Photosynthetic Duration Research Articles

Genetic Architecture, Studies to Identify the Selection Criteria for Yield in Long Duration Rice Lines (Oryza sativa L.)

Genetic Architecture, Studies to Identify the Selection Criteria for Yield in Long Duration Rice Lines (Oryza sativa L.)

Characterizing stay-green in barley across diverse environments: unveiling novel haplotypes

Key messageThere is variation in stay-green within barley breeding germplasm, influenced by multiple haplotypes and environmental conditions. The positive genetic correlation between stay-green and yield across multiple environments highlights the potential as a future breeding target.Barley is considered one of the most naturally resilient crops making it an excellent candidate to dissect the genetics of drought adaptive component traits. Stay-green, is thought to contribute to drought adaptation, in which the photosynthetic machinery is maintained for a longer period post-anthesis increasing the photosynthetic duration of the plant. In other cereal crops, including wheat, stay-green has been linked to increased yield under water-limited conditions. Utilizing a panel of diverse barley breeding lines from a commercial breeding program we aimed to characterize stay-green in four environments across two years. Spatiotemporal modeling was used to accurately model senescence patterns from flowering to maturity characterizing the variation for stay-green in barley for the first time. Environmental effects were identified, and multi-environment trait analysis was performed for stay-green characteristics during grain filling. A consistently positive genetic correlation was found between yield and stay-green. Twenty-two chromosomal regions with large effect haplotypes were identified across and within environment types, with ten being identified in multiple environments. In silico stacking of multiple desirable haplotypes showed an opportunity to improve the stay-green phenotype through targeted breeding. This study is the first of its kind to model barley stay-green in a large breeding panel and has detected novel, stable and environment specific haplotypes. This provides a platform for breeders to develop Australian barley with custom senescence profiles for improved drought adaptation.

Observations of Satellite Land Surface Phenology Indicate That Maximum Leaf Greenness Is More Associated With Global Vegetation Productivity Than Growing Season Length

AbstractVegetation green leaf phenology directly impacts gross primary productivity (GPP) of terrestrial ecosystems. Satellite observations of land surface phenology (LSP) provide an important means to monitor the key timing of vegetation green leaf development. However, differences between satellite‐derived LSP proxies and in situ measurements of GPP make it difficult to quantify the impact of climate‐induced changes in green leaf phenology on annual GPP. Here, we used 1,110 site‐years of GPP measurements from eddy‐covariance towers in association with time series of satellite LSP observations from 2000 to 2014 to show that while satellite LSP explains a large proportion of variation in annual GPP, changes in green‐leaf‐based growing season length (GSL, leaf development period from spring to autumn) had less impact on annual GPP by ∼30% than GSL changes in GPP‐based photosynthetic duration. Further, maximum leaf greenness explained substantially more variance in annual GPP than green leaf GSL, highlighting the role of future vegetation greening trends on large‐scale carbon budgets. Site‐level variability contributes a substantial proportion of annual GPP variance in the model based on LSP metrics, suggesting the importance of local environmental factors altering regional GPP. We conclude that satellite LSP‐based inferences regarding large‐scale dynamics in GPP need to consider changes in both green leaf GSL and maximum greenness.

Effects of nitrogen reduction combined with organic materials on crop yield, photosynthetic characteristics, and product quality of corn-cabbage rotation system

