In order to apply photoperiodic response functions developed in controlled environments to field grown maize (Zea mays L.), the timing of photoperiod sensitivity must be known. The timing of photoperiod sensitivity is also necessary for defining effective photoperiods for determining the effect of latitude and planting date on development of photoperiod‐ sensitive maize cultivars. The objective of this study was to determine the time when maize is sensitive to photoperiod. Two photoperiod sensitive cultivars of maize were grown in controlled environment chambers at the Duke Univ. Phytotron in photoperiods of 10, 12.5, 15, and 17.5 h. A third cultivar was grown in 12.5 and 17.5 h photoperiods. Plants were moved at different times between chambers having different photoperiods. Time from seedling emergence to tassel emergence was recorded. The length of this development interval was compared to the same interval for control plants that remained in a constant photoperiod for the entire time. The timing of photoperiod sensitivity was determined by knowing when the plants were moved and assessing which photoperiod affected them. Plants were insensitive to photoperiod immediately after seedling emergence. They remained insensitive until 4 to 8 days prior to the date of tassel initiation in short photoperiods. The insensitive, juvenile phase, therefore ended at least 4 days prior to tassel initiation in short photoperiods. The sensitivity continued until tassel initiation or shortly thereafter. The photoperiod affecting development in the field, considering the slow rate of change of photoperiod, is the value at the time of tassel initiation.

Shoots of 16-day-old soybeans (Glycine max L. Merr. cv Ransom) were chilled to 10°C for 7 days and monitored for visible signs of damage, ultrastructural changes, perturbations in fluorescence of chlorophyll (Chl), and quantitative changes in Chi a and b and associated pigments. Precautions were taken to prevent the confounding effects of water stress. A technique for the separation of lutein and zeaxanthin was developed utilizing a step gradient with the high performance liquid chromatograph. Visible losses in Chl were detectable within the first day of chilling, and regreening did not occur until the shoots were returned to 25°C. Ultrastructurally, unstacking of chloroplast grana occurred, and the envelope membranes developed protrusions. Furthermore, the lipids were altered to the point that the membranes were poorly stabilized by a glutaraldehyde/osmium double-fixation procedure. Chl fluorescence rates were greatly reduced within 2 hours after chilling began and returned to normal only after rewarming. The rapid loss of Chl that occurred during chilling was accompanied by the appearance of zeaxanthin and a decline in violaxanthin. Apparently a zeaxanthin-violaxanthin epoxidation/de-epoxidation cycle was operating. When only the roots were chilled, no substantial changes were detected in ultrastructure, fluorescence rates, or pigment levels.A large number of crop plants originated in the tropics or subtropics and begin to show deleterious responses as the temperature is lowered below 20°C. If there is physiological and structural damage, it is referred to as 'chilling injury.' The phenomenon has been well described (5,12,13).Among chilling sensitive plants such as Gossypium, Paspalum, Phaseolus vulgaris, and Glycine max, the chloroplast is the first of the organelles to show ultrastructural damage from chilling (1,11,29,35). Changes in chloroplast function have been tabulated for five chilling-sensitive species (26) and decline in photoreductive activity is common (10,14). Since changes in photosynthetic electron transfer activity can be monitored fluorometrically in intact leaves (20,26), changes in fluorescence of Chl have the potential for serving as a very early sign of chilling injury. This expectation is tested herein.Aside from the light energy transfer role of the carotenoids in photosynthetic membranes, they are known protectants of Chl against photooxidation. Apparently under chilling conditions, the equilibrium is shifted in the direction of excessive photooxidation (16, 29

A general quantitative description of maize (Zea mays L.) response to photoperiod for use with widely different genotypes and environments is important in order to accurately evaluate stages in maize ontogeny. This paper describes a method for combining photoperiod and maturity information into a relatively simple model to estimate the interval from seedling emergence to tassel initiation and tassel emergence in maize. Twenty contrasting cultivars of maize were grown to tassel emergence in the Duke Univ. Phytotron at 25°C under photoperiods ranging from 10 to 17.5 h. Dates of tassel initiation were determined destructively and tassel emergence nondesrructively. An apparently general linear relationship was found for estimating the thermal time (degree days) between seedling emergence and tassel initiation from the thermal time between seedling emergence and tassel emergence. This implies that the timing of tassel initiation can be estimated with reasonable accuracy when the temperature and date of tassel emergence are known. The time to tassel initiation was not influenced by photoperiod for all cultivars studied. Those that were photoperiod sensitive experienced a delay in rime to tassel initiation only when the photoperiod was greater than a critical threshold value ranging from 10 to 13.5 h. Below the threshold photoperiod, plants appeared to be insensitive to photoperiod. For photoperiods above the threshold value, the delay in thermal time to tassel initiation was described as a linear function of photoperiod above the threshold value. For the cultivars studied, the photoperiod sensitivity ranged from zero for six insensitive early maturity cultivars to 36 degree days per hour increase in photoperiod for one late maturity cultivar.

Ethylene-stimulated germination of witchweed [Striga luteaLour. =S. asiatica(L.) O. Ktze.] seed first occurred after 13, 10, 6, 3, and 2 days of conditioning in moist sand at day/night temperatures of 20/14, 23/17, 26/20, 29/23, and 32/26 C, respectively. Maximum germination percentages in these regimes were 0.5, 3, 20, 24, and 37%, respectively. No germination occurred at 17/11 C. Witchweed seed survived in sand frozen for 7 weeks at −7 or −15 C and subsequently germinated in response to ethylene or in the presence of corn (Zea maysL., ‘DeKalb B73 × Mo.17H′) roots. The parasites emerged from the soil and flowered when maintained at 29/23 C after the termination of the freezing treatments. In other experiments, witchweed parasitized corn and/or sorghum [Sorghum bicolor(L.) Moench ‘DeKalb E-59+′] root systems in a sandy loam under 26/17, 26/20, 29/20, 32/23, and 32/26 C day/night regimes. Witchweed emerged from the soil with 26/20, 29/20, 32/23, and 32/26 C and flowered with 26/20, 32/23, and 32/26 C day/night regimes. Underground development and subsequent emergence of the parasites were substantially reduced with day/night temperatures below 29/20 C. Winter soil temperatures and growing season soil and air temperatures are unlikely to limit the spread of witchweed into important corn- and sorghum-producing areas of the United States.