versão impressa ISSN 0100-2945
Rev. Bras. Frutic. v.23 n.3 Jaboticabal dez. 2001
LEAF GAS EXCHANGE CHARACTERISTICS OF FOUR PAPAYA GENOTYPES DURING DIFFERENT STAGES OF DEVELOPMENT1
ABSTRACT - In this research, was used four papaya (Carica papaya L.) genotypes: three from the 'Solo ( Sunrise Solo TJ, Sunrise Solo 72/12 and Baixinho de Santa Amália) group and one from the 'Formosa' group (Know-You 01). They were grown in plastic pots containing a sandy-clay-loam soil subjected to pH correction and fertilization, under greenhouse conditions. Throughout the experimental period plants were subjected to periodic irrigation to maintain the soil humitidy around field capacity. The experiment was conducted 73 days after sowing. In all genotypes, leaf gas exchange characteristics were determined. The net photosynthetic rate (A, mmol m-2 s-1 ), stomatal conductance (gs mol m-2 s-1), leaf temperature (TI, 0C) and intercellular carbon dioxide concentration (ci, mL L-1) on the 4th, 5th, 6th, 7th, 8th and 9th leaves from the plant apex were determined. No significant differences were observed for A, gs, ci, or Tl either among the leaves sampled from any of the genotypes. A was positively correlated with gs and in the other hand TI and gs were negatively correlated. The results suggest that, for 73 DAP, all the sampled papaya leaves functioned as sources of organs.
Index terms: photosynthesis, Carica papaya, stomatal conductance, leaf temperature
TROCAS GASOSAS EM FOLHAS DE DIFERENTES ESTÁDIOS DE DESENVOLVIMENTO EM GENÓTIPOS DE MAMOEIRO (Carica papaya L.)
RESUMO - Neste trabalho, utilizaram-se quatro genótipos de mamoeiro, sendo três genótipos do grupo 'Solo' (Sunrise Solo TJ, Sunrise Solo 72/12 e Baixinho de Santa Amália) e um do grupo 'Formosa' (Know-You 01). As plantas foram cultivadas em potes plásticos, sob condição de casa de vegetação, num solo franco-argilo-arenoso, corrigido, adubado e submetidas a irrigações periódicas até a capacidade de campo. Em todos os genótipos, a taxa fotossintética líquida (A, mmol m-2 s-1), a condutância estomática (gs, mol m-2 s-1), a temperatura foliar (Tl, 0C), a concentração interna de CO2 (ci, mL L-1) no mesofilo foliar foram determinadas aos 73 dias após o plantio (DAP), na 4a, 5a, 6a, 7a, 8a e 9a folhas, contadas a partir do ápice, por meio de um analisador de gás no infravermelho modelo LI-6200. Entre as folhas amostradas e entre os genótipos estudados, não foram observadas diferenças significativas nos valores de A, gs, ci e Tl. Foi observado que elevados valores de gs proporcionaram valores elevados de A em todos os genótipos estudados. Contudo, para a característica Tl, as relações com gs foram inversas, evidenciando a fundamental função do processo transpiratório no resfriamento foliar. Os resultados indicaram que, em todos os genótipos estudados, as folhas avaliadas funcionaram como órgãos fontes.
Termos para indexação: fotossíntese, Carica papaya, condutância estomática, temperatura foliar
During leaf expansion the net photosynthetic rate (A) reaches a maximum value before maximum leaf area is reached and decreases after leaf maturity. The increase in the net photosynthesis rate is related to several events linked to leaf development: increase in leaf area, leaf thickness, surface and volume of mesophyll cells, leaf inner surface and the dimensions of leaf chloroplasts (Catský & Sesták, 1997). The pigment contents, stomatal mesophyll conductance increases during leaf development. Also during leaf expansion, the activity of photosynthetic enzymes increases along with the photochemical processes such as light harvest, electron transport chain activity and phosphorylation. The leaf photosynthetic apparatus is completely developed and the maximum A value is reached when the leaf has 35 to 100% of its maximum area. The decrease in A, at senescence, is associated to the decrease in both stomatal and mesophyll conductance (Catský & Sesták, 1997).
Growth of very young leaves depends on photosynthate import from chronologically older leaves (Dickmann & Kozlowski, 1968; Dougherty et al., 1979; Hanson et al., 1988). During leaf development, the photosynthesis rate decrease perhaps due to carbohydrate accumulation and/or to a limitation in the photosynthetic capability (Suzuki et al., 1987).
According to Tetley & Thimann (1974) the reduced metabolic activity of chronologically older leaves is due to the amino acid export and is linked to protein degradation. Satler & Thimann (1977) postulated that stomatal closure could be the primary agent of the beginning of senescence in older leaves. Nevertheless, Okatan et al. (1981) reported that protein and chlorophyll degradation, in soybean, could take place even on absence of yellow leaves.
In relation to papaya that presents a spiral leaf insertion pattern, determining the leaf of greater metabolic activity, related gas exchange, is important for future determinations. This work aimed to determine the gas exchange characteristics, net photosynthesis rate, stomatal conductance, and intercellular CO2 concentration and leaf temperature, of leaves of four papaya genotypes at different stages of development.
