Selection of Nicotiana tabacum Haploids of High Photosynthetic Efficiency

Abstract

Photosynthetic efficiency is an important factor for crop productivity. It has been suggested that crop production could be increased by improving the photosynthetic efficiency (19,28).Although in some cases there is not a clear correlation between net photosynthesis and crop yield (9, 12), some reports show a clear positive correlation between net CO2 assimilation and dry weight accumulation (7,21,29).A great variability in photosynthetic ability is found in different species as well as within different cultivars of the same species.Differences in the CO2 assimilation rate have been observed in soybean (8), maize (14), tobacco (28), alfalfa, and cotton. This variability indicates the possibility of selecting plants with higher photosynthetic efficiency.The rate of CO2 assimilation can be increased either by improving the conditions under which photosynthesis is performed (CO2 and 02 concentrations, temperature, illumination, and nutrition, etc.), or by breeding and selection of lines with higher photosynthetic efficiency (1, 13).Photosynthetic ability of a plant can be increased in many different ways, which are directly or indirectly related to the photosynthetic processes. For instance, selection for characteristics to improve the plant's ability to assimilate CO2 such as increased leaf surface or altered canopy structure (15, 26), delayed senescence (17), higher Chl content (1 1), or longer periods of storage of assimilates in the grain (10) has been done with varied success.

Leaf Photosynthesis and Respiration of High CO2-Grown Tobacco Plants Selected for Survival under CO2 Compensation Point Conditions

“…The growth responses of the selected lines to high CO2 were better than those ofthe parental line, and this is remarkable because these plants were initially selected for survival under low CO2. We have confirmed the absence of important differences in photosynthesis rates per unit leaf area and CO2 compensation point, but significant reduction of mature leaf respiration per unit dry weight was observed in selected lines which could Self-pollinated, doubled-haploid tobacco (Nicotiana tabacum) lines (SP422, SP432, SP435, and SP45 1), selected as haploids by survival in a low CO2 atmosphere (near the CO2 compensation point [19]), and the parental cv Wisconsin-38 were grown from seed in a growth room kept at high CO2 levels (600-700 ppm). The substrate used was vermiculite.…”

“…Accordingly, the CO2 compensation point was about 50 ppm under the conditions used in all lines studied. Thus, the evidence suggests that the selection procedure based on survival of haploid plants at CO2 levels slightly higher than the CO2 compensation point (19) (10,21,24,26). Small differences in the CO2 compensation point associated with the component of respiration in the dark (6) may have occurred (see later) but could not be detected with our gas exchange system.…”

“…The reduced respiration rate per unit carbon (see above) could have significantly contributed to the saving of carbohydrates and other substrates during the initial selection period with very low CO2 availability for growth (19), helping to keep a more positive carbon balance and, hence, increase the survival chance under such drastic starvation conditions.…”

“…However, the alternative pathway was not normally active in these leaves in the absence of cyanide (Table III), even when respiration was stimulated by the uncoupler CCCP (not shown), suggesting that the cytochrome pathway was predominantly used in respiration. Thus, the alternative oxidase of adult leaves seems not to play a significant role in the determination of the carbon balance of the selected tobacco lines in the growth conditions used, but this pathway might have been important for survival during the low CO2 selection period (19).…”

Rapid Isolation of Loss‐of‐Function Mutations for Dominant Traits: A Case Study Using Photoperiod Sensitivity Trait in Pima Cotton

“…Haploids have been used for mutant isolation due to the ease of detecting recessive mutations in the M 1 generation and the ability to rapidly fix mutations via chromosome doubling (Walker, 1998; Szarejko and Forster, 2007). This approach has been successfully used to isolate mutants in many crops such as tobacco ( Nicotiana tabacum L.; Medrano and Primo‐Millo, 1985), canola ( Brassica napus L.; Swanson et al, 1987), and rice ( Oryza sativa L.; Chen et al, 2001). In wheat ( Triticum aestivum L.), plants hemizygous for a specific chromosome were used to isolate deletion mutations using X‐rays (Sears, 1977).…”