| About 90 to 95% of crop biomass is derived from photosynthetic products. Leaves are the main photosynthetic organs of crops. However, many other plant organs(including the ear, awn, and sheath of wheat; the bract of maize; the panicle of rice, and the developing fruit of tomato) contain or produce chlorophyll and are also photosynthetically active. Photosynthesis in these plant parts can make a significant contribution to yield. Greater understanding about the photosynthetic function of non-leaf green organs could help scientists to increase both whole plant photosynthetic potential and crop yield. This research was conducted using different irrigation treatments under field conditions. One of the aims was to learn more about the effect of water supply on canopy function. Specifically, we measured how water supply affected(i) the surface area distribution of each green organ(leaves, bracts, capsule walls, and stems) within a cotton(Gossypium hirsutum L.) canopy,(ii) canopy architecture and the distribution of photosynthetically active radiation(PAR) within the canopy, and(iii) the photosynthesis contribution of leaves and non-leaf organs at different locations within the canopy to total canopy photosynthesis. A second aim of this study was to determine the effects of three irrigation amounts on physiological characteristics related to photosynthesis, PSII activity, and protective mechanisms in leaf and non-leaf organs of cotton. To accomplish this aim, we measured gas-exchange, chlorophyll content, chlorophyll fluorescence, soluble protein content, antioxidant enzyme activity, and malondialdehyde(MDA) content of leaf and non-leaf organs during boll development. This research may help to increase the photosynthesis potential of whole cotton plants growing under stressful environments. The research also provides a theoretical basis for high-yielding cotton production and drought-tolerant cotton breeding. The main results of this study are listed below.1. We investigated the effects of water supply on the total surface area of each green organ, the PAR distribution within the canopy, and the architecture of different canopy layers. The results showed that leaf surface area declined when the amount of irrigation was reduced, with the largest changes occurring in the upper third of the canopy. Non-leaf organs accounted for approximately 40% of the total plant surface area in the deficit irrigation treatment. The mean toliage tilt angle(MTA) and canopy openness(DIFN) values in the upper third of the canopy were higher in the water deficit treatment than in the conventional irrigation treatment. The PAR transmission in different canopy layers was higher in the slight and moderate deficit irrigation treatments than in the conventional irrigation treatment. Deficit irrigation increased PAR transmission into the middle and lower layers of the cotton canopy, thus increasing the effective photosynthetic area of these layers.2. We measured the canopy photosynthetic rates of each green organ in different parts of the canopy and then calculated their relative contribution to total canopy photosynthesis. A reduction in water supply caused large declines in leaf canopy photosynthetic rates. For example, moderate deficit irrigation significantly reduced leaf canopy photosynthetic rates by 25%. However, deficit irrigation had less effect on bract and stalk photosynthesis. The relative contribution of bracts and stalks to canopy photosynthesis increased under water deficit conditions. Deficit irrigation increased the contribution of both leaves and non-leaf organs in the middle third of the canopy to total canopy photosynthesis. There was no significant difference in total canopy photosynthetic rate between the slight deficit irrigation treatment and the conventional irrigation treatment(P<0.05).3. The gas-exchange, chlorophyll content, soluble protein content, and chlorophyll fluorescence characteristics of non-leaf organs of cotton were measured in the three water treatments during different growth stages. Water-deficit caused a reduction in the net photosynthetic rate, stomatal conductance, and chlorophyll content of cotton leaves. The net photosynthetic rates of leaves declined rapidly beginning 25 days post-anthesis. In contrast, the net photosynthetic rates of bracts and capsule walls were relatively insensitive to soil moisture stress, decreasing by only a small amount between 25 and 45 days post-anthesis. On a surface area basis, the net photosynthetic rates and soluble protein content of non-leaf organs(bracts and capsule walls) were lower than those of leaves; however, the chlorophyll content, soluble protein content, stomatal conductance, and photosynthetic rates declined slightly more in bracts and capsule walls than in leaves as the plants matured. None of the water treatments damaged the photosynthetic apparatus in the leaves, bracts and capsule walls of cotton. The