A study on photosynthetic characteristics and fruiting performance of three lemon varieties in dryhot valley regions in Yunnan province

Abstract: 【Objective】Lemon can be produced year-round in dry-hot valley regions in Yunnan province.Lemon has high economic values. Photosynthesis of plants is the decisive factor of yield and quality. In or⁃der to provide theoretical basis for scientific cultivation and selection drought tolerant lemon varieties fordry-hot valley regions, the photosynthetic characteristics and bearing performance of three lemon variet⁃ies were studied.【Methods】Four -year -old grafted seedlings of three lemon varieties‘(Yunning 1’,‘Femminello’and‘Fumilainuo S1’) on Poncirus trifoliata L. were used as the experimental materials.The varieties had been introduced and cultivated in the dry-hot valley regions in Yunnan province, wherelemon is frequently influenced by high temperature and drought. Fully mature and south side leaves in themiddle and upper parts of the crown of a lemon tree were selected for the measurements. A LCpro-SD por⁃table photosynthesis system was used to measure the parameters related to photosynthetic characteristics,including net photosynthetic rate (P_n), transpiration rate (T_r), intercellular CO₂ concentration (C_i), leaf tem⁃perature (Tleaf), photosynthetically active radiation (PAR) and stomatal conductance (G_s). Water use efficien⁃cy (WUE), stomatal limitation value (L_s) and mesophyll carboxylation efficiency (MCE) were calculated.The chlorophyll content of three lemon varieties was measured by a handheld chlorophyll meter. Yield perplant, and fruit transverse and longitudinal diameters were measured by an electronic balance and a verni⁃er caliper, respectively. Peel thickness, pulp weight, fruit juice weight, soluble solids, titratable acid, vita⁃min C and edible rate of mature fruit were tested. Furthermore, correlation analysis between photosynthetic factors and environment factors was conducted.【Results】The diurnal variation of PAR and Tleaf followeda mono peak curve in October, and the peak occurred between 12:00 am and 14:00 pm. The maximum val⁃ue of PAR and Tleaf were 1 754.93 mmol·m^-2·s^-1 and 41.36 ℃ respectively. Temperature influenced photo⁃synthesis of lemon. High temperature at midday led to desiccation and metabolism disorder. As a result, Pnshowed midday depression. The diurnal variation of Pn in the three lemon varieties showed a typical bi⁃modal curve, which was apparently regulated by non-stomatal limitation. The first and second peaks occurred at about 10:00 am and 16:00 pm respectively. The highest net photosynthesis rate (Pnmax) in‘Yun⁃ning 1’,‘Femminello’and‘Fumilainuo S1’during day time was 10.11, 9.27, and 8.11 μmol·m^-2·s^-1, re⁃spectively. The diurnal changes in Tr of‘Femminello’and‘Fumilainuo S1’lemons was single-peakcurve while that of‘Yunning1’a double-peak curve. The highest diurnal value of Tr followed a descend⁃ing order of‘Femminello’>‘Fumilainuo S1’>‘Yunning 1’. The change trend of Gs in‘Yunning 1’and‘Fumilainuo S1’was a bimodal curve and similar to their Pn. The highest values of Gs in a day was higherin‘Yunning 1’than in‘Femminello’and‘Fumilainuo S1’. The maximum Gs occurred at 10:00 am. How⁃ever, in order to adapt to the stress of high temperature and drought, Gs decreased sharply from midday, amechanism to stop water loss. Correlation analysis showed that Pn of these three lemon varieties was signif⁃icantly and positively related to their Tr and Gs, while there was a significantly negative correlation be⁃tween Ci and Pn. The chlorophyll content of the three lemon varieties had significant difference in themorning, but no significant difference between‘Femminello’and‘Fumilainuo S1’in the afternoon. May⁃be the chlorophyll content was influenced by temperature. Field test showed that the yield per plant andyield per acre of‘Yunning1’and‘Femminello’were significantly different from those of‘FumilainuoS1’. The yield in‘Yunning1’was higher than those in‘Femminello’and‘Fumilainuo S1’. There wasslight difference in fruit quality among the three lemon varieties and they did not have significant differ⁃ence in peel thickness, pulp weight and fruit juice weight, but soluble solids was significantly differentand was in a order of‘Yunning1’>‘Femminello’>‘Fumilainuo S1’.【 Conclusion】Diurnal photosynthetic parameters of the three lemon varieties displayed similar trends but had differences. Gs was one of themain factors affecting Pn and Tr, but not the restriction factor. Comprehensive comparison of photosynthet⁃ic and bearing performances showed that‘Yunning1’seems a promising variety for large scale productionin the dry–hot valley regions in Yunnan province. Our results confirmed that photosynthesis is the basisof fruit tree productivity and thus a key factor affecting fruit quality and yield.