Drought severely affects citrus growth and development. In order to explore the mechanism of drought response of citrus, two cultivars (Sanhuhongju and Sanhuhuahong) that differing in drought tolerance were used as materials. The drought and rewatering treatment was conducted in pot experiments, with leaf photosynthesis, chlorophyll fluorescence, and root architecture being measured. The results showed that drought significantly decreased net photosynthetic rate (Pn), stomatal conductance (gs), transportation rate (Tr), and intercellular CO2 concentration (Ci) of both cultivars, but Sanhuhongju generally showed less reduction. After rewatering, photosynthetic parameters were partly recovered but still lower than that in control. The water use efficiency (WUE) of Sanhuhongju was significantly increased after drought stress for 15 d, but the WUE of Sanhuhuahong was decreased except at the 15 day of drought stress. In addition, the maximum photosynthesis efficiency of PS II (Fv/Fm) was increased in both cultivars, but the photochemical quantum yield of PS II [Y(II)] was increased in Sanhuhuahong under drought. Both the apparent electron transport rate (ETR) and photochemical quenching (qP) were inhibited in the treated seedlings. The non-photochemical quenching (NPQ) was decreased in Sanhuhongju while increased in Sanhuhuahong under drought and rewatering conditions. Drought stress resulted in the decrease of root surface area and volume of both cultivars, and it inhibited root elongation of Sanhuhuahong while improved the root length and root tip number of Sanhuhongju. The length of first lateral roots of Sanhuhongju was increased after drought stress 10 d, but did not change at the drought stress prophase of Sanhuhuahong, and then significantly decreased after 20 d. Furthermore, drought stress inhibited all lateral roots development except the tertiary lateral root of Sanhuhongju, and root growth could not be recovered by rewatering except root tip number. In conclusion, Sanhuhongju showed less reduction in leaf photosynthesis than Sanhuhuahong, with higher WUE and light use efficiency under drought stress. The increases of root tip number and lateral root length would help improve water uptake ability in Sanhuhongju.
干旱严重影响柑橘的生长和发育.为探索柑橘对干旱胁迫的响应机制,本试验以抗旱性不同的三湖红橘和三湖化红为材料,通过盆栽控水进行干旱胁迫和复水处理,研究处理后植株叶片光合、叶绿素荧光和根系构型的变化.结果表明: 干旱显著降低了两种柑橘幼苗的净光合作用速率、气孔导度、蒸腾速率和胞间CO2浓度,而三湖红橘的下降幅度更小;复水后,光合参数均有所恢复,但仍低于对照.三湖红橘水分利用率在干旱15 d后开始显著高于对照,而三湖化红除干旱15 d外的其他处理时间均低于对照.干旱提高了两种幼苗的PSII最大光合效率,但抑制了三湖化红的PSII实际光合效率.干旱到一定程度后,两种幼苗的PSII电子传递速率和光化学淬灭均下降,干旱和复水后非光化学淬灭在三湖红橘中下降,但在三湖化红中上升.根系构型分析表明,干旱导致两种幼苗的根表面积和根体积下降,同时抑制了三湖化红的总根长,但能够提高三湖红橘的总根长和总根尖数.进一步分析不同直径的侧根长度发现,三湖红橘的一类侧根长度在干旱胁迫10 d后开始增加,而三湖化红的一类侧根长度在干旱前期没有变化,干旱20 d时显著下降;除三湖红橘的三类侧根外,两种幼苗其余直径等级侧根的生长均受干旱抑制.除总根尖数外,复水后根系生长各参数均没有恢复.干旱对三湖红橘光合性能的影响小于三湖化红,并且前者能够维持更高的水分和光能利用率.干旱后三湖红橘根尖数和细根长度增加,可能有助于提高其对水分的吸收能力.
Keywords: chlorophyll fluorescence; citrus seedling; drought; photosynthesis; root architecture.