Judith F. Thomas

Response of agronomic and forest species to elevated atmospheric carbon dioxide

The effects of atmospheric carbon dioxide on corn, soybeans, loblolly pine, and sweetgum were studied in the field during a growing season. The plants were exposed to a range of concentrations of carbon dioxide day and night in open-topped, flow-through chambers. At a mean daytime carbon dioxide concentration of 910 parts per million, increases in total biomass ranged from 157 to 186 percent of the control values. Seed yield and wood volume increased and there were changes in plant anatomy and form. Net photosynthesis increased with increasing carbon dioxide concentration in soybeans and sweetgum, but was unaffected in corn. Water use efficiency also increased in corn, soybeans, and sweetgum.

Reversion of flowering in Glycine Max (Fabaceae).

Photoperiodic changes, if occurring before a commitment to flowering is established, can alter the morphological pattern of plant development. In this study, Glycine max (L.) Merrill cv. Ransom plants were initially grown under an inductive short-day (SD) photoperiod to promote flower evocation and then transferred to a long-day (LD) photoperiod to delay flower development by reestablishing vegetative growth (SD-LD plants). Some plants were transferred back to SD after 4-LD exposures to repromote flowering (SD-LD-SD plants). Alterations in organ initiation patterns, from floral to vegetative and back to floral, are characteristic of a reversion phenomenon. Morphological features that occurred at the shoot apical meristem in SD, LD, SD-LD, and SD-LD-SD plants were observed using scanning electron microscopy (SEM). Reverted plants initiated floral bracts and resumed initiation of trifoliolate leaves in the two-fifths floral phyllotaxy prior to terminal inflorescence development. When these plants matured, leaf-bract intermediates were positioned on the main stem instead of trifoliolate leaves. Plants transferred back to a SD photoperiod flowered earlier than those left in LD conditions. Results indicated that in plants transferred between SDs and LDs, photoperiod can influence organ initiation in florally evoked, but not committed, G. max plants.

Growth and Localized Energy Status in Phosphorus-Stressed Soybean

Abstract In plants experiencing phosphorus (P) stress, ATP concentrations can be reduced significantly and shoot growth is strongly restricted, raising the possibility that energy availability is responsible for the growth inhibition. Experiments were conducted to investigate the relationship between P deprivation and energy availability in tissues involved in the growth response. Young soybean (Glycine max [L.] Merr. cv. Ransom) plants were deprived of P for 32 days. Leaf initiation and individual leaf expansion were followed along with localized P and ATP concentrations. Tissue analyses revealed preferential distribution of P to the root, which accompanied a decline in the shoot to root dry weight ratio. Even though P concentrations in all shoot tissues dropped sharply, ATP concentrations and energy charge in the shoot meristem region were maintained similar to controls for an extended period when leaf initiation slowed. In the first trifoliolate leaf, ATP and energy charge remained at control levels during the expansion phase, but expansion was inhibited by 50%. Furthermore, ATP levels in root tips were decreased almost 30%, yet growth of the root system was equal to or greater than the control. The absence of a positive correlation between ATP levels and growth responses in the different tissues suggests that energy availability is not a primary factor limiting growth under P stress conditions. The results, along with others from previous experiments, support the notion that a signaling mechanism, as yet unidentified, controls down regulation of cell division in shoot growth regions.

Meristematic activity and leaf initiation in the shoot apex of Nicotiana tabacum during floral transition.

Meristematic activity in shoot apices of NCTG-22, a short-day (SD) sensitive cultivar of Nicotiana tabacum, was monitored throughout the 17-d period of transition from vegetative to reproductive growth The analysis included changes in (1) size of the apical meristem, (2) rate of leaf initiation, (3) amount of nuclear DNA, and (4) percentage of cells in division. Plants grown in controlled environment chambers set at 26/22 C day/night temperatures were exposed either to an inductive short-day photoperiod or a noninductive long-day (LD) photoperiod after a 6-wk juvenile period. The first morphological change associated with floral onset was an acceleration in the rate of leaf initiation following three inductive cycles, which continued and resulted in the eventual production of three more leaves by SD plants than by vegetative LD plants Apices were identical in size until seven inductive cycles were completed, after which SD apices began to enlarge, finally becoming more than twice as wide as LD apices and assuming a domed shape. At the cellular level the percentage of cells in division increased in SD apices after one inductive cycle and generally remained significantly higher (1%-2.5%) than in LD apices during the entire transition period. A majority of the population of cells in LD apices favored the G1 condition. After exposure to one and three SD cycles, however, 12%-14% more cells were in the G1 phase in SD apices than in LD. During SDs 8-15, concurrent with dome enlargement, 10%-26% more cells again were in G1 than in LD apices. These results and previous observations on the floral transition in Nicotiana suggest that there may be two different processes controlling meristematic activity during two different phases of the transition period.