Jiao-Lin Zhang

Stem hydraulic traits and leaf water-stress tolerance are co-ordinated with the leaf phenology of angiosperm trees in an Asian tropical dry karst forest

BACKGROUND AND AIMS The co-occurring of evergreen and deciduous angiosperm trees in Asian tropical dry forests on karst substrates suggests the existence of different water-use strategies among species. In this study it is hypothesized that the co-occurring evergreen and deciduous trees differ in stem hydraulic traits and leaf water relationships, and there will be correlated evolution in drought tolerance between leaves and stems. METHODS A comparison was made of stem hydraulic conductivity, vulnerability curves, wood anatomy, leaf life span, leaf pressure-volume characteristics and photosynthetic capacity of six evergreen and six deciduous tree species co-occurring in a tropical dry karst forest in south-west China. The correlated evolution of leaf and stem traits was examined using both traditional and phylogenetic independent contrasts correlations. KEY RESULTS It was found that the deciduous trees had higher stem hydraulic efficiency, greater hydraulically weighted vessel diameter (D(h)) and higher mass-based photosynthetic rate (A(m)); while the evergreen species had greater xylem-cavitation resistance, lower leaf turgor-loss point water potential (π(0)) and higher bulk modulus of elasticity. There were evolutionary correlations between leaf life span and stem hydraulic efficiency, A(m), and dry season π(0). Xylem-cavitation resistance was evolutionarily correlated with stem hydraulic efficiency, D(h), as well as dry season π(0). Both wood density and leaf density were closely correlated with leaf water-stress tolerance and A(m). CONCLUSIONS The results reveal the clear distinctions in stem hydraulic traits and leaf water-stress tolerance between the co-occurring evergreen and deciduous angiosperm trees in an Asian dry karst forest. A novel pattern was demonstrated linking leaf longevity with stem hydraulic efficiency and leaf water-stress tolerance. The results show the correlated evolution in drought tolerance between stems and leaves.

Photosynthetic characteristics, dark respiration, and leaf mass per unit area in seedlings of four tropical tree species grown under three irradiances

We investigated the effect of growth irradiance (I) on photon-saturated photosynthetic rate (Pmax), dark respiration rate (RD), carboxylation efficiency (CE), and leaf mass per unit area (LMA) in seedlings of the following four tropical tree species with contrasting shade-tolerance. Anthocephalus chinensis (Rubiaceae) and Linociera insignis (Oleaceae) are light-demanding, Barringtonia macrostachya (Lecythidaceae) and Calophyllum polyanthum (Clusiaceae) are shade-tolerant. Their seedlings were pot-planted under shading nets with 8, 25, and 50 % daylight for five months. With increase of I, all species displayed the trends of increases of LMA, photosynthetic saturation irradiance, and chlorophyll-based Pmax, and decreases of chlorophyll (Chl) content on both area and mass bases, and mass-based Pmax, RD, and CE. The area-based Pmax and CE increased with I for the light-demanders only. Three of the four species significantly increased Chl-based CE with I. This indicated the increase of nitrogen (N) allocation to carboxylation enzyme relative to Chl with I. Compared to the two shade-tolerants, under the same I, the two light-demanders had greater area- and Chl-based Pmax, photosynthetic saturation irradiance, lower Chl content per unit area, and greater plasticity in LMA and area- or Chl-based Pmax. Our results support the hypothesis that light-demanding species is more plastic in leaf morphology and physiology than shade-tolerant species, and acclimation to I of tropical seedlings is more associated with leaf morphological adjustment relative to physiology. Leaf nitrogen partitioning between photosynthetic enzymes and Chl also play a role in the acclimation to I.

