Carbohydrate metabolism in a heat-girdled maize source leaf

Heat-girdling in maize leaves was used for blocking assimilate export and p roducing alterations in carbohydrate metabolism. A rapid drop (< 24 h) in C O2 fixation and a large increase of sucrose and starch content were observe d. Reduction in photosynthetic assimilation was not due to stomatal limitat ion since the girdled leaves remained turgid and their photosynthetic rate was not restored by 4% CO2. After 24 h, the girdled leaves became red throu gh accumulation of anthocyanins and reduction of chlorophylls. A possible r elationship between carbohydrate accumulation in girdled leaves and gene ex pression was examined through transcript levels of the ribulose-1,5-bisphos phate carboxylase oxygenase small-subunit (rbcS), the leaf-specific ADPgluc ose pyrophosphorylase small-subunit (Agp-l2), and Bz1, an anthocyanin pathw ay-specific gene. The mRNA levels in girdled leaves were also compared with those in excised leaves placed on sucrose or sorbitol solutions. rbcS mRNA disappeared 8 h after the girdling treatment and after 1 day on sucrose. A gp-l2 expression was reduced by two-thirds with girdling or sucrose feeding while Bz1 expression was strikingly enhanced by both treatments. Thus, car bohydrate accumulation produced a short-term feedback inhibition of photosy nthesis and some long-term modifications in gene expression.