Based on the curvilinear relationship between leaf nitrogen content and the initial slope of the response of CO(2) assimilation (A:) to intercellular CO(2) concentrations (C:(i)) in apple, it is hypothesized that Rubisco activation state decreases with increasing leaf N content and this decreased activation state accounts for the curvilinear relationship between leaf N and CO(2) assimilation. A range of leaf N content (1.0-5.0 g m(-2)) was achieved by fertilizing bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees for 45 d with different N concentrations, using a modified Hoagland's solution. Analysis of A:/C:(i) curves under saturating light indicated that CO(2) assimilation at ambient CO(2) fell within the Rubisco limitation region of the A:/C:(i) curves, regardless of leaf N status. Initial Rubisco activity showed a curvilinear response to leaf N. In contrast, total Rubisco activity increased linearly with increasing leaf N throughout the leaf N range. As a result, Rubisco activation state decreased with increasing leaf N. Both light-saturated CO(2) assimilation at ambient CO(2) and the initial slope of the A:/C:(i) curves were linearly related to initial Rubisco activity, but curvilinearly related to total Rubisco activity. The curvatures in the relationships of both light-saturated CO(2) assimilation at ambient CO(2) and the initial slope of the A:/C:(i) curves with total Rubisco activity were more pronounced than in their relationships with leaf N. This was because the ratio of total Rubisco activity to leaf N increased with increasing leaf N. As leaf N increased, photosynthetic N use efficiency declined with decreasing Rubisco activation state.
Bark storage proteins (BSPs) accumulate in the inner bark parenchyma of many woody plants during autumn and winter. We investigated the effect of a short-day (SD) photoperiod on the accumulation of the 32-kilodalton bark storage protein of poplar (Populus deltoides Bart. ex Marsh.) under controlled environmental and natural growing conditions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and protein gel blot analysis revealed that 10 days of SD exposure (8 hours of light) resulted in a 20% increase in the relative abundance of the 32-kilodalton bark storage protein of poplar. After 17 days of SD exposure, the 32-kilodalton bark storage protein accounted for nearly one-half of the soluble bark proteins. In natural field conditions, accumulation of the 32-kilodalton bark storage protein was observed to start by August 18 (daylength 14.1 hours). Immunoprecipitation of in vitro translation products with anti-BSP serum revealed that the SD protein accumulation was correlated with changes in the pool of translatable mRNA. A survey of poplar clones from different geographic origins revealed the presence of the 32-kilodalton BSP in the dormant bark of all the clones tested. These results demonstrate that a SD photoperiod induces, whether directly or indirectly, rapid changes in woody plant gene expression, leading to the accumulation of BSP.
In spring, nitrogen (N) uptake by apple roots begins about 3 weeks after bud break. We used 1-year-old 'Fuji' Malus domestica Borkh on M26 bare-root apple trees to determine whether the onset of N uptake in spring is dependent solely on the growth stage of the plant or is a function of soil temperature. Five times during early season growth, N uptake and total amino acid concentration were measured in trees growing at aboveground day/night temperatures of 23/15 degrees C and belowground temperatures of 8, 12, 16 or 20 degrees C. We used (15NH4)(15NO3) to measure total N uptake and rate of uptake and found that both were significantly influenced by both soil temperature and plant growth stage. Rate of uptake of 15N increased with increasing soil temperature and changed with plant growth stage. Before bud break, 15N was not detected in trees growing in the 8 degrees C soil treatment, whereas 15N uptake increased with increasing soil temperatures between 12 and 20 degrees C. Ten days after bud break, 15N was still not detected in trees growing in the 8 degrees C soil treatment, although total 15N uptake and uptake rate continued to increase with increasing soil temperatures between 12 and 20 degrees C. Twenty-one days after bud break, trees in all temperature treatments were able to acquire 15N from the soil, although the amount of uptake increased with increasing soil temperature. Distribution of 15N in trees changed as plants grew. Most of the 15N absorbed by trees before bud break (approximately 5% of 15N supplied per tree) remained in the roots. Forty-six days after bud break, approximately one-third of the 15N absorbed by the trees in the 12-20 degrees C soil temperature treatments remained in the roots, whereas the shank, stem and new growth contained about two-thirds of the 15N taken up by the roots. Total amino acid concentration and distribution of amino acids in trees changed with plant growth stage, but only the amino acid concentration in new growth and roots was affected by soil temperature. We conclude that a combination of low soil temperature and plant developmental stage influences the ability of apple trees to take up and use N from the soil in the spring. Thus, early fertilizer application in the spring when soil temperatures are low or when the aboveground portion of the tree is not actively growing may be ineffective in promoting N uptake.
Bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees were fertilized with a nutrient solution (fertigation) containing 0, 2.5, 5, 7.5, 10, 15 or 20 mM nitrogen (N) in a modified Hoagland's solution from June 30 to September 1. In mid-October, half of the trees in each N treatment were sprayed twice with 3% urea, 1 week apart. The remaining trees served as controls. All trees were harvested after leaf fall and stored at 2 degrees C over winter. One group of trees from each treatment was destructively sampled before bud break to determine amounts of reserve N and total nonstructural carbohydrates (TNC); the remaining trees were transplanted to N-free medium in the spring. These trees were supplied with Hoagland's solution with or without 10 mM N (from 15N-depleted NH4NO3) for 60 days, starting from bud break. With increasing N supply from fertigation, tree N concentration increased, whereas TNC concentration decreased. Foliar urea applications increased tree N concentration and decreased TNC concentration in each N fertigation treatment. There was a negative linear relationship between tree N concentration and TNC concentration. Irrespective of whether N was provided the following spring, trees with high N reserves but low carbohydrate reserves produced a larger total leaf area at the end of the regrowth period than trees with low N reserves but high carbohydrate reserves. The pooled data on reserve N used for new growth showed that, regardless of the spring N supply, there was a linear relationship between total N accumulated in the tree during the previous season and the amount of reserve N remobilized for new shoot and leaf growth. About 50% of tree N content was remobilized to support new shoot and leaf growth over the range of tree N status examined. We conclude that the initial growth of young apple trees in the spring is determined mainly by reserve N, not reserve carbohydrates. The amount of reserve N remobilized for new growth in spring was proportional to tree N status and was unaffected by current N supply.
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