Drought is a major environmental constraint affecting growth and production of coffee. The effects of water supply on growth, biomass allocation, water relations, and gas exchange in two coffee progenies representing drought-tolerant (Siriema) and drought-sensitive (Catucaí) genotypes were compared. They were grown in 12-L pots until 4-months old, when they were submitted to two watering treatments for 60 d: plants receiving either 100% transpired water (control plants) or a fraction (about 40%) of the amount of water transpired by control plants (drought-stressed plants). Under control conditions, Siriema grew faster than Catucaí. Regardless of the watering regimes and progenies, relative growth rate (RGR) was positively correlated both with net assimilation rate (NAR) and long-term water-use efficiency (WUE), but not with differences in biomass allocation. Both progenies responded to drought stress through (i) similar decreases in both RGR and NAR with marginal, if any, changes in allocation; (ii) decreases in leaf water potential, which occurred to a greater extent in Catucaí than in Siriema, even though they have showed similar abilities to adjust osmotically and elastically; (iii) similar reductions in net photosynthesis due mainly to nonstomatal factors; and (iv) decreases in transpiration rate coupled with increased long-term WUE. However, the lower transpiration rate and the higher long-term WUE as found in Siriema relative to Catucaí under control conditions persisted under drought conditions. Overall, the major differences between these progenies were largely associated with differences in plant water use, which was likely related to the improved water status of Siriema. The possible implications of selecting coffee genotypes for high WUE are discussed.

Summary• Perturbations of the source-sink balances were performed in field-grown coffee (Coffea arabica) trees to investigate the possible role of carbohydrates in feedback regulation of photosynthesis.• Four treatments were applied at the whole-plant level: (i) complete defruiting and maintenance of the full leaf area, (ii) the half crop load and full leaf area, (iii) the full crop load and full leaf area and (iv) the full crop load and half leaf area. Sampling and measurements were performed twice during the phase of dry matter accumulation of fruits. Gas exchange, chlorophyll a fluorescence, carbon isotope labelling and steady-state metabolite measurements were assessed in source leaves.• The average rate of net photosynthetic rate (A) and stomatal conductance (g s ) were larger (> 50%), and carbon isotope composition ratio was lower, in trees with a full crop load and half leaf area than in defruited trees, with individuals of the other two treatments showing intermediate values. However, differences in A seem unlikely to have been caused either by photochemical impairments or a direct end-product-mediated feedback down-regulation of photosynthesis.• It is proposed that the decreased A in defruited coffee trees was independent of carbon metabolism and was rather directly related to a lower CO 2 availability coupled to lower g s .

Coffee (Coffea arabica L.) plants were grown in small (3-L), medium (10-L) and large (24-L) pots for 115 or 165 d after transplanting (DAT), which allowed different degrees of root restriction. Effects of altered source : sink ratio were evaluated in order to explore possible stomatal and non-stomatal mechanisms of photosynthetic down-regulation. Increasing root restriction brought about large and general reductions in plant growth associated with a rising root : shoot ratio. Treatments did not affect leaf water potential or leaf nutrient status, with the exception of N content, which dropped significantly with increasing root restriction even though an adequate N supply was available. Photosynthesis was severely reduced when plants were grown in small pots; this was largely associated with non-stomatal factors, such as decreased Rubisco activity. At 165 DAT contents of hexose, sucrose, and amino acids decreased in plants grown in smaller pots, while those of starch and hexose-P increased in plants grown in smaller pots. Photosynthetic rates were negatively correlated with the ratio of hexose to free amino acids, but not with hexose content. Activities of acid invertase, sucrose synthase, sucrose-P synthase, fructose-1,6-bisphosphatase, ADP-glucose pyrophosphorylase, starch phosphorylase, glyceraldehyde-3-P dehydrogenase, PPi : fructose-6-P 1-phosphotransferase and NADP : glyceraldehyde-3-P dehydrogenase all decreased with severe root restriction. Glycerate-3-P : Pi and glucose-6-P : fructose-6-P ratios decreased accordingly. Photosynthetic downregulation was unlikely to have been associated directly with an end-product limitation, but rather with decreases in Rubisco. Such a down-regulation was largely a result of N deficiency caused by growing coffee plants in small pots.

Limitations to photosynthesis were explored in leaves from four canopy positions of field-grown, unshaded coffee (Coffea arabica L.), a tropical tree species classified as shade-obligatory. Overall, compared to shade (lower) leaves, sun (upper) leaves had higher net carbon assimilation rate (A) (4.5 against 2.0 micromol m(-2)s(-1) at most) associated with higher electron transport rate (due to a greater irradiance availability) but unrelated to stomatal and mesophyll conductances, which were similar regardless of leaf position. Neither physiological variable directly involved with photosynthetic carbon gain nor those involved with light capture were able to adjust themselves to match the capacity of the photosynthetic machinery to the light supply. We concluded that: (i) there was no major difference in photosynthetic capacity between sun and shade leaves; (ii) the intrinsic low A in coffee was greatly associated with remarkable low diffusive limitations rather than with biochemical or photochemical constraints; and (iii) morphological (e.g., variations in specific leaf area and leaf inclination) or anatomical plasticity should be of greater acclimative value than physiological plasticity as a mean of coffee leaves to respond to changing irradiance.

