Carbon assimilation, partitioning and export in mature cladophylls of two asparagus (Asparagus officinalis) cultivars with contrasting yield.

To assess the physiological aspects underpinning cultivar difference in asparagus (Asparagus officinalis L.) yield, diel carbon exchange, carbon partitioning and export and sucrose phosphate synthase (SPS) activity were examined in mature cladophyll tissue of two asparagus cultivars (ASP-69 and ASP-03) under field conditions. Both cultivars exhibited similar diel patterns in carbon exchange rate (CER) and carbohydrate partitioning. Rates of carbon export estimated from CER and dry mass changes were the highest at midday and coincided with maximum assimilation rate. Carbon export accounted for about 74% of carbon fixation during the photoperiod. No diel fluctuations were observed in SPS activity in either cultivar. A positive correlation between day CER and carbon export rate (r2 = 0.87) was found and this relationship did not differ between the two cultivars studied. The greater carbon export rate measured in the high-yielding cultivar (ASP-69) was associated with significantly higher CER in comparison to the low-yielding cultivar (ASP-03). However, a correlation between sucrose concentration and carbon export rate did not exist. Biochemical evidence indicated that the greater CER in ASP-69 was associated with a significantly greater SPS activity (P 0.05). Phloem 14C exudate analysis revealed that 14C flux out of cladophyll tissue in ASP-69 was significantly greater than in ASP-03. These results indicate a feed-forward effect of rate of photosynthesis on assimilate export in the two cultivars studied.