ABSTRACT
Germination responses of wild-type (MM), abscisic acid (ABA)-deficient (sitw), and gibberellin (GA)-deficient (gib-1) mutant tomato (Lycopersicon esculentum Mill. cv Moneymaker) seeds to ABA, GA4+7, reduced water potential ([psi]), and their combinations were analyzed using a population-based threshold model (B.R. Ni and K.J. Bradford [1992] Plant Physiol 98: 1057-1068). Among the three genotypes, sitw seeds germinated rapidly and completely in water, MM seeds germinated more slowly and were partially dormant, and gib-1 seeds did not germinate without exogenous GA4+7. Times to germination were inversely proportional to the differences between the external osmoticum, ABA, or GA4+7 concentrations and the corresponding threshold levels that would either prevent ([psi]b, log[ABAb]) or promote (log[GAb]) germination. The sensitivity of germination to ABA, GA4+7, and [psi] varied widely among individual seeds in the population, resulting in a distribution of germination times. The rapid germination rate of sitw seeds was attributable to their low mean [psi]b (-1.17 MPa). Postharvest dormancy in MM seeds was due to a high mean [psi]b (-0.35 MPa) and a distribution of [psi]b among seeds such that some seeds were unable to germinate even on water. GA4+7 (100 [mu]M) stimulated germination of MM and gib-1 seeds by lowering the mean [psi]b to -0.75 MPa, whereas ABA inhibited germination of MM and sitw seeds by increasing the mean [psi]b. The changes in [psi]b were not due to changes in embryo osmotic potential. Rather, hormonal effects on endosperm weakening opposite the radicle tip apparently determine the threshold [psi] for germination. The analysis demonstrates that ABA- and GA-dependent changes in seed dormancy and germination rates, whether due to endogenous or exogenous growth regulators, are based primarily upon corresponding shifts in the [psi] thresholds for radicle emergence. The [psi] thresholds, in turn, determine both the rate and final extent of germination within the seed population.
ABSTRACT
Comparative responses of net photosynthesis (A) to water stress in woody species from a variety of habitats were studied to assess the relationship between photosynthetic attributes and drought tolerance. Stomatal and nonstomatal limitations to A were compared in three-month-old white oak (Quercus alba L.), post oak (Quercus stellata Wangenh.), sugar maple (Acer saccharum Marsh.), and black walnut (Juglans nigra L.) seedlings during a drying cycle. Relative stomatal limitation of photosynthesis (I) was less than 50% in all species except for Q. stellata seedlings subjected to severe water stress. No significant changes in I were observed in Q. alba and J. nigra before, during, and after drought. In A. saccharum, I was generally low and decreased significantly under water stress. Under well-watered conditions, A was highest in Q. stellata, intermediate in Q. alba, and lower in A. saccharum and J. nigra. High A in well-watered Q. stellata was associated with high stomatal conductance and carboxylation efficiency, whereas low A was associated with low stomatal conductance and carboxylation efficiency in A. saccharum and low stomatal conductance, low carboxylation efficiency, and high CO(2) compensation point in J. nigra. Under severe water stress, A, carboxylation efficiency, and stomatal conductance decreased substantially in all species; however, Q. stellata had the highest carboxylation efficiency and lowest CO(2) compensation point under these conditions. After 5 days at high soil moisture after drought, stomatal and mesophyll components of A in A. saccharum and J. nigra had not recovered to predrought levels, whereas they had completely recovered in Q. stellata and Q. alba. The photosynthetic apparatus, especially mesophyll components, of drought-tolerant Quercus species showed either less inhibition under water stress, superior recovery to predrought capacity, or both. Exposure of the leaves to (14)CO(2) indicated apparent asymmetric stomatal closure for mildly water-stressed seedlings, but not for leaves of well-watered, severely stressed, or rehydrated plants. These results suggest that patchy stomatal closure under mild water stress might be important for water stress-induced inhibition of photosynthesis, but not under the more severe water stress imposed in this study.
ABSTRACT
Mathematical models were developed to characterize the physiological bases of the responses of tomato (Lycopersicon esculentum Mill. cv T5) seed germination to water potential (psi) and abscisic acid (ABA). Using probit analysis, three parameters were derived that can describe the germination time courses of a seed population at different psi or ABA levels. For the response of seed germination to reduced psi, these parameters are the mean base water potential ( psi(b), MPa), the standard deviation of the base water potential among seeds in the population (sigma(psib), MPa), and the "hydrotime constant" (theta(H), MPa.h). For the response to ABA, they are the log of the mean base ABA concentration ([unk]ABA(b), m), the standard deviation of the base ABA concentration among seeds in the population (sigma(ABA) (b), log[m]), and the "ABA-time constant" (theta(ABA), log[m].h). The values of psi(b) and [unk]ABA(b) provide quantitative estimates of the mean sensitivity of germination rate to psi or ABA, whereas sigma(psi) (b) and sigma(ABA) (b) account for the variation in sensitivity among seeds in the population. The time constants, theta(H) and theta(ABA), indicate the extent to which germination rate will be affected by a given change in psi or ABA. Using only these parameters, germination time courses can be predicted with reasonable accuracy at any medium psi according to the equation probit(g) = [psi - (theta(H)/t(g)) - psi(b)]/sigma(psib), or at any ABA concentration according to the equation probit(g) = [log[ABA] - (theta(ABA)/t(g)) - log[[unk]ABA(b)]]/sigma(ABA) (b), where t(g) is the time to radicle emergence of percentage g, and ABA is the ABA concentration (m) in the incubation solution. In the presence of both ABA and reduced psi, the same parameters can be used to predict seed germination time courses based upon strictly additive effects of psi and ABA in delaying the time of radicle emergence. Further analysis indicates that ABA and psi can act both independently and interactively to influence physiological processes preparatory for radicle growth, such as the accumulation of osmotic solutes in the embryo. The models provide quantitative values for the sensitivity of germination to ABA or psi, allow evaluation of independent and interactive effects of the two factors, and have implications for understanding how ABA and psi may regulate growth and development.
ABSTRACT
Responses of net photosynthesis (A), leaf conductance to water vapor (g(wv)) and instantaneous water use efficiency (WUE) to decreasing leaf and soil water potentials (Psi(l), Psi(s)) were studied in three-month-old white oak (Quercus alba L.), post oak (Q. stellata Wangenh.), sugar maple (Acer saccharum Marsh.), and black walnut (Juglans nigra L.) seedlings. Quercus seedlings had the highest A and g(wv) when plants were well watered. As the soil was allowed to dry, both A and g(wv) decreased; however, trace amounts of A were observed at a Psi(l) as low as -2.9 MPa in Q. stellata and -2.6 MPa in Q. alba and A. saccharum. Photosynthesis was not measurable at Psi(l) lower than -2.2 MPa in J. nigra and water stress-induced leaflet senescence was observed in this species. Within each species, g(wv) showed a similar relationship to soil and leaf Psi, but the response to Psi(l) was shifted to more negative values by 1.2 to 1.6 MPa. As Psi(s) declined below -1 MPa, the difference between soil and leaf Psi diminished because of the suppression of transpiration. There was no indication that Psi(s) had a more direct influence on g(wv) than did Psi(l). Water use efficiency showed an initial increase as the soil dried, followed by a decline under severe water stress. Water use efficiency was highest in J. nigra, intermediate in Quercus species and lowest in A. saccharum. There was an evident relationship between gas exchange characteristics and natural distribution in these species, with the more xeric species showing higher A and g(wv) under both well-watered and water-stressed conditions. There was no trend toward increased efficiency of water use in the more xeric species.
