Shade Can Be More Positive On The Eastern Side: A Study Of Photosynthetic Characteristics On The Grass Phragmites Communis Shaded By Their Shrub Neighbor Tamarix Chinensis In Desert

Shade is thought to be a kind of negative effect for it reduces light intensity to limit photosynthesis and growth of plants. But in hot arid environments, it can also be beneficial to the shaded plants in terms of reduced leaf temperatures, transpiration and photoinhibition. Although there have been many researches about shade effect, few are concerned to the grass shaded by shrub neighbor in the desert, part-day shade and sunfleck habitat. So we hypothesized: In the high light intensity and high temperature desert environment, shade is a positive effect to the shaded plants. Plants in different orientations of their neighbors live in different sunlight circumstances, which should lead them to the varied adaptation ways. These positive shade effect and adaption strategies should be embodied in plant photosynthetic characteristics.To test our hypothesis, in the desert of Linze in Northwest China, we chose the grass Phragmites communis shaded by their shrub neighbor Tamarix chinensis to carry out our experiments in July, 2006. To compare with the plant fully exposed to ambient sunlight all day, different orientations of the grass position to their neighbor were selected, which were the easterly orientation position (plant was exposed to ambient sunlight from 7:00 to 12:00 and shaded from 13:00 to 19:00) and the westerly orientation position (plant was shaded from 7:00 to 11:00 and exposed to ambient sunlight from 12:00 to 19:00). Gas exchange and chlorophyll fluorescence techniques were used to evaluate the diurnal course of photosynthetic and chlorophyll characteristics.The results showed that shade decreased sunlight intensity above shaded plant leaves and their leaf temperatures, which buffered the high light and high temperature in desert. Caused by the sunflecks through the canopy of T. gentaensis, during shade time, the photon flux density(PFD) values above P. communis leaves showed significant fluctuations, which had a great impact to the net photosynthetic rate(Pn), stomatal conductance (Gs) and transpiration rate(E) of the shaded plants. Meanwhile, the light fluctuations showed a little impact to the vapor pressure deficit(VPD), leaf temperature(Tl) and water use efficiency(WUE), as the same as the chlorophyll fluorescence parameters.Our study showed that different positions made different part-day shade types and different photosynthetic physiological adaptations of the shaded plants. Both of the eastern side (east) and western side (west) plants had some similar characteristics: During the shade time, their maximum photochemical efficiency (Fv/Fm) and real photochemical efficiency (ΦPSII) were higher compared with the plant fully exposed to sunlight (light). During the light time, their Fv/Fm andΦPSII were lower which indicated higher photoinhibition, and their non-photochemical quenching (NPQ) were at a high level. The recovery at nightfall indicated that both of their photoinhibition was caused by the reversible photo protection mechanism rather than photo damage. At the same time, their electron transport rate (ETR) were close to the light contrast which revealed they raised their electron capture efficiency of photo reaction center and their photorespiration to maintain the electron transport rate and protect the photosystem. Compared with the light contrast, photosystem of shaded plants was more sensitive facing the sunlight variation. Their Pn, Gs and E had a rapid response to the PFD fluctuations.Beside the same characteristics, the east and west plant had their own traits: Pn of east plant could reach a high level rapidly at morning and maintain it till noon, and it could reach the secondary peak afternoon with the effective usage of the sunflecks. The photosynthesis allocation of whole day revealed its physiological adaption to the afternoon shade. The west plant could also use the sunflecks effectively in the morning, and its Pn maintained steadily around noon time and reached the peak at 15:00. It showed its physiological adaption to the morning shade.Compared with the light contrast, shaded plants suffered worse photoinhibition at their light time. It may caused by their adaptation to less PFD absorbed whole day (Light>East>West). But they raised their Gs and E (Both were East>West>Light) and sacrificed WUE (Light>West>East) to reach a higher photosynthesis level at light time, which compensated and exceeded the loss at shade time (East>West>Light).From the analysis of gas exchange and chlorophyll characteristics, our study indicated that shade was positive effect for it buffered the high light and high temperature in desert environment, and it made the shaded plants gain higher whole-day photosynthesis, which should be facilitative to plant growth and production.