Litter and its interaction with standing vegetation affect seedling recruitment in Tibetan alpine grasslands

Background: Seedling recruitment is widely recognised to be important in the maintenance of diversity and coexistence of species. It is not clear how local abiotic factors mediated by litter and biotic interactions influence plant species recruitment in alpine grasslands.

Aims: To determine how litter and standing vegetation affected seedling recruitment in plant communities in Tibetan alpine grasslands.

Methods: Seedling recruitment was quantified in response to experimental treatments: (1) removal of standing vegetation and litter; (2) removal of standing vegetation with litter intact; (3) intact standing vegetation and removal of litter and (4) control: intact standing vegetation and litter.

Results: Litter removal increased seedling numbers, while standing vegetation removal had no effect. An interaction between litter and standing vegetation removal marginally increased seedling number. Species richness of seedlings was not related to either litter removal or standing vegetation removal. Species composition of seedling differed significantly between removal of both litter and standing vegetation and control.

Conclusions: Litter and its interaction with standing vegetation has an important role in affecting plant species recruitment in this alpine plant community. In contrast, biotic interactions, such as competition or facilitation from standing vegetation, appear to have only minor effects on recruitment.     

Potentiality of Different Seed Priming Agents to Mitigate Cold Stress of Winter Rice Seedling

Seed priming has proved to be an effective pre-germination seed invigoration technique for different crops to improve seed and seedling performance under different abiotic stresses. In Bangladesh, winter rice is very often exposed to cold waves just after sowing in the nursery bed resulting in poor seed germination and seedling emergence, yellowish and thin seedlings production, and a very low survival rate. Seed priming may mitigate the cold stress during seed germination and seedling emergence and helps in the quality seedling production of winter rice. To evaluate the efficacy of different seed priming techniques in increasing seedling emergence, growth, vigor and survivability of winter rice cultivars under cold stress, a pot experiment was conducted at the Department of Agronomy, Bangladesh Agricultural University during December 2018 to January 2019. The experiment comprised two factors, (A) Winter rice variety namely, i) BRRI dhan29 and ii) BRRI dhan36; (B) Seed priming agent namely i) Control (no priming), ii) 20000 ppm NaCl, iii) 30000 ppm NaCl, iv) 20000 ppm KCl, v) 30000 ppm KCl, vi) 20000 ppm CaCl₂, vii) 30000 ppm CaCl₂, viii) 50 ppm CuSO₄, ix) 75 ppm CuSO₄, x) 10000 ppm ZnSO₄, xi) 15000 ppm ZnSO₄, xii) 2 ppm Na₂MoO₄, xiii) 3 ppm Na₂MoO₄, xiv) 100 ppm PEG (Polyethylene glycol 4000) and xv) 150 ppm PEG. Seeds were sown on two different dates viz., 1st December and 1st January so that seedlings are exposed to cold stress at different stages. The experiment was laid out in a completely randomized design (CRD) with three replications. Results indicated that (in most of the cases) seed priming has a positive impact on seedling emergence rate (%), root length, shoot length, root shoot ratio, root dry weight, shoot dry weight, seedling dry weight and survival rate (%). Among the priming agents, KCl and CaCl₂ performed best; while priming with NaCl and PEG showed no advantages over no priming for both the sowing dates. In general, BRRI dhan36 performed better than BRRI dhan29 in terms of seedling growth because of its higher tolerance to cold stress. But, both the varieties performed similarly in terms of emergence rate and survival rate. Thus, priming is an effective tool to increase seed germination, better seedling growth, and higher seedling survivability of winter rice under cold stress, and KCl (20000 ppm) or CaCl₂ (20000 ppm) can be considered as a viable priming agent.     

Does a latitudinal gradient in seedling survival favour larger seeds in the tropics?

The mean size of seeds produced by plants at the equator is two to three orders of magnitude higher than the mean size of seeds produced by plants at 60°. We compiled data from the literature to assess the possibility that this latitudinal gradient in seed size allows species to cope with more difficult seedling establishment conditions in tropical environments. We found no relationship between latitude and seedling survival through 1 week (P = 0.27, n = 112 species). There was also no evidence that a larger seed mass is required to gain a given level of seedling survival in tropical environments (P = 0.37, n = 112 species), and no relationship between latitude and the duration of the juvenile period (P = 0.57, n = 132 species). Thus, our results are not compatible with the idea that seedling establishment is more difficult in the tropics.     

Seed mass and seedling establishment after fire in Ku‐ring‐gai Chase National Park,Sydney, Australia

Abstract Relationships between seed mass and several aspects of plant regeneration ecology were investigated in a post‐fire environment in Ku‐ring‐gai Chase National Park near Sydney, Australia. We found a significant positive relationship between seed mass and time to seedling emergence (P < 0.001) and a strong negative relationship between seed mass and time between emergence and production of the first true leaf (P < 0.001). Surprisingly, we found no relationship between seed mass and seedling establishment (P = 0.21). It seems most likely that this lack of relationship is a result of the many stochastic factors affecting seedling establishment during any given recruitment episode at any given site. A cause of mortality was assigned to 56% of the 781 seedlings that died during the present study. There was no relationship between cause of death and seed mass (P = 0.28). Of the seedlings for which the cause of death was known, 57% were killed by herbivory and 21% were killed by drought. Seedling–seedling competition affected only one species.     

