Variations In Photosynthetic Characteristics And Associated Factors Of Larix Gmelinii From Diverse Origins

To understand responses and adaptation of tree photosynthesis to environmental changes, we conducted a provenance experiment on Larix gmelinii Rupr. - The dominant boreal tree species in northeastern China. Seeds, collected from six provenances (Genhe, Hebei, Sanzhan, Tahe, Wuyiling, Zhongyangzhan) with significantly different geographical and climatic conditions, were planted 26 years ago at a homogenous site in the southern boundary (Maoer Hill) of the distribution range of this species. The gas exchange characters, chlorophyll fluorescence parameters and photosynthetic associated factors were measured with a LI-6400 photosynthetic system as well as other instruments in the year 2006 and 2007. Our objectives were to examine variations in the photosynthetic characters and related variables among the six provenances, and to explore effects of provenances on changes in these variables with canopy positions and months. The results suggest:(1) In the year 2006 and 2007, the area-based light-saturated net photosynthetic rate (P_max), mass-based light-saturated net photosynthetic rate (P_maxm),carboxylation efficiency (CE), photorespiratory rate (R_p) were significantly different among the provenances, varied from 4.75-8.78μnol CO₂·m^-2·s^-1, 0.0569-0.0920μmol CO₂·g^-1·s^-1, 0.0272-0.0521 mol·m^-2·s^-1, 3.30-5.46μmol CO₂·m^-2·s^-1. Area-based respiratory rate (R_d), mass-based respiratory rate (R_dm), light compensation point (LCP) and CO₂ compensation point (ω) did not differ significantly among the provenances, averaging 2.34μmol CO₂·m^-2·s^-1/l.64μnol CO₂·m^-2·s^-1, 0.0247μmol CO₂·g^-1·s^-1/0.0221μmol CO₂·g^-1·s^-1, 61.2μmol photon-s·m^-2·s^-1/62.0μmol photons·m^-2·s^-1 and 138.7μmol·mol^-1. Apparent quantum yield (AQY) displayed significant provenance difference in the year 2006, varied 0.0325-0.0427μmol CO₂·μmol^-1photons, averaging 0.0305μmol CO₂·μmol^-1photons in 2007.There were significant differences in minimal fluorescence in light adaptation (Fo'), maximal fluorescence in light adaptation (Fm') and steady fluorescence (Fs) among provenances, while the quantum yield of PSâ…¡electron transport (Fv'/Fm'), actual efficiency of primary conversion of light energy of PSâ…¡(PhiPS2) and apparent photosynthetic electron transport rate (rETR) did not demonstrated obvious distinction, averaging 0.3886, 0.1250 and 78.77 respectively.Area-based nitrogen content (N_a), area-based carbon content (C_a), mass-based nitrogen content (N_m), mass-based carbon content (C_m) and leaf mass per area (LMA) were significantly different among the 6 provenances, and showed the same arrange pattern during 2006 and 2007, ranging 1.09-2.26 g·m^-2, 33.91-53.00 g·m^-2, 14.34-21.96 mg·g^-1, 451.78-474.65 mg·g^-1, 66.32-114.30 g·m^-2 respectively. Significant distinction about mass-based chlorophyll content (Chl_m) and the ratio (ChlA/B) among provenances were also observed, nevertheless, the arrange patterns of the provenances differed in the two periods, varied 0.82-1.39 mg·g^-1, 3.09-3.87. The starch contents were not obviously different among provenances, averaging 9.12%, while the soluble sugar contents had significantly distionction, ranging 6.99-9.07%.Significant positive correlations were found among P_max, Chl_a, N_a and LMA, as well as P_maxm with Chl_m, N_m and Chl_m.(2) Most gas exchange characters, chlorophyll fluorescence parameters and other photosynthetic related factors exhibited clear patterns among months. R_dm, R_d, LCP,ωand starch content were higher in the early section of the growing season and decreased gradually, whereas P_max, ChlA/B, Chl_a, Chl_m, N_a, LMA, C_a and soluble sugar content showed the opposite trend. Compared with July, Fo', Fm' and Fs were significantly lower in June and September. PhiPS2 and ETR reached the peak in September. The provenances significantly affected the photosynthetic capacity and related photosynthetic variables among months.(3) With canopy positions from bottom to top, the P_max, R_d, LCP, PhiPS2, rETR, N_a, C_a, C_m, Chl_a, ChlA/B, LMA, starch and soluble sugar content tended to increase gradually, while AQY, P_maxm, R_dm, Chl_m showed the opposite trend, and CE, R_p,ω, Fo', Fm', Fs, Fv'/Fm and N_m did not demonstrated distinct canopy diversity.The Sanzhan and Zhongyangzhan provenances, which lied in the central section of the distribution area, demonstrated high LMA, CE and N_a. This may be the reason of why they had higher P_max, C_a and soluble suger content. The significant variations in the photosynthetic capacity and related photosynthetic factors from the six L. gmelinii origins illustrated an evident ecophysiological adaptation of the photosynthetic apparatus to their original environmental conditions. The seasonal variation patterns of photosynthetic characters and associated factors were related with the local climate condition and corresponding tree growing rhythm, and the vertical variation patterns of the photosynthetic characters and correlated factors in the canopy were the adaption results of the radiation condition imposed on leaves.