Source‐Sink Manipulations and Carbohydrate Metabolism in Maize

Abstract

Grain filling is largely dependent on C export from source leaves, but little attention has been given to the use of leaf carbohydrate metabolism criteria for evaluating grain‐filling capacities. This study was conducted to test the effect of source‐sink manipulations on the relationship between starch accumulation in grain and leaf sucrose‐phosphate synthase activity for different maize genotypes. Field‐grown maize (Zea mays L.) plants were subjected to ear removal or to leaf excision at pollination, to examine the effect on regulation of C fixation and partitioning in source leaves during grain filling. At the plant level, the two treatments produced unbalanced effects: ear removal led to accumulation of dry matter in stalks, leaf sheaths, and blades and accelerated senescence of vegetative parts, whereas excision of most leaves hardly affected the remaining leaf and stalk, but drastically reduced the number of developing grains. Modifications in source leaves were estimated from leaf disks punched out in the first leaf above the ear. In this leaf, ear removal caused an increase in soluble sugars 10 days after pollination (DAP), a decrease in sucrose‐P synthase activity (20 DAP), an increase in adenosine dipbosphate glucose pyrophosphorylase activity (30 DAP), a decline of phosphoenolpyruvate (PEP) carboxylase (30 DAP) and of ribulose‐bisphosphate carboxylase (40 DAP). The onset of senescence was marked by an earlier increase in glucose and fructose, and in nonspecific phosphatases. In contrast, leaf excision did not affect senescence compared with control plants and the feedforward effect on source leaves was limited to a transient stimulation of PEP carboxylase (I0 DAP) and a doubling sucrose‐P synthase activity for 40 DAP. Leaf soluble sugars were only slightly depressed, but massive kernel abortion occurred and few grains filled normally. Aborting grains were shown to lack the usual maximum in soluble sugar at 30 DAP. Collectively, the results show that at the grain‐filling stage, sucrose‐P synthase activity more accurately reflected assimilate demand than did metabolite level or enzyme activity for C fixation.