Response Of Cucurbit Crops To The Changes In Temperature And Light: Light Energy Utilization And ROS Metabolism

High light may induce photoinhibition in higher plants. Plants have evolved several kinds of mechanisms to dissipate the excess light energy. Few studies have been done to compare the operation of different photoprotection pathways in different plant species with different resistance to temperature and light condiitons. Meanwhile, plants growing in greenhouses may often encounter sub- or supra- optimal root temperature, which significant affects their production and quality. A significant difference in root temperature tolerance among various plant species have been discovered, but the physiological mechanism behind has not been studied in detail. So the present study was taken to compare the differences in the mechanisms of dissipating excess light energy in cucurbit crops and to study the differences in both the ROS metabolism in root system and the photosynthesis among six cucurbit crops with different temperature tolerance growing at different root-temperatures. The main results are as follows:1,The species specific response of three cucurbit crops with different heat-tolerance: warm-adapted cucumber, heat-tolerant sponge gourd and heat-sensitive figleaf gourd to diurnal changes in air temperature and irradiance intensity were investigated in a typical summer sunny day. Three cucurbit species were compared using parameters in gas exchange, chlorophyll fluorenscence, photon allocation and photosynthetic electron flux. Compared to figleaf gourd, cucumber and sponge gourd showed higher net CO₂ assimilation (P_N), actual quantum yield of PSII electron transport (Φ_PSII), photochemical quenching coefficient (q_p), the efficiency of excitation capture by open PSII centers (Fv'/Fm'), rate of energy dissipation via light-independent thermal processes and fluorescence (J_f,D), energy flux via linear electron transport (J_PSII) and electron flux for photosynthetic carbon reduction (Jc) all the time during the measuring day. Among three crops, sponge gourd showed the lowest non-photochemical quenching (NPQ), and the highest electron flux for photorespiratory carbon oxidation (Jo). Rate of energy dissipation via light-dependent thermal processes (J_NPQ) was highest in figleaf gourd leaves. The results suggest that higher photorespiration rate in sponge gourd might contribut to its higher heat tolerance than cucumber and figleaf gourd.2,There were significant difference responses in ROS metabolism, respiration and the viability of root tip cells between cucumber and figleaf gourd after exposure to the suboptimal root temperature conditions. Under suboptimal root temperature, plant growth was more inhibited in cucumber plant than figleaf gourd. Cucumber roots growing in 14℃always showed higher O₂^·- generating rate, H₂O₂ and MDA concentration, and anti-oxidant enzyme activities than those in 24℃. These parameters did not differ from each other in figleaf gourd's two treatments. The root temperature of 14℃didn't induce significant changes in the total respiration rate (Vt) in both species but caused an increase of the alternative respiration pathway (Valt) and a decrease of the cytochrome respiration pathway (Vcyt), and more dead cells in root tips in cold-sensitive cucumber roots. Exposure to 14℃didn't change the values of Valt and Vcyt in figleaf gourd root systems. This demonstrates the inhibited growth in cucumber crops is partly due to the accumulation of ROS in cucumber root induced by the low root temperature.3,To investigate the response of plant growth and antioxidant system to changes of root temperatures, roots of six cucurbit plants were exposed to three different root-zone temperatures: low (14℃), middle (24℃) and high (34℃), while aerial parts of plants were maintained at the same natural ambient temperatures (23-33℃). The six cucurbit plant species could be classified into three groups with figleaf gourd and turban squash being I group (heat-sensitive but cold-tolerant), cucumber and melon being II group (heat- and cold-sensitive) and bitter melon and wax gourd being III group (heat-tolerant but cold-sensitive), i.e. the I, II and III group plants exhibited the greatest growth rate and the lowest malonylaldehyde (MDA) contents at 14, 24 and 24-34℃, respectively. Superoxide dismutase (SOD) activity was the highest in roots after being exposed to unoptimal growth temperatures, while catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (G-POD) operated coordinately in a complex manner to prevent the accumulation of reactive oxygen species (ROS) in cells. Moreover, all of I, II and III group plants responded to the unsuitable growth temperature by increasing synthesis of ascorbate and glutathione as well as by reducing the redox ratio.4,Figleaf gourd and turban squash plants showed the highest dry weights, light-saturated rates of the CO₂ assimilation (P_Nsat) and stomatal conductances (g_s) at 14℃. Successively lower values for these measurements were observed at temperatures of 24℃and 34℃respectively. In bitter melon and wax gourd, the dry weight, P_Nsat and g_s values at the 14℃temperature treatment, were however correspondingly and significantly lower than at the 24℃and 34℃ root temperature treatments. In contrast, cucumber and melon plants showed the highest dry weight, P_Nsat and g_s at 24°C. The decreases in P_Nsat were mainly due to the decrease in stomatal conductance. Changing root temperatures did not induce photoinhibition but induced a down-stream regulation of the quantum efficiency of PSII (Φ_PSII) due to decreasing of photochemical quenching (q_p). Meanwhile, roots played an important role in the regulation of stomatal behavior by delivering increased amount of abscisic acid (ABA) to shoots at suboptimal root temperatures. Our results also indicate that cooling or heating the root system would be useful for improving shoot growth at supraoptimal or suboptimal temperatures.