Partitioning Pattern, Process and Regulation Mechanisms of Photosynthate of Winter Wheat under Different Water Treatments

Abstract: In 2005-2006, two experiments were carried out to study the effect of drought stress on photosynthate partitioning patterns in winter wheat (Triticum aestivum L.), as well as ecophysiological responses of winter wheat to drought and rewatering, at the Fengqiu Agro-ecological Experimental Station, Chinese Academy of Sciences. Jimai20 and Zhoumai18, two winter wheat cultivars differing in sensitivity to drought stress, were grown in the field under a retractable awning. In experiment 1, plants were subjected to three levels of drought stress: W_1 (control, well-watered), W_2 (slight drought) and W_3 (severe drought). In experiment 2, plants were subjected to severe drought stress at the reviving stage, and then rewatered respectively at the jointing stage (W_(4-1)), the booting stage (W_(4-2)) and the flowering stage (W_(4-3)). Plants of the W_0 treatment group were never watered, while those of the control group were kept well-watered throughout the experiment. Destructive sampling was carried out at the jointing, booting, flowering, grain filling and harvest stages in experiment 1, and at the booting stage (W_(4-1)), the flowering stage (W_(4-2)) and the grain filling stage (W_(4-3)) in experiment 2, to determine whole plant biomass, leaf water potential, chlorophyll content, root activity, carboxylase activity, soluble saccharide/carbonhydrate content and soluble protein content. Meanwhile the photosynthesis characteristics were measured with a Li-6400 portable photosynthesis system, and hyperspectral reflectance data were collected using an ASD Fieldspec HandHeld spectrometer.Results of experiment 1:1 Photosynthate partitioning patterns and dynamics of winter wheat underdifferent water treatments1.1 Drought stress led to decreased plant height, leaf area and biomass accumulation. Slight drought at the pre-jointing stage stimulated root growth, resulting in greater root biomass and root: shoot ratio. Severe drought stress throughout the growing season resulted in a reduction in total root biomass; it however stimulated root elongation and induced root penetration into deep soil layers, thus leading to an increase in deep-root fraction. Drought stress at the various growth stages boosted photosynthate translocation to the corresponding growth centers, increased photosynthate accumulation in roots during the early stages of vegetative growth, promoted photosynthate translocation to the stem and leaf sheaths as well as accumulation of water-soluble carbonhydrates in temporary storage tissues at the jointing stage, and induced greater post-flowering reallocation of photosynthate from temporary storage organs to grains.1.2 Grain yield was somewhat correlated with growth parameters, among which total biomass at the booting stage was especially relevant (R=0.92), dry leaf biomass, dry root biomass and leaf area at the flowering stage were also important (R>0.90). Drought stress led to decreased individual yield, mainly due to a reduction in both number of spikes and number of kernels per spike. Jimai20 had a lower 1000 kernel weight than Zhoumai18. Drought stress did not affect 1000 kernel weight significantly. Harvest index of Jimai20 was linearly correlated with water supply levels, while harvest index of Zhoumai18 was insensitive to drought stress.2 Ecophysiological responses of winter wheat to different levels of drought stress2.1 Leaf water potential and chlorophyll content of winter wheat decreased as the level of drought stress climbed. Slight drought stress led to heightened RuBPCase activity while severe drought stress did the opposite. The ability to regulate soluble carbohydrates was enhanced under slight drought stress. Severe drought stress resulted in decreased Pn. Under slight drought stress, changes in Pn were caused by stomatal changes; while under severe drought stress, Pn was largely influenced by non-stomatal factors. There was a strong positive correlation between RuBPCase activity and Pn. Transpiration rate (Tr) decreased as the level of drought stress increased. Water use efficiency peaked under slight drought stress. WUE was relatively low when water was well supplied due to high levels of transpiration rate, and under severe drought stress due to low levels of photosynthetic rate. Flag leaf net photosynthetic rate was positively correlated with PAR; the light compensation point was 50-60μmol·m~(-2)·s~(-1) and the light saturation point was 1050-1100μmol·m~(-2)·s~(-1). Tr was linearly correlated with PAR (R= 0.886). Flag leaf Pn was positively correlated with Ta and Tl. The optimal Ta was 25-30℃, Tl was approximately 2℃higher than Ta, and the optimal Rh was circa 60%.2.2 At the grain filling stage, spectral reflectance of winter wheat peaked at the boundary between visible and near-infrared light. Drought stress led to increased reflectance in the visible region and decreased reflectance in the NIR region under drought stress. The first derivative spectra were used to determine the wavelength of the red-edge, the results showed that the red edge was at 728-730nm at the booting and flowering stages, then shifted to 734 nm at the filling stage. Drought stress led to decreased Dλ_(red). Chlorophyll content was positively correlated with Dλ_(730):Dλ_(702) and Dλ_(730):Dλ_(718); LWC was positively correlated with Dλ_(red) and Dλ_(718); LAI was positively correlated with Dλ_(718), Dλ_(red), and S_(red); the correlation coefficients were all greater than 0.5 (P<0.05). Chlorophyll content and Dλ_(730):Dλ_(702), LWC and Dλ_(red) were linearly correlated (R~2=0.87); LAI and S_(red) were quadratically correlated (R~2=0.68).Results of experiment 21 Influences of drought and rewatering on winter wheat developmentRewatering had compensatory effects on losses in winter wheat biomass and leaf area induced by drought stress. Rewatering at the jointing stage had the greatest stimulating effect on biomass and leaf area. Compensatory growth however did not lead to a full recovery of leaf area lost during the drought period. Rewatering at first suppressed root growth, and then stimulated it. Rewatering at either the jointing, the booting or the flowering stage stimulated root growth and induced root penetration into deep soil layers. Rewatering at the flowering stage stimulated root elongation significantly. Drought stress led to reduced photosynthate allocation in leaves and spikes, and increased photosynthate allocation in the stem, leaf sheaths and roots. Rewatering at the jointing stage, the booting stage and the flowering stage boosted photosynthate accumulation in leaves, the stem and grains respectively. Rewatering at all stages allowed full recovery of drought induced yield loss, by increasing number of spikelets at the jointing stage or 1000 kernel weight at the booting and flowering stages.2 Ecophysiological responses of winter wheat to drought and rewateringPost-drought rewatering led to increased leaf chlorophyll content. Rewatering at the jointing-booting stage resulted in chlorophyll super-compensation and heightened RuBPCase activity. Rewatering tended to have increasingly greater effect on RuBPCase activity as the plant progressed to later growth stages. Post-drought rewatering also increased root activity; plants rewatered at the jointing-booting had higher root activity than plants of the control group. Rewatering at all stages had clear compensatory effects on Pn, largely due to increases in RuBPCase activity and chlorophyll content…
Key words: wheat gluten; rheological behaviors; biaxial extensional deformation; uniaxial extensional deformation