Nitrogen reduction combined with organic materials is an important measure to achieve or even increase crop yield retention at the background of fertilizer reduction. We conducted a pot experiment to explore the effects of nitrogen reduction combined with organic materials on yield, photosynthetic characteristics, and product quality of agricultural products of maize-cabbage rotation system in yellow soil area of Guizhou. There were five treatments, including no fertilizer (CK), conventional fertilizer (CF), nitrogen reduction (20%, the same below) combined with biochar (RF+B), nitrogen reduction combined with rapeseed cake (RF+O), and nitrogen reduction combined with both biochar and rapeseed cake (RF+BO). Leaf photosynthetic characteristics were measured in maize (seedling stage, jointing stage, heading stage, and mature stage) and cabbage (seedling stage, growing stage and harvest stage). The biological characters, yield and quality indices were investigated in the harvest period. Compared with CF, RF+BO significantly enhanced the yield of corn and cabbage by 9.7% and 39.2%, respectively, while RF+O had no effect, and RF+B did not affet maize yield. RF+BO improved the biological properties of maize and cabbage, including the 100-kernel weight of maize, and plant height, maximum leaf length and total biomass of cabbage. Furthermore, the green holding period and high photosynthetic duration of maize and cabbage were prolonged, among which, maize leaf SPAD was increased respectively by 42.7%, 11.0%, 12.8%, and 30.3% at seedling, jointing, heading, and mature stages, the cabbage leaf SPAD was increased by 13.5%, 9.2%, and 30.3% in seedling, growing and harvest stages, respectively. The net photosynthetic rate (Pn) of maize was increased by 11.1%, 10.9%, and 119.8% in seedling, jointing, and mature stages, while that of cabbage was increased by 12.7% and 14.6% in growing and harvest stages, respectively. The stomatal conductance (gs) of maize was increased by 58.3% and 41.7% in jointing and harvest stages, while that of cabbage was increased by 10%, 64.7%, and 19.2% in seedling, growing, and harvest stages, respectively. The transpiration rate (Tr) of maize was increased by 55.0%, 10.6%, 14.0%, and 143.9% in seedling, jointing, heading, and mature stages, respectively, while that of cabbage was increased by 26.1% in growing stage. The nutritional quality of maize and cabbage was significantly improved. The contents of reducing sugar, starch, and crude protein in maize were increased by 16.2%, 3.5% and 20.3%. The contents of Vc, amino acid, and reducing sugar in cabbage were increased by 26.3%, 21.0% and 27.8%, separately. In conclusion, 20% nitrogen reduction combined with biochar and rapeseed cake had positive effects on crop growth, yield increase, green retention period, high photosynthetic duration, and agricultural product quality improvement in Guizhou yellow soil maize-cabbage rotation system, the overall effect of which was the best. Nitrogen reduction combined with single organic material overally did not affect crop yield, photosynthetic characteristics, and quality.

Improving photosynthesis to increase grain yield potential: an analysis of maize hybrids released in different years in China

In this work, we sought to understand how breeding has affected photosynthesis and to identify key photosynthetic indices that are important for increasing maize yield in the field. Our 2-year (2017–2018) field experiment used five high-yielding hybrid maize cultivars (generated in the 1970s, 2000s, and 2010s) and was conducted in the Xinjiang Autonomous Region of China. We investigated the effects of planting density on maize grain yield, photosynthetic parameters, respiration, and chlorophyll content, under three planting density regimens: 75,000, 105,000, and 135,000 plants ha−1. Our results showed that increasing planting density to the medium level (105,000 plants ha−1) significantly increased grain yield (Y) up to 20.32% compared to the low level (75,000 plants ha−1). However, further increasing planting density to 135,000 plants ha−1 did not lead to an additional increase in yield, with some cultivars actually exhibiting an opposite trend. Interestingly, no significant changes in photosynthetic rate, dark respiration, stomatal density, and aperture were observed upon increasing planting density. Moreover, our experiments revealed a positive correlation between grain yield and the net photosynthetic rate (Pn) upon the hybrid release year. Compared to other cultivars, the higher grain yield obtained in DH618 resulted from a higher 1000-kernel weight (TKW), which can be explained by a longer photosynthetic duration, a higher chlorophyll content, and a lower ratio of chlorophyll a/b. Moreover, we found that a higher leaf area per plant and the leaf area index (HI) do not necessarily result in an improvement in maize yield. Taken together, we demonstrated that higher photosynthetic capacity, longer photosynthetic duration, suitable LAI, and higher chlorophyll content with lower chlorophyll a/b ratio are important factors for obtaining high-yielding maize cultivars and can be used for the improvement of maize crop yield.

