RESUMO
Leaf and canopy photosynthesis of cotton (Gossypium hirsutum L.) declines as the crop approaches cutout, just as the assimilate needs for reproductive growth are peaking. Our objective with this study was to determine whether this decline is due to remobilization of leaf components to support the reproductive growth or due to some cue from the changing environmental conditions during the growing season. Field studies were conducted in 1995-1996 at Stoneville, Mississippi, using six cotton genotypes and two planting dates (early and late), which produced two distinctly different cotton populations reaching cutout at different times. Among the six genotypes were a photoperiod sensitive line (non-flowering) and its counter part which had photoperiod insensitive genes backcrossed four times to the photoperiod sensitive line (flowering). This pair was used to assess the degree that the photosynthetic decline could be attributed to reproductive sink development. Leaf CO(2)-exchange rate (CER) and chlorophyll (Chl) fluorescence measurements were taken in mid-August, a period corresponding to cutout for the early planted plots, and those leaves were collected. Leaf Chl level, soluble protein level, various soluble carbohydrate levels and Rubisco activities were assayed on those leaves. Averaged across years, leaf CER and soluble protein levels were reduced approximately 14% and 18%, respectively, for the early planted compared to the late planted cotton. Neither leaf Chl levels or Chl fluorescence Fv/Fm values for Photosystem II yield were altered by the planting date. In 1996, leaves from the non-flowering line had 12% greater Chl and 20% greater soluble protein levels than the flowering line. However, in 1996, the CER of the early planted non-flowering line was reduced 10% compared to the late planted. Although remobilization of leaf N to reproductive growth appears to be the principle component causing the cutout photosynthetic decline, the data also indicate that environmental factors can play a small role in causing the decline.
RESUMO
Canopy CO2-exchange rates (CER), air temperatures, and dew points were measured throughout ten days during the 1987 growing season for cotton (Gossypium hirsutum L.), grain sorghum [Sorghum bicolor (L) Moench], and five soybean [Glycine max (L) Merr.] cultivars, and throughout seven days in 1988, on maize (Zea maize L.). The objective was to determine if the decline in CER per unit light during the afternoon is associated with a vapor pressure deficit (VPD) increase. Some of the soybean and maize plots were kept as dry as possible. A VPD term significantly contributed (P≤0.05) to a canopy CER regression model in 54 of 80 data sets in 1987. Grain sorghum was less sensitive than the well-watered soybean genotypes to an increasing VPD (P≤0.05) on three of the ten measurement days and less sensitive than cotton (P≤0.05) on only one day. Cotton demonstrated less VPD sensitivity than soybean (P≤0.05) on one day. The moisture stressed soybean plots showed a greater CER sensitivity to VPD (P≤0.05) than the well-watered soybean plots. In 1988, the frequently irrigated maize plots were less sensitive to VPD (P≤0.05) than the rain-fed plots early in the season, before the rain-fed plots were excessively damaged by moisture stress. These results indicate that the afternoon declines in canopy CER found in a number of different species are associated with increases in the VPD; recent work of others suggests that this may be due to partial stomatal closure.
