Use of field observations to characterise genotypic flowering responses to photoperiod and temperature: a soyabean exemplar.

Thirty-nine accessions of soyabean [Glycine max (L.) Merrill] and 1 of wild annual soyabean (Glycine soja L.) were sown at two sites in Taiwan in 1989 and 1990 and on six occasions during 1990 at one site in Queensland, Australia. On two of the occasions in Australia additional treatments extended natural daylengths by 0.5 h and 2 h. The number of days from sowing for the first flower to appear on 50% of the plants in each treatment was recorded (f), and from these values the rate of progress towards flowering (1/f) was related to temperature and photoperiod. In photoperiod-insensitive accessions it was confirmed that the rate is linearly related to temperature at least up to about 29°C. In photoperiod-sensitive genotypes this is also the case in shorter daylengths but when the critical photoperiod (P c) is exceeded flowering is delayed. This delay increases with photoperiod until a ceiling photoperiod (P ce) is reached. Between P c and P ce, 1/f is linearly related to both temperature (positive) and photoperiod (negative), but in photoperiods longer than P ce there is no further response to either factor. The resulting triple-intersecting-plane response surface can be defined by six genetically-determined coefficients, the values of which are environment-independent but predict time to flower in any environment, and thus quantify the genotype x environment interaction. By this means the field data were used to characterise the photothermal responses of all 40 accessions. The outcome of this characterisation in conjunction with an analysis of the world-wide range of photothermal environments in which soyabean crops are grown lead to the following conclusions: (1) photoperiod-insensitivity is essential in soyabean crops in temperate latitudes, but such genotypes flower too rapidly for satisfactory yields in the tropics; (2) photoperiod-sensitivity appears to be essential to delay flowering sufficiently to allow adequate biomass accumulation in the warm climates of the tropics; (3) contrary to a widely held view, some degree of photoperiod-sensitivity is also needed in the tropics if crop-duration homeostasis is required where there is variation in sowing dates (this is achieved through a photoperiod-controlled delay in flowering which counteracts the seasonal increase in temperature that is correlated with increase in day-length); and (4) a greater degree of photoperiod-sensitivity is necessary to provide maturity-date homeostasis for variable sowing dates - a valuable attribute in regions of uncertain rainfall. Since the triple-intersecting-plane response model used here also applies to other species, the use of field data to characterise the photothermal responses of other crops is discussed briefly.

Characterization of photothermal flowering responses in maturity isolines of soyabean [Glycine max (L.) Merrill] cv. Clark.

All eight isolines of three maturity genes (E(1)/e(1), E(2)/e(2), and E(3)/e(3)) of soyabean [Glycine max (L.) Merrill] cv. Clark were grown in widely different combinations of photoperiod and temperature. Under the more inductive conditions, i.e. in a warm mean temperature (30 degrees C) when daylengths were less than the critical value (i.e. less than about 13 h), the isolines flowered at similar times (23-24 d). The responses of all isolines to temperature were also similar, if not identical. Increase in daylength above the critical photoperiod progressively delayed flowering until the time taken to flower (f) reached a maximum at the ceiling photoperiod. The relations between the rate of progress towards flowering (1/f) and photoperiod (between the critical and ceiling values) were linear. The coefficient characterizing the slope of the response (photoperiod sensitivity) varied amongst the isolines. These responses could be grouped into three categories of increasing sensitivity: (1) least sensitive, e(1)e(2)e(3), e(1)E(2)e(3), e(1)e(2)E(3); (2) intermediate, E(1)e(2)e(3), e(1)E(2)E(3), and (3) most sensitive, E(1)E(2)e(3), E(1)e(2)E(3), E(1)E(2)E(3). Thus, in the Clark cultivar genetic background, E(1) induces greater photoperiod sensitivity but neither E(2) nor E(3) on their own have any effect. However, both E(2) and E(3) together induce photoperiod sensitivity comparable to that induced by E(1) alone. Furthermore, in addition to this epistasis, either E(2) or E(3) has considerable epistatic effect on E(1), further increasing photoperiod sensitivity. The effects of these genes and their epistasis were also reflected in the extent of the maximum delays to flowering which occur when the ceiling photoperiod is exceeded, and also possibly in earliness in circumstances when photoperiods were below the critical value.

Variation in the durations of the photoperiod-sensitive and photoperiod-insensitive phases of development to flowering among eight maturity isolines of soyabean [Glycine max (L.) Merrill].

In soyabean [Glycine max (L.) Merrill] the period between sowing and flowering is comprised of three successive developmental phases--pre-inductive, inductive and post-inductive--in which the rate of development is affected, respectively, by temperature only, by photoperiod and temperature, and then again by temperature only. A reciprocal-transfer experiment (carried out at a mean temperature of 25 degrees C) in which cohorts of plants were transferred successively between short and long photoperiods and vice-versa showed that eight combinations of three pairs of maturity alleles (E(1)/e(1), E(2)/e(2), E(3)/e(3)) had their greatest effect on the duration of the inductive phase in long days. This phase was increased with the increasing photoperiod sensitivity induced by the different gene combinations, and ranged from about 27 to 54 d according to genotype. In a short day regime (11.5 h d(-1)), less than the critical photoperiod, the duration of the inductive phase was brief-requiring about 11 photoperiodic cycles in the less photoperiod-sensitive genotypes and only about seven cycles in the more sensitive ones. The maturity genes also affected the duration of the two photoperiod-insensitive phases; these durations were positively correlated with the photoperiod-sensitivity potential of the gene combinations. The largest effect was on the pre-inductive phase which varied from 3 to 11 d, while the post-inductive phase varied from about 13 to 18 d. As a consequence of these nonphotoperiodic effects of the maturity genes, even in the most inductive regimes (daylengths less than the critical photoperiod) the time taken to flower by the less photoperiod-sensitive combinations of maturity genes was somewhat less than in the more sensitive combinations-ranging from about 28 to 34 d. The genetic and practical implications of these findings are discussed.