Impact of pre-anthesis photosynthetic traits on yield of wheat cultivars under in vivo condition: insight based on biochemical models

Photosynthesis could be improved by increasing photosynthetic rate per unit leaf area and by optimizing light interception and utilization through modified canopy architecture, and also by modifying photosynthetic duration. The yield potential of wheat is determined by the accumulated photosynthates over the entire growing season or the fraction of the total biomass partitioned into grain at harvest. Significant correlation between grain yield and net photosynthesis rate was found under controlled environmental condition (in vitro) but similar relationship under open field conditions is not established. However, phenotyping of photosynthetic traits based on biochemical models can provide accurate physiological information, and thus connecting those traits with yield at crop level would only make crop improvement feasible involving photosynthetic manipulations. At this perspective, the present investigation was undertaken with ten diverse wheat genotypes to characterize pre-anthesis photosynthetic traits and its possible relationship with wheat productivity under in vivo condition. Genotypes were found to have high natural variations for pre-anthesis photosynthetic traits under in vivo condition. Genotypes like DPW-621-50, K 0307, HW 2044 were having high value of Vcmax, Jmax and Cc and thus expected to have better photosynthetic efficiency under ambient atmospheric CO2 condition. Rubisco specificity factor (τ) was found high in KRL-210, HD 2009, KRL-19 and thus expected to have better carboxylase activity under normal condition. Maximum genetic distance was recorded between KRL-210 and DPW-621-50. Rubisco specificity factor (τ) was found as highly correlated with fraction of photorespiration (F) and chloroplast CO2 concentration (Cc). Positive correlation of Cc with fraction of photorespiration (F), mesophyll conductance (gm) and Rubisco specificity factor (τ) indicated high value of CO2 compensation point (Г*). However, correlation between flag leaf photosynthesis rate and grain or biological yield was found non-significant in the present wheat genotypes.

Maize/peanut intercropping increases photosynthetic characteristics, 13C-photosynthate distribution, and grain yield of summer maize

Intercropping is used widely by smallholder farmers in developing countries to increase land productivity and profitability. We conducted a maize/peanut intercropping experiment in the 2015 and 2016 growing seasons in Shandong, China. Treatments included sole maize (SM), sole peanut (SP), and an intercrop consisting of four rows of maize and six rows of peanut (IM and IP). The results showed that the intercropping system had yield advantages based on the land equivalent ratio (LER) values of 1.15 and 1.16 in the two years, respectively. Averaged over the two years, the yield of maize in the intercropping was increased by 61.05% compared to that in SM, while the pod yield of peanut was decreased by 31.80% compared to SP. Maize was the superior competitor when intercropped with peanut, and its productivity dominated the yield of the intercropping system in our study. The increased yield was due to a higher kernel number per ear (KNE). Intercropping increased the light transmission ratio (LTR) of the ear layer in the maize canopy, the active photosynthetic duration (APD), and the harvest index (HI) compared to SM. In addition, intercropping promoted the ratio of dry matter accumulation after silking and the distribution of 13C-photosynthates to grain compared to SM. In conclusion, maize/peanut intercropping demonstrated the potential to improve the light condition of maize, achieving enhanced photosynthetic characteristics that improved female spike differentiation, reduced barrenness, and increased KNE. Moreover, dry matter accumulation and 13C-photosynthates distribution to grain of intercropped maize were improved, and a higher grain yield was ultimately obtained.

Response of biomass accumulation in wheat to low-temperature stress at jointing and booting stages

Climate change has resulted in a continuous increase in the frequency, intensity, and duration of extreme low-temperature events and poses a serious threat to wheat production in China. To better understand the effects of low temperature on wheat photosynthetic production and yield formation, two-year temperature controlled experiments in phytotrons were performed with two different cold-sensitive winter wheat cultivars at five daily maximum/minimum temperature levels and three temperature durations—2, 4 and 6 days—at both jointing and booting stages. Except for its effect on mean net assimilation rate (MNAR) at jointing, low temperature level had significant negative effects on mean leaf area index (MLAI), mean net assimilation rate (MNAR), harvest index (HI), biomass per plant (BPPM) and grain yield per plant (GYPP), while photosynthetic duration (D) (except for Yangmai16 at jointing) was significantly positively affected. Moreover, low temperature duration and its interaction with low temperature level had negative effects on MLAI, MNAR, HI, BPPM and GYPP, but only the effects of low temperature duration on HI, BPPM and GYPP and the interaction effect on BPPM and GYPP reached a significant level. In addition, BPPM and GYPP were more sensitive to low temperature at booting than at jointing stage. Furthermore, significant negative linear relationships were observed between the accumulated cold degree days (ACDD) and MLAI, MNAR, HI, BPPM and GYPP, while a significant positive linear relationship was observed between ACDD and D in both cultivars. The contribution of BPPM to the variation of GYPP was greater than that of HI in both cultivars. However, for biomass, from the first day of low-temperature treatment to maturity (BPPT-M), the major variation was caused by MLAI when low temperature occurred at jointing and by MNAR when low temperature occurred at booting. These results could support the improvement of crop model algorithms under cold stress and assist in the wheat breeding with higher cold tolerance.

