Plant Physiol, November 2002, Vol. 130, pp. 1573-1583

Interactions of Nitrate and CO₂ Enrichment on Growth, Carbohydrates, and Rubisco in Arabidopsis Starch Mutants. Significance of Starch and Hexose¹

Institute of Biological Chemistry (J.S., K.M.G., T.W.O., G.E.E.) and School of Biological Sciences (J.S., O.K., K.M.G., G.E.E.), Washington State University, Pullman, Washington 99164

Wild-type (wt) Arabidopsis plants, the starch-deficient mutant TL46, and the near-starchless mutant TL25 were grown in hydroponics under two levels of nitrate, 0.2 versus 6 mM, and two levels of CO₂, 35 versus 100 Pa. Growth (fresh weight and leaf area basis) was highest in wt plants, lower in TL46, and much lower in TL25 plants under a given treatment. It is surprising that the inability to synthesize starch restricted leaf area development under both low N (N_L) and high N (N_H). For each genotype, the order of greatest growth among the four treatments was high CO₂/N_H > low CO₂/N_H, > high CO₂/N_L, which was similar to low CO₂/N_L. Under high CO₂/N_L, wt and TL46 plants retained considerable starch in leaves at the end of the night period, and TL25 accumulated large amounts of soluble sugars, indicative of N-limited restraints on utilization of photosynthates. The lowest ribulose-1,5-bisphosphate carboxylase/oxygenase per leaf area was in plants grown under high CO₂/N_L. When N supply is limited, the increase in soluble sugars, particularly in the starch mutants, apparently accentuates the feedback and down-regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase, resulting in greater reduction of growth. With an adequate supply of N, growth is limited in the starch mutants due to insufficient carbohydrate reserves during the dark period. A combination of limited N and a limited capacity to synthesize starch, which restrict the capacity to use photosynthate, and high CO₂, which increases the potential to produce photosynthate, provides conditions for strong down-regulation of photosynthesis.