Wangfeng Zhang

Two distinct strategies of cotton and soybean differing in leaf movement to perform photosynthesis under drought in the field

This paper reports an experimental test of the hypothesis that cotton and soybean differing in leaf movement have distinct strategies to perform photosynthesis under drought. Cotton and soybean were exposed to two water regimes: drought stressed and well watered. Drought-stressed cotton and soybean had lower maximum CO2 assimilation rates than well-watered (control) plants. Drought reduced the light saturation point and photorespiration of both species - especially in soybean. Area-based leaf nitrogen decreased in drought-stressed soybean but increased in drought-stressed cotton. Drought decreased PSII quantum yield (ΦPSII) in soybean leaves, but increased ΦPSII in cotton leaves. Drought induced an increase in light absorbed by the PSII antennae that is dissipated thermally via ΔpH- and xanthophylls-regulated processes in soybean leaves, but a decrease in cotton leaves. Soybean leaves appeared to have greater cyclic electron flow (CEF) around PSI than cotton leaves and drought further increased CEF in soybean leaves. In contrast, CEF slightly decreased in cotton under drought. These results suggest that the difference in leaf movement between cotton and soybean leaves gives rise to different strategies to perform photosynthesis and to contrasting photoprotective mechanisms for utilisation or dissipation of excess light energy. We suggest that soybean preferentially uses light-regulated non-photochemical energy dissipation, which may have been enhanced by the higher CEF in drought-stressed leaves. In contrast, cotton appears to rely on enhanced electron transport flux for light energy utilisation under drought, for example, in enhanced nitrogen assimilation.

Alternative electron sinks are crucial for conferring photoprotection in field-grown cotton under water deficit during flowering and boll setting stages

To clarify the photoprotective mechanisms of cotton leaves under water deficit in the field, leaf gas exchange, chlorophyll a fluorescence as well as the corresponding physiological responses were examined in cotton (Gossypium hirsutum L.) to evaluate electron flux distribution. With increasing water deficit, net photosynthetic rate (Pn) significantly decreased, the total electron flux through PSII [Je(PSII)] gradually decreased and the fraction of electron flux required to sustain CO2 assimilation [Je(PCR)] markedly declined. Simultaneously, the ratio of quantum efficiency of PSII [Φ(PSII)] to the quantum efficiency of CO2 fixation [Φ(CO2)] increased, accompanied by an increase in the alternative electron flux (Ja). The enhanced alternative electron flux of O2-dependent Ja(O2-dependent) indicated that electrons had been transported to O2 in the Mehler-peroxide reaction (MPR) and that the remaining alternative electron flux Ja(O2-independent) had been used for nitrate reduction, as indicated by an increase in nitrate reductase (NR) and glutathinone reductase (GR) activities. In addition, mild water deficit increased the proportion of electron flux for the photorespiratory carbon oxidation [Je(PCO)]. Water deficit significantly increased surperoxide radical production rate (O2-•) and hydrogen peroxide content (H2O2), and the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POD) and catalase (CAT) in cotton leaves also increased under water deficit. Therefore, the Mehler-peroxidation reaction, photorespiration and nitrate reduction helped to dissipated excess light energy, being important photoprotective mechanisms for adapting the photosynthetic apparatus to mild and moderate water deficit in cotton.

Rapid recovery of photosynthetic rate following soil water deficit and re-watering in cotton plants (Gossypium herbaceum L.) is related to the stability of the photosystems ☆

The responses of gas exchange, chlorophyll fluorescence and the anti-oxidative system of cotton leaves were studied during water deficit and recovery. The results show that water deficit led to a reversible reduction in the photosynthetic rate. This reduction was mainly accompanied by stomatal limitation. The activity of photosystem II (PSII) and photosystem I (PSI) was relatively stable during water deficit and recovery. Water deficit caused an enhanced production of reactive oxygen species (ROS) and increased lipid peroxidation. Proline accumulation and the anti-oxidative enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX) and peroxidase (POD), along with the antioxidant ascorbate (AsA), increased during water deficit. On re-watering, the ROS generation rate, anti-oxidative enzymes activities and the extent of the lipid peroxidation returned to near control values. Overall, rapid recovery of the photosynthetic rate is related to the stability of the photosystems which appears to be a critical mechanism allowing cotton plants to withstand and survive drought environments.