Expression of a rice soluble starch synthase gene in transgenic wheat improves the grain yield under heat stress conditions

Wheat (Triticum aestivum L.) is a temperate cereal with an optimum temperature range of 15–22°C during the grain filling stage. Heat stress is one of the major environmental constraints for wheat production worldwide. Temperatures above 25°C during the grain filling stage significantly reduced wheat yield and quality. This reduction was reported due to the inactivation of the soluble starch synthase, a key heat-labile enzyme in starch transformation of wheat endosperm. To improve wheat productivity under heat stress, the rice soluble starch synthase I, under the control of either a constitutive promoter or an endosperm-specific promoter, was expressed in wheat and the transgenic lines were monitored for expression and the effects on yield-related traits. The results showed that the transgenic wheat events expressed rice soluble starch synthase I at a high level after four generations, and transgenic plants produced grains of greater weight during heat stress. Under heat stress conditions, the thousand kernel weight increased 21–34% in T2 and T3 transgenic plants compared to the non-transgenic control plants. In addition, the photosynthetic duration of transgenic wheat was longer than in non-transgenic controls. This study demonstrated that the engineering of a heat tolerant soluble starch synthase gene can be a potential strategy to improve wheat yield under heat stress conditions.

Photosynthetic duration, carboxylation efficiency and stomatal limitation of sun and shade leaves of different ages in field-grown grapevine ( Vitis vinifera L.)

The relationship of photosynthesis (A) of grapevine ( Vitis vinifera L.) sun and shade leaves of primary and secondary (lateral) shoots to insertion level was investigated over two seasons in the field. The leaf plastochron index (LPI) was used to denote leaf position on the shoot. Additionally, laboratory and field measurements of the response of A to CO 2 were conducted. An empirical model was developed to estimate carboxylation efficiency (CE) and stomatal limitations (1) of A. In sun leaves, the relationship of A to LPI changed little until the end of the season (October), whereas stomatal conductance (g) and the intercellular partial pressure of CO 2 (c i ) increased. Leaves acclimated to low light and leaves older than LPI 5 had 30 % lower A and were operating at a slightly higher ci as sun leaves. During September and October, lateral leaves had highest rates of CO 2 assimilation and CE. In mid-October, A and g decreased rapidly and simultaneously for all leaf types, leaf positions and both treatments (sun and shade). Photosynthesis responded similar to individual leaf age as to leaf position. A was linearly related to c i up to non-limiting conductances for sun and shade leaves, for all ages and at all times during the season. The CE and 1 were highest at the beginning of the season and strongly dependent on leaf position. Stomatal limitation declined continuously from about 55 % at the beginning to about 23 and 18 % for sun and shade leaves, respectively, at the end of the season.

Lower Canopy Temperature is Associated with Higher Cytokinin Concentration in the Flag Leaf of Wheat

ABSTRACTCanopy temperature difference (CTD) is used to select wheat (Triticumaestivum L.) varieties tolerant to drought and heat stresses. As CTD reflects the stomatal conductance and transpiration of wheat leaves, and root‐synthesized cytokinin has been shown to be distributed into the aerial parts by the transpiration stream, we therefore investigated the associations of CTD with trans‐zeatin (Z) and trans‐zeatin riboside (ZR) concentration and tolerance of a leaf to high light and high temperature (HLHT) stresses in the flag leaf of different wheat genotypes in field trials. The measurements at 14 d postanthesis (DPA) in 2009 and at 7, 14, 21, and 28 DPA in 2010 showed that Z plus ZR level increased as canopy temperature decreased. This could be at least partially explained by the fact that the flag leaves of low canopy temperature (LT) genotypes received higher delivery rates per unit leaf area of Z plus ZR from the roots and had lower cytokinin oxidase activities than those of high canopy temperature (HT) genotypes. The flag leaves of LT genotypes also exhibited longer photosynthetic durations, higher activities of antioxidant enzymes, and stronger tolerance to HLHT stresses than those of HT genotypes, especially at the late grain filling stage. Hence, CTD shows the potential to be an index for selecting wheat with high cytokinin level of the flag leaf favorable for longer photosynthetic duration and stronger tolerance to HLHT stresses.

