Photosynthetic Characteristics And Strategies Of Acclimation Of Non-foliar Organs In Cotton (Gossypium Spp.) Respond To Water Deficit

Although leaf was considered as the main photosynthetic organ, many parts of crop suchas non-foliar organs which contain chlorophyll can also performance photosynthesis,contributing to carbon acquisition and yield. This research was conducted from threeperspectives:1) changes in photosynthetic capacity of non-foliar organs in cotton and relativecontribution to yield during growth stages;2) differences in photosynthesis charactertics ofnon-foliar organs in cotton and3) the photosynthetic adaptations of green organs in cottonrespond to water stress. On the basis of the three points above, photosynthetic charactertics,relative contribution to its yield and adaptation respond to water stress of non-foliar organs incotton were studied to elucidate a) the changes in photosynthetic rate of green organs incotton during growth stages and the important photosynthetic contribution of non-foliar organto yield formation especially at later growth stage, b) differences in photosynthesischaractertic and photoprotection mechanism of non-foliar organ, and c) effects of waterdeficit on physiological process and the relative contribution of each organ to yield in cottonwas explored. This research may help to increase the potential of photosynthate production,thereby improving cotton yield. Additionally, the results obtained can provide theoreticalbasis for high-yielding cotton production and drought-tolerant cotton breeding. Below are themain results in this thesis:1. The relative photosynthetic contribution of leaves, main stem, bracts and capsule wallsin cotton was revealed by measuring their time-course of surface area development, O2evolution capacity and photosynthetic enzyme activity. Because of the early senescence ofleaves, non-foliar organs increased their surface area up to38.2%of total at late growth stage.Bracts and capsule wall showed less ontogenetic decrease in O2evolution capacity per areaand photosynthetic enzyme activity than leaves at the late growth stage. The total capacity forO2evolution of stalks and bolls （bracts plus capsule wall） was12.7%and23.7%, respectively,as estimated by multiplying their surface area by their O2evolution capacity per area. We alsokept the bolls （from15days after anthesis） or main stem （at the early full bolling stage） indarkness for comparison with non-darkened controls. Darkening the bolls and main stemreduced the boll weight by24.1%and9%, respectively. Thus, we conclude that non-foliarorgans significantly contribute to the yield at the late growth stage.2. The photosynthetic charactertics of non-foliar organs in super-high-yielding cottonduring growth stages were explored. Compared with traditional cotton Xinluzao33, thephotosynthetic rate of the main leaf of hybrid cotton Xinluzao43and Shiza2was higherduring5-15days after anthesis, which reason for faster dry matter accumulation rate ofXinluzao43and Shiza2cotton during early growth stage （Full Flowering）. However, more decrease in photosynthetic rate of green organs of Shiza2during the growth stages, whichcaused its early senescence. There was no significance in photosynthetic rate of main stemamong these three cultivars. Thus, we suggested that the high height and larger surface area ofhybrid cotton result in their better canopy photosynthetic rate of stalks at later growth stage.The photosynthetic rate of capsule wall in hybrid cotton Xinluzao43and Shiza2was higherthan that in traditional cotton Xinluzao33, and maintain stable photosynthetic rate during latergrowth stage, company with their larger surface area which all contribution their highercanopy photosynthetic rate of fruits at later growth stage.3. The rapid respiration rate of cotton （Gossypium hirsutum L.） fruits （bolls） produces aconcentrated CO₂micro-environment around the bolls and bracts. Elucidation of themechanisms by which plants adapt to elevated CO₂is needed. However, most studies of themechanisms investigated the response of plants adapted to current atmospheric CO₂. Weobserved that the intercellular CO₂concentration of a whole fruit （bract and boll） ranged from500to1300μmol mol1depending on the irradiance, even in ambient air. Arguably, this CO₂micro-environment has existed for at least1.1million years since the appearance of tetraploidcotton. Therefore, we hypothesized that the mechanisms by which cotton bracts have adaptedto elevated