甜瓜幼苗叶片光合变化特性

Abstract

为探讨甜瓜光响应变化特性与环境因子的关系,选择光响应曲线适宜测定的时段,以甜瓜幼苗为试材,将1 d分为3个时段:10:00-12:00、12:00-15:00和15:00-17:00,每个叶位叶片测定1 d,并采用直角双曲线修正模型拟合光响应曲线,研究不同时段下甜瓜叶片光响应曲线、光响应参数的变化趋势和不同叶位叶片光响应参数特性。结果表明:当环境中光合有效辐射增强,叶面温度(<em>Tl</em>)升高,空气相对湿度(<em>RH</em>)降低;当环境中光合有效辐射减弱,<em>Tl</em>降低,<em>RH</em>升高。10:00-12:00光响应曲线和12:00-15:00的第1-4叶光响应曲线呈双曲线,在强光下趋向饱和状况,12:00-15:00的第5叶光合速率和15:00-17:00光合速率在强光下出现明显的光抑制现象。1 d的不同时段均表现为10:00-12:00最大净光合速率(<em>P</em>_max)和光饱和点(<em>LSP</em>)最高,12:00-17:00降低;12:00-15:00光补偿点(<em>LCP</em>)和暗呼吸速率(<em>R</em>_d)较高,其它两个时段较低,10:00-17:00光补偿点量子效率(<em>φ</em>_c)、气孔导度(<em>G</em>_s)和胞间CO₂浓度(<em>C</em>_i)总体呈降低趋势,气孔限制值(<em>L</em>_s)升高。10:00-15:00相同时段测得的不同叶位叶片光响应参数,以第4-5叶光合性能较好,15:00-17:00以第3叶<em>P</em>_max最高,第5叶次之;10:00-17:00 <em>G</em>_s和<em>C</em>_i以第5叶较低,第1叶较高,<em>L</em>_s以第5叶较高,第1叶较低。<em>RH</em>为影响<em>P</em>_max的主要决策因子,测定时段、叶面饱和蒸汽压亏缺(<em>Vpdl</em>)和<em>Tl</em>为主要限制因子。10:00-12:00适宜进行光响应曲线测定,气孔限制为不同时段光合作用不同的主要因素,非气孔限制为影响不同叶位叶片光合作用的主要因素。;Plant light response curves reflect dynamic charateristics of photosynthesis with the photosynthetic active radiation (<em>PAR</em>). The data collection for the curves is time-consuming and the photosynthesis is sensitively affected by surrounding environmental factors, so it is vital to establish suitable time interval for data determination in one day. Muskmelon is top ten most popular fruit in the world. However, researches on dynamic characteristics of photosynthesis in muskmelon seedling leaves were rarely reported. In order to probe into the relationships between characteristics of muskmelon photosynthesis in light response and environmental factors, suitable time intervals for measuring light response curves were optimized with potted muskmelon seedlings as test material, and the light response curves and several parameters of photosynthesis for different position leaves and environmental factors were measured by LI-6400 portable photosynthesis system from 18 October to 23 October, 2009. Each leaf position was measured at 3 intervals of time:10:00-12:00, 12:00-15:00 and 15:00-17:00 in one day. All of light response curves were fitted with the modified rectangular hyperbola model, which was widely applied and could fit light response curves that photosynthesis rate decreased obviously under intensive irradiation. Maximum net photosynthetic rate (<em>P</em>_max) was selected as an indicator of photosynthetic capacity. Direct and indirect effects of environmental factors on <em>P</em>_max were calculated through the path analysis. The results show that when photosynthetic active radiation in environment increased, leaf temperature (<em>Tl</em>) rose, and air relative humidity (<em>RH</em>) decreased; photosynthetic active radiation in environment decreased, <em>Tl</em> decreased, and <em>RH</em> increased. Light response curves between 10:00-12:00 and light response curves of leaf 1-4 between 12:00-15:00 were characterized by hyperbolic curves and the photosynthetic capability approached saturation under intensive irradiation, while photosynthesis rate of leaf 5 between 12:00-15:00 and photosynthesis rate between 15:00-17:00 decreased obviously under intensive irradiation. <em>P</em>_max and light saturation point (<em>LSP</em>) were the highest between 10:00-12:00 and declined between 12:00-17:00 during one day. Light compensation point (<em>LCP</em>) and dark respiration (<em>R</em>_d) were higher between 12:00-15:00 than that under the other two time intervals. Quantum efficiency at light compensation point (<em>φ</em>_c), stomatal conductance (<em>G</em>_s), and intercellular CO₂ concentration (<em>C</em>_i) all generally displayed a declining trend, however, stomatal limitation value (<em>L</em>_s) increased from 10:00 to 17:00. Stomatal limitation was the key factor for different time intervals influencing photosynthesis. Light responsive parameters of leaf 4-5 were positive at the same time interval between 10:00-15:00, the <em>P</em>_max of leaf 3 was the highest, leaf 5 was the second between 15:00-17:00. <em>G</em>_sand <em>C</em>_i of leaf 5 were lower than that of leaf 1, while <em>L</em>_s of leaf 5 was higher than that of leaf 1 between 10:00-17:00. Non-stomatal limitation was the major factor of influencing photosynthesis for different position leaves. <em>RH</em> was the primary determining factor that influenced <em>P</em>_max; measuring time (Time), vapor pressure deficit at the leaf surface (<em>Vpdl</em>) and <em>Tl</em> were main limiting factors, so it was helpful to improve <em>P</em>_max by increasing <em>RH</em>, limiting Time, <em>Vpdl</em>, and <em>Tl</em>. The suitable time period for measuring light response curves was between 10:00-12:00.