Plant Material and Growth Conditions. Four papaya (Carica papaya L.) genotypes: three from the 'Solo ( Sunrise Solo TJ, Sunrise Solo 72/12 and Baixinho de Santa Amália) group and one from the 'Formosa' group (Know-You 01) were grown in 6 L plastic pots containing a sandy-clay-loam soil subjected to pH correction and fertilization according to Marin et al. (1993), under greenhouse conditions. During the experimental period the maximum air temperature, the minimum air temperature and the soil temperature were 30.2 ± 3.1 ºC, 19.1± 2.1 ºC, and 33.5 ± 2.2 ºC, respectively. On the 7th day after planting, two plants were eliminated leaving one plant per pot. At 60 DAP, nitrogen fertilization (50mg N kg-1 soil) was applied (Marin et al. 1993). Throughout the experimental period plants were subjected to periodic irrigation to maintain the soil humidity around field capacity.
Physiological Characteristics. The photosynthetic rate (A), stomatal conductance (gs), leaf temperature (Tl) and intercellular CO2 concentration (ci), were determined at the 73th DAP using the 4, 5, 6, 7, 8 and 9th leaves, counted down from the plant apex, using a Portable Photosynthesis System, Model LI-6200 (LI-COR, USA, Lincoln, NE). Four measurements were performed for each leaf of each genotype, and a 10 cm2 leaf area was used for measurements in a photosynthetic chamber of one liter. The leaves were irradiated by a photosynthetic active radiation (PAR) of 1600 ± 100 mmol m-2 s-1 emitted by two tungsten-halogen lamps of 500 W. The lamps were set above a transparent glass cuvette containing a 6 cm layer of continuous-flow tap water to filter the heat radiation emitted by the lamps. The leaves were acclimatized under the established radiation level for 20 minutes before measurements. During measurements of the CO2 concentration, the vapor pressure and the air temperature in the photosynthetic chamber were 370 ± 10 mL L-1, 2.0 ± 0.2 kPa e 28.8 ± 0.6 ºC, respectively.
Data analysis. Results were statistically analyzed as a randomized block design with four replications per treatment. Analysis of variance was determined using the statistical package of Statistica (Statsoft, Inc., Tulsa, OK, USA). Proc ANOVA/MANOVA was used to obtain means and standard deviations.
RESULTS AND DISCUSSION
All leaves from the four papaya genotypes showed net photosynthetic rates above 10 mmoles m-2 s-1 (Fig.1) which suggest that all analyzed leaves possibly behaved as source organs. The gas exchange was not determined on the 1st, 2nd and 3rd leaves because the size of these leaves since the leaf area was too small to be introduced inside the photosynthetic chamber device and because of the apical position of the leaves which does not leave enough room for measurements.
The 7th leaf of Know-You and Sunrise Solo TJ genotypes presented higher A than the other leaves (Fig.1). The same response was observed on the 6th leaf of Sunrise Solo 72/12. However, papaya leaves analyzed in the experiment, had active gas exchange and net photosynthesis, including the 8th and 9th leaves, which were chronologically older than the others. The gs of all genotypes behaved similarly to A (Fig.2). Figure 3 shows the relationship between these two characteristics. The good correlation between A and gs indicates a dependence of A upon the stomatal conductance on leaves of all genotypes analyzed. The stomatal conductance determines the input of CO2 in the leaf affecting the partial pressure of that gas at RUBISCO carboxylation sites (Farquhar & Sharkey, 1982). Therefore, higher gs values may cause higher A values. This statement becomes true if other factors do not limit the process. For the Sunrise Solo 72/12 genotype, the maximum gs value was 0.4 mol m-2 s-1, while the other genotypes showed higher values for this characteristic, for example the Sunrise Solo TJ presented values around 0.6 mol m-2 s-1.
The intercellular CO2 concentration (ci) was similar among leaves and genotypes (Fig. 4). This characteristic is the result of the balance between the gas assimilation through net photosynthesis rate and its income from the leaf external environment through stomata (Field et al., 1989, Long & Hallgren, 1993; Mansfield et al., 1990). Possibly, the low variation in ci could be due to the relationship between A and gs.
The relationships between A and Tl and Tl and gs are shown in Figures 5 and 6. In general, higher gs values were associated to lower Tl values (Fig.6). Probably, the transpiration rate changes caused significative changes in leaf temperature. The transpiration process is the main responsible for the reduction of leaf temperature (Farquhar & Sharkey, 1982; Nobel, 1991). Although Figure 5 shows little dependence of A upon Tl, the Tl values were affected by gs (Fig. 6). Therefore, the variations observed in A were affected mainly by the variations in gs rather than by variations in Tl, as no great variations were observed for the air temperature values during the measurements.
All evaluated papaya leaves at the 73th DAP showed higher gas exchange rates. At this time, these leaves were functioning as source organs in all four genotypes. No differences were observed for the gas exchange characteristics between chronologically younger or older leaves. The variations in A value were caused by variations in gs rather than in Tl values. The Tl values were lower for leaves that showed higher gs values. Future research should be carried out to determine, for the same genotypes, the drain-source transition stage for the leaves that were not observed (1st, 2nd and 3rd) in this study.
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1 (Trabalho 048/2001). Recebido: 19/02/2001. Aceito para publicação: 13/09/2001.
2 Engº Agr. Dr. Produção Vegetal, Setor de Fisiologia Vegetal, UENF/CCTA-Av. Alberto Lamego, 2000, 28015620, Campos/RJ
3 Eng. Agr. PhD. Fisiologia de Fruteiras, FAV/UnB, Caixa Postal 04508, Brasília, DF
4 Eng. Agr. Dr. Fisiologia Vegetal, DBV, UFV, 36579000, Viçosa, MG