Can stress turn trees hair white? Hair covering of stems improves resilience of corticular photosynthesis against heat-stress

The hypothesis was tested that hair covering of stems improves resilience of corticular photosynthesis against heat stress. Hairy and non-hairy outer bark ofQuercus ilexL. andQuercus roburL. trees was removed and optical properties measured. Additionally, structural bark traits and chlorophyll fluorescence parameters during heat stress treatment were studied. Optical analysis revealed a protective role of hairy outer bark (OB) against overheating of the underlying cortex of the stems. Hairiness decreased OB transmittance and increased thermal insulation of stems by an increased absorptance and reflectance of OB in the visible (380–720 nm) and an increased reflectance in the infrared part of the spectrum (720–900 nm). Simple linear regression analysis revealed no significant effect of stem structural traits (OB thickness (OBT), cortex density (Dcortex), cortex water content (Wcortex)) on corticular photochemistry (PScort), while optical traits of outer bark were significantly (P < 0.01) correlated with PScort. OB reflectance explained up to 91% of the variation in PSII quantum yield under heat stress. At high temperatures (> 45 °C) PScortof the hairy species showed a higher resilience and a better post-stress-recovery as compared to the non-hairy one. It is concluded that stem hairs play a physiologically significant role in modulating the stem energy balance due to a close interaction between optical characteristics of hairy OB and stem photochemical processes.

Activation of the Transcription of BrGA20ox3 by a BrTCP21 Transcription Factor Is Associated with Gibberellin-Delayed Leaf Senescence in Chinese Flowering Cabbage during Storage

Several lines of evidence have implicated the involvement of the phytohormone gibberellin (GA) in modulating leaf senescence in plants. However, upstream transcription factors (TFs) that regulate GA biosynthesis in association with GA-mediated leaf senescence remain elusive. In the current study, we report the possible involvement of a TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) TF BrTCP21 in GA-delayed leaf senescence in Chinese flowering cabbage. Exogenous GA3 treatment maintained a higher value of maximum PSII quantum yield (Fv/Fm) and total chlorophyll content, accompanied by the repression of the expression of senescence-associated genes and chlorophyll catabolic genes, which led to the delay of leaf senescence. A class I member of TCP TFs BrTCP21, was further isolated and characterized. The transcript level of BrTCP21 was low in senescing leaves, and decreased following leaf senescence, while GA3 could keep a higher expression level of BrTCP21. BrTCP21 was further found to be a nuclear protein and exhibit trans-activation ability through transient-expression analysis in tobacco leaves. Intriguingly, the electrophoretic mobility shift assay (EMSA) and transient expression assay illustrated that BrTCP21 bound to the promoter region of a GA biosynthetic gene BrGA20ox3, and activated its transcription. Collectively, these observations reveal that BrTCP21 is associated with GA-delayed leaf senescence, at least partly through the activation of the GA biosynthetic pathway. These findings expand our knowledge on the transcriptional mechanism of GA-mediated leaf senescence.

Limiting-stress-elimination hypothesis: approach to increase savanna cowpea productivity by stress reduction

We propose and test the Limiting-Stress-Elimination Hypothesis (LSEH), as a decision axiom to guide in determining the optimal intervention strategy towards yield allocation in a savanna legume. We considered osmotic stress and soil nitrogen (N) limitation, which both characterize Guinea savanna agro-ecozone. We hypothesized that biomass allocation will increase when the limiting stress is eliminated at least until the next limiting stress impacts yield. We assessed responses of Vigna unguiculata (L.) Walp (cowpea) to osmotic stress treatments (non-hormonal biostimulant and exogenous metabolite) and N input by leaf level physiology, N-fixing capacity and biomass. The relative increase in biomass (%), and pod yields reveal that osmotic stress (45%) more than nitrogen (13%) is limiting to cowpea growth under Guinea savanna conditions, although N fertilization increased nodulation and maximized PSII quantum yield. In the Sprengel-Liebig’s decrement from the maximum concept, the decrement from the maximum for each stressor must be minimized in order to produce the absolute maximum production. However, this may not be economically feasible in many situations. Conversely, LSEH demonstrates that significant productivity is attainable by eliminating a relatively more limiting stress, osmotic stress, regardless of the limitation and natural demand for the relatively less limiting N in leguminous cowpea in the savanna.

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.

Mixed field plantation of native and exotic species in semi-arid Brazil

An afforestation of mixed plantation was proposed to provide wood supply and minimise exploratory actions in a biological reserve. Eucalyptus plants were indicated for this purpose. This study was carried out to test the effect of volatile oils extracted from the leaves of Eucalyptus camaldulensis Dehn. and E. grandis Hill ex Maiden on the growth of Enterolobium contortisiliquum (Vell.) Morong. Under laboratory conditions, the Enterolobium plants showed tolerance to E. grandis oil, whereas E. camaldulensis oil caused loss of leaves, inhibition of height and diameter growth and a concomitant decrease in effective PSII quantum yield and the reduction of photosynthetic electron-transport chains. The field growth of E. contortisiliquum was not modified by intercropped E. grandis plants, confirming its tolerance. Inoculation with rhizobia and or mycorrhizal fungi significantly improved the height and diameter growth of these species. The results showed that E. contortisiliquum plants could be intercropped with E. grandis for reforestation and agroforestry systems.