茄子嫁接苗与自根苗光合特性比较

对茄子嫁接苗与自根苗的光合生理特性进行了比较研究。结果表明,不同时期嫁接苗功能叶片的净光合速率（Pn）均显著高于自根苗;茄子嫁接苗与自根苗功能叶片的Pn日变化均呈“双峰”曲线,但嫁接苗的“午休”程度较自根苗轻;嫁接苗比自根苗有较低的光补偿点（LCP）和CO2补偿点（CCP）,较高的光饱和点（LSP）和CO2饱和点（CSP）;有较低的光合冷限温度和较高的光合热限温度,光合最适温度两者之间没有明显的差别;有较高的光饱和、CO2饱和及最适温度时的Pn;嫁接苗Pn高是因为嫁接苗胞间的光合反应的底物浓度大（Ci高）,表观量子效率（AQY）、羧化效率（CE）和光合能力（A350）高的缘故。     

不同育苗方式对移栽后侧柏和白榆幼苗根系生长的影响

不同类型苗木,具有不同的根系结构特征,其根系结构也将影响林木的生长和恢复生态系统的稳定性。以侧柏和白榆为研究对象,对移栽18个月后的种基盘苗与营养钵苗根系的生长进行了调查。结果表明:侧柏种基盘苗的总根长和平均直径比营养钵苗分别增加了316.20 cm和0.05mm,白榆苗则分别增加了651.54 cm和0.88mm。侧柏和白榆种基盘苗的根系表面积比营养钵苗分别增加了40.05%和73.04%。侧柏种基盘苗的根系总体积与营养钵苗的差异不显著,而白榆种基盘苗的根系总体积则比营养钵苗增加了54.70%。侧柏和白榆营养钵苗的一级侧根数量大于种基盘苗,增幅分别为42.31%和30.65%。对于侧柏来说,营养钵苗的根尖总数比种基盘苗的增加324个,但白榆苗差异不显著。各种处理的幼苗总根长与根系表面积都有显著相关性,但与根体积不具有显著相关性。营养钵苗的根系平均直径和根尖数量具有相对独立性,而种基盘的根系平均直径与总根长以及根体积均表现为显著相关。种基盘苗能提高侧柏、白榆幼苗的根冠比,促进幼苗株高、地径和主根的生长。采用种基盘苗进行植被恢复,由于其具有较大的根表面积和根长度,林木便具有了较大的吸收水分和营养的能力,以及较高的固结表层土壤能力。     

Cotyledon venation patterns in the Leguminosae: Gaesalpinioideae

Cotyledon venation patterns are described for 93 species representing all tribes of the Caesalpinioideae. Patterns are grouped into a series of levels of complexity according to the number of primary veins, nine, seven, six, five, three or one, but five- and three-vein patterns predominate. The number of petiolar vascular strands varies from one to eight but most species have two or four strands. It is proposed that all the patterns have been derived from one in which four strands and a protoxylem trace in the petiole branch and anastomose to form seven primary veins in the lamina. Venation patterns show correlations with cotyledon anatomy, size and shape and with taxonomic grouping at tribal, infratribal and generic levels. Each tribe is characterized by a particular set of related patterns, pattern frequencies and evolutionary trends.     

Restoration of a Restinga Sandy Coastal Plain in Brazil: Survival and Growth of Planted Woody Species

In this article we report the results of an experiment introducing 17 native shrub and tree species into a Brazilian restinga (i.e., coastal sandy plain vegetation). Restingas have been affected by human impact for about 8,000 years, and human occupation for housing, tourism, and land speculation has recently increased in such a way that there is a need for conservation of remnant patches and restoration of degraded areas throughout the coast to protect biodiversity. Our study site is a remnant located in Rio de Janeiro, the second largest city in the country, and has been subjected in the past to deforestation, man‐made fire, and sand extraction. Although trees and shrubs predominantly compose natural restinga vegetation, local vegetation after impact was replaced by an exotic grass cover, which meant a drastic reduction in species richness. Thus, in this experiment we removed the grass cover, introduced shrub and tree species, and monitored survival and growth of 20 plants per species for 2 years. Despite the adversities imposed by the nutrient‐poor sandy soil, 70% of the species showed high survival percentage and considerable growth. This report on restoration initiatives in the restingas points out the viability of shrub and tree plantation following exotic grass removal as a strategy to restore Brazilian coastal vegetation.     

四种沙埋深度对羊草种子出苗和幼苗生长的影响

模拟羊草种子自然埋藏深度状况,设计了0、1、2和3cm共4种沙埋深度实验,研究了其出苗率及幼苗生长对沙埋深度的响应。结果表明,羊草种子出苗率随埋深的增加呈下降趋势,0cm时出苗率最高为89.3%,1和2cm时分别为81.5%和78.5%,3cm时最低(73.5%),与前三者具有显著差异。埋深对地上株高生长动态的影响主要表现在,播种后25d,0～2cm处理的羊草幼苗地上株高均高于3cm处理;播种后36d,4种处理的地上株高没有显著差异。此外,随着埋深的增加,羊草幼苗的根长、根数、根冠比呈先上升后下降趋势,叶片数呈下降趋势,绝对株高则呈上升趋势。表明1～2cm沙埋深度更适宜于羊草种子出苗及幼苗生长,而0或3cm埋深不利于其出苗和幼苗生长。