Seasonal Changes In Leaf Cuticular Wax In Herbs From Low Altitude Of Chongqing Area

As the outmost surface of plant, cuticular wax directly contacts with the environment, and its peculiar hydrophobic organic compounds form a natural protective layer. It plays an important role in helping plant adapt to a variety of external environments. Studies have shown that wax crystal, wax synthesis and distribution may be influenced by the external environments (humidity, temperature, etc.), which thus might alter plants adaptation to certain environment. To date, the studies related to the variations of leaf cuticular wax mainly derived from controlled experiments, and mainly on cultivated plants and trees (shrub). The wide herbs are different from cultivated plants; however, the seasonal variation of cuticular wax in these herbs under natural environment is less studied.In our research, 31 herbaceous species belonging to 10 families were sampled from different seasons in Southwest university campus. The leaves were extracted for cuticular waxes, and the chemical compositions and contents of cuticular wax were analyzed by GC-MS (gas chromatography-mass spectrometry). The crystal structures were investigated using SEM (scanning electron microscope). In this study, the changes of wax crystal structures, the distribution features of n-alkane, n-fatty acid and primary alcohol, the average chain length (ACL) and the carbon preference index (CPI) and waxy characteristics in different families, genera and species collected in different seasons will be discussed, to explore the relationship between them and the nature environment factors. The main results were as follows:The smooth wax film was covered on leaf surface of 23 herbs in 31 tested plants, and the wax crystal structures had two types, granular and flat. The predominant wax crystal type of Sonchus oleraceus, Chenopodium glaucu and Gramineae except Setaria plicata was flat, that of Solanum nigru was granular. Seasons lead to the change of wax crystal. The wax coverage of leaf surface was higher in spring and lower in summer and autumn with the change of crystal structure; and winter wax deposition obviously. The dense flat crystal of Oplismenus undulatifolius, Sonchus oleraceus, Chenopodium glaucu, Digitaria sanguinalis and Setaria viridis appeared melting phenomenon with varying degrees in the summer and autumn. The crystal structure of Oplismenus undulatifolius changed from flat into granular and tiled in leaf surface. These results indicated that the cuticular wax of plants might respond to environment by changing in the distribution and structure of wax, and self-regulation to protect them from harm.The leaf n-alkanes showed a distribution in carbon number ranging from C17 to C33. The contents of the odd number carbon, C29, C31 and C33, had an absolute advantage and accounted for 53% to 95% of the total contents of n-alkanes. CPI values of n-alkanes ranged from 1.29 to 20.21, showing a strong odd-even predominance (CPI>1) of the distributions of n-alkanes. And the ACL ranged from 27.02 to 31.11, indicating they have long carbon chain length of n-alkanes. The carbon of n-alkane of the most herbs at four seasons was C31 as the dominant component, whereas Oplismenus undulatifolius was C33; Chenopodium glaucum, Sonchus oleraceus and Humulus scandens appeared C29. CPI values of n-alkanes from the herbs collected in different seasons gradually decreased from spring to autumn. However, the CPI values of Sonchus oleraceus and Conyza canadensis rose in autumn. The long chain length of n-alkanes also showed a strong odd-even predominance (CPI>1). But the ACL values of n-alkanes had a smaller changes with the different seasons, ranging between 0.00-1.75 and no seasonal pattern. The dominant component of herbs had not changed with the changing season, except Sonchus oleraceus and Humulus scandens. Different plants had their own unique distribution of n-alkanes, the physiological processes of plant leaves has played an important role in the way of synthesizing n-alkanes.The carbon chain length distribution of n-fatty acids and primary alcohols of herbs ranged from C16 to C28 and from C18 to C30, all with even carbons. ACL values of them ranged from 17.05 to 25.82 and from 23.82 to 29.68. The distribution of n-fatty acids dominated by C16, C18, C26 or C28, but Humulus scandens by C22. The distribution of primary alcohols dominated by C26, C28 or C30, but Oplismenus undulatifolius, Kalimeris indica, Plantago asiatica and Humulus scandens by C22 or C24. Herbs had different dominant carbons in different seasons, except n-fatty acids and primary alcohols of Sonchus oleraceus, where C26 acid and C26 alcohol were the dominant in all seasons. ACL values of them in summer were higher than in winter, but ACL values of n-fatty acides of Chenopodium glaucum, Calystegia headracea and Humulus scandens and primary alcohols of Solanum nigrum were the opposite. These results suggested that the distribution of leaf cuticular wax carbon chain would be influenced by environment, and plants might improve their adaptation by regulating carbon chain distribution.It is complicated the composition of leaf waxes. And the components of cuticular wax of herbs in our study were consisted of alkanes, fatty acids, primary alcohols, secondary alcohols, ketones, esters and unidentified components. The main components of cuticualr waxes of most herbs were alkanes, accounting for about 75%, followed by fatty acids, primary alcohols, esters. The contents of secondary alcohols and esters were less. The dominant components of waxes would appear difference in different plants. The dominant components of Sonchus oleraceus were fatty acids and primary alcohols, accounting for about 90%. And the dominant components of Humulus scandens were fatty acids, primary alcohols and esters, accounting for about 86% in total amount. The dominant components of Chenopodium glaucum was only primary alcohols, accounting for about 74%. The dominant component of Senecio scandens was esters, accounting for about 33% in total amount. The dominant component of Gramineae plants were primary alcohols without Oplismenus undulatifolius dominated the n-alkane, while there existed difference in the contents of primary alcohols among plant species.The total amount of cuticular wax changed among different seasons. Overall, plants in summer had the lowest leaf cuticular wax content, while the highest appered in different seasons. The seasonal variations of alkanes, fatty acides and primary alcohols showed similar trend as the total wax amount. The contents of the rest components had less, so the seasonal variations of them were smaller. These results indicated that the wild herbs altered wax deposition in different seasons, to improve their adaptation to changing environments.