Variações diurnas da fotossíntese e efeitos do anelamento de ramos sobre a fotossíntese e o metabolismo do carbono em café arábica / Diurnal changes in photosynthesis and effects of branch girdling on photosynthesis and carbon metabolism in arabica coffee

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

This study was carried out in order to examine (i) the diurnal changes in photosynthesis rates under low atmospheric evaporative demand, and (ii) the relationship between the rate of net photosynthesis (A) and carbon (C) metabolism in experimentally-manipulated branches through girdling. Two experiments were separately conducted and so analyzed. In both experiments, plants of arabica coffee (Coffea arabica L.) were grown in 12-L pots during approximately eight months. Then, gas exchanges (measured using an infrared gas analyzer), the rate of uptake of 14CO2 and the partitioning of the recently fixed 14C through the major photosynthetic routes were analyzed; leaf material was also harvested for biochemical assays. In Experiment I, plants were grown in a greenhouse under semi-controlled conditions (diurnal values of vapor pressure deficit, VPD, ranging from 1.0 to 1.6 kPa). There was a decrease (20%) in A paralleling a reduction (35%) in stomatal conductance (gs) at 16:00 h as compared with the values of these traits obtained at 08:00 and 12:00 h. The narrow diurnal fluctuations in A were apparently coupled with the maintenance of low VPD throughout the day. Simple and canonical correlations evidenced a lack of feedback inhibition to photosynthesis, a fact further supported by the (i) lack of diurnal changes in carbohydrate and amino acid levels and (ii) unchanging rate of 14CO2 uptake and the partitioning of the recently fixed 14C during the day. In addition, the activity of ADP-glucose pyrophosphorylase and the initial and total activities of Rubisco (as well as its activation state) did not change during the day. In fact, diurnal variations in A were largely explained by diffusive limitations. In Experiment II, gas exchange was measured at 08:00 h during 11 days in girdled and non-girdled (control) branches. There was a remarkable decrease in A and especially in gs. In girdled branches, starch levels increased by 271% and 203%, respectively at the 4th and 10th days after applying the treatments. The hexose-to-amino acid ratio increased in girdled branches at the 10th day of girdling, but the concentrations of other soluble sugars and amino acids did no change in response to the treatments, as also did the activity of ADP-glucose pyrophosphorylase and the initial and total activities of Rubisco. Moreover, the potential photosynthetic capacity also remained unchanged after girdling, suggesting that changes in A were marginally affected, if so, by biochemical constraints to the photosynthetic machinery. As in Experiment I, changes in A were largely explained by diffusive limitations. In summary, it is proposed that diurnal oscillations in A during the day, as well as between leaves from girdled and non-girdled branches were merely a consequence from diffusive, rather than from biochemical, limitations to photosynthesis.

ASSUNTO(S)

anelamento fisiologia de plantas cultivadas coffea arabica coffee vapor pressure deficit fotossíntese coffea arabica déficit de pressão de vapor photosynthesis girdling café arábica

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