Growth Analysis of Biomass Production in Sole‐Crop and Double‐Crop Corn Systems

ABSTRACTIncreased biomass productivity could be achieved through double‐cropping if extended growth duration could be realized with minimal reductions in growth efficiency relative to sole‐cropping. To test this hypothesis, functional growth analysis was used to assess the relative importance of photosynthetic duration and efficiency in determining biomass production by sole‐crop corn (Zea mays L.; SC) and double‐crop triticale (×Triticosecale Wittmack)–double‐crop corn (DT–DC). Aboveground dry matter and leaf area were measured weekly, and net dry matter production was assessed for each crop at harvest. Over 2 yr, average harvested dry matter was 25% greater for DT–DC (22.7 Mg ha−1) than for SC (18.2 Mg ha−1), despite greater maximum leaf area index and greater maximum crop growth rate for SC relative to DT–DC. Leaf area duration was increased by 23% for DT–DC compared with SC, while maximum net assimilation rate and seasonal net assimilation rate did not differ between cropping systems. Across systems, variation in yield was positively related to maximum crop growth rate, maximum leaf area index, and leaf area duration but was not associated with maximum or seasonal net‐assimilation rate. Therefore, leaf duration was more important than leaf efficiency in determining productivity in both cropping systems, and greater biomass yield for DT–DC was the outcome of photosynthesis occurring over an extended duration. These results suggest that potential exists to increase biomass productivity by expanding the seasonal interval of photosynthesis, and that in the case of double‐cropping, expansion of leaf duration is not necessarily associated with reductions in leaf efficiency.

Effect of intercropping summer and spring maize on photosynthesisand yield of summer maize

Three cultivars of maize (the early maturity cultivar of "Luyuandan14", the medium maturity cultivar of "Yedan22" and the late maturity cultivar of "Yedan13") were selected to study the effect of maize cultivars on the photosynthetic characteristics and yield of summer maize intercropped with spring maize and monocropped one. The results show that late maturity spring maize cultivar reduces leaf area, light interception rate, chlorophyll content, nitrate reductase activity, and the rate of photosynthesis of summer maize after anthesis under intercropping. To further analyze the relationship between photosynthetic characteristics and the yield of summer maize, the concept of "photosynthetic productivity product (PPP)" was developed. PPP is the product of two or more factors including photosynthetic leaf area, photosynthetic capacity and photosynthetic duration. Compared with single photosynthetic character, PPP is more significantly correlated with yield. Furthermore, the arithmetic product of leaf area, ear leaf chlorophyll contentand days after anthesis, and that of leaf area, photosynthetic rate of ear leaf and days after anthesis are more closely related to yield than the arithmetic product of leaf area and days after anthesis. In conclusion therefore, the differences in yield under different cropping systems are the common actions of photosynthetic leaf area, capacity and duration. Thus medium maturity cultivars of spring maize and late maturity cultivars of summer maize are beneficial to total yield under intercropping. PPP provides a new and efficient analytical dimension for yield and quality superiority in intercropping.

Elevated CO<sub>2</sub> concentration enhances the role of the ear to the flag leaf in determining grain yield of wheat

Net photosynthetic rate (P N) of ear and flag leaf during grain filling stage and grain yield of plants with non-darkened or darkened flag leaf or darkened ear were examined in two different CO2 concentrations: ambient (AC) and AC+200 µmol mol−1 (EC). Ear showed much higher enhancement (56 %) of P N than flag leaf (23 %) under EC. Moreover, CO2 enrichment shortened the photosynthetic duration of flag leaf relative to ear. In this way the ratio of ear to flag leaf contribution to grain yield increased from 1.18 (AC) to 1.39 (EC).