photochemical activity of leaves was higher than that of non-leaf organs. The electron transport rate(ETR) of leaves was significantly lower in the moderate deficit irrigation treatment than in the control treatment, whereas the slight deficit irrigation treatment had relatively little effect on the ETR of leaves. The ETR of non-leaf organs was significantly lower than that of leaves under different light intensities. The ETR of non-leaf organs was not affected by water treatment. Overall, the photosynthetic rates declined rapidly when the cotton leaves began to senesce; however, the photosynthetic rates of non-leaf organs remained relatively steady. These results suggest that leaf photosynthesis is supplemented by photosynthesis in stems, bracts, and capsule walls. This is especially important when leaf photosynthesis capacity declines due to aging or water stress.4. We estimated the canopy photosynthetic rate of leaves, bracts, and capsule walls in each canopy layer by multiplying the net photosynthetic rate(Pn) per unit area of each organ by its total surface area in the layer. The result was then multiplied by the number of plants per unit area. The Pn rates of leaves and bracts were significantly and positively correlated with the canopy photosynthetic rates of each canopy layer(leaves: R2 = 0.7047, P<0.01; bracts: R2 = 0.7148, P<0.01). In contrast, there was no significant correlation between the Pn rates of capsule walls and the canopy photosynthetic rates. There was also significant positive correlation between the measured canopy photosynthetic rates and the estimated canopy photosynthetic rate of leaves and bracts in different canopy layers. Because of the relationship between the Pn and the canopy photosynthetic rate of each green organ, we could use the Pn of different canopy layers to reliably estimate the total canopy photosynthetic rate.5. Water deficit causes the closure of leaf stomates in plants, reducing the carbon assimilation rate and producing large amounts of surplus light energy. This results in photoinhibition of the thylakoid membrane in PSII. The green organs of plants have a variety of protective mechanisms to minimize the negative effects caused by light inhibition. Therefore, we measured the effects of water treatment on the Car/Chl ratio, PSII chlorophyll fluorescence, MDA content, and antioxidant enzyme activity in the leaf and non-leaf cotton organs. The results showed that the antioxidant enzyme activity and MDA values of capsule walls were lower than those of leaves, bracts, and stems. The non-photochemical quenching(NPQ) of capsule walls and leaves in the moderate water deficit treatment was significantly higher than that in the conventional irrigation treatment. These results suggest that under water deficit conditions, capsule walls have little damage to the cell plasma membrane, low antioxidant enzyme activity, and high light-independent non-photochemical quenching. The Car/Chl ratio was higher in capsule walls than in leaves, bracts, or stems. This showed that that capsule walls quenched excess light energy by accumulating carotenoids. Water deficit significantly increased the superoxide dismutase(SOD) activity in leaves and bracts. These results showed that SOD activity in cotton leaves and bracts was relatively sensitive to water deficit. Stems mainly rely on the high activity of antioxidative enzymes to protect their photosynthetic apparatus.6. The relative photosynthetic contribution of bolls(capsule walls plus bracts) and stems was assessed by darkening either the bolls or the stems during fruit development. Cotton seed weight in the conventional irrigation treatment decreased by 10.1%–29.7% when the bolls(capsule walls plus bracts) were darkened and by 5.3%–9.9% when the stalks were darkened. In conclusion, photosynthesis in non-leaf organs can make a significant contribution to cotton seed yield formation, especially when water stress occurs during late fruit development. Water deficit reduced both the number of bolls per plant and the boll weight. Water deficit caused declines in the number of bolls per plant in the upper canopy layer. In contrast, slight deficit irrigation caused slight increases in boll weight in both the upper and mid-canopy layer. Slight deficit irrigation caused a slight increase in seed yield; however, seed yield decreased by 25%–49% in the moderate deficit irrigation treatments. Hence, water-saving irrigation methods can be used to alter the distribution of PAR within cotton canopies. Water-saving irrigation can also affect the photosynthetic activity of each green organ in the canopy. It is important to maintain high leaf photosynthetic rates. It is also important to develop the potential photosynthetic capacity of non-leaf organs, especially by increasing their surface area and PAR transmission into the middle and lower canopy layers. |
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