供氮水平对落叶松幼苗光合作用的影响

Effects of nitrogen supply on photosynthesis in larch seedlings

为探讨落叶松光合作用对外界供氮环境变化的响应规律,采用砂培方法在温室内设置了4种供氮浓度(1、4、8mmol/L和16mmol/L),对生长在不同供氮水平下落叶松(Larixgmelinii)1年生幼苗的气体交换参数、叶绿素荧光参数以及一些生化指标进行了测定。结果显示,随供氮水平的提高,落叶松幼苗针叶氮含量、叶绿素含量、类胡罗卜素含量、可溶性蛋白(TSP)含量和光饱和净光合速率(Pmax)均显著增加,同时伴随着叶磷含量和胞间与外界CO2浓度之比(Ci/Ca)的降低。然而,当供氮水平超过8mmol/L增至16mmol/L时,TSP含量及Pmax不再增加,反而略有下降。供氮不足显著降低了幼苗针叶光系统最大光能转换效率(Fv/Fm)、光系统量子效率(ΦPS)和光化学猝灭系数(qP),却增加了非光化学猝灭系数(NPQ)。而增加供氮可使Fv/Fm和ΦPS回升,同时qP升高,NPQ下降,但当供氮水平超过8mmol/L后,各叶绿素荧光参数变化幅度较小。结果表明,增加供氮可显著提高落叶松幼苗的光合能力,增加光系统天线色素捕获的光能用于光化学电子传递的份额,减缓光抑制。然而,16mmol/L已经超过落叶松幼苗最适的供氮水平,过量供氮引起的负面效应可能主要与过低的叶磷含量导致的营养失衡有关。

Nitrogen (N) is the most important nutrients for plants. Detailed studies on the influence of N supply on photosynthesis have been reported in many plant species, but seldom in larch, which is a significant forestation species in Northeast China. In this paper, we report the characteristics of photosynthesis in larch seedlings that have been provided with different N supplies. The experiment was conducted in greenhouse from April 2003 to September 2003. On April 5, one-year-old larch (Larix gmelinii) seedlings were transplanted into plastic pot (diameter 30 cm, height 27 cm) containing sterilized and washed quartz sand after extensive washing and sterilizing of the roots. The seedlings were supplied with a complete nutrient solution for 40 days: This solution contained 4 mmol/L of NH_4NO_3, 1 mmol/L of KH_2PO_4, 1 mmol/L of KCl, 1 mmol/L of CaCl_2, 0.6 mmol/L of MgSO_4, 0.02 mmol/L of FeCl_3, 6 μmol/L of MnCl_2, 0.016 mmol/L of H_3BO_3, 0.3 μmol/L of ZnCl_2, 0.3 μmol/L of CuCl_2, 0.3 μmol/Lof NaMoO_4. On the May 15, the seedlings were treated with four different N concentrations (1, 4, 8 and 16 mmol/L, respectively). After 15-weeks of treatment, gas exchange, chlorophyll fluorescence and some biochemical parameters were determined. The results showed that with an increase in N supply, the levels of both N and chlorophyll in the needles were up significantly, while the phosphorus content declined. The light-saturated net photosynthetic rate (P_~max ) of the needles showed a linear increase when N supply was increased form 1 mmol/L to 8 mmol/L. Meanwhile, total soluble protein (TSP) content and stomatal conductance (g_s) were also increased, whereas internal to ambient CO_2 concentration ratio (C_i/C_a) was decreased. When the N supply exceeded 8 mmol/L and reached 16 mmol/L, the P_~max and the TSP content exhibited a non-significant decease. N deficiency caused a significant decline in the maximum quantum efficiency of photosystem Ⅱ (PSⅡ) photochemistry (F_v/F_m), quantum efficiency of PSⅡ (Φ_~PSⅡ ) and the photochemical quenching co-efficient (qP). These changes were accompanied by an increase in both the non-photochemical quenching co-efficient (NPQ) and the carotenoid/chlorophyll ratio (Car/Chl). Although increasing the N supply increased F_v/F_m, Φ_~PSⅡ and qP, the chlorophyll fluorescence parameters showed little change when the N level exceeded 8 mmol/L. These results suggested that when plants grow in a N deficiency condition, increasing the N supply can enhance their photosynthetic capability by improving photochemistry quantum efficiency of PSⅡ and alleviating photoinhibition. Regarding the results of 16 mmol/L condition, although this condition may have exceeded the optimum N level for larch seedlings, the negative effect of excess N supply might be mainly due to nutrient imbalance.