Higher salinity tolerance cultivars of winter wheat relieved senescence at reproductive stage

In order to probe into the potential of relieving senescence in the new cultivars, we have compared gas exchanges and related physiological parameters of two cultivars of winter wheat (DK961, salt-tolerant; JN17, salt-sensitive) under a series of NaCl concentrations. Through out the whole reproductive period, it was noted that net photosynthetic rate, stomatal conductance, pigment contents, ions contents, leaf area index, leaf area duration, leaf relative water content and dry matter accumulation of spikes decreased in both cultivars with saline concentrations increasing. However, the salt-tolerant cultivars showed none significant reductions in those parameters compared with control under 0.3% and 0.5% salt concentrations, with only considerable decrease happening when soil salt concentration exceeded to 0.7%. Sharply contrast to salt-tolerant cultivars, the salt-sensitive cultivars appeared linear reductions in physiological parameters under a series of salt concentrations (0.3%, 0.5% and 0.7%), with the photosynthetic duration being evidently shorter. Significantly positive correlations among K +/Na +, reproductive growth period and total growth period were noted in salt-sensitive cultivars, however, none significant relations appeared among those parameters in salt-tolerant cultivars, indicating a strong tolerant behavior happened. Our result suggested that higher salinity tolerance cultivars of winter wheat could relieve senescence at the reproductive stage.

Carbon Remobilization and Grain Filling in Japonica/Indica Hybrid Rice Subjected to Postanthesis Water Deficits

Poor grain filling, as in unfilled grains, and unused carbohydrate in straws are two major problems in japonica/indica (J/I) hybrid rice (Oryza sativa L.). This study investigated if an early senescence, induced by controlled postanthesis soil drying, could enhance the remobilization of reserved C to the grains, and therefore improve the grain filling. Two J/I hybrids and one indica/indica (I/I) hybrid were field‐grown. Three levels of soil water potential (ψsoil)—well watered (ψsoil = 0), moderate water deficit (ψsoil = −0.025 MPa), and severe water deficit (ψsoil = −0.05 MPa)—were imposed 9 d after anthesis to maturity. As a result of water deficit treatment, chlorophyll concentrations and photosynthetic rates of flag leaves declined more quickly, indicating an enhanced senescence. The remobilized C reserve from the culm and sheath during grain filling was increased by 47 to 61% for the J/I hybrids and 12 to 26% for the I/I hybrid. Partitioning of 14C from the flag leaves to the grains was increased by 18 to 28%. Active grain‐filling period was shortened by 2.7 to 8.7 d, and grain‐filling rate increased by 0.14 to 0.36 mg d−1 grain−1. Water deficits reduced the grain yield of the I/I hybrid by 2.9 to 16.4% but increased that of the two J/I hybrids by 4.4 to 13.3%. We conclude that if water deficit is properly controlled for J/I, the gain from enhanced remobilization of stored C may outweigh the loss due to shortened photosynthetic duration, leading to a fast grain filling and high grain yield in cases where senescence is unfavorably delayed.

Effect of iron and zinc fertilization on senescence in French bean (phaseolus vulgaris L.)

Abstract In greenhouse experiments, applications of Fe and/or Zn in the form of ferrous sulphate and zinc sulphate induced increased chlorophyll ‘a’ and ‘b’ concentrations, indole‐3‐acetic acid, nitrate reductase activity, and dry matter yield of french bean (Phaseoius vulgaris L.) plants. Iron and Zn in combination were more effective in delaying the leaf senescence compared to controls. Chlorophyll destruction was more rapid at the post‐flowering stage in control plants which led to early senescence and reduced photosynthetic duration. A rapid decline in nitrate reductase activity and indole‐3‐acetic acid concentration in control plants after the 50‐day growth stage further induced senescence. Iron alone and in combination of Zn appeared to contribute to the inhibition of senescence in french bean plants.

Diurnal Pattern of Translocation and Carbohydrate Metabolism in Source Leaves of Beta vulgaris L.

Transitions in carbohydrate metabolism and translocation rate were studied for evidence of control of export by the sugar beet (Beta vulgaris L. Klein E.) source leaf. Steady-state labeling was carried out for two consecutive 14-hour light periods and various quantities related to translocation were measured throughout two 24-hour periods. Starch accumulation following illumination was delayed. Near the end of the light period, starch stopped accumulating, whereas photosynthesis rate and sucrose level remained unchanged. At the beginning of the dark period there was a 75-minute delay before starch was mobilized. The rate of import to the developing sink leaves at night was similar to that during the day, whereas export decreased considerably at night.Starch accumulation and degradation seemed to be initiated in response to the level of illumination. Cessation of starch accumulation before the end of the light period was initiated endogenously. Exogenous control appeared to be mediated by the level of sucrose in the source leaf while endogenous control seemed to be keyed to photoperiod or photosynthetic duration.