Water-use advantage for lianas over trees in tropical seasonal forests

Lianas exhibit peak abundance in tropical forests with strong seasonal droughts, the eco-physiological mechanisms associated with lianas coping with water deficits are poorly understood. We examined soil water partitioning, sap flow, and canopy eco-physiological properties for 99 individuals of 15 liana and 34 co-occurring tree species in three tropical forests that differed in soil water availability. In the dry season, lianas used a higher proportion of deep soil water in the karst forest (KF; an area with severe seasonal soil water deficit (SSWD)) and in the tropical seasonal forest (TSF, moderate SSWD), permitting them to maintain a comparable leaf water status than trees in the TSF or a better status than trees in the KF. Lianas exhibited strong stomatal control to maximize carbon fixation while minimizing dry season water loss. During the dry period, lianas significantly decreased water consumption in the TSF and the KF. Additionally, lianas had a much higher maximum photosynthetic rates and sap flux density in the wet season and a lower proportional decline in photosynthesis in the dry season compared with those of trees. Our results indicated that access to deep soil water and strong physiological adjustments in the dry season together with active wet-season photosynthesis may explain the high abundance of lianas in seasonally dry forests.

Differences in the responses of photosystem I and photosystem II of three tree species Cleistanthus sumatranus, Celtis philippensis and Pistacia weinmannifolia exposed to a prolonged drought in a tropical limestone forest

Drought stress can induce closure of stomata, thus leading to photoinhibition. The effects of prolonged severe drought under natural growing conditions on photosystem I (PSI), photosystem II (PSII) and cyclic electron flow (CEF) in drought-tolerant tree species are unclear. In spring 2010, southwestern China confronted severe drought that lasted several months. Using three dominant evergreen species, Cleistanthus sumatranus (Miq.) Muell. Arg. (Euphorbiaceae), Celtis philippensis Bl. (Ulmaceae) and Pistacia weinmannifolia J. Poisson ex Franch. (Anacardiaceae) that are native to a tropical limestone forest, we investigated the influence of this stress on PSI and PSII activities as well as light energy distribution in the PSII and P700 redox state. By the end of the drought period, predawn leaf water potential (Ψ(pd)) largely declined in each species, especially in C. sumatranus. Photosystem I activity strongly decreased in the three species, especially in C. sumatranus which showed a decrease of 65%. The maximum quantum yield of PSII after dark adaptation remained stable in P. weinmannifolia and C. philippensis but significantly decreased in C. sumatranus. Light response curves indicated that both linear electron flow and non-photochemical quenching were severely inhibited in C. sumatranus along with disappearance of CEF, resulting in deleterious excess light energy in PSII. We conclude that PSI is more sensitive than PSII to prolonged severe drought in these three drought-tolerant species, and CEF is essential for photoprotection in them.

Sustained diurnal photosynthetic depression in uppermost-canopy leaves of four dipterocarp species in the rainy and dry seasons: does photorespiration play a role in photoprotection?

Diurnal and seasonal changes in gas exchange and chlorophyll fluorescence of the uppermost-canopy leaves of four evergreen dipterocarp species were measured on clear days. The trees, that were growing in a plantation stand in southern Yunnan, China, had canopy heights ranging from 17 to 22 m. In the rainy season, Dipterocarpus retusus Bl. had higher photosynthetic capacity (A(max)) than Hopea hainanensis Merr. et Chun, Parashorea chinensis Wang Hsie and Vatica xishuangbannaensis G.D. Tao et J.H. Zhang (17.7 versus 13.9, 11.8 and 7.7 micromol m(-2) s(-1), respectively). In the dry season, A(max) in all species decreased by 52-64%, apparent quantum yield and dark respiration rate decreased in three species, and light saturation point decreased in two species. During the diurnal courses, all species exhibited sustained photosynthetic depression from midmorning onward in both seasons. The trees were able to regulate light energy allocation dynamically between photochemistry and heat dissipation during the day, with reduced actual photochemistry and increased heat dissipation in the dry season. Photorespiration played an important role in photoprotection in all species in both seasons, as indicated by a continuous increase in photorespiration rate in the morning toward midday and a high proportion of electron flow (about 30-65% of total electron flow) allocated to oxygenation for most of the day. None of the species suffered irreversible photoinhibition, even in the dry season. The sustained photosynthetic depression in the uppermost-canopy leaves of these species could be a protective response to prevent excessive water loss and consequent catastrophic leaf hydraulic dysfunction.

