Photosynthetic Acclimation, Biomass Allocation, and Water Use Efficiency of Garlic in Response to Carbon Dioxide Enrichment and Nitrogen Fertilization

Garlic (Allium sativum) is a commercially and culturally important crop worldwide. Despite the importance of garlic, there have been few studies investigating how garlic growth and development will be affected by the atmospheric enrichment of carbon dioxide (CO2). A split-plot experiment with CO2 concentrations as main plot and nitrogen (N) fertilization as subplot was carried out to examine the effects of elevated CO2 at (mean ± sd) 745 ± 63 µmol·mol−1 across three levels of N: high-N (16.0 mm), mid-N (4.0 mm), and low-N (1.0 mm). Three hypotheses were tested: 1) garlic plants will allocate proportionally more biomass to bulb when grown in elevated CO2 compared with the plants grown in ambient CO2; 2) plants will sustain improved photosynthesis without downregulation in elevated CO2, irrespective of N; and 3) elevated CO2 will improve plant water use efficiency (WUE) across N fertilization levels. We found that proportional biomass allocation to bulb was not significantly enhanced by CO2 enrichment in garlic. Overall biomass accumulation represented by leaf, stem, and bulb did not respond significantly to CO2 enrichment but responded strongly to N treatments (P < 0.001). Contrary to our hypothesis, photosynthetic downregulation was apparent for garlic plants grown in elevated CO2 with a decrease in Rubisco capacity (P < 0.01). Instantaneous leaf WUE improved in response to elevated CO2 (P < 0.001) and also with increasing N fertilization (P < 0.001). Finally, our results indicate that bulbing ratio is likely to remain unchanged with CO2 or N levels and may continue to serve as a useful nondesctructive metric to estimate harvest timing and bulb size.

Photosynthetic downregulation in the conifer Metasequoia glyptostroboides growing under continuous light: the significance of carbohydrate sinks and paleoecophysiological implications

During the Eocene (ca. 45 Ma) a temperate climate at high northern latitudes provided an environment unlike any that currently exists on Earth. The growing season was characterized by long (up to 4 months) periods of continuous, low- to moderate-intensity illumination. While this remarkable light regime offered opportunities for substantial growth, it also imposed physiological challenges consequential to potential carbon sink–source imbalance and resulting downregulation of photosynthetic capacity. To better understand the physiology of adaptation to a continuous-light (CL) environment, we experimentally investigated the effects of CL and carbon sink–source relationships in the deciduous conifer Metasequoia glyptostroboides Hu et Cheng, an extant representative of a genus that was the dominant tree component of many Eocene high-latitude forests. We tested the importance of branch-level and whole-plant sinks in curtailing feedback inhibition and the specific roles of starch and sugars in that process using manipulative experiments. Trees growing under either normal day–night cycles or continuous illumination were subjected to reduction of local, branch-level sinks or both local and whole-tree sinks. Reduction in sink strength led to downregulation of photosynthetic capacity, as evidenced by reduction of photosynthetic rates, carboxylation capacity, and electron transfer capacity. Our results suggest that photosynthetic downregulation is minimized by maintenance of both whole-tree sinks and local sinks. downregulation showed a greater correlation with starch than with sugar content, and ultrastructural evidence indicated that foliar starch accumulated only in chloroplasts, and was accompanied by reduction in functional chloroplast grana, but showed no evidence of physical disruption of thylakoids.