No abstract

It has long been held that the regulation of photosynthesis in source leaves may be controlled by carbohydrates. The mechanisms that govern the diurnal fluctuation of photosynthesis and the potential role of feedback regulation by carbohydrates during photosynthesis in coffee (Coffea arabica) leaves were investigated in three independent and complementary experiments. An integrative approach using gas exchange measurements in addition to carbon isotope labelling and steady-state carbohydrate and amino acid analysis was performed. Canonical correlation analysis was also performed. In fieldgrown plants under naturally fluctuating environmental conditions (Experiment I), the overall pattern of gas exchange was characterised by both low stomatal conductance (g s ) and net carbon assimilation rate (A) in the afternoon; no apparent signs of photoinhibition were observed. Under conditions of low air evaporative demand (Experiment II), only slight decreases (*20%) in A were observed at the end of the day, which were associated with a reduction (*35%) in g s . For both conditions, any increase in carbohydrate and amino acid pools over the course of the day was small. In leaves from girdled branches (Experiment III), a remarkable decrease in A and particularly in g s was observed, as were increases in starch but not in hexoses and sucrose pools. Furthermore, the rate of 14 CO 2 uptake (assessed under saturating CO 2 conditions) and the partitioning of recently fixed 14 C were not affected by girdling. It is proposed that the diurnal oscillations in A and the differences in A in leaves from girdled and non-girdled branches were merely a consequence of diffusive limitations rather than from photochemical constraints or direct metabolite-mediated down-regulation of photosynthesis.

The coffee plant is native to shaded environments and its seedlings are often produced in shaded nurseries. However, some nursery managers, in an effort to improve the acclimation of seedlings to field conditions after transplantation, produce seedlings in full sun exposure. In this study, the morphological and physiological parameters of arabica coffee (Coffea arabica) seedlings produced in full sun (T1) and in shade (T2) were examined. The biomass accumulation and relative growth rate of T1 and T2 seedlings were similar. The T1 seedlings had less biomass allocation to shoots, a lower leaf mass ratio and a lower leaf area ratio; however, they had a greater net assimilation rate (rate of increase in plant mass per unit leaf area), which was associated with a greater net photosynthetic rate. There were no alterations in the concentrations of total chlorophylls or in the chlorophyll a/b ratio when comparing T1 and T2 seedlings. No indications of photoinhibition or photooxidative damage were observed in the T1 plants, which were shown to have a more robust antioxidant system than the T2 plants. Seedlings transferred from shade to full sun (T3) were not capable of utilising the incident extra light to fix CO 2 . These seedlings showed a remarkable nocturnal retention of zeaxanthin and a significantly increased deepoxidation state of the xanthophyll cycle, even at predawn, but the activity of antioxidant enzymes was lower than in the T1 and T2 plants. Despite the acclimation capacity of T3 seedlings to the new light environment, they exhibited chronic photoinhibition and considerable photooxidative damage throughout the seven days following the transfer to full sun exposure. We further discuss the practical implications of producing coffee seedlings in full sunlight and under shade.

O objetivo deste trabalho foi avaliar as trocas gasosas, os parâmetros de fluorescência da clorofila a, a concentração de pigmentos fotossintéticos e o potencial osmótico em plantas de tomateiro submetidas a crescentes concentrações salinas. Para esta finalidade, plantas de tomateiro do cultivar Santa Clara foram cultivadas em vasos de 5 dm3 contendo solução nutritiva de Hoagland e submetidas aos tratamentos de 0, 50, 100 e 150 mmol L-1 de NaCl. O experimento foi realizado em casa de vegetação no Campus da Universidade Federal de Viçosa, num delineamento inteiramente casualizado com quatro repetições. Cada unidade experimental foi constituída por um vaso contendo duas plantas. Os resultados mostraram que o potencial osmótico nas folhas e nas raízes apresentaram reduções significativas à medida que se aumentava a concentração de NaCl na solução nutritiva, detectando o aumento do estresse sobre as plantas. A utilização da energia para os processos fotossintéticos e a capacidade de dissipação do excesso de energia na maquinaria fotossintética foi comprometida pelas crescentes concentrações salinas impostas pelos tratamentos, reduzindo a fotossíntese e o rendimento fotoquímico, demonstrado pelas trocas gasosas e pelos parâmetros de fluorescência de clorofila a, evidenciando que houve dano ao aparato fotossintético na fase bioquímica. As concentrações de pigmentos fotossintéticos diminuíram significativamente com o aumento na concentração salina, limitando a absorção da radiação fotossinteticamente ativa.