Field evaluation of a model of photothermal flowering responses in a world lentil collection.

A model to predict flowering time in diverse lentil genotypes grown under widely different photothermal conditions was developed in controlled environments. The present study evaluated that model with a world germ plasm collection of 369 accessions using two field environments in Syria and two in Pakistan. Photoperiod alone accounted for 69% of the variance in 1/f, the reciprocal of time (d) from sowing to flower. In contrast, temperature alone did not account for a significant proportion of variation in flowering time due to the exposure of plants to supra-optimal temperatures in the late-sown Syrian trial. With the model mean pre-flowering values of photoperiod and temperature combined additively to account for 90.3% of the variance of 1/f over accessions. The correlation of field-derived estimates of temperature sensitivity of accessions to glass-house-derived estimates was significant at P = 0.05, but the equivalent correlation for estimates of photoperiodic sensitivity was higher at P < 0.01. Flowering in the field was better measured as time from sowing to 50% plants in flower rather than time to first bloom or its node number. Dissemination of the lentil crop following domestication in West Asia to the lower latitudes such as Ethiopia and India has depended on selection for intrinsic earliness and reduced sensitivity to photoperiod. Movement from West Asia to the higher latitudes accompanied by spring sowing has resulted in a modest reduction in photoperiod sensitivity and an increase in temperature sensitivity.

Characterization of responses to temperature and photoperiod for time to flowering in a world lentil collection.

The times from sowing to first flowering (f) of 231 accessions of lentil (Lens culinaris Medik.), comprising germ plasm from eight countries and breeding lines from ICARDA in Syria, were recorded in four glasshouse environments; two photoperiods (16 and 13 h/day) combined with warmer (24°/13°C) and cooler (18°/9°C) day/night temperatures. The linear model 1/f=a+bT + cP (where T is mean diurnal temperature and P is photoperiod) provided an average fit over the 231 accessions of r (2)=0.852. Since there is no interaction term in this linear model, the flowering responses of an accession to temperature and photoperiod are independent. The values of the constants b and c indicate relative responsiveness of rate of progress towards flowering (1/f) to temperature and photoperiod, respectively. Comparison among the 231 accessions showed a weak, but significant, negative correlation between the values of b and c (r=-0.291, P<0.01). Since the proportion of the variance of b not attributed to its linear regression on c was >0.91, we conclude that these phenological responses are under separate control and that there is considerable scope for selection of any combination of sensitivities to temperature and photoperiod in lentil. Just as a large proportion of the variation among accessions in mean time to first flowering was attributed to country of origin, so also was variability in the values of the constants a, b, and c. In particular, sensitivity to photoperiod (i.e., the value of constant c) was dependent upon latitude of origin. Breeding lines from ICARDA were equally variable in a, b, and c as were germ plasm accessions from elsewhere, while the mean values were similar to those of accessions from neighboring Jordan. A single accession of wild lentil (L. culinaris subsp. orientalis) from Turkey showed flowering responses to T and P similar to the mean value of accessions of cultivated lentil from that country. Results from diverse environments for the Argentinian cv Precoz show that the use of this linear model facilitates predictions of time to flowering in any environment (within wide limits) of known mean temperature and photoperiod. The model, then, minimizes the need for multisite evaluations of phenology, since predictions of pre-flowering duration in any environment, and characterization of flowering responses to photoperiod and temperature, can now be achieved by screening germ plasm in a few, carefully selected locations.

Temperature and seed germination.

Temperature can affect the percentage and rate of germination through at least three separate physiological processes. 1. Seeds continuously deteriorate and, unless in the meanwhile they are germinated, they will ultimately die. The rate of deterioration depends mainly on moisture content and temperature. The Q10 for rate of loss of viability in orthodox seeds consistently increases from about 2 at -10 degrees C to about 10 at 70 degrees C. 2. Most seeds are initially dormant. Relatively dry seeds continuously lose dormancy at a rate which is temperature-dependent. Unlike enzyme reactions, the Q10 remains constant over a wide range of temperature at least up to 55 degrees C, and typically has a value in the region of 2.5-3.8. Hydrated seeds respond quite differently: high temperatures generally reinforce dormancy or may even induce it. Low temperatures may also induce dormancy in some circumstances, but in many species they are stimulatory (stratification response), especially within the range -1 degree C to 15 degrees C. Small, dormant, hydrated seeds are usually also stimulated to germinate by alternating temperatures which typically interact strongly and positively with light (and often also with other factors including nitrate ions). The most important attributes of alternating temperatures are amplitude, mean temperature, the relative periods spent above and below the median temperature of the cycle (thermoperiod) and the number of cycles. 3. Once seeds have lost dormancy their rate of germination (reciprocal of the time taken to germinate) shows a positive linear relation between the base temperature (at and below which the rate is zero) and the optimum temperature (at which the rate is maximal); and a negative linear relation between the optimal temperature and the ceiling temperature (at and above which the rate is again zero). The optimum temperature for germination rate is typically higher than that required to achieve maximum percentage germination in partially dormant or partially deteriorated seed populations. None of the sub-cellular mechanisms which underlie any of these temperature relations are understood. Nevertheless, the temperature responses can all be quantified and are fundamental to designing seed stores (especially long term for genetic conservation), prescribing germination test conditions, and understanding seed ecology (especially that required for the control of weeds).