Effects Of Leaf Removal On Photosynthetic Capacity And Yield Of Summer Maize Under High Density And Its Physiological Mechanisms

The remarkable increase in grain yield of maize(Zea mays L.) has been attributed mainly to an increase in plant density. However, due to increased plant density, solar radiation intercepted by lower strata leaves decreases, and per-plant crop growth rate are also suppressed. How to coordinate the relationship between canopy apparent photosynthesis(CAP) and individual leaf photosynthesis capacity under high plant density is an important problem we are facing. Additionally, there exists leaf redundancy when maize crops were planted at high density. Leaves of the upper canopy shade leaves at the middle strata, especially under high plant density. Partially removing several leaves above the ear leaf is beneficial for improved photosynthesis per plant. Field experiments were conducted during the growing season of 2012 and 2013 at Experimental Farm of Shandong Agricultural University and State Key Laboratory of Crop Biology. Two summer maize(Zea mays L.) cultivars, Zhengdan958(a compact cultivar) and Jinhai5(a semi-compact cultivar), were planted at a plant density of 105,000 plants ha–1. Plants were subjected to the removal of two(S2), four(S4) or six leaves(S6) from top of a plant at 3 days after anthesis(DAA), with no leaf removal as control(S0). We examined whether canopy apparent photosynthesis and individual photosynthetic capacity were coordinated, and whether leaf senescence were delayed and 13C-photosynthate distribution to grain were increased, and finally achieved higher grain yield due to leaf removal, which will provide theoretical basis and technical supports for super-high-yield cultivation and breeding. The main results were as follows: 1. Effects of leaf removal on grain yield of two plant types of maize under high densityDifferent levels of leaf removal above the ear leaf led to clear differences in the grain yield and yield components of two maize crops under high density. Excising two leaves from top of a plant increased grain yield significantly, while the magnitude of this regulation varied between cultivars. For Jinhai5, excising two leaves led to an increase in grain yield by 15.41% and 11.67% in 2012 and 2013 compared to control, respectively, while it increased 9.60% and 9.81% for Zhengdan958, indicating that the effects of leaf removal on Jinhai5 were more apparent than Zhengdan958. However, the grain yield was greatly impaired in four or six leaves removal treatments due to significantly reduced source size. For example, in S4 treatment, Jinhai5 exhibited 25.79% and 27.59% decreases in grain yield, respectively, while it decreased by 56.50% and 55.62% in S6 treatment, respectively. The yield components were also affected by different levels of leaf removal. No significant difference was found between S2 and S0 in KNP, while KW in S2 was significantly heavier than that in other treatments. Take Jinhai5 for instance, excising two leaves led to an increase in KW by 11.85 and 2.95% in 2012 and 2013 growing seasons compared to the control, respectively; while for S4 treatment, it decreased by 6.31 and 5.50 %, respectively, and S6 treatment lead to an decrease in KW by 12.85% and 7.25%. In addition, excising two leaves led to more harvested ears at physiological maturity. 2. Effects of leaf removal on dry matter accumulation and distribution in two planttypes of maize under high densityThe effects of leaf removal, year and their interaction were significant on total plant dry matter at physiological maturity. Plants in S2 had the significantly higher dry matter weight, and it was greatly reduced in S4 and S6 treatments. Meanwhile, it also increased biomass accumulation amount from 4DAS to R6 and the ratio of dry matter accumulation after 4DAS to R6. In addition, leaf removal also affected dry matter partitioning to different plant organs at R6. There were significant effects of leaf removal and year on the amount of dry matter in stem tissues(P < 0.001), and there was also a significant interaction of leaf removal × year for Jinhai5(P < 0.05). At physiological maturity, excising two leaves from top of a plant led to a reduction of dry matter in stem and promoted the distribution of it to grain. Nevertheless, excessive leaf removal(four or six uppermost leaves removed) increased the retention of dry matter in the stem, which did not favor yield formation. 3. Effects of leaf removal on canopy apparent photosynthetic characteristics of twoplant types of maize under high densityThere exist significant effects of leaf removal on the capacity of canopy apparent photosynthesis of two plant type corns under high density. Since leaf-removal treatment application, a longer duration of canopy apparent photosynthesis(CAP) and green leaf area index(LAI) were obtained in S2 treatment. For example, at the dough stage, Zhengdan958 and Jinhai5 plants in S2 exhibited a 23.85% and 38.08% increase, respectively, in CAP compared to control. However, it decreased sharply in S4 and S6 treatments. At the dough stage, Zhengdan958 exhibited 31.43% and 42.14% decreases, respectively, in CAP and Jinhai5 decreased by 26.36% and 44.77%, respectively. The results indicated that two leaves removal was beneficial to obtain higher CAP during grain filling, and and the effects of leaf removal on Jinhai5 were more apparent than Zhengdan958. In addition, leaf removal above the cob markedly improved light conditions within the canopy during grain filling, while excessive leaf removal(S4 and S6) caused poor utilization ratio of light resource due to increased light leakage losses during late grain filling. 