IMPROVING PLANT-PRODUCTION BY SELECTION FOR SURVIVAL AT LOW CO2 CONCENTRATIONS

Attempts to select C-3 plants with slow rates of photorespiration and increased rates of net photosynthesis have met with little success. This review analyses the properties of mutant genotypes of tobacco (Nicotiana tabacum L. cv. Wisconsin), derived from selection of haploid plants (produced by in vitro mutagenesis of anthers) which survived in CO2 concentrations close to the compensation point, Survivors were diploidized and doubled-haploid plants were self-pollinated to obtain seeds (the selected genotypes). Several glasshouse and field experiments showed that the method of selection at low CO2 concentrations gave genotypes with increased capacity for total dry matter accumulation; increases were similar (mean 24%; range 14-36%) in different conditions for two selected genotypes (SP422 and SP451) when compared to the parental genotype Wisconsin-38. This increase was related to a greater leaf area per plant (mean increase 19%; range 9-43%), to faster photosynthetic rates in mature and old leaves and to similar rates of dark respiration per unit leaf area, but smaller rates per unit dry matter. These changes were related to a greater number of mesophyll cells of smaller size in the selected genotypes. However, the increased productivity could not be related to reduced photorespiration rate or CO2 compensation point nor to improved Rubisco properties (e.g. increased specificity factor) which the selection method was designed to achieve. Selection by survival at low CO2 produced genotypes able to invest more assimilate in growing larger leaves and to maintain a better leaf carbon balance than the parent genotype. These features improved light capture and carbon accumulation and thus increased dry matter production.