Cotton bracts are adapted to a microenvironment of concentrated CO2 produced by rapid fruit respiration

BACKGROUND AND AIMS Elucidation of the mechanisms by which plants adapt to elevated CO2 is needed; however, most studies of the mechanisms investigated the response of plants adapted to current atmospheric CO2. The rapid respiration rate of cotton (Gossypium hirsutum) fruits (bolls) produces a concentrated CO2 microenvironment around the bolls and bracts. It has been observed that the intercellular CO2 concentration of a whole fruit (bract and boll) ranges from 500 to 1300 µmol mol(-1) depending on the irradiance, even in ambient air. Arguably, this CO2 microenvironment has existed for at least 1·1 million years since the appearance of tetraploid cotton. Therefore, it was hypothesized that the mechanisms by which cotton bracts have adapted to elevated CO2 will indicate how plants will adapt to future increased atmospheric CO2 concentration. Specifically, it is hypothesized that with elevated CO2 the capacity to regenerate ribulose-1,5-bisphosphate (RuBP) will increase relative to RuBP carboxylation. METHODS To test this hypothesis, the morphological and physiological traits of bracts and leaves of cotton were measured, including stomatal density, gas exchange and protein contents. KEY RESULTS Compared with leaves, bracts showed significantly lower stomatal conductance which resulted in a significantly higher water use efficiency. Both gas exchange and protein content showed a significantly greater RuBP regeneration/RuBP carboxylation capacity ratio (Jmax/Vcmax) in bracts than in leaves. CONCLUSIONS These results agree with the theoretical prediction that adaptation of photosynthesis to elevated CO2 requires increased RuBP regeneration. Cotton bracts are readily available material for studying adaption to elevated CO2.

Different strategies of acclimation of photosynthesis, electron transport and antioxidative activity in leaves of two cotton species to water deficit

To better understand the adaptation mechanisms of the photosynthetic apparatus of cotton plants to water deficit conditions, the influence of water deficit on photosynthesis, chlorophyll a fluorescence and the activities of antioxidant systems were determined simultaneously in Gossypium hirsutum L. cv. Xinluzao 45 (upland cotton) and Gossypium barbadense L. cv. Xinhai 21 (pima cotton). Water deficit decreased photosynthesis in both cotton species, but did not decrease chlorophyll content or induce any sustained photoinhibition in either cotton species. Water deficit increased ETR/4-AG, where ETR/4 estimates the linear photosynthetic electron flux and AG is the gross rate of carbon assimilation. The increase in ETR/4-AG, which represents an increase in photorespiration and alternative electron fluxes, was particularly pronounced in Xinluzao 45. In Xinluzao 45, water deficit increased the activities of antioxidative enzymes, as well as the contents of reactive oxygen species (ROS), which are related to the Mehler reaction. In contrast, moderate water deficit particularly increased non-photochemical quenching (NPQ) in Xinhai 21. Our results suggest that Xinluzao 45 relied on enhanced electron transport such as photorespiration and the Mehler reaction to dissipate excess light energy under mild and moderate water deficit. Xinhai 21 used enhanced photorespiration for light energy utilisation under mild water deficit but, when subjected to moderate water deficit, possessed a high capacity for dissipating excess light energy via heat dissipation.

Effects of cotton field management practices on soil CO2 emission and C balance in an arid region of Northwest China

Changes in both soil organic C storage and soil respiration in farmland ecosystems may affect atmospheric CO2 concentration and global C cycle. The objective of this field experiment was to study the effects of three crop field management practices on soil CO2 emission and C balance in a cotton field in an arid region of Northwest China. The three management practices were irrigation methods (drip and flood), stubble managements (stubble- incorporated and stubble-removed) and fertilizer amendments (no fertilizer (CK), chicken manure (OM), inorganic N, P and K fertilizer (NPK), and inorganic fertilizer plus chicken manure (NPK+OM)). The results showed that within the C pool range, soil CO2 emission during the whole growing season was higher in the drip irrigation treatment than in the corresponding flood irrigation treatment, while soil organic C concentration was larger in the flood irrigation treatment than in the corresponding drip irrigation treatment. Furthermore, soil CO2 emission and organic C concentration were all higher in the stubble-incorporated treatment than in the corresponding stubble-removed treatment, and larger in the NPK+OM treatment than in the other three fertilizer amendments within the C pool range. The combination of flood irrigation, stubble incorporation and application of either NPK+OM or OM increased soil organic C concentration in the 0-60 cm soil depth. Calculation of net ecosystem productivity (NEP) under different management practices indicated that the combination of drip irrigation, stubble incorporation and NPK+OM increased the size of the C pool most, followed by the combination of drip irrigation, stubble incorporation and NPK. In conclusion, management practices have significant impacts on soil CO2 emission, organic C concentration and C balance in cotton fields. Consequently, appropriate management practices, such as the combination of drip irrigation, stubble incorporation, and either NPK+OM or NPK could increase soil C storage in cotton fields of Northwest China.

