Photosynthetic Characteristics And ¹⁴C-assimilate Transportation During Middle And Later Filling In Wheat Spike Under Water Deficit

Drought is one of the main abiotic stresses limiting crop growth. Wheat non-leaf organs（ear bracts, peduncle, sheath, stems, etc.） hold higher stress tolerance, exhibiting relatively higher photosynthetic characteristics and contribution to grain filling under water defict. Pot experiment were performed using two different drought resistance of wheat varieties. The net photosynthetic rate（Pn） of spike and flag leaves was detected in the later grown stage and the chloroplast ultrastructure was observed at 24 day after anthesis（DAA）. ¹⁴CO₂ isotope labeling technique was carried out to study the wheat spike ¹⁴C-assimilates accumulation and transportation during grain filling middle and late period under water deficit condition. The main result were list as follow:1. Pn of two different drought resistance wheat flag leaf were declined after anthesis under both water treatments, while spike Pn reached a peak at 6 DAA. Compared to normal condition, flag leaf Pn in drought-resistant cultivar Pubing 143 were decreased 33.7% and drought-sensitive cultivar Zhengyin 1 were decreased 45.3% under water deficit. The rate of decline in water-sensitive cultivar was higher than that in drought-resistant cultivar. Spike Pn in Pubing 143 and Zhengyin 1 were decreased 16.9% and 23.7%（P<0.05） under water deficit. It suggested that the water-sensitive cultivar is more sensitive to water deficit. Compared to the flag leaf, water deficit less impacted on spike Pn and spike posess persistent photosynthesis, showing photosynthetic advantage.2. TEM observation showed that the chloroplast of flag leaf in Zhengyin 1 and Pubing143 were deformed under water deficit and the size of chloroplasts in flag leaves gradually expanded to round and large number of plastoglobuli were rapidly accumulated. The envelope membrane was invariably ruptured, the stacks of thylakoids disappeared with only a few grana remaining, and the number of plastoglobuli obviously increased in flag leaves of drought-sensitive cultivar. The volume of chloroplasts in spike bracts, glumes lemma and awn increased slightly, and their shape was longer and narrower than flag leaves. During later grain filling, the cells of spike bracts had well-differentiated chloroplasts, containing fully developed grana with numerous layers and well-developed stroma lamellae at a higher density under normal water supply in Zhengyin 1 and Pubing 143. The thylakoid membrane system were still abundant in spike bracts comparison to the complete disruption of that in flag leaves. All of these changes indicate that drought accelerate the aging of flag leaves and the degradation of chloroplast. The impact of drought on Zhe is greater than on Pub. The results suggest that the onset of the chloroplast of flag leaves more degradation and senescence than spikes.3. ¹⁴CO₂ isotope labeling experiment indicated that ¹⁴C-assimilates in seeds was largely accumulated at 15²0 ADD, and then reached a stable maximum value at 25 DAA. At 30DAA（on harvest） the ¹⁴C-assimilates distribution rate in grain of Pubing 143 were increased by 1.9% and in Zhengyin 1 were 1.65% under water deficit. Water deficit promoted assimilates transport from spike bracts. Whereas ¹⁴C-assimilates in spike bracts（glume, lemma and awn） was rapidly transported at 15²0 ADD, followed completely transferred at 25 DAA. On harvest, ¹⁴C-assimilates of grain under water deficit condition was significantly higher than that under normal water condition and grain ¹⁴C-assimilates in drought-resistant cultivar Pubing143 was higher than that in water-sensitive cultivar Zhengyin1, indicating that the photosynthetic capacity of spike organs cannot be ignored.4. The weight of grains per ear and 1000-kernel weight in Pubing 143 declined by 30.14% and 9.55%, in Zhengyin 1 declined by 39.58% and 16.2%. The decreasing rang of HI in Zhengyin 1（14.9%） was higher than in Pubing143（5.56%） under water deficit. The WUE in Pubing143 increased by 10.3, whereas in Zhengyin 1 droped 11.1%（P<0.05）. It shows the effect of moderate soil drought on wheat yield and harvest index in drought-resistant cultivar less than in drought-sensitive cultivar. This may be related to the photosynthetic delays related to persistence and aging.