Photosynthetic base of reduced grain yield by shading stress during the early reproductive stage of two wheat cultivars

Abstract

the young microspore (YM) stage is the most sensitive period for wheat grain formation to abiotic stress. Shading stress during YM stage reduces grain yield mainly due to grain number decrease. However, the photosynthetic base for grain number decrease is still unclear. In this study, 100% (control), 40% (S1), and 10% (S2) of natural light were applied for 1, 3, 5, and 7 days (D1, D3, D5 and D7) during YM stage of two wheat cultivars (Henong825, Kenong9204). The results showed that grain number in Henong825 and Kenong9204 was reduced by − 3.6 to 33.3% and 14.2-72.7%, respectively. The leaf photosynthetic rate (Pn) in Henong825 and Kenong9204 was deducted by 4.5-93.9% and 26.4-99.0%. Under S1-D1, the leaf Pn of Henong825 reducing was mainly due to the reduction of light intensity. With shading intensity and duration increasing, the reasons for leaf Pn decrease were the low light intensity, the low Gs (stomatal conductance) and chlorophyll content, the damage of ultrastructure of chloroplast and photosynthetic system. Under S2-D7, the chlorophyll content, Fv/Fm (maximal photochemical efficiency of photosystem II) and Jmax (maximum electron transport) were reduced by 19.6%, 5.2% and 28.8% in Henong825, and by 29.9%, 7.8% and 33.1% in Kenong9204. After shading removal, the leaf Pn of Kenong9204 under D5 and D7 could not reach to the level of CK. This study indicated that the reduction of leaf Pn was mainly due to the low light intensity under short shading duration (shorter than 3 days), and due to low light intensity and damage of the leaf photosynthetic system under longer shading duration (longer than 5 days), especially for Kenong9204 (shade-sensitive cultivar). Light is necessary for plant growth. Solar radiation gradually decreased with climate change. According to the Intergovernmental Panel on Climate Change (IPCC) 1 , the global radiation reaching the earth's surface from 1960 to 2000 has been reduced by 1.3% on average per decade. In most parts of China, the solar radiation has declined by more than 6% per decade 2 , especially in South China and the North China Plain 3. The solar radiation of this region from 1961 to 2003 annually decreased by 19.6 MJ m −2 a −14,5. Moreover, the reduction of solar radiation during wheat-growing season ranged from 11 to 21% in 2010-2012 6. Thus, in order to ensure food security in the shading conditions in the future, it is necessary to understand and assess how solar radiation affects yield variation in wheat. Grain yield reduction due to shading depends on shading intensity and duration, growth stage and variety characteristics of plant 7,8. From jointing (Z31) 9 to maturity (Z90), moderate shading intensity (≥ 85% of full radiation) augmented the grain yield of shade-tolerant cultivars, but more than 22% shading intensity (≤ 78% of full radiation) significantly reduced the yield 10,11. Shading at approximately 20 days pre-anthesis of wheat, grain yield decreased the most primarily because of the decrease in kernel numbers 12. Fischer 13 also found that at 15 days pre-ant...

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