Limitations of Gas Exchanges in Intact Leaves during Ontogeny of Field-grown Chickpeas

The ontogenic changes in several component processes of photosynthesiswere measured in chickpeas. Gas exchange characteristics ofintact leaves were studied to analyse the effects of ambientconditions under which chickpeas are usually grown. The CO₂assimilation rate per unit leaf area remained fairly high duringthe vegetative stage, reaching a peak at early pod-fill anddeclining subsequently throughout pod development. The intercellularCO₂ partial pressure (C₁) remained more or less constant (195µbar) during vegetative growth and the early stages ofseed-filling. With falling RWC and PAR interception, the stomatalconductance declined more rapidly than the CO₂ assimilationrate resulting in a value of C₁ less than that normally existingunder ambient conditions. From the A/C₁-analysis, CO₂ assimilationduring pod-filling appears to be limited by the RuBP-regenerationcapacity because the carboxylation efficiency and in vitro RuBPCaseactivity were initially unaffected. However, as leaves aged,the carboxylation efficiency and in vitro RuBPCase activitydecreased abruptly with increasing leaf temperatures above 30°C, and the C₁ was greater than normally existing values(195 µbar), suggesting an increased mesophyll limitationof photosynthesis. It is suggested that a decline in the CO₂assimilation rate of leaves during pod development and an acceleratedsenescence are induced by adverse ambient conditions, particularlyplant water stress and high leaf temperature. Key words: Cicer arietinum L., gas exchange, photosynthesis, ribulose-1,5-bisphosphate carboxylase     

Muscle and Muscle Fiber Type Transformation In Clawed Crustaceans

SYNOPSIS. The first pair of thoracic limbs in many crustaceansis elaborated into claws in which the principal muscle is thecloser. Changes in the fiber composition of the closer muscleduring claw development, regeneration and reversal are reviewedhere and the hypothesis is advanced that such changes are nerve-dependent.In adult lobsters, Homarus amencanus, the paired claws and closermuscles are bilaterally asymmetric, consisting of a minor orcutter claw with predominantly fast fibers and a small ventralband of slow and a major or crusher claw with 100% slow fibers.Yet in the larval and early juvenile stages the paired clawsand closer muscles are symmetric consisting of a central bandof fast fibers sandwiched by slow. Differentiation into a cutteror crusher muscle during subsequent juvenile development isby appropriate fiber type transformation. Experimental manipulationof the claws or the environment in early juvenile stages whenthe claws are equipotent revealed that the determination ofclaw and closer muscle asymmetry is dependent on the convergenceof neural input from the paired claws: the point of convergencemost likely being the CNS. Bilaterally symmetrical input resultsin the development of paired cutter claws while bilaterallyasymmetric input gives rise to dimorphic, cutter and crusherclaws. In the northern crayfish, Orconectes rusticus, wherethe paired claws are bilaterally similar, the closer muscletransforms its central band of fast fibers to slow, both duringprimary development and regeneration. Whether these fiber typetransformations are nerve-dependent is unknown. In adult snappingshrimps, Alpheus sp., the paired claws and closer muscles areasymmetric: the minor or pincer claw has a central band of fastfibers flanked by slow while the major or snapper claw has 100%slow fibers. Claw reversal occurs with removal of the snapperresulting in the transformation of the existing pincer to asnapper and the regeneration of a new pincer at the old snappersite. Transformation of the closer muscle from pincer to snappertype is by degeneration of the fast fiber band and hypertrophyof the slow fibers. Claw transformation can be either preventedif the pincer nerve is sectioned at the time of snapper removalor promoted if the snapper nerve is sectioned: both resultsimplicating a neural basis for muscle transformation.     

A Senescence Factor and Foliar Abscission in Catharanthus roseus

Abscission-accelerating activity was found in naturally senescentpetioles, and senescent leaves of Catharanthus indicating thepresence of a senescence factor (SF) in these tissues. No SFactivity was observed in non-senescent petioles, or non-senescentleaves at various defined developmental stages. The resultsare discussed in relation to the auxin-auxin balance theoryfor the control of leaf abscission and the auxin-SF balancetheory.