Photosynthetic compensation of maize in heterogeneous light is impaired by restricted photosynthate export.

When a plant is exposed to heterogeneous light, the photosynthesis of unshaded leaves is often stimulated to compensate for the decline in photosynthesis of shaded leaves, i.e., photosynthetic compensation. However, a decline of photosynthesis in unshaded leaves, which means an impairment of photosynthetic compensation, has also been widely reported. Herein, two cultivars of maize (Zea mays L.), 'Rongyu1210' (RY) and 'Zhongdan808' (ZD), were studied comparatively. Both cultivars performed evident photosynthetic compensation under heterogeneous light (HL) as the light phase begins (8:30 a.m.). However, as the light phase continues (10:30 a.m.), an impairment of photosynthetic compensation took place in HL-treated ZD, but not in HL-treated RY. For both cultivars, nitrogen content of unshaded leaves was higher under HL, indicating a preferential nitrogen distribution towards unshaded leaves. This is related to the photosynthetic compensation but not the cause of the impairment. In addition, no obvious difference was found in the response of photosynthates (sucrose and starch) to HL between cultivars at 8:30 a.m. However, at 10:30 a.m., the content of photosynthates decreased significantly in unshaded leaves of HL-treated RY, along with increased abundances of both sucrose transporters (SUTs) and H+-ATPase (EC 7.1.2.1). In contrast, it increased along with decreased abundances of SUTs and H+-ATPase in HL-treated ZD. These results suggest that the photosynthetic compensation is impaired when photosynthates export of unshaded leaves is restricted. This suggestion is further confirmed by the results of 13C labeling and dry weight detection on young leaves as 'sink'.

Potential importance of malate diffusion in the response of maize photosynthesis to heterogeneous light.

It is well known that the photosynthetic performance of a leaf is highly dependent on the systemic regulation from distal parts within a plant under light heterogeneity. However, there are few studies focusing on C4-specific processes. In the present study, two cultivars of maize (Zea mays L.), 'Rongyu 1210' (RY) and 'Zhongdan 808' (ZD), were treated with heterogeneous light (HL). The net photosynthetic rate (Pn) of newly developed leaves was found to increase in HL-treated RY, while it decreased in HL-treated ZD. Result also showed a negative correlation between the Pn and the content of malate, a key metabolite in C4 photosynthesis, in these two cultivars. In HL-treated ZD, malate content increased with a decline in the abundance of NADP-malic enzyme (EC 1.1.1.40), suggesting that less malate was decarboxylated. Moreover, a restriction of malate diffusion is proposed in HL-treated ZD, since the interface length between mesophyll cells (MC) and bundle sheath cells (BSC) decreased. In contrast, malate diffusion and subsequent decarboxylation in HL-treated RY should be stimulated, due to an increase in the abundance of NADP-malate dehydrogenase (EC 1.1.1.82) and a decline in the content of malate. In this case, malate diffusion from MC to BSC should be systemically stimulated, thereby facilitating C4 photosynthesis of a maize leaf in heterogeneous light. While if it is systemically restricted, C4 photosynthesis would be suppressed.

Comparative study on the different responses of maize photosynthesis to systemic regulation under light heterogeneity.

Photosynthetic performance of a leaf is widely recognized to be systemically regulated by distal parts within the same plant. However, the effects of systemic regulation on different plant materials cannot be generalized. In this work, two cultivars of maize (Zea mays L.), 'Rongyu 1210' (RY) and 'Zhongdan 808' (ZD), were selected for a comparative study on the different responses of photosynthesis to light-dependent systemic regulation. After the growth of plants in heterogeneous light, the net photosynthetic rate of newly developed leaves increased in RY but decreased in ZD. A distinct capacity of CO2 fixation and assimilation between these two cultivars is also suggested. In ZD, the area of vascular bundles declined obviously, suggesting a restriction on carbohydrate export, which is also indicated by an increase in starch content. Resulting excessive accumulation of carbohydrates is proposed to inhibit the carbon assimilation, and eventually the photosynthesis. A decline in the area of bundle sheath cells also suggests a restriction on carbon assimilation. In contrast, these restrictions were unlikely to present in RY. This study reveals that the response of leaf photosynthetic performance to light heterogeneity is largely dependent on the systemic regulation of carbon assimilation, as well as carbohydrate export in maize.

Involvement of carbohydrates in long-term light-dependent systemic regulation on photosynthesis of maize under light heterogeneity.

It is widely recognized that different parts of a plant can communicate with each other via light-dependent long-distance signaling under heterogeneous light conditions. However, the mechanism of such systemic signaling has not been revealed yet. Our studies on different species suggest the involvement of carbohydrates in light-dependent systemic regulation between different parts of a plant under both short- and long-term light heterogeneity. Leaves exposed to better light condition perform enhanced photosynthetic capacity, and act to compensate for the decline in photosynthesis of other leaves under bad light condition within the same plant. This kind of compensatory photosynthesis has a close relationship to the distribution of carbohydrates, and can be regarded as an integrative strategy to make efficient use of sunlight at the whole-plant level.