4. Effects of leaf removal on photosynthetic capacity of individual plant of two planttypes of maize under high densityLeaf removal after anthesis differentially affected Pn, gs and Ci of ear leaves in the two cultivars. Net photosynthetic rate(Pn), gs and chlorophyll content of the remaining ear leaves in the six-leaf removal plants significantly increased prior to 26 DAA compared to control plants, whereas significant reductions in these variables were observed thereafter. In addition, plants in the S2 treatment had an advantage in terms of individual photosynthetic capacity since 26 DAA. Under S2 at 52 DAA, Pn in Zhengdan958 and Jinhai5 increased by 22.58% and 26.67%, respectively, compared to control. However, under S4 and S6, Pn in Zhengdan958 decreased by 8.80% and 28.37%, respectively, whereas it in Jinhai5 decreased by 3.65% and 22.70%, respectively. In contrast, Ci exhibited a trend opposite of Pn. The results indicated that excising two leaves made plants of both maize cultivars more tolerant to high plant density of 105 000 plants ha-1, resulting in a better coordination between population and individuals. In addition, relative to Zhengdan 958, the effects of leaf removal on Jinhai 5 were more apparent. 5. Effects of leaf removal on leaf senescence characteristics and chloroplastultrastructure of two plant types of maize under high densitySOD activity of ear leaf was significantly affected by different levels of leaf removal. SOD activity in S2 treatment increased beginning at 26 DAA, whereas values declined rapidly in the S4 and S6 treatments. At 52 DAA in 2012, the activity of SOD increased by 9.97% in the ear leaves of S2 in Zhengdan958 and by 16.34% in Jinhai5. Meanwhile, MDA content in S2 treatment was significantly lower than that in control, indicating that the removal of two leaves decreased the degree of membrane lipid peroxidation of the ear leaves in both cultivars. The results indicated that leaf senescence was delayed by excising two leaves under high plant densities and the effects of excising two leaves on senescence were greater for Zhengdan958 than for Jinhai5. However, excessive leaf removal accelerated the senescence of the remaining ear leaf. In addition, excising two leaves from top of a plant made the ear leaf maintain relative better chloroplast ultrastructure, which delayed leaf senescence. With the increased intensity of leaf removal, chloroplast ultrastructure became worsely, and the chloroplast membrane structure incompleted gradually. Finally, the grana was dissolved, which accerated the senescence of ear leaf. 6. Effects of leaf removal on 13C-photosynthates distribution in different organs at fourdays after silking(4DAS) and physiological maturity(R6)Modification of the number of leaves above the ear leaf led to different patterns of allocation of 13C-photosynthates. At 4DAS, when the grain was a weak sink, we found the maximum ratio of distribution of labeled photosynthates was recorded in the stem, followed by other leaves and ear bracts, which accounted for more than 50% of labeled photosynthates. After that, they were retranslocated to the grains during the latter part of the grain filling period. At R6, excising two leaves led to a reduction of 13C-photosynthates in stem and other leaves and promoted the distribution of 13C-photosynthates to grain. Relative to control, the distribution of 13C-photosynthates in grain of S2 treatment increased by 15.57%. However, that allocation decreased by 13.91% in S4 and 22.00% in S6, respectively, which can be interpreted as a result of increased retention of 13C-photosynthates in the stem and ear bracts, which was not favorable for yield formation. 7. Effects of leaf removal on N accumulation and distribution of two plant types ofmaize under high densityN accumulation amount and distribution in different plant organs at four days after silking and physiological maturity was affected by leaf removal treatments during both growing seasons. At four days after silking(4DAS), when the grain was a weak sink, we found N distribution was mainly in stem and other leaves, which accounted for more than 70% of total N accumulation amount. At physiological maturity, excising two leaves increased total N accumulation amount per plant. Take Jinhai 5 for example, relative to the control, plants in S2 obtained 14.66% and 11.17% higher N accumulation amount in 2012 and 2013, respectively. Meanwhile, excising two leaves decreased N accumulation in stem, which was favorable for N remobilization from stem to grain, leading to higher N accumulation amount in grain. However, excising four leaves or six leaves above the ear leaf decreased the N accumulation in grain due to increased retention of N in the stem and ear bracts, which was not favorable for N remobilization from vegetative organs to grain. 8. Effects of leaf removal on hormone content in ear leaves of two plant types of maizeunder high densityThe results indicated that the contents of IAA, ABA, ZR and GA in ear leaves of two plant types of maize under high plant density was obviously regulated by different levels of leaf removal above the ear leaf. Between 0 to 13 days after anthesis, the above mentioned hormone contents in leaf removal treatments were significantly higher than that in non-leaf removal treatment, while it exhibited different patterns between treatments. The contents of IAA and ZR in two leaves removal treatment were significantly higher than that in control, while those in four or six leaves removal treatments were significantly reduced, indicating that proper leaf removal could delay leaf senescence. Meanwhile, excising two leaves decreased ABA content of ear leaves, delayed leaf senescence and pronged the functional time, which was favorable for improving the photosynthetic capacity. Nevertheless, excessive leaf removal increased the accumulation of ABA, leading to a relatively higher ABA content in ear leaves, which accelerated the leaf senescence. In addition, leaf removal had significant effects on hormone content in different parts of grains. The contents of IAA, ABA, ZR in ear leaves of two leaves removal treatment were significantly higher than those in other treatments. Moreover, the lower content of IAA, ABA and ZR and the higher content of GA in the upper grains can be accounted for the grain abortion.