Photosynthesis, non‐photochemical pathways and activities of antioxidant enzymes in a resilient evergreen oak under different climatic conditions from a valley‐savanna in Southwest China

Plants in the savanna-valleys in Southwest China are annually exposed to different combinations of multiple stresses from the hot-rainy, to chill-dry, and to warm-dry seasons. This study monitored seasonal changes in photosynthesis and photoprotection in an evergreen oak (Cyclobalanopsis helferiana) from one of these valleys for four years during which usual and abnormal drought occurred. In general, during the study period with decreasing xylem water potential (Psix), photosynthetic gas exchange, quantum yield of photosystem II (PSII) photochemistry and activities of most of the measured antioxidant enzymes decreased, while activities of the xanthophyll cycle and associated non-photochemical energy dissipation and glutathione peroxidase (GP) (EC 1.11.1.9) increased. In a fairly severe chill period, high concentration of reactive oxygen species induced high activities of most of the antioxidant enzymes and relatively stronger decrease in gas exchange. In the most severe dry period, even when predawn Psix decreased down to -4 MPa, considerable Pn (maximum photosynthetic rate) (4 micromol m(-2) s(-1)) was still maintained in midmorning. At this time, most of the antioxidant enzyme activities decreased to the lowest values, whereas the xanthophyll cycle and associated non-photochemical energy dissipation and GP activities increased to their highest levels. High predawn antheraxanthin and zeaxanthin contents were observed in the severe and very severe drought periods. Superoxide dismutase maintained high and fairly constant activity (1500-1800 U mg(-1) protein) and predawn maximum photochemistry efficiency of PSII was always above 0.8 throughout the whole study period. These results indicated that the photosynthetic apparatus of the oak leaves was highly capable of maintaining its function under the multiple stresses in different seasons in the present valley-savanna.

Leaf Photosynthetic Rate of Tropical Ferns Is Evolutionarily Linked to Water Transport Capacity

Ferns usually have relatively lower photosynthetic potential than angiosperms. However, it is unclear whether low photosynthetic potential of ferns is linked to leaf water supply. We hypothesized that there is an evolutionary association of leaf water transport capacity with photosynthesis and stomatal density in ferns. In the present study, a series of functional traits relating to leaf anatomy, hydraulics and physiology were assessed in 19 terrestrial and 11 epiphytic ferns in a common garden, and analyzed by a comparative phylogenetics method. Compared with epiphytic ferns, terrestrial ferns had higher vein density (Dvein), stomatal density (SD), stomatal conductance (gs), and photosynthetic capacity (Amax), but lower values for lower epidermal thickness (LET) and leaf thickness (LT). Across species, all traits varied significantly, but only stomatal length (SL) showed strong phylogenetic conservatism. Amax was positively correlated with Dvein and gs with and without phylogenetic corrections. SD correlated positively with Amax, Dvein and gs, with the correlation between SD and Dvein being significant after phylogenetic correction. Leaf water content showed significant correlations with LET, LT, and mesophyll thickness. Our results provide evidence that Amax of the studied ferns is linked to leaf water transport capacity, and there was an evolutionary association between water supply and demand in ferns. These findings add new insights into the evolutionary correlations among traits involving carbon and water economy in ferns.

Nutrient resorption is associated with leaf vein density and growth performance of dipterocarp tree species

