The Relationship between Sucrose and Starch during ‘Dark’ Export from Leaves of Uniculm Barley

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Abstract
Uniculm barley plants were grown in 8 h photoperiods at a quantum flux density of 655 μE m−2 s−1. Groups of plants were transferred to four different light environments for one 8 h photoperiod (106, 270, 665, and 975 μE m−2 s−1) and harvested at intervals throughout the succeeding dark period for subsequent carbohydrate analysis of the youngest mature leaf. Sucrose was the predominant carbohydrate in the leaves (attaining a level of c. 100 mg dm−2 after 8 h at 975 μE m−2 s−1) but starch was also of significance (20 mg dm−2 after 8 h at 975 μE m−2 s−1). During the dark period, following a photoperiod at the three highest light levels (270, 665, and 975 μE m−2 s−1), sucrose was exported first while the starch level remained fairly constant. When the-sucrose level fell to 15–20 mg dm−2 starch degradation began. This critical sucrose level was reached earlier in those plants subjected to lower quantum flux densities during the preceding photoperiod. The delay in the remobilization of starch suggests an important regulatory mechanism which may be dependent upon the sucrose level. At 106 μE m−2 s−1 the sucrose level rose to only 10 mg dm−2. Here there was no discernible delay in the depletion of sucrose or starch.