CO₂will indicate how plants will adapt to future increased atmospheric CO₂concentration. To test the hypothesis, the morphological and physiological traits of bracts andleaves of cotton were measured, including stomatal density, gas-exchange and proteincontents. Compared with leaves, bracts showed significantly smaller stomatal conductancewhich resulted in a significantly higher water use efficiency. Both gas-exchange and proteincontent showed a significantly greater RuBP regeneration/RuBP carboxylation capacity ratio(J_max/V_cmax) in bracts than in leaves. These results agree with the theoretical prediction thatadaptation of photosynthesis to elevated CO₂requires increased RuBP regeneration. Cottonbracts are readily-available material for studying adaption to elevated CO₂.4. Photoinactivation of Photosystem II （PS II） during photosynthesis can lead to the lossof photochemical efficiency and decrease in crop yield. Plants have evolved variousphotoprotective strategies to ameliorate photoinactivation of PS II. Non-leaf organs of cottonalso contribute to carbon gain, but it is not clear how they photoprotect themselves. This studyinvestigated the photoprotective mechanisms in the leaf, bract, main stem and capsule wall ofcotton. Our results suggested that the bract mainly relies on high activities of antioxidativeenzymes and high ΔpH-and xanthophyll-regulated thermal dissipation (Φ_NPQ) forphotoprotection. The main stem preferentially dissipated its absorbed light energy vialight-regulated as well as light-independent non-photochemical quenching, aided by themoderately high activities of antioxidative enzymes. The capsule wall was less able to removereactive oxygen species due to lower activities of antioxidative enzymes, and less able todissipate energy via heat due to its lower Φ_NPQ. Its main photoprotective mechanisms seem tobe （a） direct quenching of the energy by abundant carotenoids and （b） light-independentconstitutive thermal dissipation via Φf,D. The green organs of cotton have different ways touse or dissipate energy. 5. Here we report on a whole-tissue determination of the rate coefficient ofphotoinactivation ki, and that of repair k_rin cotton leaf discs. The P700kinetics area, directlyproportional to the oxygen yield per single-turnover, saturating flash, was used to obtain bothkiand k_r. Changes in kiand k_rof two orientations （abaxial/adaxial surface contacting water）were investigated in this study. The value of ki, directly proportional to irradiance, wasslightly higher when CO₂diffusion into the abaxial surface （richer in stomata） was blocked bycontact with water. The value of k_r, sizable in darkness, changed in the light depending onwhich surface was blocked by contact with water. When the abaxial surface was blocked, k_rfirst peaked at moderate irradiance and then decreased at high irradiance. When the adaxialsurface was blocked, k_rfirst increased at low irradiance, then plateaued, before increasingmarkedly at high irradiance. At the highest irradiance, k_rdiffered by an order of magnitudebetween the two orientations, attributable to different extents of oxidative stress affectingrepair. The P700kinetics area is a rapid, whole-tissue and in vivo measurement of relativeassay of functional PSII content, can help to explore the differences and underlyingmechanisms in photosynthetic capacity of leaf in cotton and also provide technical basis forexploring the differences and underlying mechanisms in photosynthetic capacity of variousgreen organs in cotton.6. Water deficit is one of the most important causes of decreased yield in cultivatedplants. The physiological response of each organ to water deficit has not beencomprehensively studied in relation to the water status, photosynthetic rate, photosyntheticenzyme activity, antioxidant enzymes. Water deficit significantly decreased the surface area ofeach organ, but to a lesser extent in non-foliar organs. Non-foliar organs （bracts and capsulewall） showed less ontogenetic decrease in O2evolution capacity and in RuBPC activity （perdry weight） as well as better antioxidant systems than leaves at various days after anthesis. Addition ally, the relative contribution of biomass accumulation of non-foliar organs to thewhole cotton plant was estimated from the surface area, photosynthetic duration and the O2evolution capacity per area. Our result s showed that the relative contribution of biomassaccumulation of non-foliar organs increased under water deficit.