Sugars and Starch Activities and Change in 13C in Maize
Different light environments significantly regulated the various sugars and starch levels in maize leaves (Fig. 3 ). Generally, all the treatments showed a similar trend of initial increase up to 10-20 days followed by a decline in later days, except the initial decrease of soluble starch content and sucrose synthase activity in PS leaves. However, partial light conditions significantly induced the content of sucrose, soluble sugar, and relative enzymes than normal light conditions (days from 20 to 40). Notably, unlike NL treatments (NF and NS), PL treatments (PF and PS) differed significantly. PS conditions significantly increased 2.0, 1.6, 1.4, 1.2 fold activities of α-amylase, β-amylase, starch phosphorylase, and DBE than PF, while the similar activities of all enzymes were observed in NF and NS. Interestingly, despite the initial highest content under PF conditions, PS conditions retained the highest amount of soluble sugars, soluble starch, and sucrose after 20 days. Furthermore, sucrose synthase and sucrose phosphate activities were highest in PF than PS, NF, and NS. These results may indicate that PS leaves degrade starch and PF leaves enhance sucrose synthesis to make up for the deficiency of sugar caused by shading in the early stage, and both PS and PF leaves increase the supply of sugar in the later stage by delaying senescence.
To determine whether an increase in carbon activity played a major role in delaying leaf senescence in maize, we measured the activities of13C movement and location in maize plants under different light treatments. We found that 13C abundance significantly increased by 16.9% in PL than in NL and 38.1% in PS than in PF on day 1 up to day 10, respectively. Decrease in13C abundance started from day 20 up to day 40 without significant differences in their levels under PL and NL (Fig. 4A). The abundance of 13C in different leaves showed that partial shade increased 13C accumulation in the ML (middle leaves) and BL (bottom leaves) than in the TL (top leaves) on day 1 before it started to decrease from day 10 up to day 40 (Fig. 4B). Different light treatments on different maize leaves showed that SM (middle leaves of shade side) and SB (bottom leaves of shade side) increased 57.1% and 20.8% of 13C abundance under PL before day 10 compared to NL before starting to decrease from day 20 up to day 40 (Fig. 4C). Analysis of different parts of maize plants showed that 13C abundance increased by 18.2 % in the ear of PL leaves compared to NL with the senescence progress from day 20 up to day 40. These increases mainly came from roots and stem, compared to NL,13C abundance reduced by 34.7% and 7.8% in roots and stem of PL, respectively (Fig. 4D). Interestingly, 13C abundance in the stem decreased more than in other tissues in the later stage (days from 20 to 40), even in the leaves. The above results showed that carbon supplement in the middle and bottom leaves of the shade side at the early period and mobilization of various organs (especially reducing carbon accumulation in stem and roots) to promote carbon accumulation in-ears at the later period are the key characteristics of improving carbon utilization in maize under partial shade.