Effect of leaf removal on photosynthetically active radiation distribution in maize canopy and stalk strength

Abstract The objectives of this study were to determine how the distribution of photosynthetically active radiation (PAR) in a maize canopy affected basal internode strength and stalk lodging. The distributions of PAR within the canopies of two maize cultivars (Zhongdan 909 and Xinyu 41) were altered by removing whole leaves or half leaves in different canopy layers. The results showed that removing whole leaves or half leaves above the three-ear-leaves (R AE and R AE/2 ) at flowering significantly increased PAR at the ear and interception of PAR (IPAR) from the ear to middle of the ear and soil surface. These changes increased the structural carbohydrate content and rind penetration strength (RPS) of the third basal internode by 5.4–11.6% and reduced lodging by 4.2–7.8%. Removal of the first three leaves below the three-ear-leaves (R BE ) before flowering significantly reduced IPAR from the ear to half way below the ear. This reduced the structural carbohydrate content and the RPS of the third basal internode by 9.1–17.4% and increased lodging by 7.0–11.2%. Removal of the three lowest green leaves (R B ) in the canopy before flowering increased PAR at the bottom of the canopy, but had no effect on the structural carbohydrate content of the basal internode, the RPS, and the lodging rate. Overall, the results indicated that the key factors affecting the basal internode strength formation and lodging were PAR at the ear and IPAR from the ear to halfway below the ear. Increasing PAR at the ear and IPAR from the ear to halfway below the ear could enhance lodging resistance by increasing the structural carbohydrate content and mechanical strength of the basal internode.

Whole-tissue determination of the rate coefficients of photoinactivation and repair of photosystem II in cotton leaf discs based on flash-induced P700 redox kinetics

Using radioactively labelled amino acids to investigate repair of photoinactivated photosystem II (PS II) gives only a relative rate of repair, while using chlorophyll fluorescence parameters yields a repair rate coefficient for an undefined, variable location within the leaf tissue. Here, we report on a whole-tissue determination of the rate coefficient of photoinactivation ki, and that of repair kr in cotton leaf discs. The method assays functional PS II via a P700 kinetics area associated with PS I, as induced by a single-turnover, saturating flash superimposed on continuous background far-red light. The P700 kinetics area, directly proportional to the oxygen yield per single-turnover, saturating flash, was used to obtain both ki and kr. The value of ki, directly proportional to irradiance, was slightly higher when CO2 diffusion into the abaxial surface (richer in stomata) was blocked by contact with water. The value of kr, sizable in darkness, changed in the light depending on which surface was blocked by contact with water. When the abaxial surface was blocked, kr first peaked at moderate irradiance and then decreased at high irradiance. When the adaxial surface was blocked, kr first increased at low irradiance, then plateaued, before increasing markedly at high irradiance. At the highest irradiance, kr differed by an order of magnitude between the two orientations, attributable to different extents of oxidative stress affecting repair (Nishiyama et al., EMBO J 20: 5587–5594, 2001). The method is a whole-tissue, convenient determination of the rate coefficient of photoinactivation ki and that of repair kr.

Characteristics of Photosystem II Behavior in Cotton （Gossypium hirsutum L.） Bract and Capsule Wall

Abstract Though bract and capsule wall of boll in cotton ( Gossypium hirsutum L.) have different photosynthetic capacities, the features of photosystem II (PS II) in these organs are scarce. In this paper, chlorophyll a fluorescence emission was measured to investigate the difference in the photosynthetic apparatus of dark-acclimated (JIP-test) and light-acclimated (light-saturation pulse method) bract and capsule wall. Compared with leaves, the oxygen evolving system of non-foliar organs had lower efficiency. The pool size of PS II electron acceptor of non-foliar organs was small, and the photochemical activity of leaves was higher than that of the bract and capsule wall. In regard to the photosystem I (PS I) electron acceptor side, the pool size of end electron acceptors of leaves was larger, and the quantum yield of electron transport from Q A (PS II primary plastoquinone acceptor) further than the PS I electron acceptors of leaves was higher than that of bract and capsule wall. In all green organs, the actual quantum yield of photochemistry decreased with light. The thermal dissipation fraction of light absorbed by the PS II antennae was the highest in bract and the lowest in capsule wall relative to leaves. Compared with leaves, capsule wall was characterized by less constitutive thermal dissipation and via dissipation as fluorescence emission. These results suggested that lower PS II photochemical activity in non-foliar organs may be result from limitations at the donor side of PS II and the acceptor sides of both photosystems.

The Relative Contribution of Non-Foliar Organs of Cotton to Yield and Related Physiological Characteristics Under Water Deficit

Abstract Water deficit is one of the most important causes of decreased yield in cultivated plants. Non-foliar green organs in cotton play an important role in yield formation at the late growth stage. Although better photosynthetic performance was observed in a non-foliar organ (bract) compared with leaves under water deficit. However, the physiological response of each organ in cotton to water deficit has not been comprehensively studied in relation to the water status and photosynthesis characteristics. We studied the maintenance of water status of each organ in cotton by measuring their relative water content, proline content and stomatal characteristics. Water deficit significantly decreased the surface area of each organ, but to a lesser extent in non-foliar organs. Our results showed that the relative contribution of biomass accumulation of non-foliar organs increased under water deficit. Non-foliar organs (bracts and capsule wall) showed less ontogenetic decrease in O2 evolution capacity and in RuBPC activity (per dry weight) as well as better antioxidant systems than leaves at various days after anthesis. We conclude that the photosynthesis from non-foliar organs is important for increasing cotton yield especially under water deficit conditions.