Nutrient resorption is important for the nutrient budget of plants, but little is known about which plant traits mediate nutrient resorption, how resorption efficiency is associated with other leaf traits and whether nutrient resorption has an impact on plant growth. In this study, 17 dipterocarp tree species were compared in a common garden experiment. N and P resorption efficiencies were regressed against suites of traits associated with phloem transport capacity (i.e. leaf vein density; D-vein), leaf nutrient conservation traits (e.g. leaf mass per area; LMA) and species growth rate. Across the dipterocarp species studied, N resorption efficiency (percentage N resorbed) was positively correlated with D-vein and leaf thickness. N resorption efficiency was also correlated with D-vein after considering phylogenetic effects. N resorption proficiency (N remaining in senesced leaves) was negatively correlated with D-vein, LMA, leaf thickness and palisade and spongy mesophyll thickness. Senesced-leaf N concentration was still negatively correlated with LMA and leaf thickness after considering phylogenetic effects. N resorption efficiency was positively correlated with both height and diameter growth rates. After considering phylogenetic effect, N resorption efficiency was marginally correlated with diameter growth rate. Green-leaf N concentration was positively correlated with height growth rate after considering phylogenetic effect. P resorption efficiency and proficiency were not related to any of the leaf morphological and anatomical traits, or to species growth rates.Synthesis. These results indicate that higher phloem transport capacity of the dipterocarp species is positively correlated with greater N resorption efficiency and that N resorption proficiency is closely linked with leaf nutrient conservation traits. Growth rates of the dipterocarps are more likely governed by photosynthetic rates associated with green-leaf N concentration than N resorption rates per se. Although P is generally deficient in tropical soils, it appears that N rather than P availability is the key limiting factor for the growth of the dipterocarp species.

Differentiation of water-related traits in terrestrial and epiphytic Cymbidium species

Epiphytes that grow in the canopies of tropical and subtropical forests experience different water regimes when compared with terrestrial plants. However, the differences in adaptive strategies between epiphytic and terrestrial plants with respect to plant water relations remain poorly understood. To understand how water-related traits contrast between epiphytic and terrestrial growth forms within the Cymbidium (Orchidaceae), we assessed leaf anatomy, hydraulics, and physiology of seven terrestrial and 13 epiphytic species using a common garden experiment. Compared with terrestrial species, epiphytic species had higher values for leaf mass per unit area (LMA), leaf thickness (LT), epidermal thickness, saturated water content (SWC) and the time required to dry saturated leaves to 70% relative water content (T70). However, vein density (Dvein), stomatal density (SD), and photosynthetic capacity (Amax) did not differ significantly between the two forms. T70 was positively correlated with LT, LMA, and SWC, and negatively correlated with stomatal index (SI). Amax showed positive correlations with SD and SI, but not with Dvein. Vein density was marginally correlated with SD, and significantly correlated with SI. Overall, epiphytic orchids exhibited substantial ecophysiological differentiations from terrestrial species, with the former type showing trait values indicative of greater drought tolerance and increased water storage capacity. The ability to retain water in the leaves plays a key role in maintaining a water balance in those epiphytes. Therefore, the process of transpiration depends less upon the current substrate water supply and enables epiphytic Cymbidium species to adapt more easily to canopy habitats.

Photosynthetic thermotolerance of woody savanna species in China is correlated with leaf life span

BACKGROUND AND AIMS Photosynthetic thermotolerance (PT) is important for plant survival in tropical and sub-tropical savannas. However, little is known about thermotolerance of tropical and sub-tropical wild plants and its association with leaf phenology and persistence. Longer-lived leaves of savanna plants may experience a higher risk of heat stress. Foliar Ca is related to cell integrity of leaves under stresses. In this study it is hypothesized that (1) species with leaf flushing in the hot-dry season have greater PT than those with leaf flushing in the rainy season; and (2) PT correlates positively with leaf life span, leaf mass per unit area (LMA) and foliar Ca concentration ([Ca]) across woody savanna species. METHODS The temperature-dependent increase in minimum fluorescence was measured to assess PT, together with leaf dynamics, LMA and [Ca] for a total of 24 woody species differing in leaf flushing time in a valley-type savanna in south-west China. KEY RESULTS The PT of the woody savanna species with leaf flushing in the hot-dry season was greater than that of those with leaf flushing in the rainy season. Thermotolerance was positively associated with leaf life span and [Ca] for all species irrespective of the time of flushing. The associations of PT with leaf life span and [Ca] were evolutionarily correlated. Thermotolerance was, however, independent of LMA. CONCLUSIONS Chinese savanna woody species are adapted to hot-dry habitats. However, the current maximum leaf temperature during extreme heat stress (44·3 °C) is close to the critical temperature of photosystem II (45·2 °C); future global warming may increase the risk of heat damage to the photosynthetic apparatus of Chinese savanna species.