The Regulation of Carbon Transport and the Carbon Balance of Mature Tomato Leaves

Rates of carbon transport from a single mature tomato leaf inthe light period (day transport) and the dark period (nighttransport) were estimated from the rate of carbon fixation inthe light period, the rate of respiration in the dark periodand the changes in carbon contents over these two periods. Plantswere grown initially at 40 W m^–2 light intensity witheither 350 vpm (nonenriched plants) or 1000 vpm CO₂ (enrichedplants). Various light flux densities or CO₂ concentrationswere then applied to the experimental leaves in the light periodduring the experiment When leaves were temporarily exposed to contrasting light fluxdensities both day transport and night transport were linearlyrelated to the rate of carbon fixation. If leaves were shadedbelow the light compensation point for up to five days, or transferredto contrasting CO₂ concentrations for up to ten days, the linearrelationship between carbon fixation and carbon transport nolonger held. During acclimatization, therate of wbon fixationwas simply related to thecurrent light flux density and CO₂concentration, but the rate of carbon transport changed withtime. Day and night transports responded differently to changesin environment: night transport was more related to the contentof reserve, particularly starch, than to the rate of concurrentwbon fixation. It is concluded that the rate of carbon transport of a maturetomato leaf in a single photoperiod is regulated not merelyby the rate of concurrent carbon fixation but by the contentof reserve in the leaf. The latter results from previous cumulativewbon fixation and carbon transport. As a result of changingthe rate of carbon transport, a balance of carbon input andoutput was achieved within 10 days of acclimatization in a maturetomato leaf.